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Higgs boson
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In particle physics, the Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model.
The Higgs boson is the only Standard Model particle that has not yet been observed. Experimental detection of the Higgs boson would help explain how massless elementary particles can have mass. More specifically, 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.
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Encyclopedia
In particle physics, the Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model.
The Higgs boson is the only Standard Model particle that has not yet been observed. Experimental detection of the Higgs boson would help explain how massless elementary particles can have mass. More specifically, 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.
The Large Hadron Collider (LHC) at CERN, which came online on 10 September 2008 and is scheduled to become fully operational by late 2009, is expected to provide experimental evidence confirming or refuting the Higgs boson's existence. But with an accident in September 2008 putting the LHC out of commission temporarily, the US Fermilab may be the ones to detect the Higgs boson first. The American team says the odds of its Tevatron accelerator detecting the famed particle first are now 50-50 at worst, and up to 96% at best.
Origin
The Higgs mechanism, which gives mass to vector bosons, was theorized in 1964 by François Englert and Robert Brout ("boson scalaire"); in October of the same year by Peter Higgs, working from the ideas of Philip Anderson; and independently by Gerald Guralnik, C. R. Hagen, and Tom Kibble, who worked out the results by the spring of 1963. 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. Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The electroweak theory predicts a neutral particle whose mass is not far from that of the W and Z bosons.
Theoretical overview
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 which should have mass, 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 which travel through the field, converting them into particles with mass which form, for example, the components of atoms.
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 around one 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.
Experimental search
As of early 2009, the Higgs boson has yet to be observed experimentally, despite large efforts invested in accelerator experiments at CERN and Fermilab. This nonobservation suggests an experimental lower bound for the mass of the Standard Model Higgs boson of at 95% confidence level. Experiments at the LEP collider at CERN have recorded a small number of events that could be interpreted as resulting from Higgs bosons, but the evidence is inconclusive. The Large Hadron Collider (LHC), due to begin proper experimentation in 2009 after initial calibration, is expected to be able to confirm or reject the existence of the Higgs boson. Full operational mode has been delayed until late September 2009, because of problems 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 August 2008, combined data from CDF and D0 experiments at the Tevatron were finally sufficient to exclude the Higgs boson at at the 95% confidence level. Continued data collection is aimed at raising this lower bound.
It may be possible to estimate the mass of the Higgs Boson indirectly. In the Standard Model, the Higgs has a number of indirect effects; most notably, Higgs loops result in tiny corrections to W and Z masses. Precision measurements of electroweak parameters, such as Fermi constant and masses of W/Z bosons, can be used to constrain the mass of the Higgs. Current estimates exclude a Standard Model Higgs boson having a mass greater than at 95% CL, and estimate its mass to be . (138 proton masses)
Some have argued that there exists potential evidence of the Higgs Boson, but to date no such evidence has convinced the physics community.
Alternatives to the Higgs mechanism 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.
- Abbott-Farhi models of composite W and Z vector bosons.
- Top quark condensate
Popular culture
The Higgs boson is frequently referred to, by non-physicists, as "the God particle," after the title of Leon Lederman's book for lay readers. This name is frequently derided by particle physicists, since while the discovery of the Higgs boson would be a groundbreaking stage in the story of electroweak unification, it would leave remaining the question of unification with QCD and gravity and the ultimate origins and early evolution of the universe.
See also
Further reading
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