Higgs boson

The Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model in particle physics. At present there are no known fundamental scalar particles in nature.

The Higgs boson is the only Standard Model particle that has not been observed. Experimental detection of the Higgs boson would help explain the origin of mass in the universe. The Higgs boson would explain the difference between the massless photon, which mediates electromagnetism and the massive W and Z bosons, which mediate the weak force. If the Higgs boson exists, it is an integral and pervasive component of the material world.

The Large Hadron Collider (LHC) at CERN, which ran briefly on September 10, 2008 and was scheduled to become operational by late November 2009 ^[1] is expected to provide experimental evidence as to the Higgs boson's existence or non-existence. An accident in September 2008 damaged the LHC; experiments at Fermilab continue previous attempts at detection (although hindered by the lower energy of the Fermilab Tevatron accelerator). It has been reported that Fermilab physicists suggest the odds of Tevatron detecting the Higgs boson are between 50% and 96%, depending on its mass.^[2]

Origin

The Higgs mechanism, which gives mass to vector bosons, was theorized in 1964 by François Englert and Robert Brout ("boson scalaire");^[3] in October of the same year by Peter Higgs,^[4] working from the ideas of Philip Anderson; and independently by Gerald Guralnik, C. R. Hagen, and Tom Kibble,^[5] who worked out the results by the spring of 1963.^[6] The three papers written on this discovery by Guralnik, Hagen, Kibble, Higgs, Brout, and Englert were each recognized as milestone papers during Physical Review Letters 50th anniversary celebration.^[7] While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL Symmetry Breaking papers is noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.^[8] 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.

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, 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 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 mass of the Higgs boson. If that mass 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 1.4 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.

Supersymmetric extensions of the Standard Model (so called SUSY) predict the existence of whole families of Higgs bosons, as opposed to a single Higgs particle of the Standard Model. Among the SUSY models, in the Minimal Supersymmetric extension (MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two Higgs doublets, leading to the existence of a quintet of (i.e. five) scalar particles: two CP-even neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H^±.

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.

A Feynman diagram of another way the Higgs boson may be produced at the LHC. Here, two quarks each emit a W or Z boson, which combine to make a neutral Higgs.

As of November 2009^[update], the Higgs boson has yet to be observed experimentally, despite large efforts invested in accelerator experiments at CERN and Fermilab. The data gathered at the LEP collider at CERN allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of 114.4 GeV/c² at 95% confidence level. The same experiment has produced a small number of events that could be interpreted as resulting from Higgs bosons with mass just above said cutoff—around 115 GeV—but the number of events was insufficient to draw definite conclusions.^[9] The LEP was shut down in 2000 due to construction of its successor, the Large Hadron Collider (LHC). The 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. Full operational mode has been delayed until mid-November 2009, because of a serious fault discovered with a number of magnets during the calibration and startup phase.^[10][11]

At the Fermilab Tevatron, there are ongoing experiments searching for the Higgs boson. As of March 2009^[update], combined data from CDF and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range between 160 GeV/c² and 170 GeV/c² at the 95% confidence level.^[12] Continued data collection is aimed at raising this lower bound.

It may be possible to estimate the mass of the Higgs Boson indirectly. In the Standard Model, the Higgs has a number of indirect effects; most notably, Higgs loops result in tiny corrections to W and Z masses. Precision measurements of electro-weak parameters, such as the Fermi constant and masses of W/Z bosons, can be used to constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusion of a Standard Model Higgs boson having a mass greater than 285 GeV/c² at 95% CL, and estimated its mass to be 129+74−49 GeV/c² (approximately 138 proton masses).^[13] As of August 2009, the Standard Model Higgs boson is excluded by electroweak measurements above 186 GeV at 95% CL. However, it should be noted that these indirect constraints make the assumption that the SM is correct. One may still discover a Higgs boson above 186 GeV if it is accompanied by other particles between Standard Model and GUT scale.

Some have argued that there already exists potential evidence,^[14][15] but to date no such evidence has convinced the physics community.

Alternatives for electroweak symmetry breaking

In the years since the Higgs boson was proposed, several alternatives to the Higgs mechanism have been proposed. 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^[16] is a class of models that attempts to mimic the dynamics of the strong force as a way of breaking electroweak symmetry.
• Extra dimensional Higgsless models where the role of the Higgs field is played by the fifth component of the gauge field.^[17]
• Abbott-Farhi models of composite W and Z vector bosons.^[18]
• Top quark condensate.

In popular culture

The Higgs boson is often referred to as "the God particle" by the media,^[19] after the title of Leon Lederman's book, The God Particle: If the Universe Is the Answer, What Is the Question?.^[20] While use of this term may have contributed to increased media interest in particle physics and the Large Hadron Collider^[20] it is disliked by scientists as overstating the importance of the particle.^[19]

See also

• Overview and Differences of 1964 PRL Symmetry Breaking papers
• List of particles
• Yukawa interaction
• ZZ diboson
• Quantum triviality

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
• The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles
• The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles (PDF)
• Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia
• History of Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia
• God Particle Overview
• Fermilab 'closing in' on the God particle