What is the Higgs Boson?
The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. At present there are no known elementary scalar bosons (spin-0 particles) in nature, although many composite spin-0 particles are known. The existence of the particle is postulated as a means of resolving inconsistencies in current theoretical physics, and attempts are being made to confirm the existence of the particle by experimentation, using the Large Hadron Collider (LHC) at CERN and the Tevatron at Fermilab. Other theories exist that do not anticipate the Higgs boson, described elsewhere as the Higgsless model. The Higgs boson is the only Standard Model particle that has not been observed. It is a consequence of the so-called Higgs mechanism which is the part of the Standard Model which explains how most of the known elementary particles become massive. For example, the Higgs boson would explain the difference between the massless photon, which mediates electromagnetism, and the massive W and Z bosons, which mediate the weak force. If the Higgs boson exists, it is an integral and pervasive component of the material world.
Relatively model-independent arguments suggest that any mechanism which generates the masses of the elementary particles must be visible below 1.4 TeV. Therefore the Large Hadron Collider is expected to provide experimental evidence of the existence or non-existence of the Higgs boson. Experiments at Fermilab also continue previous attempts at detection, albeit hindered by the lower energy of the Tevatron accelerator, although it theoretically has the necessary energy to produce the Higgs boson.
When discussing the origin of the Higgs boson concept, it is important to distinguish between the Higgs mechanism and the Higgs boson. The Higgs mechanism (or "Englert-Brout-Higgs-Guralnik-Hagen-Kibble") is a mechanism by which vector bosons can get a mass. It was proposed in 1964 independently and almost simultaneously by three groups of physicists: François Englert and Robert Brout, by Peter Higgs, (who was inspired by the ideas of Philip Anderson); and by Gerald Guralnik, C. R. Hagen, and Tom Kibble.
The three papers written on this discovery by Guralnik, Hagen, Kibble, Higgs, Brout, and Englert were each recognized as milestone papers during Physical Review Letters 50th anniversary celebration. While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL Symmetry Breaking papers are noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.
Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts the ratio of the W boson and Z boson masses as well as their couplings among themselves and with the Standard Model quarks and leptons. Many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.
Out of the three seminal papers on the Higgs mechanism, only the paper by Peter Higgs mentioned, in a closing sentence, possible existence of the Higgs boson ("<...> an essential feature of the type of theory which has been described in this note is the prediction of incomplete multiplets of scalar and vector bosons."). Peter Higgs added this sentence when he was revising the paper after it was rejected by Physics Letters, and before resubmitting it to Physical Review Letters. The first detailed description of the Higgs boson properties was given in 1966, also by Peter Higgs.
In fact, the Higgs boson existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson exists in some but not all theories which use the Higgs mechanism. For example, Higgs boson exists in the Standard Model and the Minimal Supersymmetric Standard Model. Yet it is not expected to exist in Technicolor models or, more generally, Higgsless models. All of these models realize various forms of the Higgs mechanism. A major goal of the LHC experiments is to distinguish among these models and determine if the Higgs boson exists or not.
The Higgs boson particle is one quantum component of the theoretical Higgs field. In empty space, the Higgs field has an amplitude different from zero; i.e., a non-zero vacuum expectation value. The existence of this non-zero vacuum expectation plays a fundamental role: it gives mass to every elementary particle that couples to the Higgs field, including the Higgs boson itself. In particular, the acquisition of a non-zero vacuum expectation value spontaneously breaks electroweak gauge symmetry, which scientists often refer to as the Higgs mechanism. This is the simplest mechanism capable of giving mass to the gauge bosons while remaining compatible with gauge theories. In essence, this field is analogous to a pool of molasses that "sticks" to the otherwise massless fundamental particles that travel through the field, converting them into particles with mass that form, for example, the components of atoms. Prof. David J. Miller of University College London provided a simple explanation of the Higgs Boson, for which he won an award.
In the Standard Model, the Higgs field consists of two neutral 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 the massive Higgs boson. Since the Higgs field is a scalar field, the Higgs boson has no spin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even.
The Standard Model does not predict the mass of the Higgs boson. 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 (1016 TeV). Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model. 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. Many models of supersymmetry predict that the lightest Higgs boson (of several) will have a mass only slightly above the current experimental limits, at around 120 GeV or less. Supersymmetric extensions of the Standard Model (so called SUSY) predict the existence of whole families of Higgs bosons, as opposed to a single Higgs particle of the Standard Model. Among the SUSY models, in the Minimal Supersymmetric extension (MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two Higgs doublets, leading to the existence of a quintet of scalar particles: two CP-even neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±.
There are over a hundred theoretical Higgs-mass predictions.
As of December 2010, the Higgs boson has yet to be confirmed experimentally, despite large efforts invested in accelerator experiments at CERN and Fermilab.
Prior to the year 2000, the data gathered at the LEP collider at CERN allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of 114.4 GeV/c2 at 95% confidence level. The same experiment has produced a small number of events that could be interpreted as resulting from Higgs bosons with mass just above said cutoff—around 115 GeV—but the number of events was insufficient to draw definite conclusions. The LEP was shut down in 2000 due to construction of its successor, the Large Hadron Collider which is expected to be able to confirm or reject the existence of the Higgs boson. Full operational mode was delayed until mid-November 2009, because of a serious fault discovered with a number of magnets during the calibration and startup phase.
At the Fermilab Tevatron, there are 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 between 158 GeV/c2 and 175 GeV/c2 at the 95% confidence level. Data collection and analysis in search of Higgs are intensifying since March 30, 2010 when the LHC began operating at 3.5 Tev and is rapidly approaching in its design range of 7 Tev, well above that at which detection should occur.
It may be possible 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 constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusion of a Standard Model Higgs boson having a mass greater than 285 GeV/c2 at 95% CL, and estimated its mass to be 129+74 −49 GeV/c2 (the central value corresponds to approximately 138 proton masses). As of August 2009, the Standard Model Higgs boson is excluded by electroweak measurements above 186 GeV at 95% CL. However, it should be noted that these indirect constraints make the assumption that the Standard Model is correct. One may still discover a Higgs boson above 186 GeV if it is accompanied by other particles between Standard Model and GUT scales. Some have argued that there already exists potential evidence, but to date no such evidence has convinced the physics community.
In a 2009 preprint, it was suggested (and reported under headlines such as Higgs could reveal itself in Dark-Matter collisions) that the Higgs Boson might not only interact with the above-mentioned particles of the Standard model of particle physics, but also with the mysterious WIMPs ("weakly interacting massive particles") of the Dark matter, playing a most-important role in recent astrophysics.
In principle, a relation between the Higgs particle and the Dark matter would be "not unexpected", since, (i), the Higgs field does not directly couple to the quanta of light (i.e. the photons), while at the same time, (ii), it generates mass. However, "dark matter" is a metonym for the discrepancy between the apparent observed mass of the universe and that given by the standard model and is not a component of any known theory of physics. Consequently, the usefulness of this conjecture is limited.
Barring discovery during current intensive efforts, it will be sometime after the end of the current physics fill at the LHC in 2011 and some further months or years of analysis of the collected data before scientists can confidently believe that the Higgs Boson does not exist.
Alternatives for electroweak symmetry breaking
In the years since the Higgs boson was proposed, several alternatives to the Higgs mechanism have been proposed. All of the alternative mechanisms use strongly interacting dynamics to produce a vacuum expectation value that breaks electroweak symmetry. A partial list of these alternative mechanisms are
Technicolor is 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.
Abbott-Farhi models of composite W and Z vector bosons.
Top quark condensate.
Braid model of Standard Model particles by Sundance Bilson-Thompson, compatible with loop quantum gravity and similar theories.
"The God particle"
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