Higgs boson

Infobox Particle
name = Higgs boson

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composition = Elementary particle
family = Boson
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status = Hypothetical
theorized = F. Englert, R. Brout, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble 1964
discovered =
symbol =
mass =
decay_time =
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spin = 0
num_spin_states =

The Higgs boson 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"); [cite web|url=http://link.aps.org/abstract/PRL/v13/p321|title=Broken Symmetry and the Mass of Gauge Vector Mesons] in October of the same year by Peter Higgs, [cite web|url=http://link.aps.org/abstract/PRL/v13/p508|title=Broken Symmetries and the Masses of Gauge Bosons] working from the ideas of Philip Anderson; and independently by Gerald Guralnik, C. R. Hagen, and Tom Kibble, [cite web|url=http://prola.aps.org/abstract/PRL/v13/i20/p585_1|title=Global Conservation Laws and Massless Particles] who worked out the results by the spring of 1963. [ [http://chep.het.brown.edu/stlouis-v4.pdf A Physics History of roles in the Theory of Spontaneous Symetry Breaking anf Gauge particles] ] 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. [ [http://prl.aps.org/50years/milestones Physical Review Letters - 50th Anniversary Milestone Papers] ] 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

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.

Experimental search

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. [ [http://pdg.lbl.gov/2006/reviews/higgs_s055.pdf Searches for Higgs Bosons] (pdf), from cite journal | author=W.-M. Yao "et al."| title=Review of Particle Physics | journal=J Phys. G | year=2006 | volume=33 | issue= | pages=1 | url=http://pdg.lbl.gov| doi=10.1088/0954-3899/33/1/001 ] 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.

Experiments searching for the Higgs boson are ongoing at the Fermilab Tevatron. As of August 2008, combined data from CDF and D0 experiments at the Tevatron were finally sufficient to exclude the Higgs boson at 170 GeV/c² at the 95% confidence level. [cite web |url=http://www.physorg.com/news137076565.html |title=Tevatron experiments double-team Higgs boson] Continued data-taking will allow to expand the excluded range.

There have been optimistic articles about potential evidence of the Higgs Boson, [Potential Higgs Boson discovery: [http://www.newscientist.com/channel/fundamentals/mg19325934.600-higgs-boson-glimpses-of-the-god-particle.html Higgs Boson: Glimpses of the God particle] ] but no evidence is yet compelling enough to convince the scientific community as a whole.

In addition to direct searches, it may be possible to estimate mass of the Higgs Boson indirectly. In the Standard Model, 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 mass of the Higgs. Current estimates exclude a Standard Model Higgs boson with mass above 285 GeV at 95% CL and estimate its mass to be 129 ⁺⁷⁴_-49 GeV. [ [http://pdglive.lbl.gov/popupblockdata.brl?nodein=S055HEW&inscript=Y&fsizein=1 H⁰ Indirect Mass Limits from Electroweak Analysis] ]

Alternatives to the Higgs mechanism for electroweak symmetry breaking

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 [ cite journal | author= S. Dimopoulos and L. Susskind| title=Mass Without Scalars| journal=Nucl.Phys.B| year=1979 | volume=155 | pages=237–252 | doi= 10.1016/0550-3213(79)90364-X] 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. [ cite journal | author= L. F. Abbott and E. Farhi| title=Are the Weak Interactions Strong? | journal=Phys.Lett.B| year=1981 | volume=101 | pages=69| doi=10.1016/0370-2693(81)90492-5 ]
*Top quark condensate

Popular culture

The Higgs boson is frequently referred to as 'the god particle', a name adopted after Leon Lederman's which enjoyed wide popularity. In fact this is a code name for the familiar expression 'the goddamn particle' which working physicists frequently used in desperation. [ Lederman L., The God Particle: If the Universe Is the Answer, What Is the Question?, New York:Dell, 1993]

ee also

*Yukawa interaction
*List of particles
*ZZ diboson

Notes

References

* [http://lepewwg.web.cern.ch/LEPEWWG/ "The LEP Electroweak Working Group"]
* [http://pdg.lbl.gov/2005/reviews/contents_sports.html#hyppartetc Particle Data Group: Review of searches for Higgs bosons]
* "The God Particle: If the Universe Is the Answer, What Is the Question?", by Leon Lederman, Dick Teresi, hardcover ISBN 0-395-55849-2, paperback ISBN 0-385-31211-3, Houghton Mifflin Co; (January 1993)
* [http://www.spacedaily.com/news/physics-04s.html "Fermilab Results Change Estimated Mass Of Postulated Higgs boson"]
* [http://physicsweb.org/article/news/4/9/2/1 "Higgs boson on the horizon"]
* [http://www.sciencenews.org/articles/20001104/fob6.asp "Signs of mass-giving particle get stronger"]
* [http://pprc.qmul.ac.uk/~lloyd/epp/higgs3.html "Higgs boson: One page explanation"] :: "In 1993, the UK Science Minister, William Waldegrave, challenged physicists to produce an answer that would fit on one page to the question "What is the Higgs boson, and why do we want to find it?" "
* [http://www.pparc.ac.uk/ps/bbs/bbs_mass_hm.asp "Higgs mechanism/boson simple explanation via cartoon"]
* [http://www.quark.lu.se/~atlas/thesis/egede/thesis-node6.html "Higgs physics at the LHC"]
* [http://www.newscientist.com/news/news.jsp?id=ns99995095 "Quark experiment predicts heavier Higgs"]
* [http://www.wired.com/wired/archive/12.04/grid_pr.html "The God Particle and the Grid" by Richard Martin]
* [http://www.exploratorium.edu/origins/cern/ideas/higgs.html "The Higgs boson" by the CERN exploratorium]
* [http://www.bbc.co.uk/radio4/history/inourtime/inourtime_20041118.shtml "BBC Radio 4: In Our Time " Higgs Boson - the search for the God particle"]

Further reading

External links

* [http://www.nytimes.com/2007/07/24/science/24ferm.html At Fermilab, the Race Is on for the 'God Particle']
* [http://physicsworld.com/cws/article/print/11353 Physics World, Introducing the little Higgs]
* [http://www.hep.ucl.ac.uk/~djm/higgsa.html A quasi-political Explanation of the Higgs Boson]
* [http://www.theatomsmashers.blogspot.com/ "The Atom Smashers", a blog about the making of a documentary about the search for the Higgs boson]
* [http://cerncourier.com/cws/article/cern/32522 In CERN Courier, Steven Weinberg reflects on spontaneous symmetry breaking]
* [http://www.pas.rochester.edu/urpas/news/Hagen_030708 Steven Weinberg Praises Teams for Higgs Boson Theory]
* [http://prl.aps.org/50years/milestones Physical Review Letters - 50th Anniversary Milestone Papers]
* [http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_13-6-2008-12-42-20?newsid=38514 Imperial College London on PRL 50th Anniversary Milestone Papers]
* [http://ngm.nationalgeographic.com/2008/03/god-particle/achenbach-text "The God Particle", from National Geographic Magazine]
* [http://www.physorg.com/news137076565.html "Tevatron experiments double-team Higgs boson", sets lower bound at 170GeV]

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