Higgs boson

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Higgs boson
Composition: Elementary particle
Family: Boson
Status: Hypothetical
Theorized: Peter Higgs, 1964
Spin: 0

The Higgs boson 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, in particular the difference between the massless photon and the very 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.

As of 2006, no experiment has directly detected the existence of the Higgs boson, but there is some indirect evidence for it. The Higgs boson was first theorized in 1964 by the British physicist Peter Higgs, working from the ideas of Philip Anderson, and independently by others.

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[edit] Theoretical details

The particle called Higgs boson is in fact 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. This 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 only known 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. One of the neutral and the charged component fields are Goldstone bosons, which are massless and unphysical. They become 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. This means that this particle has no intrinsic angular momentum and that a collection of Higgs bosons satisfies the Bose-Einstein statistics.

The Standard Model does not predict the value of the Higgs boson mass. It has been argued that if the mass of the Higgs boson lies between about 130 and 190 GeV, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV). However, most theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on some 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.

[edit] Experimental search

A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two gluons decay into a top/anti-top pair which then combine to make a neutral Higgs.
Enlarge
A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two gluons decay into a top/anti-top pair which then combine to make a neutral Higgs.

As of 2006, 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 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 [1]. It is expected among physicists that the Large Hadron Collider, currently under construction at CERN, will be able to confirm or deny the existence of the Higgs boson. Precision measurements of electroweak observables indicate that the Standard Model Higgs boson mass has an upper bound of 166 GeV at the 95% confidence level as of July, 2006 (using an updated measurement of the top quark mass). Searches for the Higgs boson are ongoing at experiments at the Fermilab Tevatron. The limits set by these searches are now less than a factor of 5 away from Standard Model predictions in some Higgs mass regimes[1].

[edit] Alternatives

Some alternative models to the (Standard Model) Higgs mechanism are:

[edit] In fiction

Main article: Higgs boson (fiction)

Mentions of the Higgs boson 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.

[edit] See also

[edit] Further reading

[edit] References

  1. ^ Searches for Higgs Bosons (pdf), from W.-M. Yao et al. (2006). "Review of Particle Physics". J Phys. G 33: 1.
In 1993, the UK Science Minister, William Waldegrave, challenged physicists to produce an answer that would fit on one page to the question "What is the Higgs boson, and why do we want to find it?"
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Particles in physics - elementary particles
Fermions: Quarks: (Up · Down · Strange · Charm · Bottom · Top) | Leptons: (Electron · Muon · Tau · Neutrinos)
Gauge bosons: Photon | W and Z bosons | Gluons
Not yet observed: Higgs boson | Graviton | Other hypothetical particles
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