One possible signature of a Higgs boson from a simulated collision between two protons. It decays almost immediately into two jets of hadrons and two electrons, visible as lines.Note 1
|Status||A Higgs boson of mass ~ 125 GeV has been tentatively confirmed by CERN on 14 March 2013,123 although unclear as yet which model the particle best supports or whether multiple Higgs bosons exist.2
(See: Current status)
|Theorised||R. Brout, F. Englert, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble (1964)|
|Discovered||Previously unknown boson confirmed to exist on 4 July 2012, by the ATLAS and CMS teams at the Large Hadron Collider; tentatively confirmed as a Higgs boson of some kind on 14 March 2013 (see above).|
|Mass||125.3 ± 0.4 (stat) ± 0.5 (sys) GeV/c2,4 126.0 ± 0.4 (stat) ± 0.4 (sys) GeV/c25|
|Mean lifetime||1.56×10−22 sNote 2 (predicted in the Standard Model)|
|Decays into||(observed) W and Z bosons, two photons. (Others still being studied)|
|Spin||0 (tentatively confirmed at 125 GeV)1|
|Parity||+1 (tentatively confirmed at 125 GeV)1|
The Higgs boson or Higgs particle is an elementary particle initially theorised in 1964,67 and tentatively confirmed to exist on 14 March 2013.8 The discovery has been called "monumental"910 because it appears to confirm the existence of the Higgs field,1112 which is pivotal to the Standard Model and other theories within particle physics. In this discipline, it explains why some fundamental particles have mass when the symmetries controlling their interactions should require them to be massless, and—linked to this—why the weak force has a much shorter range than the electromagnetic force. Its existence and knowledge of its exact properties are expected to impact scientific knowledge across a range of fields, and should eventually allow physicists to determine whether the Standard Model or a competing theory is more likely to be correct, guide other theories and discoveries in particle physics, and—as with other fundamental discoveries of the past—potentially over time lead to developments in "new" physics,13 and new technologies.1415
This unanswered question in fundamental physics is of such importance1112 that it led to a search of over 40 years for the Higgs boson and finally the construction of one of the most expensive and complex experimental facilities to date, the Large Hadron Collider,16 able to create and study Higgs bosons and related questions. On 4 July 2012, a previously unknown particle with a mass between 125 and 127 GeV/c2 was announced as being detected, which physicists suspected at the time to be the Higgs boson.171018 By March 2013, the particle had been proven to behave, interact and decay in many of the expected ways predicted by the Standard Model, and was also tentatively confirmed to have + parity and zero spin,1 two fundamental criteria of a Higgs boson, making it also the first known scalar particle to be discovered in nature,19 although a number of other properties were not fully proven and some partial results do not yet precisely match those expected; in some cases data is also still awaited or being analyzed.2 As of March 2013 it is still uncertain whether its properties (when eventually known) will exactly match the predictions of the Standard Model, or whether additional Higgs bosons exist as predicted by some theories.3
The Higgs boson is named after Peter Higgs, one of six physicists who, in 1964, proposed the mechanism that suggested the existence of such particle. Although Higgs' name has become ubiquitous in this theory, the resulting electroweak model (the final outcome) involved several researchers between about 1960 and 1972, who each independently developed different parts. In mainstream media the Higgs boson is often referred to as the "God particle," from a 1993 book on the topic; the sobriquet is strongly disliked by many physicists, who regard it as inappropriate sensationalism.2021
In the Standard Model, the Higgs particle is a boson with no spin, electric charge, or color charge. It is also very unstable, decaying into other particles almost immediately. It is a quantum excitation of one of the four components of the Higgs field, constituting a scalar field, with two neutral and two electrically charged components, and forms a complex doublet of the weak isospin SU(2) symmetry. The field has a "Mexican hat" shaped potential with nonzero strength everywhere (including otherwise empty space) which in its vacuum state breaks the weak isospin symmetry of the electroweak interaction. When this happens, three components of the Higgs field are "absorbed" by the SU(2) and U(1) gauge bosons (the "Higgs mechanism") to become the longitudinal components of the now-massive W and Z bosons of the weak force. The remaining electrically neutral component separately couples to other particles known as fermions (via Yukawa couplings), causing these to acquire mass as well. Some versions of the theory predict more than one kind of Higgs fields and bosons. Alternative "Higgsless" models would have been considered if the Higgs boson were not discovered.
|Standard model of particle physics|
In particle physics, elementary particles and forces give rise to the world around us. Nowadays, physicists explain the behaviour of these particles and how they interact using the Standard Model—a widely accepted and "remarkably" accurate22:22 framework based on gauge invariance and symmetries, believed to explain almost everything in the world we see, other than gravity.23
But by around 1960 all attempts to create a gauge invariant theory for two of the four fundamental forces had consistently failed at one crucial point: although gauge invariance seemed extremely important, including it seemed to make any theory of electromagnetism and the weak force go haywire, by demanding that either many particles with mass were massless or that non-existent forces and massless particles had to exist. Scientists had no idea how to get past this point.
Work done on superconductivity and "broken" symmetries around 1960 led physicist Philip Anderson to suggest in 1962 a new kind of solution that might hold the key. In 1964 a theory was created by 3 different groups of researchers, that showed the problems could be resolved if an unusual kind of field existed throughout the universe. It would cause existing particles to acquire mass instead of new massless particles being formed. By 1972 it had been developed into a comprehensive theory and proved capable of giving "sensible" results. Although there was not yet any proof of such a field, calculations consistently gave answers and predictions that were confirmed by experiments, including very accurate predictions of several other particles,Note 3 so scientists began to believe this might be true and to search for proof whether or not a Higgs field exists in nature.
If this field did exist, this would be a monumental discovery for science and human knowledge, and is expected to open doorways to new knowledge in many fields. If not, then other more complicated theories would need to be explored. The easiest proof whether or not the field existed was by searching for a new kind of particle it would have to give off, known as "Higgs bosons" or the "Higgs particle" (after Peter Higgs who first predicted them in 1964). These would be extremely difficult to find, so it was only many years later that experimental technology became sophisticated enough to answer the question.
While several symmetries in nature are spontaneously broken through a form of the Higgs mechanism, in the context of the Standard Model the term "Higgs mechanism" almost always means symmetry breaking of the electroweak field. It is considered proven, but the exact cause has been exceedingly difficult to prove. The Higgs boson's existence would finally after 50 years confirm that the Standard Model is essentially correct and allow further development, while its non-existence would confirm that other theories are needed instead.
A simple explanation – what are the Higgs mechanism, field and boson? Symmetries and forces In the Standard Model, fundamental forces arise from laws of nature called symmetries, and are transmitted by means of particles known as gauge bosons. The symmetry governing the weak force is expected to lead to the weak force having massless gauge bosons, but experiments actually showed it having very massive and short-ranging gauge bosons (now called W and Z bosons).Note 4 Their short range – a result of their mass – makes structures like atoms and stars possible, but explaining their unexpected mass posed an exceedingly difficult problem. Higgs mechanism Some symmetry laws only fully apply under specific conditions.Note 5 This means that circumstances could exist, in theory, where a given symmetry law might not be followed (or 'obeyed'). The Higgs mechanism is an explanation devised in 1964 of why and how gauge bosons for an interaction could be massive despite their governing symmetry, if the conditions for the symmetry were 'broken' by an unusual type of field. Higgs field The Standard Model states that a field (the Higgs field) exists throughout space which breaks certain symmetry laws of the electroweak interaction.Note 6 The field's existence triggers the Higgs mechanism, and therefore the gauge bosons corresponding to these symmetries—those responsible for the weak force—are massive, and consequently have a very short range.Note 4
Some years after the original theory, scientists realised that the same field would also explain, in a different way, why other fundamental constituents of matter (including electrons and quarks) have mass.
Higgs boson The existence of the Higgs field can be proven by searching for a matching particle associated with it, which should also exist—the "Higgs boson". Detecting Higgs bosons would automatically prove the Higgs field exists, and that the Standard Model is essentially correct—the crucial question.1112 If found, further testing should show which version of the theory best matches the results of experiments. In 2012, scientists confirmed that they had found a new particle while searching. They suspect that it may turn out to be the Higgs boson, but it will take time to be certain.1718
Various analogies have also been invented to describe the Higgs field and boson, including analogies with well-known symmetry breaking effects such as the rainbow and prism, electric fields, ripples, and resistance affecting some people moving through crowds or some objects moving through syrup or molasses. Analogies based on simple resistance to motion are inaccurate as the Higgs field does not work by resisting motion.
As yet, there are no known immediate technological benefits of finding the Higgs particle. However observers in both media and science point out that when fundamental discoveries are made about our world, their practical uses can take decades to emerge, but are often world-changing when they do.141525 A common pattern for fundamental discoveries is for practical applications to follow later, once the discovery had been explored further, at which point they become the basis for social change and new technologies.
For example, in the first half of the 20th century nobody expected that quantum mechanics would make possible transistors and microchips, mobile phones and computers, lasers and M.R.I. scanners.26 Radio waves were described by their co-discoverer in 1888 as "an interesting laboratory experiment" with "no useful purpose" whatsoever,27 and are now used in innumerable ways (radar, weather prediction, medicine, television, wireless computing and emergency response), positrons are used in hospital tomography scans, and special and general relativity which explain black holes also enable satellite-based GPS and satellite navigation ("satnav").26 Electric power generation and transmission, motors, and lighting, all stemmed from previous theoretical work on electricity and magnetism; air conditioning and refrigeration resulted from thermodynamics. It is impossible to predict how seemingly esoteric knowledge may affect society in the future.1425
Other observers highlight technological spin-offs from this and related particle physics activities, which have already brought major developments to society. For example, the World Wide Web as used today was created by physicists working in global collaborations on particle experiments at CERN to share their results, and the results of massive amounts of data produced by the Large Hadron Collider have already led to significant advances in distributed and cloud computing, now well established within mainstream services.15
If proven to exist, the Higgs field and evidence of its properties would be extremely significant scientifically, for many reasons. (The Higgs boson's importance is largely that it is able to be examined using existing knowledge and experimental technology, as a way to confirm and study the entire Higgs field theory).1112 Proof that the Higgs field and boson do not exist would also be significant. Its relevance includes:
|Validating the Standard Model, or choosing between extensions and alternatives||Does the Higgs field exist, which fundamentally validates the Standard Model? If it does, then which more advanced extensions are suggested or excluded based upon measurements of its properties? What else can we learn about this fundamental field, now that we have the experimental means to study its behavior and interactions? Alternatively, if the Higgs field doesn't exist, which alternatives and modifications to the Standard Model are likely to be preferred? Will the data suggest an extension, or a completely different approach (such as supersymmetry or string theory)?
Related to this, a belief generally exists among physicists that there is likely to be "new" physics beyond the Standard Model—the Standard Model will at some point be extended or superseded. The Higgs field and related issues present a promising "doorway" to understand better the places where the Standard Model might become inadequate or fail, and could provide considerable evidence guiding researchers into future enhancements or successors.
|Finding how symmetry breaking happens within the electroweak interaction||Below an extremely high temperature, electroweak symmetry breaking causes the electroweak interaction to manifest in part as the short-ranged weak force, which is carried by massive gauge bosons. Without this, the universe we see around us could not exist, because atoms and other structures could not form, and reactions in stars such as our Sun would not occur. But it is not clear how this actually happens in nature. Is the Standard Model correct in its approach, and can it be made more exact with actual experimental measurements? If not the Higgs field, then what is breaking symmetry in its place?|
|Finding how certain particles acquire mass||Electroweak symmetry breaking (due to a Higgs field or otherwise) is believed proven responsible for the masses of fundamental particles such as elementary fermions (including electrons and quarks) and the massive W and Z gauge bosons. Finding how this happens is pivotal to particle physics. But it is not responsible for all the mass we see around us. For example, about 99% of the mass of baryons (composite particles such as the proton and neutron) is due instead to the kinetic energy of quarks and to the energies of (massless) gluons of the strong interaction inside the baryons.28
The Standard Model shows how the energy of the Higgs field and vacuum can manifest, in the right conditions, as the property we call 'mass'. But the Higgs field is not actually "creating" mass miraculously out of nothing (which would violate the law of conservation of energy). In Higgs-based theories, mass is a manifestation of potential energy transferred to the particle during interactions ("coupling") with the Higgs field, which had contained that mass in the form of energy.29
|Evidence whether or not scalar fields exist in nature, and "new" physics||Proof of a scalar field such as the Higgs field would be hard to over estimate: "[The] verification of real scalar fields would be nearly as important as its role in generating mass". 10 Rolf-Dieter Heuer, director general of the LHC project, stated in a 2011 talk on the Higgs field:30
|Insight into cosmic inflation||There has been considerable scientific research on possible links between the Higgs field and the inflaton – a hypothetical field suggested as the explanation for the expansion of space during the first fraction of a second of the universe (known as the "inflationary epoch"). Some theories suggest that a fundamental scalar field might be responsible for this phenomenon; the Higgs field is such a field and therefore has led to papers analysing whether it could also be the inflaton responsible for this exponential expansion of the universe during the Big Bang. Such theories are highly tentative and face significant problems related to unitarity, but may be viable if combined with additional features such as large non-minimal coupling, a Brans-Dicke scalar, or other "new" physics, and have received treatments suggesting that Higgs inflation models are still of interest theoretically.|
|Insight into the nature of the universe, and its possible fates||
For decades, scientific models of our universe have included the possibility that it exists as a long-lived, but not completely stable, sector of space, which could potentially at some time be destroyed upon 'toppling' into a more stable vacuum state.3233343536 If the masses of the Higgs boson and top quark are known more exactly, and the Standard Model provides a correct description of particle physics up to extreme energies of the Planck scale, then it is possible to calculate whether the universe's present vacuum state is stable or merely long-lived.3738 (This was sometimes misreported as the Higgs boson "ending" the universe42). A 125 – 127 GeV Higgs mass seems to be extremely close to the boundary for stability (estimated in 2012 as 123.8 – 135.0 GeV31) but a definitive answer requires much more precise measurements of the top quark's pole mass.31
If measurements of the Higgs boson suggest that our universe lies within a false vacuum of this kind, then it would imply – more than likely in many billions of years43Note 7 – that the universe's forces, particles, and structures could cease to exist as we know them (and be replaced by different ones), if a true vacuum happened to nucleate.43Note 8 It also suggests that the Higgs self-coupling λ and its βλ function could be very close to zero at the Planck scale, with "intriguing" implications, including theories of gravity and Higgs-based inflation.31:218 A future electron-positron collider would be able to provide the precise measurements of the top quark needed for such calculations.31
|Insight into the 'energy of the vacuum'||More speculatively, the Higgs field has also been proposed as the energy of the vacuum, which at the extreme energies of the first moments of the Big Bang caused the universe to be a kind of featureless symmetry of undifferentiated extremely high energy. In this kind of speculation, the single unified field of a Grand Unified Theory is identified as (or modeled upon) the Higgs field, and it is through successive symmetry breakings of the Higgs field or some similar field at phase transitions that the present universe's known forces and fields arise.45|
|Link to the 'cosmological constant' problem||The relationship (if any) between the Higgs field and the presently observed vacuum energy density of the universe has also come under scientific study. As observed, the present vacuum energy density is extremely close to zero, but the energy density expected from the Higgs field, supersymmetry, and other current theories are typically many orders of magnitude larger. It is unclear how these should be reconciled. This cosmological constant problem remains a further major unanswered problem in physics.|
Particle physicists study matter made from fundamental particles whose interactions are mediated by exchange particles known as force carriers. At the beginning of the 1960s a number of these particles had been discovered or proposed, along with theories suggesting how they relate to each other, some of which had already been reformulated as field theories in which the objects of study are not particles and forces, but quantum fields and their symmetries.citation needed However, attempts to unify known fundamental forces such as the electromagnetic force and the weak nuclear force were known to be incomplete. One known omission was that gauge invariant approaches, including non-abelian models such as Yang–Mills theory (1954), which held great promise for unified theories, also seemed to predict known massive particles as massless.46 Goldstone's theorem, relating to continuous symmetries within some theories, also appeared to rule out many obvious solutions,47 since it appeared to show that zero-mass particles would have to also exist that were "simply not seen".48 According to Guralnik, physicists had "no understanding" how these problems could be overcome.48
Particle physicist and mathematician Peter Woit summarised the state of research at the time:
- "Yang and Mills work on non-abelian gauge theory had one huge problem: in perturbation theory it has massless particles which don’t correspond to anything we see. One way of getting rid of this problem is now fairly well-understood, the phenomenon of confinement realized in QCD, where the strong interactions get rid of the massless “gluon” states at long distances. By the very early sixties, people had begun to understand another source of massless particles: spontaneous symmetry breaking of a continuous symmetry. What Philip Anderson realized and worked out in the summer of 1962 was that, when you have both gauge symmetry and spontaneous symmetry breaking, the Nambu-Goldstone massless mode can combine with the massless gauge field modes to produce a physical massive vector field. This is what happens in superconductivity, a subject about which Anderson was (and is) one of the leading experts." [text condensed] 46
The Higgs mechanism is a process by which vector bosons can get rest mass without explicitly breaking gauge invariance, as a byproduct of spontaneous symmetry breaking.4950 The mathematical theory behind spontaneous symmetry breaking was initially conceived and published within particle physics by Yoichiro Nambu in 1960,51 the concept that such a mechanism could offer a possible solution for the "mass problem" was originally suggested in 1962 by Philip Anderson,52:4–553 and Abraham Klein and Benjamin Lee showed in March 1964 that Goldstone's theorem could be avoided this way in at least some non-relativistic cases and speculated it might be possible in truly relativistic cases.
These approaches were quickly developed into a full relativistic model, independently and almost simultaneously, by three groups of physicists: by François Englert and Robert Brout in August 1964;54 by Peter Higgs in October 1964;55 and by Gerald Guralnik, Carl Hagen, and Tom Kibble (GHK) in November 1964.56 Higgs also wrote a short but important57 response published in September 1964 to an objection by Gilbert,58 which showed that if calculating within the radiation gauge, Goldstone's theorem and Gilbert's objection would become inapplicable.Note 9 (Higgs later described Gilbert's objection as prompting his own paper.59) Properties of the model were further considered by Guralnik in 1965,60 by Higgs in 1966,61 by Kibble in 1967,62 and further by GHK in 1967.63 The original three 1964 papers showed that when a gauge theory is combined with an additional field that spontaneously breaks the symmetry, the gauge bosons can consistently acquire a finite mass.495064 In 1967, Steven Weinberg65 and Abdus Salam66 independently showed how a Higgs mechanism could be used to break the electroweak symmetry of Sheldon Glashow's unified model for the weak and electromagnetic interactions67 (itself an extension of work by Schwinger), forming what became the Standard Model of particle physics. Weinberg was the first to observe that this would also provide mass terms for the fermions.68 Note 10
However, the seminal papers on spontaneous breaking of gauge symmetries were at first largely ignored, because it was widely believed that the (non-Abelian gauge) theories in question were a dead-end, and in particular that they could not be renormalised. In 1971–72, Martinus Veltman and Gerard 't Hooft proved renormalisation of Yang-Mills was possible in two papers covering massless, and then massive, fields.68 Their contribution, and others' work on the renormalization group, was eventually "enormously profound and influential",69 but even with all key elements of the eventual theory published there was still almost no wider interest. For example, Coleman found in a study that "essentially no-one paid any attention" to Weinberg's paper prior to 197170 – now the most cited in particle physics71 – and even in 1970 according to Politzer, Glashow's teaching of the weak interaction contained no mention of Weinberg's, Salem's, or Glashow's own work.69 In practice, Politzer states, almost everyone learned of the theory due to physicist Benjamin Lee, who combined the work of Veltman and 't Hooft with insights by others, and popularised the completed theory.69 In this way, from 1971, interest and acceptance "exploded" 69 and the ideas were quickly absorbed in the mainstream.6869
The resulting electroweak theory and Standard Model have correctly predicted (among other discoveries) weak neutral currents, three bosons, the top and charm quarks, and with great precision, the mass and other properties of some of these.Note 3 Many of those involved eventually won Nobel Prizes or other renowned awards. A 1974 paper in Reviews of Modern Physics commented that "while no one doubted the [mathematical] correctness of these arguments, no one quite believed that nature was diabolically clever enough to take advantage of them".72 By 1986 and again in the 1990s it became possible to write that understanding and proving the Higgs sector of the Standard Model was "the central problem today in particle physics." 7374
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The three papers written in 1964 were each recognised as milestone papers during Physical Review Letters's 50th anniversary celebration.64 Their six authors were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.75 (A controversy also arose the same year, because in the event of a Nobel Prize only up to three scientists could be recognised, with six being credited for the papers.76 ) Two of the three PRL papers (by Higgs and by GHK) contained equations for the hypothetical field that eventually would become known as the Higgs field and its hypothetical quantum, the Higgs boson.5556 Higgs's subsequent 1966 paper showed the decay mechanism of the boson; only a massive boson can decay and the decays can prove the mechanism.citation needed
In the paper by Higgs the boson is massive, and in a closing sentence Higgs writes that "an essential feature" of the theory "is the prediction of incomplete multiplets of scalar and vector bosons".55 In the paper by GHK the boson is massless and decoupled from the massive states.56 In reviews dated 2009 and 2011, Guralnik states that in the GHK model the boson is massless only in a lowest-order approximation, but it is not subject to any constraint and acquires mass at higher orders, and adds that the GHK paper was the only one to show that there are no massless Goldstone bosons in the model and to give a complete analysis of the general Higgs mechanism.4877 All three reached similar conclusions, despite their very different approaches: Higgs' paper essentially used classical techniques, Englert and Brout's involved calculating vacuum polarization in perturbation theory around an assumed symmetry-breaking vacuum state, and GHK used operator formalism and conservation laws to explore in depth the ways in which Goldstone's theorem may be worked around.57
In addition to explaining how mass is acquired by vector bosons, the Higgs mechanism also predicts the ratio between the W boson and Z boson masses as well as their couplings with each other and with the Standard Model quarks and leptons.citation needed Subsequently, many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus overwhelmingly confirming that some kind of Higgs mechanism does take place in nature,78 but the exact manner by which it happens has not yet been discovered.citation needed The results of searching for the Higgs boson are expected to provide evidence about how this is realized in nature.citation needed
Gauge invariance is an important property of modern particle theories such as the Standard Model, partly due to its success in other areas of fundamental physics such as electromagnetism and the strong interaction (quantum chromodynamics). However, there were great difficulties in developing gauge theories for the weak nuclear force or a possible unified electroweak interaction. Fermions with a mass term would violate gauge symmetry and therefore cannot be gauge invariant. (This can be seen by examining the Dirac Lagrangian for a fermion in terms of left and right handed components; we find none of the spin-half particles could ever flip helicity as required for mass, so they must be massless.Note 11) W and Z bosons are observed to have mass, but a boson mass term contains terms which clearly depend on the choice of gauge and therefore these masses too cannot be gauge invariant. Therefore it seems that none of the standard model fermions or bosons could "begin" with mass as an inbuilt property except by abandoning gauge invariance. If gauge invariance were to be retained, then these particles had to be acquiring their mass by some other mechanism or interaction. Additionally, whatever was giving these particles their mass, had to not "break" gauge invariance as the basis for other parts of the theories where it worked well, and had to not require or predict unexpected massless particles and long-range forces (seemingly an inevitable consequence of Goldstone's theorem) which did not actually seem to exist in nature.
A solution to all of these overlapping problems came from the discovery of a previously un-noticed borderline case hidden in the mathematics of Goldstone's theorem,Note 9 that under certain conditions it might theoretically be possible for a symmetry to be broken without disrupting gauge invariance and without any new massless particles or forces, and having "sensible" (renormalisable) results mathematically: this became known as the Higgs mechanism.
The Standard Model hypothesizes a field which is responsible for this effect, called the Higgs field (symbol: ), which has the unusual property of a non-zero amplitude in its ground state; i.e., a non-zero vacuum expectation value. It can have this effect because of its unusual "Mexican hat" shaped potential whose lowest "point" is not at its "centre". Below a certain extremely high energy level the existence of this non-zero vacuum expectation spontaneously breaks electroweak gauge symmetry which in turn gives rise to the Higgs mechanism and triggers the acquisition of mass by those particles interacting with the field. This effect occurs because scalar field components of the Higgs field are "absorbed" by the massive bosons as degrees of freedom, and couple to the fermions via Yukawa coupling, thereby producing the expected mass terms. In effect when symmetry breaks under these conditions, the Goldstone bosons that arise interact with the Higgs field (and with other particles capable of interacting with the Higgs field) instead of becoming new massless particles, the intractable problems of both underlying theories "neutralise" each other, and the residual outcome is that elementary particles acquire a consistent mass based on how strongly they interact with the Higgs field. It is the simplest known process capable of giving mass to the gauge bosons while remaining compatible with gauge theories.79 Its quantum would be a scalar boson, known as the Higgs boson.80
In the Standard Model, the Higgs field consists of four components, two neutral ones and two charged component fields. Both of the charged components and one of the neutral fields are Goldstone bosons, which act as the longitudinal third-polarization components of the massive W+, W–, and Z bosons. The quantum of the remaining neutral component corresponds to (and is theoretically realised as) the massive Higgs boson.81 Since the Higgs field is a scalar field (meaning it does not transform under Lorentz transformations), the Higgs boson has no spin. The Higgs boson is also its own antiparticle and is CP-even, and has zero electric and colour charge.82
The Minimal Standard Model does not predict the mass of the Higgs boson.83 If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1019 GeV).84 Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model.85 The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity is violated in certain scattering processes.86
It is also possible, although experimentally difficult, to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has a number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons. Precision measurements of electroweak parameters, such as the Fermi constant and masses of W/Z bosons, can be used to calculate constraints on the mass of the Higgs. As of July 2011, the precision electroweak measurements tell us that the mass of the Higgs boson is likely to be less than about 161 GeV/c2 at 95% confidence level (this upper limit would increase to 185 GeV/c2 if the lower bound of 114.4 GeV/c2 from the LEP-2 direct search is allowed for78). These indirect constraints rely on the assumption that the Standard Model is correct. It may still be possible to discover a Higgs boson above these masses if it is accompanied by other particles beyond those predicted by the Standard Model.87
Vector boson fusion
If Higgs particle theories are correct, then a Higgs particle can be produced much like other particles that are studied, in a particle collider. This involves accelerating a large number of particles to extremely high energies and extremely close to the speed of light, then allowing them to smash together. Protons and lead ions (the bare nuclei of lead atoms) are used at the LHC. In the extreme energies of these collisions, the desired esoteric particles will occasionally be produced and this can be detected and studied; any absence or difference from theoretical expectations can also be used to improve the theory. The relevant particle theory (in this case the Standard Model) will determine the necessary kinds of collisions and detectors. The Standard Model predicts that Higgs bosons could be formed in a number of ways,888990 although the probability of producing a Higgs boson in any collision is always expected to be very small—for example, only 1 Higgs boson per 10 billion collisions in the Large Hadron Collider.Note 12 The most common expected processes for Higgs boson production are:
- Gluon fusion. If the collided particles are hadrons such as the proton or antiproton—as is the case in the LHC and Tevatron—then it is most likely that two of the gluons binding the hadron together collide. The easiest way to produce a Higgs particle is if the two gluons combine to form a loop of virtual quarks. Since the coupling of particles to the Higgs boson is proportional to their mass, this process is more likely for heavy particles. In practice it is enough to consider the contributions of virtual top and bottom quarks (the heaviest quarks). This process is the dominant contribution at the LHC and Tevatron being about ten times more likely than any of the other processes.8889
- Higgs Strahlung. If an elementary fermion collides with an anti-fermion—e.g., a quark with an anti-quark or an electron with a positron—the two can merge to form a virtual W or Z boson which, if it carries sufficient energy, can then emit a Higgs boson. This process was the dominant production mode at the LEP, where an electron and a positron collided to form a virtual Z boson, and it was the second largest contribution for Higgs production at the Tevatron. At the LHC this process is only the third largest, because the LHC collides protons with protons, making a quark-antiquark collision less likely than at the Tevatron.888990
- Weak boson fusion. Another possibility when two (anti-)fermions collide is that the two exchange a virtual W or Z boson, which emits a Higgs boson. The colliding fermions do not need to be the same type. So, for example, an up quark may exchange a Z boson with an anti-down quark. This process is the second most important for the production of Higgs particle at the LHC and LEP.8890
- Top fusion. The final process that is commonly considered is by far the least likely (by two orders of magnitude). This process involves two colliding gluons, which each decay into a heavy quark-antiquark pair. A quark and anti-quark from each pair can then combine to form a Higgs particle.8889
Quantum mechanics predicts that if it is possible for a particle to decay into a set of lighter particles, then it will eventually do so.92 This is also true for the Higgs boson. The likelihood with which this happens depends on a variety of factors including: the difference in mass, the strength of the interactions, etc. Most of these factors are fixed by the Standard Model, except for the mass of the Higgs boson itself. For a Higgs boson with a mass of 126 GeV/c2 the SM predicts a mean life time of about 1.6×10−22 s.Note 2
Since it interacts with all the massive elementary particles of the SM, the Higgs boson has many different processes through which it can decay. Each of these possible processes has its own probability, expressed as the branching ratio; the fraction of the total number decays that follows that process. The SM predicts these branching ratios as a function of the Higgs mass (see plot).
One way that the Higgs can decay is by splitting into a fermion–antifermion pair. As general rule, the Higgs is more likely to decay into heavy fermions than light fermions, because the mass of a fermion is proportional to the strength of its interaction with the Higgs.94 By this logic the most common decay should be into a top–antitop quark pair. However, such a decay is only possible if the Higgs is heavier than ~346 GeV/c2, twice the mass of the top quark. For a Higgs mass of 126 GeV/c2 the SM predicts that the most common decay is into a bottom–antibottom quark pair, which happens 56.1% of the time.93 The second most common fermion decay at that mass is a tau–antitau pair, which happens only about 6% of the time.93
Another possibility is for the Higgs to split into a pair of massive gauge bosons. The most likely possibility is for the Higgs to decay into a pair of W bosons (the light blue line in the plot), which happens about 23.1% of the time for a Higgs boson with a mass of 126 GeV/c2.93 The W bosons can subsequently decay either into a quark and an antiquark or into a charged lepton and a neutrino. However, the decays of W bosons into quarks are difficult to distinguish from the background, and the decays into leptons cannot be fully reconstructed (because neutrinos are impossible to detect in particle collision experiments). A cleaner signal is given by decay into a pair of Z-bosons (which happens about 2.9% of the time for a Higgs with a mass of 126 GeV/c2),93 if each of the bosons subsequently decays into a pair of easy-to-detect charged leptons (electrons or muons).
Decay into massless gauge bosons (i.e., gluons or photons) is also possible, but requires intermediate loop of virtual heavy quarks (top or bottom) or massive gauge bosons.94 The most common such process is the decay into a pair of gluons through a loop of virtual heavy quarks. This process, which is the reverse of the gluon fusion process mentioned above, happens approximately 8.5% of the time for a Higgs boson with a mass of 126 GeV/c2.93 Much rarer is the decay into a pair of photons mediated by a loop of W bosons or heavy quarks, which happens only twice for every thousand decays.93 However, this process is very relevant for experimental searches for the Higgs boson, because the energy and momentum of the photons can be measured very precisely, giving an accurate reconstruction of the mass of the decaying particle.94
The Minimal Standard Model as described above is the simplest known model for the Higgs mechanism with just one Higgs field. However, an extended Higgs sector with additional Higgs particle doublets or triplets is also possible, and many extensions of the Standard Model have this feature. The non-minimal Higgs sector favoured by theory are the two-Higgs-doublet models (2HDM), which predict the existence of a quintet of scalar particles: two CP-even neutral Higgs bosons h0 and H0, a CP-odd neutral Higgs boson A0, and two charged Higgs particles H±. Supersymmetry ("SUSY") also predicts relations between the Higgs-boson masses and the masses of the gauge bosons, and could accommodate a 125 GeV/c2 neutral Higgs boson.
The key method to distinguish between these different models involves study of the particles' interactions ("coupling") and exact decay processes ("branching ratios"), which can be measured and tested experimentally in particle collisions. In the Type-I 2HDM model one Higgs doublet couples to up and down quarks, while the second doublet does not couple to quarks. This model has two interesting limits, in which the lightest Higgs couples to just fermions ("gauge-phobic") or just gauge bosons ("fermiophobic"), but not both. In the Type-II 2HDM model, one Higgs doublet only couples to up-type quarks, the other only couples to down-type quarks.95 The heavily researched Minimal Supersymmetric Standard Model (MSSM) includes a Type-II 2HDM Higgs sector, so it could be disproven by evidence of a Type-I 2HDM Higgs.citation needed
In other models the Higgs scalar is a composite particle. For example, in technicolor the role of the Higgs field is played by strongly bound pairs of fermions called techniquarks. Other models, feature pairs of top quarks (see top quark condensate). In yet other models, there is no Higgs field at all and the electroweak symmetry is broken using extra dimensions.9697
||This section needs attention from an expert in physics.|
The Standard Model leaves the mass of the Higgs boson as a parameter to be measured, rather than a value to be calculated. This is seen as theoretically unsatisfactory, particularly as quantum corrections (related to interactions with virtual particles) should apparently cause the Higgs particle to have a mass immensely higher than that observed, but at the same time the Standard Model requires a mass of the order of 100 to 1000 GeV to ensure unitarity (in this case, to unitarise longitudinal vector boson scattering).98 Reconciling these points appears to require explaining why there is an almost-perfect cancellation resulting in the visible mass of ~ 125 GeV, and it is not clear how to do this. Because the weak force is about 1032 times stronger than gravity, and (linked to this) the Higgs boson's mass is so much less than the Planck mass or the grand unification energy, it appears that either there is some underlying connection or reason for these observations which is unknown and not described by the Standard Model, or some unexplained and extremely precise fine-tuning of parameters – however at present neither of these explanations is proven. This is known as a hierarchy problem.99 More broadly, the hierarchy problem amounts to the worry that a future theory of fundamental particles and interactions should not have excessive fine-tunings or unduly delicate cancellations, and should allow masses of particles such as the Higgs boson to be calculable. The problem is in some ways unique to spin-0 particles (such as the Higgs boson), which can give rise to issues related to quantum corrections that do not affect particles with spin.98 A number of solutions have been proposed, including supersymmetry, conformal solutions and solutions via extra dimensions such as braneworld models.
To produce Higgs bosons, two beams of particles are accelerated to very high energies and allowed to collide within a particle detector. Occasionally, although rarely, a Higgs boson will be created fleetingly as part of the collision byproducts. Because the Higgs boson decays very quickly, particle detectors cannot detect it directly. Instead the detectors register all the decay products (the decay signature) and from the data the decay process is reconstructed. If the observed decay products match a possible decay process (known as a decay channel) of a Higgs boson, this indicates that a Higgs boson may have been created. In practice, many processes may produce similar decay signatures. Fortunately, the Standard Model precisely predicts the likelihood of each of these, and each known process, occurring. So, if the detector detects more decay signatures consistently matching a Higgs boson than would otherwise be expected if Higgs bosons did not exist, then this would be strong evidence that the Higgs boson exists.
Because Higgs boson production in a particle collision is likely to be very rare (1 in 10 billion at the LHC),Note 12 and many other possible collision events can have similar decay signatures, the data of hundreds of trillions of collisions needs to be analysed and must "show the same picture" before a conclusion about the existence of the Higgs boson can be reached. To conclude that a new particle has been found, particle physicists require that the statistical analysis of two independent particle detectors each indicate that there is lesser than a one-in-a-million chance that the observed decay signatures are due to just background random Standard Model events—i.e., that the observed number of events is more than 5 standard deviations (sigma) different from that expected if there was no new particle. More collision data allows better confirmation of the physical properties of any new particle observed, and allows physicists to decide whether it is indeed a Higgs boson as described by the Standard Model or some other hypothetical new particle.
To find the Higgs boson, a powerful particle accelerator was needed, because Higgs bosons might not be seen in lower-energy experiments. The collider needed to have a high luminosity in order to ensure enough collisions were seen for conclusions to be drawn. Finally, advanced computing facilities were needed to process the vast amount of data (25 petabytes per year as at 2012) produced by the collisions.100 For the announcement of 4 July 2012, a new collider known as the Large Hadron Collider was constructed at CERN with a planned eventual collision energy of 14 TeV—over seven times any previous collider—and over 300 trillion (3×1014) LHC proton–proton collisions were analysed by the LHC Computing Grid, the world's largest computing grid (as of 2012), comprising over 170 computing facilities in a worldwide network across 36 countries.100101102
The first extensive search for the Higgs boson was conducted at the Large Electron–Positron Collider (LEP) at CERN in the 1990s. At the end of its service in 2000, LEP had found no conclusive evidence for the Higgs.Note 13 This implied that if the Higgs boson were to exist it would have to be heavier than 114.4 GeV/c2.103
The search continued at Fermilab in the United States, where the Tevatron—the collider that discovered the top quark in 1995—had been upgraded for this purpose. There was no guarantee that the Tevatron would be able to find the Higgs, but it was the only supercollider that was operational since the Large Hadron Collider (LHC) was still under construction and the planned Superconducting Super Collider had been cancelled in 1993 and never completed. The Tevatron was only able to exclude further ranges for the Higgs mass, and was shut down on 30 September 2011 because it no longer could keep up with the LHC. The final analysis of the data excluded the possibility of a Higgs boson with a mass between 147 GeV/c2 and 180 GeV/c2. In addition, there was a small (but not significant) excess of events possibly indicating a Higgs boson with a mass between 115 GeV/c2–140 GeV/c2.104
The Large Hadron Collider at CERN in Switzerland, was designed specifically to be able to either confirm or exclude the existence of the Higgs boson. Built in a 27 km tunnel under the ground near Geneva originally inhabited by LEP, it was designed to collide two beams of protons, initially at energies of 3.5 TeV per beam (7 TeV total), or almost 3.6 times that of the Tevatron, and upgradeable to 2 x 7 TeV (14 TeV total) in future. Theory suggested if the Higgs boson existed, collisions at these energy levels should be able to reveal it. As one of the most complicated scientific instruments ever built, its operational readiness was delayed for 14 months by a magnet quench event nine days after its inaugural tests, caused by a faulty electrical connection that damaged over 50 superconducting magnets and contaminated the vacuum system.105106107
Data collection at the LHC finally commenced in March 2010.108 By December 2011 the two main particle detectors at the LHC, ATLAS and CMS, had narrowed down the mass range where the Higgs could exist to 115–130 GeV. In addition, both experiments were starting to see hints of a new particle that could be the Higgs with a mass around 125 GeV.109110 It was therefore widely expected around the end of 2011, that the LHC would provide sufficient data to either exclude or confirm the finding of a Higgs boson by the end of 2012, when their 2012 collision data (with slightly higher 8 TeV collision energy) had been examined.111112
|Feynman diagrams showing the cleanest channels associated with the Low-Mass, ~125GeV, Higgs Candidate observed by the CMS at the LHC. The dominant production mechanism at this mass involves two gluons from each proton fusing to a Top-quark Loop, which couples strongly to the Higgs Field to produce a Higgs Boson.|
On 22 June 2012 CERN announced an upcoming seminar covering tentative findings for 2012,115116 and shortly afterwards (from around 1 July 2012 according to an analysis of the spreading rumour in social media117) rumours began to spread in the media that this would include a major announcement, but it was unclear whether this would be a stronger signal or a formal discovery.118119 Speculation escalated to a "fevered" pitch when reports emerged that Peter Higgs, who proposed the particle, was to be attending the seminar.120121 On 4 July 2012 both of the CERN experiments announced they had independently made the same discovery:122 CMS of a previously unknown boson with mass 125.3 ± 0.6 GeV/c2113114 and ATLAS of a boson with mass 126.5 GeV/c2.123124 Using the combined analysis of two interaction types (known as 'channels'), both experiments reached a local significance of 5-sigma—or less than a 1 in one million chance of error. When additional channels were taken into account, the CMS significance was reduced to 4.9-sigma.113
The two teams had been working 'blinded' from each other for some timewhen?, meaning they did not discuss their results with each other, providing additional certainty that any common finding was genuine validation of a particle.100 This level of evidence, confirmed independently by two separate teams and experiments, meets the formal level of proof required to announce a confirmed discovery.
On 31 July 2012, the ATLAS collaboration presented additional data analysis on the "observation of a new particle", including data from a third channel, which improved the significance to 5.9-sigma (1 in 588 million chance of being due to random background effects) and mass 126.0 ± 0.4 (stat) ± 0.4 (sys) GeV/c2,5 and CMS improved the significance to 5-sigma and mass 125.3 ± 0.4 (stat) ± 0.5 (sys) GeV/c2.4
As of 2012, observations remained consistent with the observed particle being the Standard Model Higgs boson. The particle decayed into at least some of the predicted channels. Moreover, the production rates and branching ratios for the observed channels broadly matched the predictions by the Standard Model within the experimental uncertainties. However, the experimental uncertainties currently still leave room for alternative explanations, meaning an announcement of the discovery of a Higgs boson would have been premature.94 To allow more opportunity for data collection, the LHC's proposed 2012 shutdown and 2013–14 upgrade were postponed by 7 weeks into 2013.125
In November 2012, in a conference in Kyoto researchers said evidence gathered since July was falling into line with the basic Standard Model more than its alternatives, with a range of results for several interactions matching that theory's predictions.126 Physicist Matt Strassler highlighted "considerable" evidence that the new particle is not a pseudoscalar negative parity particle (consistent with this required finding for a Higgs boson), "evaporation" or lack of increased significance for previous hints of non-Standard Model findings, expected Standard Model interactions with W and Z bosons, absence of "significant new implications" for or against supersymmetry, and in general no significant deviations to date from the results expected of a Standard Model Higgs boson.127 However some kinds of extensions to the Standard Model would also show very similar results;128 so commentators noted that based on other particles that are still being understood long after their discovery, it may take years to be sure, and decades to fully understand the particle that has been found.126127
These findings meant that as of January 2013, scientists were very sure they had found an unknown particle of mass ~ 125 GeV, and had not been misled by experimental error or a chance result. They were also sure, from initial observations, that the new particle was some kind of boson. The behaviours and properties of the particle, so far as examined since July 2012, also seemed quite close to the behaviours expected of a Higgs boson. Even so, it could still have been a Higgs boson or some other unknown boson, since future tests could show behaviours that do not match a Higgs boson, so as of December 2012 CERN still only stated that the new particle was "consistent with" the Higgs boson,1718 and scientists did not yet positively say it was the Higgs boson.129 Despite this, in late 2012, widespread media reports announced (incorrectly) that a Higgs boson had been confirmed during the year.135
In January 2013, CERN director-general Rolf-Dieter Heuer stated that based on data analysis to date, an answer could be possible 'towards' mid-2013,136 and the deputy chair of physics at Brookhaven National Laboratory stated in February 2013 that a "definitive" answer might require "another few years" after the collider's 2015 restart.137 In early March 2013, CERN Research Director Sergio Bertolucci stated that confirming spin-0 was the major remaining requirement to determine whether the particle is at least some kind of Higgs boson.138
On 14 March 2013 CERN confirmed that:
- "CMS and ATLAS have compared a number of options for the spin-parity of this particle, and these all prefer no spin and positive parity [two fundamental criteria of a Higgs boson consistent with the Standard Model]. This, coupled with the measured interactions of the new particle with other particles, strongly indicates that it is a Higgs boson." 1
Requirement How tested / explanation Current status (March 2013) Zero spin Examining decay patterns. Spin-1 had been ruled out at the time of initial discovery by the observed decay to two photons (γ γ), leaving spin-0 and spin-2 as remaining candidates. Spin-0 tentatively confirmed in March 2013.1 + and not - parity Studying the angles at which decay products fly apart. Negative parity was also disfavoured if spin-0 was confirmed.141 Positive parity tentatively confirmed.1139 Decay channels (outcomes of particle decaying) are as predicted The Standard Model predicts the decay patterns of a 125–126 GeV Higgs boson. Are these all being seen, and at the right rates? γ γ, WW and ZZ observed; bb, τ τ not yet confirmed.139 Some branching levels (decay rates) are a little higher than expected in preliminary results,139 in particular H → γ γ which gives a peak at ATLAS a little higher than that seen in 4-lepton decays and at CMS.142 Couples to mass
(ie, interacts with particles that have mass)
Particle physicist Adam Falkowski states that the essential qualities of a Higgs boson are that it is a spin-0 (scalar) particle which also couples to mass (W and Z bosons); proving spin-0 alone is insufficient.140 Couplings to mass strongly evidenced ("At 95% confidence level cV is within 15% of the standard model value cV=1").140 Higher energy results remain consistent After the LHC's 2015 restart at the LHC's full planned energies of 13 – 14 TeV, searches for multiple Higgs particles (as predicted in some theories) and tests targeting other versions of particle theory will take place. These higher energy results must continue to give results consistent with Higgs theories To be studied following LHC upgrade
The name most strongly associated with the particle and field is the Higgs boson143:168 and Higgs field. For some time the particle was known by a combination of its PRL author names, for example the Brout-Englert-Higgs particle, or the Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism, 145 and these are still used at times.57146 Fueled in part by the issue of recognition and a potential shared Nobel Prize,146147 the most appropriate name is still occasionally a topic of debate as at 2012.146 (Higgs himself prefers to call the particle either by an acronym of all those involved, or "the scalar boson", or "the so-called Higgs particle".147)
A considerable amount has been written on how Higgs' name came to be exclusively used. Two main explanations are offered.
Reason Basis of explanation Higgs undertook a step which was either unique, clearer or more explicit in his paper, in formally predicting and examining the particle. Of the PRL papers' authors, only the paper by Higgs explicitly offered as a prediction, that a massive particle would exist, and calculated some of its properties;143:167148 he was therefore "the first to postulate the existence of a massive particle" according to Nature.146 Author Frank Close notes that the paper by GHK was also completed after Higgs and Brout-Englert were published.143:167 However in Higgs' view, Brout and Englert did not explicitly mention the boson since its existence is plainly obvious in their work,52:6 while according to Guralnik the GHK paper was a complete analysis of the entire symmetry breaking mechanism whose mathematical rigour is absent from the other two papers, and a massive particle may exist in some solutions.77:9 Higgs' paper also provided an "especially sharp" statement of the challenge and its solution according to science historian David Kaiser.147 The name was popularised in the 1970s due to its use as a convenient shorthand or because of a mistake in citing. Many accounts (including Higgs' own52:7) credit the "Higgs" name to physicist Benjamin Lee (in Korean: Lee Whi-soh). Lee was a significant populist for the theory in its early stages, and habitually attached the name "Higgs" as a "convenient shorthand" for its components from 1972146149150151152 and in at least one instance from as early as 1966.154143:167 Although Lee clarified in his footnotes that "'Higgs' is an abbreviation for Higgs, Kibble, Guralnik, Hagen, Brout, Englert",152 his use of the term (and perhaps also Steven Weinberg's mistaken cite of Higgs' paper as the first in his seminal 1967 paper143153154) meant that by around 1975–76 others had also begun to use the name 'Higgs' exclusively as a shorthand.155
The Higgs boson is often referred to as the "God particle" by individuals outside the scientific community, from the title of a 1993 book on the Higgs boson and particle physics by Nobel Physics prizewinner and Fermilab director Leon Lederman.22 The book was written in the context of failing US government support for the Superconducting Super Collider,156 a part-constructed titanic157158 competitor to the Large Hadron Collider with planned collision energies of 2 × 20 TeV that was championed by Lederman since its 1983 inception156159160 and shut down in 1993; the book sought in part to promote awareness of the significance and need for such a project in the face of its possible loss of funding.161 While media use of this term may have contributed to wider awareness and interest,162 many scientists feel the name is inappropriate2021163 since it is sensational hyperbole and misleads readers;164 the particle also has nothing to do with God,164 leaves open numerous questions in fundamental physics, and does not explain the ultimate origin of the universe. Higgs, an atheist, was reported to be displeased and stated in a 2008 interview that he found it "embarrassing" because it was "the kind of misuse... which I think might offend some people".164165166 Science writer Ian Sample stated in his 2010 book on the search that the nickname is "universally hate[d]" by physicists and perhaps the "worst derided" in the history of physics, but that (according to Lederman) the publisher rejected all titles mentioning "Higgs" as unimaginative and too unknown.167
Lederman explains his choice with a review of the long human search for knowledge, using an analogy between the impact of the Higgs field on the fundamental symmetries at the Big Bang, and the apparent chaos of structures, particles, forces and interactions that resulted and shaped our present universe, with the biblical story of Babel in which the primordial single language of early Genesis was fragmented into many disparate languages and cultures.168
Today ... we have the standard model, which reduces all of reality to a dozen or so particles and four forces. ... It's a hard-won simplicity [...and...] remarkably accurate. But it is also incomplete and, in fact, internally inconsistent... This boson is so central to the state of physics today, so crucial to our final understanding of the structure of matter, yet so elusive, that I have given it a nickname: the God Particle. Why God Particle? Two reasons. One, the publisher wouldn't let us call it the Goddamn Particle, though that might be a more appropriate title, given its villainous nature and the expense it is causing. And two, there is a connection, of sorts, to another book, a much older one...—Leon M. Lederman and Dick Teresi, The God Particle: If the Universe is the Answer, What is the Question22 p. 22
Lederman whimsically asks whether the Higgs boson was added just to perplex and confound those seeking knowledge of the universe, and whether physicists will be confounded by it as recounted in that story, or ultimately surmount the challenge and understand "how beautiful is the universe [God has] made".169
A renaming competition by British newspaper The Guardian in 2009 resulted in their science correspondent choosing the name "the champagne bottle boson" as the best submission: "The bottom of a champagne bottle is in the shape of the Higgs potential and is often used as an illustration in physics lectures. So it's not an embarrassingly grandiose name, it is memorable, and [it] has some physics connection too."170 The name Higgson was suggested as well, in an opinion piece in the Institute of Physics' online publication physicsworld.com.171
There has been considerable public discussion of analogies and explanations for the Higgs particle and how the field creates mass,172173 including coverage of explanatory attempts in their own right and a competition in 1993 for the best popular explanation by then-UK Minister for Science Sir William Waldegrave174 and articles in newspapers worldwide.
An educational collaboration involving an LCH physicist and a High School Teachers at CERN educator suggests that dispersion of light – responsible for the rainbow and dispersive prism – is a useful analogy for the Higgs field's symmetry breaking and mass-causing effect.175
In a vacuum, light of all colours (or photons of all wavelengths) travels at the same velocity, a symmetrical situation. In some substances such as glass, water or air, this symmetry is broken (See: Photons in matter). The result is that light of different wavelengths appears to have different velocities (as seen from outside). Symmetry breaking
in particle physics
In 'naive' gauge theories, gauge bosons and other fundamental particles are all massless – also a symmetrical situation. In the presence of the Higgs field this symmetry is broken. The result is that particles of different types will have different masses.
Matt Strassler uses electric fields as an analogy:176
Some particles interact with the Higgs field while others don’t. Those particles that feel the Higgs field act as if they have mass. Something similar happens in an electric field – charged objects are pulled around and neutral objects can sail through unaffected. So you can think of the Higgs search as an attempt to make waves in the Higgs field [create Higgs bosons] to prove it’s really there.
The Higgs boson is essentially a ripple in a field said to have emerged at the birth of the universe and to span the cosmos to this day ... The particle is crucial however: it is the smoking gun, the evidence required to show the theory is right.
The Higgs field's effect on particles was famously described by physicist David Miller as akin to a room full of political party workers spread evenly throughout a room: the crowd gravitates to and slows down famous people but does not slow down others.Note 14 He also drew attention to well-known effects in solid state physics where an electron's effective mass can be much greater than usual in the presence of a crystal lattice.178
Analogies based on drag effects, including analogies of "syrup" or "molasses" are also well known, but can be somewhat misleading since they may be understood (incorrectly) as saying that the Higgs field simply resists some particles' motion but not others' – a simple resistive effect could also conflict with Newton's third law.180
There has been considerable discussion of how to allocate the credit if the Higgs boson is proven, made more pointed by its near-certain Nobel prize in future, and the very wide basis of people entitled to consideration. These include a range of theoreticians who made the Higgs mechanism theory possible, the theoreticians of the 1964 PRL papers (including Higgs himself), the theoreticians who derived from these, a working electroweak theory and the Standard Model itself, and also the experimentalists at CERN and other institutions who made possible the proof of the Higgs field and boson in reality. The Nobel prize has a limit of 3 persons to share an award, and some possible winners are already prize holders for other work, or are deceased (the prize is only awarded to persons in their lifetime). Existing prizes for works relating to the Higgs field, boson, or mechanism include:
- Nobel Prize in Physics (1979) – Weinberg and Salam (and a co-creator), for contributions to the theory of the unified weak and electromagnetic interaction between elementary particles 181
- Nobel Prize in Physics (1999) – 't Hooft and Veltman, for elucidating the quantum structure of electroweak interactions in physics 182
- Nobel Prize in Physics (2008) – Nambu (shared), for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics 51
- J. J. Sakurai Prize for Theoretical Particle Physics (2010) – Hagen, Englert, Guralnik, Higgs, Brout, and Kibble, for elucidation of the properties of spontaneous symmetry breaking in four-dimensional relativistic gauge theory and of the mechanism for the consistent generation of vector boson masses 75 (for the 1964 papers described above)
- Wolf Prize (2004) – Englert, Brout, and Higgs
Additionally Physical Review Letters' 50-year review (2008) recognized the 1964 PRL papers and Weinberg's 1967 paper A model of Leptons (the most cited paper in particle physics, as of 2012) "milestone Letters".71
The original 1964 papers' authors have not yet been awarded a Nobel Prize, nor have other theorists and experimentalists, although a further Nobel prize is widely expected to be awarded if predictions regarding the Higgs field and boson eventually prove correct and the Higgs boson's existence is proven.183184
Following reported observation of the Higgs-like particle in July 2012, several Indian media outlets reported on the supposed neglect of credit to Indian physicist Satyendra Nath Bose after whose work in the 1920s the class of particles "bosons" is named,185186 although physicists have described Bose's connection to the discovery as tenuous.187
while the field has charge +1/2 under the weak hypercharge U(1) symmetry (in the convention where the electric charge, Q, the weak isospin, I3, and the weak hypercharge, Y, are related by Q = I3 + Y).188
The Higgs part of the Lagrangian is188
where and are the gauge bosons of the SU(2) and U(1) symmetries, and their respective coupling constants, (where are the Pauli matrices) a complete set generators of the SU(2) symmetry, and and , so that the ground state breaks the SU(2) symmetry (see figure). The ground state of the Higgs field (the bottom of the potential) is degenerate with different ground states related to each other by a SU(2) gauge transformation. It is always possible to pick a gauge such that in the ground state . The expectation value of in the ground state (the vacuum expectation value or vev) is then , where . The measured value of this parameter is ~246 GeV/c2.94 It has units of mass, and is the only free parameter of the Standard Model that is not a dimensionless number. Quadratic terms in and arise, which give masses to the W and Z bosons:188
The quarks and the leptons interact with the Higgs field through Yukawa interaction terms:
where are left-handed and right-handed quarks and leptons of the ith generation, are matrices of Yukawa couplings where h.c. denotes the hermitian conjugate terms. In the symmetry breaking ground state, only the terms containing remain, giving rise to mass terms for the fermions. Rotating the quark and lepton fields to the basis where the matrices of Yukawa couplings are diagonal, one gets
where the masses of the fermions are , and denote the eigenvalues of the Yukawa matrices.188
- Standard Model
- Quantum gauge theory
- Introduction to quantum mechanics
- Noncommutative standard model and noncommutative geometry generally
- Standard Model (mathematical formulation) (and especially Standard Model fields overview and mass terms and the Higgs mechanism)
- Note that such events also occur due to other processes. Detection involves a statistically significant excess of such events at specific energies.
- In the Standard Model, the total decay width of a Higgs boson with a mass of 126 GeV/c2 is predicted to be 4.21×10−3 GeV.93 The mean lifetime is given by .
- The success of the Higgs based electroweak theory and Standard Model is illustrated by their predictions of the mass of two particles later detected: the W boson (predicted mass: 80.390 ± 0.018 GeV, experimental measurement: 80.387 ± 0.019 GeV), and the Z boson (predicted mass: 91.1874 ± 0.0021, experimental measurement: 91.1876 ± 0.0021 GeV). The existence of the Z boson was itself another prediction. Other correct predictions included the weak neutral current, the gluon, and the top and charm quarks, all later proven to exist as the theory said.
- Because the range of a force is inversely proportional to the mass of the particles transmitting it.24 – In the Standard Model, forces are carried by virtual particles, whose movement and interaction is limited by the energy-time uncertainty principle: the more massive a single virtual particle, the greater its energy and therefore the shorter the distance it can travel. This limits the range of its interactions and any force it mediates. The same argument says that massless and near-massless particles can carry long distance forces. (See also: Compton wavelength and Static forces and virtual-particle exchange) Experiments show the weak force only acts over a very short range, implying massive gauge bosons. Their masses have since been confirmed by measurement.
- It is quite common for a law of physics to only hold true under certain assumptions or conditions. For example, Newton's laws of motion only apply at speeds where relativistic effects are negligible, and laws related to conductivity, gases, and classical physics (as opposed to quantum mechanics) may only apply in certain ranges of scale, temperature, pressure, or other conditions.
- Electroweak symmetry is broken by the Higgs field in its lowest energy state, called its "ground state". At high energy levels this does not happen, and the gauge bosons of the weak force would then be expected to be massless.
- The bubble's effects would be expected to propagate across the universe at the speed of light from wherever it occurred. However space is vast – with even the nearest galaxy being over 2 million lightyears from us, and others being many billions of lightyears distant, so the effect of such an event would be unlikely to arise here for billions of years after first occurring.4344
- If the Standard Model is correct, then the particles and forces we observe in our universe exist as they do, because of underlying quantum fields. Quantum fields can have states of differing stability, including 'stable', 'unstable' and 'metastable' states (the latter remain stable unless sufficiently perturbed). If a more stable vacuum state were able to arise, then existing particles and forces would no longer arise as they presently do. Different particles or forces would arise from (and be shaped by) whatever new quantum states arose. The world we know depends upon these particles and forces, so if this happened, everything around us, from subatomic particles to galaxies, and all fundamental forces, would be reconstituted into new fundamental particles and forces and structures. The universe would potentially lose all of its present structures and become inhabited by new ones (depending upon the exact states involved) based upon the same quantum fields.
- Goldstone's theorem only applies to gauges having manifest Lorentz covariance, a condition that took time to become questioned. But the process of quantisation requires a gauge to be fixed and at this point it becomes possible to choose a gauge such as the 'radiation' gauge which is not invariant over time, so that these problems can be avoided.
- A field with the "Mexican hat" potential and has a minimum not at zero but at some non-zero value . By expressing the action in terms of the field (where is a constant independent of position), we find the Yukawa term has a component . Since both g and are constants, this looks exactly like the mass term for a fermion of mass . The field is then the Higgs field.
- In the Standard Model, the mass term arising from the Dirac Lagrangian for any fermion is . This is not invariant under the electroweak symmetry, as can be seen by writing in terms of left and right handed components:
- The example is based on the production rate at the LHC operating at 7 TeV. The total cross-section for producing a Higgs boson at the LHC is about 10 picobarn,88 while the total cross-section for a proton–proton collision is 110 millibarn.91
- Just before LEP's shut down, some events that hinted at a Higgs were observed, but it was not judged significant enough to extend its run and delay construction of the LHC.
- In Miller's analogy, the Higgs field is compared to political party workers spread evenly throughout a room. There will be some people (in Miller's example an anonymous person) who pass through the crowd with ease, paralleling the interaction between the field and particles that do not interact with it, such as massless photons. There will be other people (in Miller's example the British prime minister) who would find their progress being continually slowed by the swarm of admirers crowding around, paralleling the interaction for particles that do interact with the field and by doing so, acquire a finite mass.178179
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[A] Well, actually, they don’t. What they really care about is the Higgs field, because it is so important. [emphasis in original]"
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Q: 'If we don't know the new particle is a Higgs, what do we know about it?' We know it is some kind of boson, says Vivek Sharma of CMS [...]
Q: 'are the CERN scientists just being too cautious? What would be enough evidence to call it a Higgs boson?' As there could be many different kinds of Higgs bosons, there's no straight answer.
[emphasis in original]"
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- Adam Falkowski (writing as 'Jester') (2012-12-13). "Twin Peaks in ATLAS". Résonaances particle physics blog. Retrieved 24 February 2013.
- Close, Frank (2011). The Infinity Puzzle: Quantum Field Theory and the Hunt for an Orderly Universe. Oxford: Oxford University Press. ISBN 978-0-19-959350-7.
- Liu, G. Z.; Cheng, G. (2002). "Extension of the Anderson-Higgs mechanism". Physical Review B 65 (13). doi:10.1103/PhysRevB.65.132513.
- Other names have included: the "Anderson–Higgs" mechanism,144 "Higgs–Kibble" mechanism (by Abdus Salam)143 and "ABEGHHK'tH" mechanism [for Anderson, Brout, Englert, Guralnik, Hagen, Higgs, Kibble and 't Hooft] (by Peter Higgs).143
- Editorial (2012-03-21). "Mass appeal: As physicists close in on the Higgs boson, they should resist calls to change its name". Nature. 483, 374 (7390): 374. doi:10.1038/483374a. Retrieved 21 January 2013.
- Becker, Kate (2012-03-29). "A Higgs by Any Other Name". "NOVA" (PBS) physics. Retrieved 21 January 2013.
- "Frequently Asked Questions: The Higgs!". The Bulletin. CERN. Retrieved 18 July 2012.
- "Rochester's Hagen Sakurai Prize Announcement" (Press release). University of Rochester. 2010.
- C.R. Hagen Sakurai Prize Talk (YouTube). 2010. http://www.youtube.com/watch?v=QrCPrwRBi7E&feature=PlayList&p=BDA16F52CA3C9B1D&playnext_from=PL&index=9.
- Ian Sample (29 May 2009). "Anything but the God particle". Guardian (London)
- Peskin, M. (July 2012). "40 Years of the Higgs Boson". Presentation at SSI 2012. Standford/SSI 2012. pp. 3–5. Retrieved 21 January 2013. "quoting Lee's ICHEP 1972 presentation at Fermilab: "...which is known as the Higgs mechanism..." and "Lee's locution" – his footnoted explanation of this shorthand"
- Weinberg, Steven (2012-05-10). "The Crisis of Big Science". The New York Review of Books (footnote 1). Retrieved 12 February 2013.
- Cho, A (2012-09-14). "Particle physics. Why the 'Higgs'?". Science 337 (6100): 1287. doi:10.1126/science.337.6100.1287. PMID 22984044. Retrieved 12 February 2013. "Lee ... apparently used the term 'Higgs Boson' as early as 1966... but what may have made the term stick is a seminal paper Steven Weinberg...published in 1967...Weinberg acknowledged the mix-up in an essay in the New York Review of Books in May 2012." (See also original article in New York Review of Books153 and Frank Close's 2011 book The Infinity Puzzle143:372 (Book extract) which identified the error)
- Examples of early papers using the term "Higgs boson" include 'A phenomenological profile of the Higgs boson' (Ellis, Gaillard and Nanopoulos, 1976), 'Weak interaction theory and neutral currents' (Bjorken, 1977), and 'Mass of the Higgs boson' (Wienberg, received 1975)
- ASCHENBACH, JOY (1993-12-05). "No Resurrection in Sight for Moribund Super Collider : Science: Global financial partnerships could be the only way to salvage such a project. But some feel that Congress delivered a fatal blow". Los Angeles Times. Retrieved 16 January 2013. "'We have to keep the momentum and optimism and start thinking about international collaboration,' said Leon M. Lederman, the Nobel Prize-winning physicist who was the architect of the super collider plan"
- "A Supercompetition For Illinois". Chicago Tribune. 1986-10-31. Retrieved 16 January 2013. "The SSC, proposed by the U.S. Department of Energy in 1983, is a mind-bending project ... this gigantic laboratory ... this titanic project"
- Diaz, Jesus (2012-12-15). "This Is [The] World's Largest Super Collider That Never Was". Gizmodo. Retrieved 16 January 2013. "...this titanic complex..."
- Abbott, Charles (June 1987). "Illinois Issues journal, June 1987". p. 18. "Lederman, who considers himself an unofficial propagandist for the super collider, said the SSC could reverse the physics brain drain in which bright young physicists have left America to work in Europe and elsewhere." (direct link to article: 
- Kevles, Dan. California Institute of Technology: "Engineering & Science". 58 no. 2 (Winter 1995): 16–25 http://calteches.library.caltech.edu/568/1/ES58.2.1995.pdf
|url=missing title (help). Retrieved 16 January 2013. "Lederman, one of the principal spokesmen for the SSC, was an accomplished high-energy experimentalist who had made Nobel Prize-winning contributions to the development of the Standard Model during the 1960s (although the prize itself did not come until 1988). He was a fixture at congressional hearings on the collider, an unbridled advocate of its merits "
- Calder, Nigel (2005). Magic Universe:A Grand Tour of Modern Science. pp. 369–370. ISBN 9780191622359. "The possibility that the next big machine would create the Higgs became a carrot to dangle in front of funding agencies and politicians. A prominent American physicist, Leon lederman [sic], advertised the Higgs as The God Particle in the title of a book published in 1993 ...Lederman was involved in a campaign to persuade the US government to continue funding the Superconducting Super Collider... the ink was not dry on Lederman's book before the US Congress decided to write off the billions of dollars already spent"
- Alister McGrath, Higgs boson: the particle of faith, The Daily Telegraph, Published 15 December 2011. Retrieved 15 December 2011.
- Sample, Ian (3 March 2009). "Father of the God particle: Portrait of Peter Higgs unveiled". London: The Guardian. Retrieved 24 June 2009.
- Chivers, Tom (2011-12-13). "How the 'God particle' got its name". The Telegraph (London). Retrieved 2012-12-03.
- Key scientist sure "God particle" will be found soon Reuters news story. 7 April 2008.
- "Interview: the man behind the 'God particle'", New Scientist 13 September 2008, pp. 44–5 (original interview in the Guardian: Father of the 'God Particle', June 30, 2008)
- Sample, Ian (2010). Massive: The Hunt for the God Particle. pp. 148–149 and 278–279. ISBN 9781905264957.
- Cole, K. (2000-12-14). "One Thing Is Perfectly Clear: Nothingness Is Perfect". Los Angeles Times. p. 'Science File'. Retrieved 17 January 2013. "Consider the early universe–a state of pure, perfect nothingness; a formless fog of undifferentiated stuff ... 'perfect symmetry' ... What shattered this primordial perfection? One likely culprit is the so-called Higgs field ... Physicist Leon Lederman compares the way the Higgs operates to the biblical story of Babel [whose citizens] all spoke the same language ... Like God, says Lederman, the Higgs differentiated the perfect sameness, confusing everyone (physicists included) ... [Nobel Prizewinner Richard] Feynman wondered why the universe we live in was so obviously askew ... Perhaps, he speculated, total perfection would have been unacceptable to God. And so, just as God shattered the perfection of Babel, 'God made the laws only nearly symmetrical'"
- Lederman, p. 22 et seq:
- "Something we cannot yet detect and which, one might say, has been put there to test and confuse us ... The issue is whether physicists will be confounded by this puzzle or whether, in contrast to the unhappy Babylonians, we will continue to build the tower and, as Einstein put it, 'know the mind of God'."
- "And the Lord said, Behold the people are un-confounding my confounding. And the Lord sighed and said, Go to, let us go down, and there give them the God Particle so that they may see how beautiful is the universe I have made".
- Sample, Ian (12 June 2009). "Higgs competition: Crack open the bubbly, the God particle is dead". The Guardian (London). Retrieved 4 May 2010.
- Gordon, Fraser (5 July 2012). "Introducing the higgson". physicsworld.com. Retrieved 25 August 2012.
- Wolchover, Natalie (2012-07-03). "Higgs Boson Explained: How 'God Particle' Gives Things Mass". Huffington Post. Retrieved 21 January 2013.
- Oliver, Laura (2012-07-04). "Higgs boson: how would you explain it to a seven-year-old?". The Guardian (London). Retrieved 21 January 2013.
- Zimmer, Ben (2012-07-15). "Higgs boson metaphors as clear as molasses". The Boston Globe. Retrieved 21 January 2013.
- "The Higgs particle: an analogy for Physics classroom (section)". www.lhc-closer.es (a collaboration website of LCHb physicist Xabier Vidal and High School Teachers at CERN educator Ramon Manzano). Retrieved 2013-01-09.
- Flam, Faye (2012-07-12). "Finally – A Higgs Boson Story Anyone Can Understand". The Philadelphia Inquirer (philly.com). Retrieved 21 January 2013.
- Sample, Ian (2011-04-28). "How will we know when the Higgs particle has been detected?". The Guardian (London). Retrieved 21 January 2013.
- Miller, David. "A quasi-political Explanation of the Higgs Boson; for Mr Waldegrave, UK Science Minister 1993". Retrieved 10 July 2012.
- Kathryn Grim. "Ten things you may not know about the Higgs boson". Symmetry Magazine. Retrieved 10 July 2012.
- David Goldberg, Associate Professor of Physics, Drexel University (2010-10-17). "What's the Matter with the Higgs Boson?". io9.com "Ask a physicist". Retrieved 21 January 2013.
- The Nobel Prize in Physics 1979 – official Nobel Prize website.
- The Nobel Prize in Physics 1999 – official Nobel Prize website.
- Evans, Robert (27 July 2011). "U.S. team sets end-September target in Higgs chase". Reuters.com. Retrieved 2013-01-19.
- by Mariette le Roux and Laurent Banguet (2012-10-08). "Higgs discovery creates Nobel headache". Cosmosmagazine.com. Retrieved 2013-01-19.
- Daigle, Katy (10 July 2012). "India: Enough about Higgs, let's discuss the boson". AP News. Retrieved 10 July 2012.
- Bal, Hartosh Singh (19 September 2012). "The Bose in the Boson". New York Times. Retrieved 21 September 2012.
- Alikhan, Anvar (16 July 2012). "The Spark In A Crowded Field". Outlook India. Retrieved 10 July 2012.
- Peskin & Schroeder 1995, Chapter 20
- G.S. Guralnik, C.R. Hagen and T.W.B. Kibble (1968). "Broken Symmetries and the Goldstone Theorem". In R.L. Cool and R.E. Marshak. Advances in Physics, Vol. 2. Interscience Publishers. pp. 567–708. ISBN 978-0470170571.
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- P.W. Anderson (1963). "Plasmons, Gauge Invariance, and Mass". Physical Review 130: 439. Bibcode:1963PhRv..130..439A. doi:10.1103/PhysRev.130.439.
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|Wikimedia Commons has media related to: Higgs boson|
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- Hunting the Higgs boson at C.M.S. Experiment, at CERN
- The Higgs boson" by the CERN exploratorium.
- The Atom Smashers, a documentary film about the search for the Higgs boson at Fermilab.
- Collected Articles at the Guardian
- Video (04:38) – CERN Announcement on 4 July 2012, of the discovery of a particle which is suspected will be a Higgs boson.
- Video1 (07:44) + Video2 (07:44) – Higgs Boson Explained by CERN Physicist, Dr. Daniel Whiteson (16 June 2011).
- HowStuffWorks: What exactly is the Higgs boson?
- Carroll, Sean. "Higgs Boson with Sean Carroll". Sixty Symbols. University of Nottingham.
- The story of the Higgs theory by the authors of the PRL papers:
- Higgs, Peter (2010). "My Life as a Boson". Talk given at Kings College, London, Nov 24 2010. Retrieved 17 January 2013. (also: )
- Kibble, Tom (2009). "Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism (history)". Scholarpedia. Retrieved 17 January 2013. (also: )
- Guralnik, Gerald (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A 24 (14): 2601–2627. arXiv:0907.3466. Bibcode:2009IJMPA..24.2601G. doi:10.1142/S0217751X09045431., and Guralnik, Gerald (2011). "The Beginnings of Spontaneous Symmetry Breaking in Particle Physics. Proceedings of the DPF-2011 Conference, Providence, RI, 8–13 August 2011". arXiv:1110.2253v1 physics.hist-ph.
- Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC
- Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC
- Particle Data Group: Review of searches for Higgs bosons.
- 2001, a spacetime odyssey: proceedings of the Inaugural Conference of the Michigan Center for Theoretical Physics : Michigan, USA, 21–25 May 2001, (p.86 – 88), ed. Michael J. Duff, James T. Liu, ISBN 978-981-238-231-3, containing Higgs' story of the Higgs boson.