Higgs boson

The Higgs boson, also known as the "God Particle", is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. It is the only Standard Model particle not yet observed, but plays a key role in explaining the origins of the mass of other elementary particles,^[clarify] in particular the difference between the massless photon and the relatively heavy W and Z bosons. Elementary particle masses, and the differences between electromagnetism (caused by the photon) and the weak force (caused by the W and Z bosons), are critical to many aspects of the structure of microscopic (and hence macroscopic) matter; thus, if it exists, the Higgs boson has an enormous effect on the world around us.^{[citation needed]}

As of 2008, no experiment has directly detected the existence of the Higgs boson. The Higgs mechanism, which gives mass to vector bosons, was first theorized in 1964 by Peter Higgs, François Englert and Robert Brout, working from the ideas of Philip Anderson, and independently by G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble ^[1]. Higgs proposed that the existence of a massive scalar particle could be a test of the theory, a remark added to his Physical Review letter ^[2] at the suggestion of the referee ^[3]. 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 the W and Z bosons.

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Theoretical overview

  The particle called the Higgs boson is the quantum of one of the components of a Higgs field. In empty space, the Higgs field acquires a non-zero value, which permeates every place in the universe at all times. The vacuum expectation value (VEV) of the Higgs field is constant and equal to 246 GeV. The existence of this non-zero VEV plays a fundamental role: it gives mass to every elementary particle, including to the Higgs boson itself. In particular, the acquisition of a non-zero VEV spontaneously breaks the electroweak gauge symmetry, a phenomenon known as the Higgs mechanism. This is the simplest mechanism capable of giving mass to the gauge bosons that is also compatible with gauge theories.

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 are massless and become, respectively, the longitudinal third-polarization components of the massive 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 spin zero and has no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even.

The Standard Model does not predict the value of the Higgs boson mass. If the mass of the Higgs boson is between 115 and 180 GeV, 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 2008, the Higgs boson has not been observed experimentally, despite large efforts invested in accelerator experiments at CERN and Fermilab. The non-observation of clear signals leads to an experimental lower bound for the Standard Model Higgs boson mass of 114.4 GeV at 95% confidence level. A small number of events were recorded by experiments at LEP collider at CERN that could be interpreted as resulting from Higgs bosons, but the evidence is inconclusive.^[4] The Large Hadron Collider (LHC), currently under construction at CERN, is expected to be able to confirm or deny the existence of the Higgs boson in most circumstances.

Precision measurements of electroweak observables indicate that the Standard Model Higgs boson mass has an upper bound of 144 GeV at the 95% confidence level^[5] as of March 2007 (incorporating an updated measurement of the top quark and W boson masses). Searches for the Higgs boson are ongoing at experiments at the Fermilab Tevatron. The limits on the production cross section of the Higgs boson set by the on-going Tevatron searches are now less than a factor of 1.5 away from Standard Model predictions in the mass range where the Higgs boson primarily decays to an on-shell W boson and an off-shell W boson.^[6] There have been optimistic articles about potential evidence of the Higgs Boson,^[7] but no evidence is yet compelling enough to convince the scientific community as a whole.

Alternatives to the Higgs mechanism for electroweak symmetry breaking

Main article: Higgsless model

In the years since the Higgs boson was proposed, there have been several alternative mechanisms to the Higgs mechanism. 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^[8] 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 ^[9].
• Top quark condensate

In fiction

Main article: Higgs boson (fiction)

Mentions of the Higgs boson (sometimes referred to in popular articles as the 'God particle', after the not-all-serious title of Nobel laureate Leon Lederman's book The God Particle: If the Universe Is the Answer, What Is the Question?), occur in some works of fiction. These references mostly imbue it with fantastic properties, and of the actual theory of the particle only its unknown mass is capitalized upon.

See also

• Yukawa interaction
• List of particles
• Large Hadron Collider

References

Further reading