Higgs boson

Simulated Higgs boson decay in the ATLAS detector

The Higgs in a nutshell

The Higgs is a subatomic particle predicted to exist but that hasn't yet been confirmed
It was proposed as a mechanism to explain mass by six physicists, including Peter Higgs, in 1964
It imparts mass to other fundamental particles via the associated Higgs field
It is the last missing member of the Standard Model, which explains how particles interact

A hypothetical massive particle and the last undiscovered member of the so-called Standard Model of particle physics – the pre-eminent theory describing how particles and forces interact. The Higgs boson is hypothesized as giving all other particles mass, but the Standard Model doesn't predict what the Higgs itself weighs. The search for the Higgs boson, which has become like a Holy Grail quest among particle physicists, is nearing a conclusion. By the end of 2012, the Higgs will either have been found or demonstrated beyond reasonabe doubt not to exist. The results of experiments at the Large Hadron Collider, as of December 2011, suggested the possible existence of the Higgs at a mass of about 125 GeV (roughly equal to the mass of two copper atoms).

Background

Elementary particle masses (in particular the difference between the massless photon and the very heavy W and Z bosons), 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.

Results announced on Dec. 13, 2011, by scientists at the Large Hadron Collider, point to the possible existence of the Higgs boson. In particular, two large detectors, called ATLAS and CMS, have found indications of a particle at about the same mass (125 GeV). Further observations at the LHC in 2012 should demonstrate conclusively whether or not the Higgs exists and, if it does, with what mass.

The Higgs mechanism 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. 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

Fermilab scientist Don Lincoln describes the nature of the Higgs boson

Don Lincoln explains the latest results in the search for the Higgs boson

The particle known as 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

Evidence presented on December 13, 2011, by two teams at the Large Hadron Collider suggest the possible existence of a relatively lightweight Higgs boson with a mass in the range 124–126 GeV. Prior to this there was no strong experimental evidence, despite major efforts invested in accelerator experiments at CERN and Fermilab. The non-observation of clear signals led 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 the LEP collider at CERN (the predecessor of the Large Hasdron Collider) that could be interpreted as resulting from Higgs bosons, but the evidence was inconclusive.¹

Precision measurements of electroweak observables indicated that the Standard Model Higgs boson mass had an upper bound of 144 GeV at the 95% confidence level as of March 2007 (incorporating an updated measurement of the top quark and W boson masses). Searches for the Higgs boson were made at the Fermilab Tevatron, which was permanently shut down in Sepetember 2011. The limits on the production cross section of the Higgs boson set by the on-goinTevatron searches were less than a factor of 3.5 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.² There had been optimistic articles about potential evidence of the Higgs boson,³ but evidence compelling enough to convince the scientific community as a whole.

With the December 13, 2011 announcements from CERN, however, that situation now appears to have changed. The fact that two major detectors at the LHC, ATLAS and CMS, have found signs of a particle in the same narrow energy range has physicists eagerly anticipating further measurements and analysis in 2012 which will settle the issue once and for all. A final verdict on the existence of the Higgs is imminent.

References

Searches for Higgs Bosons (pdf), from W.-M. Yao et al. (2006). "Review of Particle Physics". J Phys. G 33: 1.
Combined DØ and CDF Upper Limits on Standard-Model Higgs-Boson Production.
Potential Higgs Boson discovery: Higgs Boson: Glimpses of the God particle

Related category

• PARTICLE PHYSICS

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Higgs boson".

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