Higgs boson
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"God particle (physics)" redirects here. For other uses, see God particle.
Higgs boson
CMS Higgs-event.jpg
A simulated event, featuring the appearance of the Higgs boson
Composition: Elementary particle
Particle statistics: Bosonic
Status: Hypothetical
Theorized: F. Englert, R. Brout, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble 1964
Mass: between 115 and 185 GeV/c2 (model-dependent upper bound[1])
Spin: 0
The Higgs boson is a hypothetical massive elementary particle predicted to exist by the Standard Model of particle physics. The existence of the particle is postulated as a means of resolving inconsistencies in current theoretical physics, and attempts are being made to confirm the existence of the particle by experimentation, using the Large Hadron Collider (LHC) at CERN and the Tevatron at Fermilab.
The Higgs boson is the only Standard Model particle that has not been observed in particle physics experiments. It is a consequence of the so-called Higgs mechanism which is the part of the Standard Model that explains how most of the known elementary particles become massive.[2] For example, 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.
If it exists, it is of a class of particles known as scalar bosons. Bosons have integer spin, and scalar bosons have spin 0. The photon is a kind of boson, and so is the less-familiar gluon, along with the W and Z particles mentioned above. But these particles are all vector bosons, with spin 1. At present there are no known elementary scalar bosons in nature, although many composite spin-0 particles are known.
Theories exist that do not anticipate the Higgs boson, described elsewhere as Higgsless models. Relatively model-independent arguments suggest that any mechanism which generates the masses of the elementary particles must be visible below 1.4 TeV.[3] Therefore the Large Hadron Collider[4] is expected to provide experimental evidence of the existence or non-existence of the Higgs boson. Experiments at Fermilab also continue previous attempts at detection, albeit hindered by the lower energy of the Tevatron accelerator, although it theoretically has the necessary energy to produce the Higgs boson.
Contents
[hide]
1 Origin of the theory
2 Theoretical overview
3 Experimental search
4 Alternatives for electroweak symmetry breaking
5 "The God particle"
6 See also
7 References
8 Further reading
9 External links
[edit] Origin of the theory
2010 APS J.J. Sakurai Prize Winners
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See also: 1964 PRL symmetry breaking papers
The Higgs mechanism (or "Englert-Brout-Higgs-Guralnik-Hagen-Kibble" [5]) is a mechanism by which vector bosons can get a mass. It was proposed in 1964 independently and almost simultaneously by three groups of physicists: François Englert and Robert Brout;[6] by Peter Higgs,[7] (who was inspired by the ideas of Philip Anderson); and by Gerald Guralnik, C. R. Hagen, and Tom Kibble.[8]
The three papers written on this discovery were each recognized as milestone papers during Physical Review Letters's 50'th anniversary celebration.[9] While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL Symmetry Breaking papers are noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.[10]
Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts the ratio of the W boson and Z boson masses as well as their couplings among themselves and with the Standard Model quarks and leptons. Many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.[11]
Out of the three seminal papers on the Higgs mechanism, only the paper by Peter Higgs mentioned, in a closing sentence, possible existence of the Higgs boson ("<...> an essential feature of the type of theory which has been described in this note is the prediction of incomplete multiplets of scalar and vector bosons."). Peter Higgs added this sentence when he was revising the paper after it was rejected by Physics Letters, and before resubmitting it to Physical Review Letters.[12] The first detailed description of the Higgs boson properties was given in 1966, also by Peter Higgs.[13]
The Higgs boson existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson exists in some but not all theories which use the Higgs mechanism. For example, Higgs boson exists in the Standard Model and the Minimal Supersymmetric Standard Model. Yet it is not expected to exist in Technicolor models or, more generally, Higgsless models. All of these models realize various forms of the Higgs mechanism. A major goal of the LHC experiments is to distinguish among these models and determine if the Higgs boson exists or not.
[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 if it is heavy enough.
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 that couples to the Higgs field, 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 that travel through the field, converting them into particles with mass that 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 scalar particles: two CP-even neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±.
There are over a hundred theoretical Higgs-mass predictions.[14]
[edit] Experimental search
Status as of August 2010, to 95% confidence interval
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 December 2010, the Higgs boson has yet to be confirmed experimentally,[15] despite large efforts invested in accelerator experiments at CERN and Fermilab.
Prior to the year 2000, 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.[16] The LEP was shut down in 2000 due to construction of its successor, the Large Hadron Collider which is expected to be able to confirm or reject the existence of the Higgs boson. Full operational mode was delayed until mid-November 2009, because of a serious fault discovered with a number of magnets during the calibration and startup phase.[17][18]
At the Fermilab Tevatron, there are ongoing experiments searching for the Higgs boson. As of July 2010, combined data from CDF and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range between 158 GeV/c2 and 175 GeV/c2 at the 95% confidence level.[19] [20] Data collection and analysis in search of Higgs are intensifying since March 30, 2010 when the LHC began operating at 3.5 Tev and is rapidly approaching in its design range of 7 Tev, well above that at which detection should occur.[21]
It may be possible to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has a number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons. Precision measurements of electroweak 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 (the central value corresponds to approximately 138 proton masses).[22] 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 Standard Model is correct. One may still discover a Higgs boson above 186 GeV if it is accompanied by other particles between Standard Model and GUT scales.
Some have argued that there already exists potential evidence,[23][24][25] but to date no such evidence has convinced the physics community.
In a 2009 preprint,[26] it was suggested (and reported under headlines such as Higgs could reveal itself in Dark-Matter collisions[27]) that the Higgs boson might not only interact with the above-mentioned particles of the Standard model of particle physics, but also with the mysterious WIMPs ("weakly interacting massive particles") of the Dark matter, playing a most-important role in recent astrophysics. In this case, it is natural to augment the above Feynman diagrams by terms representing such an interaction.
In principle, a relation between the Higgs particle and the Dark matter would be "not unexpected", since, (i), the Higgs field does not directly couple to the quanta of light (i.e. the photons), while at the same time, (ii), it generates mass. However, "dark matter" is a metonym for the discrepancy between the apparent observed mass of the universe and that given by the standard model and is not a component of any known theory of physics. Consequently, the usefulness of this conjecture is limited.
Barring discovery during current intensive efforts, it will be sometime after the end of the current physics fill at the LHC in 2011 and some further months or years of analysis of the collected data before scientists can confidently believe that the Higgs boson does not exist.
[edit] Alternatives for electroweak symmetry breaking
Mergefrom.svg
It has been suggested that Higgsless model be merged into this article or section. (Discuss)
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[28] 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.[29]
Abbott-Farhi models of composite W and Z vector bosons.[30]
Top quark condensate.
Braid model of Standard Model particles by Sundance Bilson-Thompson, compatible with loop quantum gravity and similar theories.[31]
[edit] "The God particle"
The Higgs boson is often referred to as "the God particle" by the media,[32] after the title of Leon Lederman's book, The God Particle: If the Universe Is the Answer, What Is the Question?.[33] While use of this term may have contributed to increased media interest in particle physics and the Large Hadron Collider,[33] many scientists dislike it, since it overstates the particle's importance, not least since its discovery would still leave unanswered questions about the unification of QCD, the Electroweak interaction and gravity and the ultimate origin of the universe.[32] In a renaming competition, a jury of physicists chose the name "the champagne bottle boson" as the best popular name.[34]
[edit] See also
Book: Particles of the Standard Model
Wikipedia Books are collections of articles that can be downloaded or ordered in print.
Higgs boson in fiction
Higgs mechanism
List of particles
Overview and Differences of 1964 PRL Symmetry Breaking papers
Quantum triviality
Yukawa interaction
ZZ diboson
[edit] References
^ This upper bound for the Higgs boson mass is a prediction within the minimal Standard Model assuming that it remains a consistent theory up to the Planck scale. In extensions of the SM, this bound can be loosened or, in the case of supersymmetric theories, lowered. The lower bound which results from direct experimental exclusion by LEP is valid for most extensions of the SM, but can be circumvented in special cases. [1]
^ The masses of composite particles such as the proton and neutron would only be partly due to the Higgs Mechanism and are already understood as a consequence of the strong interaction.
^ Lee, Benjamin W.; Quigg, C.; Thacker, H. B. (1977). "Weak interactions at very high energies: The role of the Higgs-boson mass". Phys. Rev. D 16: 1519–1531. doi:10.1103/PhysRevD.16.1519.
^ "Huge $10 billion collider resumes hunt for 'God particle' - CNN.com". CNN. 2009-11-11. Retrieved 2010-05-04.
^ Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia
^ Englert, François; Brout, Robert (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13: 321–23. doi:10.1103/PhysRevLett.13.321
^ Higgs, Peter (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13: 508–509. doi:10.1103/PhysRevLett.13.508
^ Guralnik, Gerald; Hagen, C. R.; Kibble, T. W. B. (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13: 585–587. doi:10.1103/PhysRevLett.13.585
^ Physical Review Letters - 50th Anniversary Milestone Papers. Physical Review Letters
^ "American Physical Society - J. J. Sakurai Prize Winners"
^ "LEP Electroweak Working Group"
^ Higgs, Peter (2007). "Prehistory of the Higgs boson". Comptes Rendus Physique 8: 970–972. doi:10.1016/j.crhy.2006.12.006
^ Higgs, Peter (1966). "Spontaneous Symmetry Breakdown without Massless Bosons". Physical Review 145: 1156–1163. doi:10.1103/PhysRev.145.1156
^ T. Schücker (2007). "Higgs-mass predictions". arXiv:0708.3344 [hep-ph].
^ Scientists present first “bread-and-butter” results from LHC collisions Symmetry Breaking, 8 June 2010
^ W.-M. Yao et al. (2006). Searches for Higgs Bosons "Review of Particle Physics". Journal of Physics G 33: 1. doi:10.1088/0954-3899/33/1/001.
^ "CERN management confirms new LHC restart schedule". CERN Press Office. 9 February 2009. Retrieved 2009-02-10.
^ "CERN reports on progress towards LHC restart". CERN Press Office. 19 June 2009. Retrieved 2009-07-21.
^ T. Aaltonen et al. (CDF and DØ Collaborations) (2010). "Combination of Tevatron searches for the standard model Higgs boson in the W+W− decay mode". arXiv:1001.4162 [hep-ex].
^ "Fermilab experiments narrow allowed mass range for Higgs boson". Fermilab. 26 July 2010. Retrieved 2010-07-26.
^ CERN Bulletin Issue No. 18-20/2010 - Monday 3 May 2010
^ "H0 Indirect Mass Limits from Electroweak Analysis."
^ Potential Higgs Boson discovery: "Higgs Boson: Glimpses of the God particle." New Scientist, 02 March 2007
^ "'God particle' may have been seen," BBC news, 10 March 2004.
^ US experiment hints at 'multiple God particles' BBC News 14 June 2010
^ arXiv:0912.0004 Higgs in Space!
^ Physics World, [2], a website supported by the British Institute of Physics
^ S. Dimopoulos and Leonard Susskind (1979). "Mass Without Scalars". Nuclear Physics B 155: 237–252. doi:10.1016/0550-3213(79)90364-X.
^ 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. doi:10.1103/PhysRevLett.92.101802. PMID 15089195. arXiv:hep-ph/0308038.
^ L. F. Abbott and E. Farhi (1981). "Are the Weak Interactions Strong?". Physics Letters B 101: 69. doi:10.1016/0370-2693(81)90492-5.
^ Bilson-Thompson, Sundance O.; Markopoulou, Fotini; Smolin, Lee (2007). "Quantum gravity and the standard model". Class. Quantum Grav. 24 (16): 3975–3993. doi:10.1088/0264-9381/24/16/002. arXiv:hep-th/0603022.
^ a b Ian Sample (29 May 2009). "Anything but the God particle". London: The Guardian. Retrieved 2009-06-24.
^ a b Ian Sample (3 March 2009). "Father of the God particle: Portrait of Peter Higgs unveiled". London: The Guardian. Retrieved 2009-06-24.
^ Sample, Ian (2009-06-12). "Higgs competition: Crack open the bubbly, the God particle is dead". The Guardian (London). Retrieved 2010-05-04.
[edit] Further reading
G.S. Guralnik, C.R. Hagen and T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13: 585. doi:10.1103/PhysRevLett.13.585.
G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A 24: 2601–2627. doi:10.1142/S0217751X09045431. arXiv:0907.3466.
F. Englert and R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13: 321. doi:10.1103/PhysRevLett.13.321.
P. Higgs (1964). "Broken Symmetries, Massless Particles and Gauge Fields". Physics Letters 12: 132. doi:10.1016/0031-9163(64)91136-9.
P. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13: 508. doi:10.1103/PhysRevLett.13.508.
P. Higgs (1966). "Spontaneous Symmetry Breakdown without Massless Bosons". Physical Review 145: 1156. doi:10.1103/PhysRev.145.1156.
Y. Nambu and G. Jona-Lasinio (1961). "Dynamical Model of Elementary Particles Based on an Analogy with Superconductivity". Physical Review 122: 345–358. doi:10.1103/PhysRev.122.345.
J. Goldstone, A. Salam and S. Weinberg (1962). "Broken Symmetries". Physical Review 127: 965. doi:10.1103/PhysRev.127.965.
P.W. Anderson (1963). "Plasmons, Gauge Invariance, and Mass". Physical Review 130: 439. doi:10.1103/PhysRev.130.439.
A. Klein and B.W. Lee (1964). "Does Spontaneous Breakdown of Symmetry Imply Zero-Mass Particles?". Physical Review Letters 12: 266. doi:10.1103/PhysRevLett.12.266.
W. Gilbert (1964). "Broken Symmetries and Massless Particles". Physical Review Letters 12: 713. doi:10.1103/PhysRevLett.12.713.
[edit] External links
Explanation for beginners
"The Higgs boson" by the CERN exploratorium.
"Particle Data Group: Review of searches for Higgs bosons."
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