The Higgs boson, also BEH Mechanism, 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. An experimental observation of it would help to explain how otherwise massless elementary particles cause matter to have mass. More specifically, the Higgs boson would explain the difference between the massless photon and the relatively massive 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 is an integral and pervasive component of the material world.

No experiment has yet directly detected the Higgs boson; the Large Hadron Collider (LHC) at CERN, which came on line on 10 September 2008, is expected to provide experimental evidence that will confirm or reject the particle's existence. The Higgs mechanism, which gives mass to vector bosons, was theorized in August 1964 by François Englert and Robert Brout ("boson scalaire");^[1] in October of the same year by Peter Higgs,^[2] working from the ideas of Philip Anderson; and independently by Gerald Guralnik, C. R. Hagen, and Tom Kibble,^[3] who worked out the results by the spring of 1963.^[4] The three papers written on this discovery by Guralnik, Hagen, Kibble, Higgs, Brout, and Englert were each recognized as milestone papers by Physical Review Letters 50th anniversary celebration.^[5] 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

A one-loop Feynman diagram of the first-order correction to the Higgs mass. The Higgs boson couples strongly to the top quark so it may decay into top anti-top quark pairs.

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 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 value of the Higgs boson mass. If the mass of the Higgs boson 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

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.

Precision measurements of electroweak observables exclude a Standard Model Higgs boson mass of 170 GeV/c² at the 95% confidence level^[7] as of August 2008 (incorporating an updated measurement of the top quark and W boson masses). Experiments searching for the Higgs boson are ongoing 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.^[8] There have been optimistic articles about potential evidence of the Higgs Boson,^[9] but no evidence is yet compelling enough to convince the scientific community as a whole.

Alternatives to the Higgs mechanism for electroweak symmetry breaking

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^[10] 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.^[11]
• Top quark condensate

Popular culture

The Higgs boson is frequently referred to as 'the god particle', a name adopted after Leon Lederman's book which enjoyed wide popularity. In fact this is a code name for the familiar expression 'the goddamn particle' which working physicists frequently used in desperation.^[12]

See also

• Yukawa interaction
• List of particles
• ZZ diboson

Notes

References

• ''The LEP Electroweak Working Group''
• Particle Data Group: Review of searches for Higgs bosons
• The God Particle: If the Universe Is the Answer, What Is the Question?, by Leon Lederman, Dick Teresi, hardcover ISBN 0-395-55849-2, paperback ISBN 0-385-31211-3, Houghton Mifflin Co; (January 1993)
• ''Fermilab Results Change Estimated Mass Of Postulated Higgs boson''
• ''Higgs boson on the horizon''
• ''Signs of mass-giving particle get stronger''
• ''Higgs boson: One page explanation'':

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?"

• ''Higgs mechanism/boson simple explanation via cartoon''
• ''Higgs physics at the LHC''
• ''Quark experiment predicts heavier Higgs''
• ''The God Particle and the Grid'' by Richard Martin
• ''The Higgs boson'' by the CERN exploratorium
• ''BBC Radio 4: In Our Time " Higgs Boson - the search for the God particle"''

Further reading

External links

• At Fermilab, the Race Is on for the 'God Particle'
• Physics World, Introducing the little Higgs
• A quasi-political Explanation of the Higgs Boson
• ''The Atom Smashers'', a blog about the making of a documentary about the search for the Higgs boson
• In CERN Courier, Steven Weinberg reflects on spontaneous symmetry breaking
• Steven Weinberg Praises Teams for Higgs Boson Theory
• Physical Review Letters - 50th Anniversary Milestone Papers
• Imperial College London on PRL 50th Anniversary Milestone Papers
• ''The God Particle'', from National Geographic Magazine
• "Tevatron experiments double-team Higgs boson", sets lower bound at 170GeV

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