Higgs boson

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Higgs boson
Composition: Elementary particle
Family: Boson
Status: Hypothetical
Theorized: F. Englert, R. Brout, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble 1964
Spin: 0

In particle physics, the Higgs boson (nicknamed the God particle) is a massive scalar elementary particle predicted to exist by the Standard Model.

The Higgs boson is the only Standard Model particle that has not yet been observed. Experimental detection of the Higgs boson would help explain how massless elementary particles can have mass. More specifically, 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 in Geneva, which came online on 10 September 2008 and is scheduled to become fully operational by late 2009, was expected to provide experimental evidence confirming or refuting the Higgs boson's existence. But, with an accident in September 2008 putting the LHC out of commission temporarily, the US Fermilab may be the ones to detect the Higgs boson first, though hindered due to its relatively weak particle accelerator compared to the LHC. The American team says the odds of its Tevatron accelerator detecting the famed particle first are now 50-50 at worst, and up to 96% at best.[1]

Contents

[edit] Origin

The Higgs mechanism, which gives mass to vector bosons, was theorized in 1964 by François Englert and Robert Brout ("boson scalaire");[2] in October of the same year by Peter Higgs,[3] working from the ideas of Philip Anderson; and independently by Gerald Guralnik, C. R. Hagen, and Tom Kibble,[4] who worked out the results by the spring of 1963.[5] 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.[6] 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.

[edit] 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 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 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 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.

[edit] 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 early 2009, 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 to set an experimental lower bound 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.[7] 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 late September 2009, because of problems discovered with a number of magnets during the calibration and startup phase.

At the Fermilab Tevatron, there are ongoing experiments searching for the Higgs boson. As of March 2009, combined data from CDF and D0 experiments at the Tevatron were sufficient to exclude the Higgs boson in the range between 160 GeV/c2 170 GeV/c2 at the 95% confidence level.[8] 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 electroweak parameters, such as 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 to exclude 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).[9] As of early 2009, Standard Model Higgs is excluded by electroweak measurements above 185 GeV at 95% CL.

Some have argued that there exists potential evidence of the Higgs Boson,[10][11] but to date no such evidence has convinced the physics community.

[edit] Alternatives to the Higgs mechanism for electroweak symmetry breaking

Main article: Higgsless model

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[12] 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. [13]
  • Abbott-Farhi models of composite W and Z vector bosons.[14]
  • Top quark condensate

[edit] In popular culture

Main article: Higgs boson in fiction

The Higgs boson is frequently referred to as "the God particle," after the title of Leon Lederman's book for lay readers.[15] However, while the discovery of the Higgs boson would be a groundbreaking stage in the story of electroweak unification, it would leave remaining the question of unification with Quantum Chromodynamics (QCD), gravity, and the ultimate origins and early evolution of the universe

[edit] See also

[edit] Notes

  1. ^ "Race for 'God particle' heats up". http://news.bbc.co.uk/2/hi/science/nature/7893689.stm. 
  2. ^ François Englert and Robert Brout, 1964, "Broken Symmetry and the Mass of Gauge Vector Mesons," Phys. Rev. Lett. 13: 321-23.
  3. ^ Peter Higgs, 1964, " Broken Symmetries and the Masses of Gauge Bosons,"Phys. Rev. Lett. 13: 508-09.
  4. ^ Gerald Guralnik, C. R. Hagen, and T. W. B. Kibble, 1964, "Global Conservation Laws and Massless Particles," Phys. Rev. Let. 13: 585-87.
  5. ^ Gerald Guralnik, 2001, "A Physics History of My Part in the Theory of Spontaneous Symmetry Breaking and Gauge particles," Text of talk presented at a Colloquium at St. Louis University.
  6. ^ Physical Review Letters - 50th Anniversary Milestone Papers
  7. ^ "Searches for Higgs Bosons (pdf)]" from W.-M. Yao et al. (2006). "Review of Particle Physics". J Phys. G 33: 1. doi:10.1088/0954-3899/33/1/001. http://pdg.lbl.gov. 
  8. ^ "Fermilab experiments constrain Higgs mass". http://www.fnal.gov/pub/presspass/press_releases/Higgs-mass-constraints-20090313.html. 
  9. ^ "H0 Indirect Mass Limits from Electroweak Analysis."
  10. ^ Potential Higgs Boson discovery: "Higgs Boson: Glimpses of the God particle."
  11. ^ "'God particle' may have been seen," BBC news.
  12. ^ S. Dimopoulos and Leonard Susskind (1979). "Mass Without Scalars". Nucl.Phys.B 155: 237–252. doi:10.1016/0550-3213(79)90364-X. 
  13. ^ C. Csaki and C. Grojean and L. Pilo and J. Terning (2004). "Towards a realistic model of Higgsless electroweak symmetry breaking". Physical Review Letters 92: 101802. http://arXiv.org/abs/hep-ph/0308038. 
  14. ^ L. F. Abbott and E. Farhi (1981). "Are the Weak Interactions Strong?". Phys.Lett.B 101: 69. doi:10.1016/0370-2693(81)90492-5. 
  15. ^ Leon Lederman, 1993. The God Particle: If the Universe Is the Answer, What Is the Question? New York: Dell.

[edit] References

[edit] Further reading

[edit] External links

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