"The God particle" if the universe is the Answer Then what is the question?
Higgs boson
"God particle" redirects here. For the book, see The God Particle: If the Universe Is the Answer, What Is the Question?.
 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] |
|
| Composition | Elementary particle |
|---|---|
| Statistics | Bosonic |
| Status | Tentatively observed – a boson "consistent with" the Higgs boson has been observed, but as of July 2012, scientists have not conclusively identified it as the Higgs boson.[1] |
| Symbol | H0 |
| Theorised | R. Brout, F. Englert, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble (1964) |
| Discovered | tentatively announced July 4, 2012 (see above), by the ATLAS and CMS teams at the Large Hadron Collider |
| Types | 1 in the Standard Model; 5 or more in supersymmetric models |
| Mass | 125.3±0.6 GeV/c2,[2] ∼126 GeV/c2[3] |
| Mean lifetime | 1 zeptosecond (10-21 s) [citation needed] |
The Higgs boson is named for Peter Higgs who, along with two other teams, proposed the mechanism that suggested such a particle in 1964[6][7][8] and was the only one to explicitly predict the massive particle and identify some of its theoretical properties.[9] In mainstream media it is often referred to as "the God particle", after the title of Leon Lederman's book on the topic (1993). Although the proposed particle is both important and elusive, the epithet is strongly disliked by physicists, who regard it as inappropriate sensationalism since the particle has nothing to do with God nor any mystical associations,[10][11] and because the term is misleading: the crucial focus of study is to learn how the symmetry breaking mechanism takes place in nature - the search for the boson is part of, and a key step towards, this goal.
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 spin, electric charge, or colour charge. It is also very unstable, decaying into other particles almost immediately. Some extensions of the Standard Model predict the existence of more than one kind of Higgs boson.
Proof of the Higgs field (by confirming its boson), and evidence of its properties, are seen as likely to greatly affect human understanding of the universe, validate the final unconfirmed part of the Standard Model as essentially correct, indicate which of several current particle physics theories are more likely correct, and open up "new" physics beyond current theories.[12] If the Higgs boson were shown not to exist, other alternative sources for the Higgs mechanism would need to be considered. On 4 July 2012, the CMS and the ATLAS experimental teams at the LHC 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 has been "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.
Overview
 |
| Standard model of particle physics |
|---|
Large Hadron Collider tunnel at CERN
|
The leading and simplest theory for how this effect takes place in nature was that if a particular kind of "field" (known as the Higgs field) existed, which in contrast to the more familiar gravitational field and electromagnetic fieldhad a constant strength everywhere, then this field would give rise to a Higgs mechanism in nature, and would therefore allow particles interacting with this field to acquire a 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 fundamental particle as the counterpart of this field. This particle would be the Higgs boson. If the Higgs boson were confirmed to exist, as the Standard Model suggested, then scientists could be satisfied that this part of the Standard Model was fundamentally correct. If the Higgs boson were proved not to exist, then other theories would need to be considered as candidates instead.
The Standard Model also made clear that the existence of the Higgs boson would be very difficult to demonstrate. 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. And even then, the particle collisions are much more likely to produce other particles than the Higgs boson. Once a Higgs bosons is created it will exist for only a tiny fraction of a second before decaying 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. However, there are many other processes that produce a similar decay signature, making the Higgs particle a proverbial "needle-in-a-haystack". 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 announced [1] 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.
Useful terminology
This article involves several related concepts with similar names.- The Higgs mechanism shows how some particles can gain mass by symmetry breaking without affecting parts of current physics theory that are believed approximately correct. The existence of some kind of symmetry breaking Higgs mechanism is believed proven, however there are a number of ways it could happen and physicists have not yet determined which of these takes place in nature, or whether the mechanism arises in some other way not yet identified.
- The Higgs field is one of several ways that this mechanism could arise, and is the current preferred theory. If this theory is correct, then a field exists throughout space that is responsible for the Higgs mechanism. The field - if it exists - would have a related particle, which would be a previously unknown type of boson. That field can be proven to exist and its properties studied, by finding and examining the boson and studying its properties. The Standard Model of particle physics is based on the theory that such a field exists, although it allows for variants of the theory where the details of the Higgs field and number of related Higgs bosons can differ. If the Higgs field does not exist then other modifications to the Standard Model exist to explain how the Higgs mechanism arises, and these would be examined instead.
- The Higgs boson is the massive and fleetingly short-lived boson associated with a Higgs field, and also the Higgs field's smallest possible excitation, or quantum. If the preferred theory is correct, then this massive boson will exist, and can be detected in experiments and tested to see whether it is a Higgs boson. If shown to be a Higgs boson, then this would prove the Higgs field exists, which in turn will confirm how the Higgs mechanism takes place and that the Standard Model is essentially correct. Further studies would be needed to test which variant of the Standard Model is accurate. At present as of 2012, a particle has been detected but not yet tested fully to show if it is a Higgs boson.
History
See also: 1964 PRL symmetry breaking papers and Higgs mechanism
  The six authors of the 1964 PRL papers, who received the 2010 J. J. Sakurai Prize for their work. From left to right: Kibble, Guralnik, Hagen, Englert, Brout. Right: Higgs. |
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 Anderson[15] 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;[7] by Peter Higgs in October 1964;[6] and by Gerald Guralnik, C. R. Hagen, and Tom Kibble (GHK) in November 1964.[8] Properties of the model were further considered by Guralnik in 1965 [16] and by Higgs in 1966.[17] 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,[18][19][20] in what became the Standard Model of particle physics.
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.[24][25]
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,[26] 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.
Theoretical properties
Main article: Higgs mechanism
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.[29][30] An anonymous person who passes through the crowd with ease would be like the interaction between the field and a massless photon. If the British prime minister, however, tried to do the same, his or her progress would be greatly held back by the swarm of admirers crowding around him/her, 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 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).[34] Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model.[35] 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.[36]
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.[26] 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.[37]
Alternative models
See also: Higgsless model
The Minimal Standard Model as described above contains the simplest
possible model for the Higgs mechanism with just one Higgs field.
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.[38]Many extensions to the Standard Model, including supersymmetry (SUSY), often contain an extended Higgs sector. Supersymmetric models predict relations between the Higgs-boson masses and the masses of the gauge bosons, and can accommodate a neutral Higgs boson with a mass around 125 GeV/c2. 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.[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.[39][40]
Production
A Higgs particle can be produced in a particle collider by taking two particles smashing them together at very high energies. The exact process depends on the details of the particles used and the energy at which they are collided. [41][42][43] But in any case the probability of producing a Higgs boson in any collision is always expected to be very small with only 1 Higgs boson being produced per 10 billion collisions.[Note 3] The most common processes are the following:
- Gluon fusion. If the collided particles are hadrons such the proton or antiproton—as is the case in the LHC and Tevatron—then its 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.[41][42]
- Higgs Strahlung. If an elemental 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 W or Z boson. This boson can then decay by emitting a Higgs boson. If the Higgs boson, were lighter than the W and Z bosons, this process would be very common. It would have the dominant production process for the Higgs at the LEP collider, which would have discovered Higgs in the late nineties. The fact, that this did not happen tells us that the Higgs is in fact heavier than the Z boson (91.2 GeV/c2). In this case, this process is only possible if the created W or Z boson is a virtual version with a much larger mass.[4] This process was the second largest contribution for Higgs production at the Tevatron. At the LHC, this process is less likely, because it collides protons with protons making a quark-antiquark collision unlikely.[41][42][43]
- 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 exampe, 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.[41][43]
- 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 in to a heavy quark-antiquark pair. A quark and anti-quark from each pair can then combine to form a Higgs particle. [41][42]
Experimental search
  |
| Feynman diagrams showing two ways the Higgs boson might be produced at the LHC. Left: two gluons convert to top/anti-top quark pairs, which combine. Right: two quarks emit W or Z bosons, which combine. |
 |
Wikimedia Commons has media related to: Large Hadron Collider |
Since the Higgs boson, if it existed, 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,[45] requiring very large worldwide computing facilities. Ultimately over 300 trillion (3 x 1014) proton-proton collisions at the LHC were analysed in confirming the July 2012 particle's discovery.[45] 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.
Comments
Post a Comment