The God Particle

The Higgs boson (The God Particle), 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 when fully operational in 2009. The Higgs mechanism, which gives mass to vector bosons, was theorized in August 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 by 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.

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 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.



photo: 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.

Experimental Search:



photo: 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, 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. 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. The LHC has had trouble with a number of magnets in its initial calibration and startup phase and has been delayed 2 months due to these problems.

Precision measurements of electroweak observables exclude a Standard Model Higgs boson mass of 170 GeV/c² at the 95% confidence level 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. There have been optimistic articles about potential evidence of the Higgs Boson, but no evidence is yet compelling enough to convince the scientific community as a whole.