Theorisation Particle physicists study
matter made from
fundamental particles whose interactions are mediated by exchange particles
gauge bosonsacting 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. However, attempts to produce quantum field models for two of the four known
fundamental forces – the
electromagnetic force and the
weak nuclear force – and then to
unify these interactions, were still unsuccessful. One known problem 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.
Goldstone's theorem, relating to
continuous symmetries within some theories, also appeared to rule out many obvious solutions, since it appeared to show that zero-mass particles known as
Goldstone bosons would also have to exist that simply were "not seen". According to
Guralnik, physicists had "no understanding" how these problems could be overcome. Initially, the mathematical theory behind spontaneous symmetry breaking was conceived and published within particle physics by
Yoichiro Nambu in 1960 (and
somewhat anticipated by
Ernst Stueckelberg in 1938), and the concept that such a mechanism could offer a possible solution for the "mass problem" was originally suggested in 1962 by Philip Anderson, who had previously written papers on broken symmetry and its outcomes in superconductivity. Anderson concluded in his 1963 paper on the Yang–Mills theory, that "considering the superconducting analog ... [t]hese two types of bosons seem capable of canceling each other out ... leaving finite mass bosons"), and in March 1964,
Abraham Klein and
Benjamin Lee showed 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; by
Peter Higgs in October 1964; and by
Gerald Guralnik,
Carl Hagen, and
Tom Kibble (GHK) in November 1964. Higgs also wrote a short, but important, which showed that if calculating within the radiation gauge, Goldstone's theorem and Gilbert's objection would become inapplicable. Higgs later described Gilbert's objection as prompting his own paper. Properties of the model were further considered by Guralnik in 1965, by Higgs in 1966, by Kibble in 1967, and further by GHK in 1967. The original three 1964 papers demonstrated that when a
gauge theory is combined with an additional charged scalar field that spontaneously breaks the symmetry, the gauge bosons may consistently acquire a finite mass. In 1967,
Steven Weinberg and
Abdus Salam 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 interactions, (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. At first, these seminal papers on spontaneous breaking of gauge symmetries were 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.was eventually "enormously profound and influential", 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 1971 and discussed by
David Politzer in his 2004 Nobel speech.and even in 1970 according to Politzer, Glashow's teaching of the weak interaction contained no mention of Weinberg's, Salam's, or Glashow's own work. adding that the theory had so far produced accurate answers that accorded with experiment, but it was unknown whether the theory was fundamentally correct. 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".
Summary and impact of the PRL papers The three papers written in 1964 were each recognised as milestone papers during
Physical Review Letters 50th anniversary celebration. (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.) 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.) In the paper by GHK the boson is massless and decoupled from the massive states. All three reached similar conclusions, despite their very different approaches: Higgs's paper essentially used classical techniques, Englert and Brout's involved calculating vacuum polarisation 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 was avoided. Some versions of the theory predicted more than one kind of Higgs fields and bosons, and alternative
"Higgsless" models were considered until the discovery of the Higgs boson.
Experimental search 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), 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 less than a one-in-a-million chance that the observed decay signatures are due to just background random Standard Model eventsi.e., that the observed number of events is more than five
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 of 2012) produced by the collisions.
Search before 4 July 2012 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. This implied that if the Higgs boson were to exist it would have to be heavier than . The search continued at
Fermilab in the United States, where the
Tevatronthe 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 and . In addition, there was a small (but not significant) excess of events possibly indicating a Higgs boson with a mass between and . 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 per beam (7 TeV total), or almost 3.6 times that of the Tevatron, and upgradeable to (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. Data collection at the LHC finally commenced in March 2010. 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 around (ATLAS) and (CMS). There had also already been a number of promising event excesses that had "evaporated" and proven to be nothing but random fluctuations. However, from around May 2011, both experiments had seen among their results, the slow emergence of a small yet consistent excess of gamma and 4-lepton decay signatures and several other particle decays, all hinting at a new particle at a mass around . It was therefore widely anticipated 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.
Discovery of candidate boson at CERN On 22 June 2012
CERN announced an upcoming seminar covering tentative findings for 2012, and shortly afterwards (from around 1 July 2012 according to an analysis of the spreading rumour in social media) 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. Speculation escalated to a "fevered" pitch when reports emerged that
Peter Higgs, who proposed the particle, was to be attending the seminar, and that "five leading physicists" had been invitedgenerally believed to signify the five living 1964 authorswith Higgs, Englert, Guralnik, Hagen attending and Kibble confirming his invitation (Brout having died in 2011). On 4 July 2012 both of the CERN experiments announced they had independently made the same discovery: CMS of a previously unknown boson with mass and ATLAS of a boson with mass . Using the combined analysis of two interaction types (known as 'channels'), both experiments independently reached a local significance of 5 sigmaimplying that the probability of getting at least as strong a result by chance alone is less than one in three million. When additional channels were taken into account, the CMS significance was reduced to 4.9 sigma. 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 obtaining at least as strong evidence by random background effects alone) and mass ,
New particle tested as a possible Higgs boson Following the 2012 discovery, it was still unconfirmed whether the particle was a Higgs boson. On one hand, observations remained consistent with the observed particle being the Standard Model Higgs boson, and 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 still left room for alternative explanations, meaning an announcement of the discovery of a Higgs boson would have been premature. 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. 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. However some kinds of extensions to the Standard Model would also show very similar results; 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. Despite this, in late 2012, widespread media reports announced (incorrectly) that a Higgs boson had been confirmed during the year. 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, 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. 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.
Confirmation of existence and status On 14 March 2013 CERN confirmed the following: CMS and ATLAS have compared a number of options for the spin-parity of this particle, and these all prefer no spin and even 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.
Findings since 2013 ({g}_{V} the absolute coupling strength). In July 2017, CERN confirmed that all measurements still agree with the predictions of the Standard Model, and called the discovered particle simply "the Higgs boson". The LHC's experimental work since restarting in 2015 has included probing the Higgs field and boson to a greater level of detail, and confirming whether less common predictions were correct. In particular, exploration since 2015 has provided strong evidence of the predicted direct decay into
fermions such as pairs of
bottom quarks (3.6
σ)described as an "important milestone" in understanding its short lifetime and other rare decaysand also to confirm decay into pairs of
tau leptons (5.9
σ). This was described by CERN as being "of paramount importance to establishing the coupling of the Higgs boson to leptons and represents an important step towards measuring its couplings to third generation fermions, the very heavy copies of the electrons and quarks, whose role in nature is a profound mystery". == Theoretical issues ==