The God Particle: Higgs Boson and the Standard Model

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Higgs boson

The Higgs boson or Higgs particle is a proposed elementary particle in the Standard Model of particle physics. The Higgs boson is named after Peter Higgs who, along with two other teams, proposed the mechanism that suggested such a particle in 1964.456 The existence of a Higgs field and its associated Higgs boson would be the simplest7 of several methods to explain why some other elementary particles have mass. According to this theory, certain elementary particles[Note 2] obtain mass by interacting with the Higgs field which has non-zero strength everywhere, even in otherwise empty space. The Higgs boson—the smallest possible excitation of this field—is predicted to exist by the same theory, and as this would be detectable, it has been the target of a long search in particle physics.

One of the primary goals of the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland—the most powerful particle accelerator and one of the most complicated scientific instruments ever built—was to test the existence of the Higgs boson and measure its properties which would allow physicists to confirm this cornerstone of modern theory.

According to the Standard Model, the Higgs particle is a boson, a type of particle that allows multiple identical particles to exist in the same place in the same quantum state. It has no intrinsic spin, no electric charge, and no colour charge. It is also very unstable, decaying into other particles almost immediately. If the Higgs boson were shown not to exist, other “Higgsless” models would be considered. In some variants of the Standard Model there can be multiple Higgs bosons.

Because of its possible role in producing a fundamental property of elementary particles, the Higgs boson has been referred to as the “God particle” in popular culture, although many scientists regard this as a hyperbole.[citation needed]

On 4 July 2012, the CMS and the ATLAS experimental teams at the Large Hadron Collider independently announced that they each confirmed the formal discovery of a previously unknown boson of mass between 125–127 GeV/c2, whose behaviour so far was “consistent with” a Higgs boson, while adding a cautious note that further data and analysis were needed before positively identifying the new particle as being a Higgs boson of some type.

[edit] Overview

The existence of the Higgs boson was predicted in 1964 to explain the Higgs mechanism (sometimes termed in the literature the Brout–Englert–Higgs, BEH or Brout–Englert–Higgs–Hagen–Guralnik–Kibble mechanism after its original proposers8)—the mechanism by which some elementary particles are given mass.[Note 2] While the Higgs mechanism is considered confirmed to exist, the boson itself—a cornerstone of the leading theory—had not been observed and its existence was unconfirmed. Its tentative discovery in July 2012 may validate the Standard Model as essentially correct, as it is the final elementary particle predicted and required by the Standard Model which had not yet been observed via particle physics experiments.9 Alternative sources of the Higgs mechanism that do not need the Higgs boson also are possible and would be considered if the existence of the Higgs boson were to be ruled out. They are known as Higgsless models.

The Higgs boson is named after Peter Higgs, who in 1964 wrote one of three ground-breaking papers alongside the work of Robert Brout and François Englert and Tom Kibble, C. R. Hagen and Gerald Guralnik covering what is now known as the Higgs mechanism and described the related Higgs field and boson.

Technically, it is the quantum excitation of the Higgs field, and the non-zero value of the ground state of this field, that give mass to the other elementary particles, such as quarks and electrons. The Standard Model completely fixes the properties of the Higgs boson, except for its mass. It is expected to have no spin and no electric or colour charge, and it interacts with other particles through the weak interaction and Yukawa-type interactions between the various fermions and the Higgs field.

Because the Higgs boson is a very massive particle and decays almost immediately when created, only a very high-energy particle accelerator can observe and record it. Experiments to confirm and determine the nature of the Higgs boson using the Large Hadron Collider (LHC) at CERN began in early 2010, and were performed at Fermilab’s Tevatron until its close in late 2011. Mathematical consistency of the Standard Model requires that any mechanism capable of generating the masses of elementary particles become visible at energies above 1.4 TeV;10 therefore, the LHC (designed to collide two 7 TeV proton beams, but currently running at 4 TeV each) was built to answer the question of whether or not the Higgs boson exists.11

On 4 July 2012, the two main experiments at the LHC (ATLAS and CMS) both reported independently the confirmed existence of a previously unknown particle with a mass of about 125 GeV/c2 (about 133 proton masses, on the order of 10−25 kg), which is “consistent with the Higgs boson” and widely believed to be the Higgs boson. They cautioned that further work would be needed to confirm that it is indeed the Higgs boson (meaning that it has the theoretically predicted properties of the Higgs boson and is not some other previously unknown particle) and, if so, to determine which version of the Standard Model it best supports.1231213

[edit] General description

In particle physics, elementary particles and forces give rise to the world around us. Physicists explain the behaviours of these particles and how they interact using the Standard Model—a widely accepted framework believed to explain most of the world we see around us.14 Initially, when these models were being developed and tested, it seemed that the mathematics behind those models, which were satisfactory in areas already tested, would also forbid elementary particles from having any mass, which showed clearly that these initial models were incomplete. In 1964 three groups of physicists almost simultaneously released papers describing how masses could be given to these particles, using approaches known as symmetry breaking. This approach allowed the particles to obtain a mass, without breaking other parts of particle physics theory that were already believed reasonably correct. This idea became known as the Higgs mechanism (not the same as the boson), and later experiments confirmed that such a mechanism does exist—but they could not show exactly how it happens.

The leading and simplest theory for how this effect takes place in nature was that if a particular kind of “field” (known as a Higgs field) happened to permeate space, and if it could interact with fundamental particles in a particular way, then this would give rise to a Higgs mechanism in nature, and would therefore create around us the phenomenon we call “mass”. During the 1960s and 1970s the Standard Model of physics was developed on this basis, and it included a prediction and requirement that for these things to be true, there had to be an undiscovered boson—one of the fundamental particles—as the counterpart of this field. This would be the Higgs boson. If the Higgs boson were confirmed to exist, as the Standard Model suggested, then scientists could be satisfied that the Standard Model was fundamentally correct. If the Higgs boson were proved not to exist, then other theories would be considered as candidates instead.

The Standard Model also made clear that the Higgs boson would be very difficult to demonstrate. It exists for only a tiny fraction of a second before breaking up into other particles—so quickly that it cannot be directly detected—and can be detected only by identifying the results of its immediate decay and analysing them to show they were probably created from a Higgs boson and not some other source. The Higgs boson requires so much energy to create (compared to many other fundamental particles) that it also requires a massive particle accelerator to create collisions energetic enough to create it and record the traces of its decay. Given a suitable accelerator and appropriate detectors, scientists can record trillions of particles colliding, analyse the data for collisions likely to be a Higgs boson, and then perform further analysis to test how likely it is that the results combined show a Higgs boson does exist, and that the results are not just due to chance.

Experiments to try to show whether the Higgs boson did or did not exist began in the 1980s, but until the 2000s it could only be said that certain areas were plausible, or ruled out. In 2008 the Large Hadron Collider (LHC) was inaugurated, being the most powerful particle accelerator ever built. It was designed especially for this experiment, and other very-high-energy tests of the Standard Model. In 2010 it began its primary research role: to prove whether or not the Higgs boson exists.

In late 2011 two of the LHC’s experiments independently began to suggest “hints” of a Higgs boson detection around 125 GeV. In July 2012 CERN announced1 evidence of discovery of a boson with an energy level and other properties consistent with those expected in a Higgs boson. Further work is necessary for the evidence to be considered conclusive (or disproved). If the newly discovered particle is indeed the Higgs boson, attention will turn to considering whether its characteristics match one of the extant versions of the Standard Model. The CERN data include clues that additional bosons or similar-mass particles may have been discovered as well as, or instead of, the Higgs itself. If a different boson were confirmed, it would allow and require the development of new theories to supplant the current Standard Model.

[edit] History

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; however, even accepted versions such as the Unified field theory were known to be incomplete. One omission was that they could not explain the origins of mass as a property of matter. Goldstone’s theorem, relating to continuous symmetries within some theories, also appeared to rule out many obvious solutions.15

The Higgs mechanism is a process by which vector bosons can get rest mass[Note 2] without explicitly breaking gauge invariance. The proposal for such a spontaneous symmetry breaking mechanism originally was suggested in 1962 by Philip Warren Anderson16 and developed into a full relativistic model, independently and almost simultaneously, by three groups of physicists: by François Englert and Robert Brout in August 1964;5 by Peter Higgs in October 1964;4 and by Gerald Guralnik, C. R. Hagen, and Tom Kibble (GHK) in November 1964.6 Properties of the model were further considered by Guralnik in 1965 17 and by Higgs in 1966.18 The papers showed that when a gauge theory is combined with an additional field that spontaneously breaks the symmetry group, the gauge bosons can consistently acquire a finite mass. In 1967, Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the breaking of the electroweak symmetry, and showed how a Higgs mechanism could be incorporated into Sheldon Glashow’s electroweak theory,192021 in what became the Standard Model of particle physics.

The three papers written in 1964 were each recognised as milestone papers during Physical Review Letters’s 50th anniversary celebration.22 Their six authors were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.23 (A dispute also arose the same year; in the event of a Nobel Prize up to three scientists would be eligible, with six authors credited for the papers.24 ) 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. 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.

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”. In the paper by GHK the boson is massless and decoupled from the massive states. 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.2526

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. 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,27 but the exact manner by which it happens has not yet been discovered. The results of searching for the Higgs boson are expected to provide evidence about how this is realized in nature.

[edit] Theoretical properties

Main article: Higgs mechanism

The Standard Model predicts the existence of a field, called the Higgs field, which has a non-zero amplitude in its ground state; i.e. a non-zero vacuum expectation value. The existence of this non-zero vacuum expectation spontaneously breaks electroweak gauge symmetry which in turn gives rise to the Higgs mechanism. It is the simplest process capable of giving mass to the gauge bosons while remaining compatible with gauge theories.[citation needed] Its quantum would be a scalar boson, known as the Higgs boson.28 In layman’s terms the Higgs field was famously imagined by physicist David Miller as akin to a room full of political party workers spread evenly throughout a room.2930 An anonymous person who passes through the crowd with ease would be like the interaction between the field and a massless photon. The British prime minister, however, walks around the room flocked by a swarm of admirers and would be more like the interaction for a particle that acquires a finite mass.

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.31 Since the Higgs field is a scalar field, 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.32

The Minimal Standard Model does not predict the mass of the Higgs boson.33 If that mass is between 120 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1018 GeV).34 Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model.[citation needed] 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.[citation needed]

In theory, the mass of the Higgs boson may be estimated 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 constrain the mass of the Higgs. As of July 2011, the precision electroweak measurements tell us that the mass of the Higgs boson is lower than about 161 GeV/c2 at 95% confidence level (CL). This upper bound increases to 185 GeV/c2 when including the LEP-2 direct search lower bound of 114.4 GeV/c2.27 These indirect constraints rely on the assumption that the Standard Model is correct. It may still be possible to discover a Higgs boson above 185 GeV/c2 if it is accompanied by other particles beyond those predicted by the Standard Model.[citation needed]

The Minimal Standard Model as described above contains only one complex isospin Higgs doublet; however, it also is possible to have an extended Higgs sector with additional doublets or triplets. 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±. The key method to distinguish different variations of the 2HDM models and the minimal SM involves their coupling and the branching ratios of the Higgs decays. The so called Type-I model has one Higgs doublet coupling 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 doesn’t couple to either fermions (fermiophobic) or gauge bosons (gauge-phobic). In the 2HDM of Type-II, one Higgs doublet only couples to up-type quarks, while the other only couples to down-type quarks.

Many extensions to the Standard Model, including supersymmetry (SUSY), often contain an extended Higgs sector. Many supersymmetric models predict that the lightest Higgs boson will have a mass only slightly above the current experimental limits, at around 120 GeV/c2 or less.[citation needed] The heavily researched Minimal Supersymmetric Standard Model (MSSM) belongs to the class of models with a Type-II two-Higgs-doublet sector and could be ruled out by the observation of a Higgs belonging to a Type-I 2HDM.

[edit] Alternative mechanisms for electroweak symmetry breaking

Main article: Higgsless model
In the years since the Higgs field and boson were proposed, several alternative models have been proposed by which the Higgs mechanism might be realised. The Higgs boson exists in some, but not all, theories. For example, it exists in the Standard Model and extensions such as the Minimal Supersymmetric Standard Model yet is not expected to exist in alternative models such as Technicolor. Models which do not include a Higgs field or a Higgs boson are known as Higgsless models. In these models, strongly interacting dynamics rather than an additional (Higgs) field produce the non-zero vacuum expectation value that breaks electroweak symmetry. A partial list of these alternative mechanisms are:

Technicolor,35 a class of models that attempts to mimic the dynamics of the strong force as a way of breaking electroweak symmetry.
Extra dimensional Higgsless models where the role of the Higgs field is played by the fifth component of the gauge field.36
Abbott-Farhi models of composite W and Z vector bosons.37
Top quark condensate theory in which a fundamental scalar Higgs field is replaced by a composite field composed of the top quark and its antiquark.
The braid model of Standard Model particles by Sundance Bilson-Thompson, compatible with loop quantum gravity and similar theories.38
A goal of the LHC and Tevatron experiments is to distinguish between these models and determine if the Higgs boson exists or not.

[edit] Experimental search

Like other massive particles (e.g. the top quark and W and Z bosons), Higgs bosons decay to other particles almost immediately, long before they can be observed directly. However, the Standard Model precisely predicts the possible modes of decay and their probabilities. This allows the creation and decay of a Higgs boson to be shown by careful examination of the decay products of collisions. The experimental search therefore commenced in the 1980s with the opening of particle accelerators sufficiently powerful to provide evidence related to the Higgs boson.

Since the Higgs boson was expected to be very massive and hard to detect, and if it existed, it could have any mass in a very wide range, a number of very advanced facilities were eventually required for the search. These included very powerful particle accelerator and detectors (in order to create Higgs bosons and detect their decay, if possible), and processing and analysis of vast amounts of data,39 requiring very large worldwide computing facilities. Ultimately over 300 trillion (3 × 1014) proton-proton collisions at the LHC were analyzed in confirming the particle’s discovery.39 Experimental techniques included examination of a wide range of possible masses (often quoted in GeV) in order to gradually narrow down the search area and rule out possible masses where the Higgs was unlikely, statistical analysis, and operation of multiple experiments and teams in order to see if the results from all were in agreement.

[edit] Exclusion of possible ranges

Prior to the year 2000, data gathered at the Large Electron–Positron Collider (LEP) at CERN had allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of 114.4 GeV/c2 at the 95% confidence level (CL). The same experiment produced a small number of events that could be interpreted as resulting from Higgs bosons with a mass just above this cut off—around 115 GeV—but the number of events was insufficient to draw definite conclusions.40 The LEP was shut down in 2000 due to construction of its successor, the Large Hadron Collider (LHC). This approach of narrowing down and excluding possible ranges continued under the Tevatron and LHC programs.

[edit] Tevatron and Large Hadron Collider

Full operation at the LHC was delayed for 14 months from its initial successful tests, on 10 September 2008, until mid-November 2009,4142 following a magnet quench event nine days after its inaugural tests that damaged over 50 superconducting magnets and contaminated the vacuum system.43 The quench was traced to a faulty electrical connection and repairs took several months;4445 electrical fault detection and rapid quench-handling systems were also upgraded.

At the Fermilab Tevatron, there were also ongoing experiments searching for the Higgs boson. As of July 2010, combined data from CDF and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range 158-175 GeV/c2 at 95% CL.4647 Preliminary results as of July 2011 extended the excluded region to the range 156-177 GeV/c2 at 95% CL.48

Data collection and analysis in search of Higgs intensified from 30 March 2010 when the LHC began operating at 3.5 TeV.49 Preliminary results from the ATLAS and CMS experiments at the LHC as of July 2011 excluded a Standard Model Higgs boson in the mass range 155-190 GeV/c250 and 149-206 GeV/c2,51 respectively, at 95% CL. All of the above confidence intervals were derived using the CLs method.

As of December 2011 the search had narrowed to the approximate region 115–130 GeV, with a specific focus around 125 GeV, where both the ATLAS and CMS experiments had independently reported an excess of events,5253 meaning that a higher than expected number of particle patterns compatible with the decay of a Higgs boson were detected in this energy range. The data was insufficient to show whether or not these excesses were due to background fluctuations (i.e. random chance or other causes), and its statistical significance was not large enough to draw conclusions yet or even formally to count as an “observation”, but the fact that two independent experiments had both shown excesses at around the same mass led to considerable excitement in the particle physics community.54

On 22 December 2011, the DØ collaboration also reported limitations on the Higgs boson within the Minimal Supersymmetric Standard Model, an extension to the Standard Model. Proton-antiproton (pp) collisions with a centre-of-mass energy of 1.96 TeV had allowed them to set an upper limit for Higgs boson production within MSSM ranging from 90 to 300 GeV, and excluding tanβ > 20–30 for masses of the Higgs boson below 180 GeV (tanβ is the ratio of the two Higgs doublet vacuum expectation values).55

At the end of December 2011, it was therefore widely expected that the LHC would provide sufficient data to either exclude or confirm the existence of the Standard Model Higgs boson by the end of 2012, when their 2012 collision data (at energies of 8 TeV) had been examined.56

Updates from the two LHC teams continued during the first part of 2012, with the tentative December 2011 data largely being confirmed and developed further.575859 Updates were also available from the team analysing the final data from the Tevatron.60 All of these continued to highlight and narrow down the 125 GeV region as showing interesting features.

On 2 July 2012, the ATLAS collaboration published additional analyses of their 2011 data, excluding boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV. They observed an excess of events corresponding to the Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 sigma.61 On the same date, the DØ and CDF collaborations announced further analysis that increased their confidence. The significance of the excesses at energies between 115–140 GeV was now quantified as 2.9 standard deviations, corresponding to a 1 in 550 probability of being due to a statistical fluctuation. However, this still fell short of the 5 sigma confidence, therefore the results of the LHC experiments were necessary to establish a discovery. They excluded Higgs mass ranges at 100–103 and 147–180 GeV.6263

[edit] Discovery of new boson

On 22 June 2012 CERN announced an upcoming seminar covering tentative findings for 2012,6566 and shortly afterwards 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.6768 On 4 July 2012 CMS announced the discovery of a previously unknown boson with mass 125.3 ± 0.6 GeV/c2264 and ATLAS of a boson with mass 126.5 GeV/c2.369 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 4.9 sigma.2

The two teams had been working ‘blinded’ from each other for some time[when?], meaning they did not discuss their results with each other, providing additional certainty that any common finding was genuine validation of a particle.39 This level of evidence, confirmed independently by two separate teams and experiments, meets the formal level of proof required to announce a confirmed discovery. CERN have been cautious, and stated only that the new particle is “consistent with” the Higgs boson, but scientists have not positively identified it as being the Higgs boson, pending further data collection and analysis.1

This announcement means that observations show the newly discovered boson could be a Higgs boson, and it is widely believed by scientists to be very likely to be a Higgs boson, but further study of this particle, now that its existence is proven, will still be required to place beyond doubt the question whether the particle is in fact confirmed as a Higgs boson.

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