Higgs boson
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The Higgs boson or BEH Mechanism, popularised 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. Experimental observation would elucidate how otherwise massless elementary particles nevertheless manage to construct mass in matter. 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.

As of yet, no experiment has directly detected the existence of the Higgs boson, but this may change as the Large Hadron Collider (LHC) at CERN produces results. 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 G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble ^[3] who worked out the results by the spring of 1963 ^[4]. The three papers written by Higgs, Brout, Englert, Guralnik, Hagen, and Kibble were each recognized as milestone papers by Physical Review Letters 50th anniversary celebration ^[5]. 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^[6] at the suggestion of the referee.^[7] 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.

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.

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 has 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 the basis of the atom.

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 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/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.

As of 2008^[update], 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 GeV/c² 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.^[8] The Large Hadron Collider (LHC), currently cooling to 1.9 kelvin, is expected to be able to confirm or reject the existence of the Higgs boson in most circumstances.

Precision measurements of electroweak observables exclude a Standard Model Higgs boson mass of 170 GeV/c² at the 95% confidence level^[9] 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.^[10] There have been optimistic articles about potential evidence of the Higgs Boson,^[11] 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^[12] 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.^[13]
• Top quark condensate.

Popular culture

Main article: Higgs boson in fiction

The Higgs boson has appeared in several works of fiction in popular culture. These references rarely reflect the expected properties of the hypothetical elementary particle, or do so only vaguely, and often imbue it with fantastic properties.

More realistically, CERN's Large Hadron Collider (and its primary quarry the Higgs Boson) is the subject of a scientifically accurate rap video featuring some of CERN's own staff: [1].

See also

• Yukawa interaction
• List of particles
• Large Hadron Collider
• ZZ diboson

Notes

References

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