Movatterモバイル変換

[0]ホーム
URL:

Jump to content
WikipediaThe Free Encyclopedia
Search

Top quark

From Wikipedia, the free encyclopedia
Type of quark

Top quark
A collision event involving top quarks
Compositionelementary particle
Statisticsfermionic
Familyquark
Generationthird
Interactionsstrong,weak,electromagnetic,gravity
Symbolt
Antiparticletop antiquark (t)
TheorizedMakoto Kobayashi andToshihide Maskawa (1973)
DiscoveredCDF andDØ collaborations (1995)
Mass172.76±0.3 GeV/c2[1]
Mean lifetime5×10−25 s
Decays intobottom quark (99.8%)
strange quark (0.17%)
down quark (0.007%)
Electric charge+⁠ 2 /3⁠e
Color chargeyes
Spin⁠ 1 /2⁠ ħ
Topness1
Weak isospinLH: +⁠ 1 /2⁠,RH: 0
Weak hyperchargeLH: +⁠1 /3⁠,RH: +⁠4/3⁠

Thetop quark, sometimes also referred to as thetruth quark, (symbol: t) is the most massive of all observedelementary particles. It derives its mass from its coupling to theHiggs field. This couplingyt is very close to unity; in theStandard Model ofparticle physics, it is the largest (strongest) coupling at the scale of the weak interactions and above. The top quark was discovered in 1995 by theCDF[2] andDØ[3] experiments atFermilab.

Like all otherquarks, the top quark is afermion withspin-1/2 and participates in all fourfundamental interactions:gravitation,electromagnetism,weak interactions, andstrong interactions. It has anelectric charge of +⁠ 2 /3⁠ e. It has amass of172.76±0.3 GeV/c2,[1] which is close to therhenium atom mass (more precisely, the average of its isotopes).[4] Theantiparticle of the top quark is thetop antiquark (symbol:t, sometimes calledantitop quark or simplyantitop), which differs from it only in that some of its properties haveequal magnitude but opposite sign.

The top quark interacts withgluons of thestrong interaction and is typically produced in hadron colliders via this interaction. However, once produced, the top (or antitop) can decay only through theweak force. It decays to aW boson and either abottom quark (most frequently), astrange quark, or, on the rarest of occasions, adown quark.[a]

The Standard Model determines the top quark'smean lifetime to be roughly5×10−25 s.[5] This is about a twentieth of the timescale for strong interactions,[b] and therefore it does notform hadrons, giving physicists a unique opportunity to study a "bare" quark (all other quarkshadronize, meaning that they combine with other quarks to formhadrons and can only be observed as such).

Because the top quark is so massive, its properties allowed indirect determination of the mass of theHiggs boson (see§ Mass and coupling to the Higgs boson below). As such, the top quark's properties are extensively studied as a means to discriminate between competing theories of new physics beyond the Standard Model. The top quark is the only quark that has been directly observed due to its decay time being shorter than the hadronization time.[b][6]

History

[edit]

In 1973,Makoto Kobayashi andToshihide Maskawa predicted the existence of a third generation of quarks to explain observedCP violations inkaondecay. The names top andbottom were introduced byHaim Harari in 1975,[7][8] to match the names of the first generation of quarks (up anddown) reflecting the fact that the two were the "up" and "down" component of aweak isospindoublet.[9][10]

The proposal of Kobayashi and Maskawa heavily relied on theGIM mechanism put forward bySheldon Glashow,John Iliopoulos andLuciano Maiani,[11] which predicted the existence of the then still unobservedcharm quark. (Direct evidence for the existence of quarks, including the othersecond generation quark, thestrange quark, was obtained in 1968; strange particles were discovered back in 1947.) When inNovember 1974 teams atBrookhaven National Laboratory (BNL) and theStanford Linear Accelerator Center (SLAC) simultaneously announced the discovery of theJ/ψ meson, it was soon after identified as a bound state of the missing charm quark with its antiquark. This discovery allowed the GIM mechanism to become part of the Standard Model.[12] With the acceptance of the GIM mechanism, Kobayashi and Maskawa's prediction also gained in credibility. Their case was further strengthened by the discovery of thetau byMartin Lewis Perl's team at SLAC between 1974 and 1978.[13] The tau announced a third generation ofleptons, breaking the newsymmetry between leptons and quarks introduced by the GIM mechanism. Restoration of the symmetry implied the existence of a fifth and sixth quark.[citation needed]

It was in fact not long until a fifth quark, the bottom, was discovered by theE288 experiment team, led byLeon Lederman atFermilab in 1977.[14][15][16] This strongly suggested that there must also be a sixth quark, the top, to complete the pair. It was known that this quark would be heavier than the bottom, requiring more energy to create in particle collisions, but the general expectation was that the sixth quark would soon be found. However, it took another 18 years before the existence of the top was confirmed.[17]

Early searches for the top quark atSLAC andDESY (inHamburg) came up empty-handed. When, in the early 1980s, theSuper Proton Synchrotron (SPS) atCERN discovered theW boson and theZ boson, it was again felt that the discovery of the top was imminent. As the SPS gained competition from theTevatron at Fermilab there was still no sign of the missing particle, and it was announced by the group at CERN that the top mass must be at least41 GeV/c2. After a race between CERN and Fermilab to discover the top, the accelerator at CERN reached its limits without creating a single top, pushing the lower bound on its mass up to77 GeV/c2.[17]

The Tevatron was (until the start ofLHC operation atCERN in 2009) the only hadron collider powerful enough to produce top quarks. In order to be able to confirm a future discovery, a second detector, theDØ detector, was added to the complex (in addition to theCollider Detector at Fermilab (CDF) already present). In October 1992, the two groups found their first hint of the top, with a single creation event that appeared to contain the top. In the following years, more evidence was collected and on 22 April 1994, the CDF group submitted their article presenting tentative evidence for the existence of a top quark with a mass of about175 GeV/c2. In the meantime, DØ had found no more evidence than the suggestive event in 1992. A year later, on 2 March 1995, after having gathered more evidence and reanalyzed the DØ data (which had been searched for a much lighter top), the two groups jointly reported the discovery of the top at a mass of176±18 GeV/c2.[2][3][17]

In the years leading up to the top-quark discovery, it was realized that certain precision measurements of the electroweak vector boson masses and couplings are very sensitive to the value of the top-quark mass. These effects become much larger for higher values of the top mass and therefore could indirectly see the top quark even if it could not be directly detected in any experiment at the time. The largest effect from the top-quark mass was on theT parameter, and by 1994 the precision of these indirect measurements had led to a prediction of the top-quark mass to be between145 GeV/c2 and185 GeV/c2.[17] It is the development of techniques that ultimately allowed such precision calculations that led toGerardus 't Hooft andMartinus Veltman winning theNobel Prize in physics in 1999.[18][19]

Properties

[edit]
  • At the final Tevatron energy of 1.96 TeV, top–antitop pairs were produced with across section of about 7 picobarn (pb).[20] TheStandard Model prediction (atnext-to-leading order withmt =175 GeV/c2) is 6.7–7.5 pb.
  • The W bosons from top quark decays carry polarization from the parent particle, hence pose themselves as a unique probe to top polarization.
  • In the Standard Model, the top quark is predicted to have a spin quantum number of⁠ 1 /2⁠ ħ and electric charge⁠++ 2 /3⁠ e. A first measurement of the top quark charge has been published, resulting in some confidence that the top quark charge is indeed ⁠++ 2 /3⁠ e.[21]

Production

[edit]

Because top quarks are very massive, large amounts of energy are needed to create one. The only way to achieve such high energies is through high-energy collisions. These occur naturally in the Earth's upper atmosphere ascosmic rays collide with particles in the air, or can be created in aparticle accelerator. In 2011, after theTevatron ceased operations, theLarge Hadron Collider atCERN became the only accelerator that generates a beam of sufficient energy to produce top quarks, with acenter-of-mass energy of 7 TeV. There are multiple processes that can lead to the production of top quarks, but they can be conceptually divided in two categories: top-pair production, and single-top production.[citation needed]

Top-quark pairs

[edit]
gluon–gluon fusion
t-channel
quark–antiquark annihilation

The most common isproduction of a top–antitop pair viastrong interactions. In a collision, a highly energeticgluon is created, which subsequently decays into a top and antitop. This process was responsible for the majority of the top events at Tevatron and was the process observed when the top was first discovered in 1995.[22] It is also possible to produce pairs of top–antitop through the decay of an intermediatephoton orZ-boson. However, these processes are predicted to be much rarer and have a virtually identical experimental signature in ahadron collider like Tevatron.[citation needed]

Single top quarks

[edit]
s-channel
t-channel
tW channel

The production of single top quarks viaweak interaction is a distinctly different process. This can happen in several ways (called channels): Either an intermediateW-boson decays into a top and antibottom quarks ("s-channel") or a bottom quark (probably created in a pair through the decay of a gluon) transforms to a top quark by exchanging a W boson with an up or down quark ("t-channel"). A single top quark can also be produced in association with a W boson, requiring an initial-state bottom quark ("tW-channel"). The first evidence for these processes was published by the DØ collaboration in December 2006,[23] and in March 2009 the CDF[24] and DØ[22] collaborations released twin articles with the definitive observation of these processes. The main significance of measuring these production processes is that their frequency is directly proportional to the|Vtb|2 component of theCKM matrix.[citation needed]

Decay

[edit]
ttbar-decay final states
All possible final states of the decay of a top-quark pair

The only known way the top quark can decay is through theweak interaction, producing aW boson and abottom quark.[a]Because of its enormousmass, the top quark is extremely short-lived, with a predicted lifetime of only5×10−25 s.[5] As a result, top quarks do not have time before they decay toform hadrons as other quarks do.[b]The absence of a hadron surrounding the top quark provides physicists with the unique opportunity to study the behavior of a "bare" quark.

In particular, it is possible to directly determine thebranching ratio:

B⁡( t→W+ b )≡Γ⁡( t→W+ b ) ∑q = b,s,dΓ⁡( t→W+ q )  .{\displaystyle \operatorname {B} \left(\ \mathrm {t} \to W^{+}\ \mathrm {b} \ \right)\equiv {\frac {\operatorname {\Gamma } \left(\ \mathrm {t} \to W^{+}\ \mathrm {b} \ \right)}{\ \sum _{q\ =\ \mathrm {b,s,d} }\operatorname {\Gamma } \left(\ \mathrm {t} \to W^{+}\ q\ \right)\ }}~.}

The best current determination of this ratio is0.957±0.034.[25] Since this ratio is equal to|Vtb|2 according to theStandard Model, this gives another way of determining the CKM element |Vtb|, or in combination with the determination of|Vtb| from single top production provides tests for the assumption that the CKM matrix is unitary.[26]

The Standard Model also allows more exotic decays, but only at one loop level, meaning that they are extremely rare. In particular, it is conceivable that a top quark might decay into another up-type quark (an up or a charm) by emitting a photon or a Z-boson.[27] However, searches for these exotic decay modes have produced no evidence that they occur, in accordance with expectations of the Standard Model. The branching ratios for these decays have been determined to be less than 1.8 in 10000 for photonic decay and less than 5 in 10000 for Z boson decay at 95%confidence.[25]

Mass and coupling to the Higgs boson

[edit]

The Standard Model generates fermion masses through their couplings to theHiggs boson. This Higgs boson acts as a field that fills space. Fermions interact with this field in proportion to their individual coupling constantsyi, which generates mass. A low-mass particle, such as theelectron has a minuscule couplingyelectron =2×10−6, while the top quark has the largest coupling to the Higgs,yt ≈ 1.

In the Standard Model, all of the quark and lepton Higgs–Yukawa couplings are small compared to the top-quark Yukawa coupling. This hierarchy in the fermion masses remains a profound and open problem in theoretical physics. Higgs–Yukawa couplings are not fixed constants of nature, as their values vary slowly as the energy scale (distance scale) at which they are measured. These dynamics of Higgs–Yukawa couplings, called "running coupling constants", are due to a quantum effect called therenormalization group.

The Higgs–Yukawa couplings of the up, down, charm, strange and bottom quarks are hypothesized to have small values at the extremely high energy scale of grand unification,1015 GeV. They increase in value at lower energy scales, at which the quark masses are generated by the Higgs. The slight growth is due to corrections from theQCD coupling. The corrections from the Yukawa couplings are negligible for the lower-mass quarks.

One of the prevailing views in particle physics is that the size of the top-quark Higgs–Yukawa coupling is determined by a unique nonlinear property of therenormalization group equation that describes therunning of the large Higgs–Yukawa coupling of the top quark. If a quark Higgs–Yukawa coupling has a large value at very high energies, its Yukawa corrections will evolve downward in mass scale and cancel against the QCD corrections. This is known as a (quasi-)infrared fixed point, which was first predicted by B. Pendleton and G.G. Ross,[28] and byChristopher T. Hill,[29] No matter what the initial starting value of the coupling is, if sufficiently large, it will reach this fixed-point value. The corresponding quark mass is then predicted.

The top-quark Yukawa coupling lies very near theinfrared fixed point of the Standard Model. The renormalization group equation is:

μ  ∂∂μ yt≈ yt 16 π2( 9 2yt2−8g32− 9 4g22− 17 20g12) ,{\displaystyle \mu \ {\frac {\ \partial }{\partial \mu }}\ y_{\mathrm {t} }\approx {\frac {\ y_{\text{t}}\ }{16\ \pi ^{2}}}\left({\frac {\ 9\ }{2}}y_{\mathrm {t} }^{2}-8g_{3}^{2}-{\frac {\ 9\ }{4}}g_{2}^{2}-{\frac {\ 17\ }{20}}g_{1}^{2}\right)\ ,}

whereg3 is the color gauge coupling,g2 is the weak isospin gauge coupling, andg1 is the weak hypercharge gauge coupling. This equation describes how the Yukawa coupling changes with energy scale μ. Solutions to this equation for large initial valuesyt cause the right-hand side of the equation to quickly approach zero, lockingyt to the QCD couplingg3.

The value of the top quark fixed point is fairly precisely determined in the Standard Model, leading to a top-quark mass of 220 GeV. This is about 25% larger than the observed top mass and may be hinting at new physics at higher energy scales.

The quasi-infrared fixed point subsequently became the basis oftop quark condensation andtopcolor theories of electroweak symmetry breaking, in which the Higgs boson is composed of a pair of top and antitop quarks. The predicted top-quark mass comes into improved agreement with the fixed point if there are additional Higgs scalars beyond the standard model and therefore it may be hinting at a rich spectroscopy of new Higgs fields at energy scales that can be probed with the LHC and its upgrades.[30][31]

See also

[edit]

Footnotes

[edit]
  1. ^abThe overwhelming majority of top quark decays produce abottom quark, whose mass is closest to the top's. On very rare occasions it may decay into astrange quark;almost never adown quark.
  2. ^abcTop quark decay is an exceptional example of aweak process that is faster than astrong interaction.

References

[edit]
  1. ^abZyla, P.A.; et al. (Particle Data Group) (2020)."2020 Review of Particle Physics".Progress of Theoretical and Experimental Physics: 083C01.
  2. ^abAbe, F.; et al. (CDF Collaboration) (1995). "Observation of top quark production inpp collisions with the Collider-Detector at Fermilab".Physical Review Letters.74 (14):2626–2631.arXiv:hep-ex/9503002.Bibcode:1995PhRvL..74.2626A.doi:10.1103/PhysRevLett.74.2626.PMID 10057978.S2CID 119451328.
  3. ^abAbachi, S.; et al. (DØ Collaboration) (1995). "Observation of the Top Quark".Physical Review Letters.74 (14):2632–2637.arXiv:hep-ex/9503003.Bibcode:1995PhRvL..74.2632A.doi:10.1103/PhysRevLett.74.2632.PMID 10057979.S2CID 42826202.
  4. ^Elert, Glenn."Quantum Chromodynamics".The Physics Hypertextbook. Retrieved23 March 2019.
  5. ^abQuadt, A. (2006)."Top quark physics at hadron colliders".European Physical Journal C.48 (3):835–1000.Bibcode:2006EPJC...48..835Q.doi:10.1140/epjc/s2006-02631-6.S2CID 121887478.
  6. ^Aubert, Jean-Jacques; Gastmans, Raymond; Gérard, Jean-Marc (6 December 2012).Particle Physics: Ideas and recent developments. Springer, Dordrecht. p. 69.ISBN 978-0-7923-6436-8. Retrieved11 June 2020.
  7. ^Harari, H. (1975). "A new quark model for hadrons".Physics Letters B.57 (3): 265.Bibcode:1975PhLB...57..265H.doi:10.1016/0370-2693(75)90072-6.
  8. ^Staley, K.W. (2004).The Evidence for the Top Quark.Cambridge University Press. pp. 31–33.ISBN 978-0-521-82710-2.
  9. ^Perkins, D.H. (2000).Introduction to High Energy Physics.Cambridge University Press. p. 8.ISBN 978-0-521-62196-0.
  10. ^Close, F. (2006).The New Cosmic Onion.CRC Press. p. 133.ISBN 978-1-58488-798-0.
  11. ^Glashow, S.L.; Iliopoulous, J.; Maiani, L. (1970). "Weak interactions with lepton–hadron symmetry".Physical Review D.2 (7):1285–1292.Bibcode:1970PhRvD...2.1285G.doi:10.1103/PhysRevD.2.1285.
  12. ^Pickering, A. (1999).Constructing Quarks: A sociological history of particle physics.University of Chicago Press. pp. 253–254.ISBN 978-0-226-66799-7.
  13. ^Perl, M.L.; et al. (1975). "Evidence for anomalous lepton production ine+
    e−
     annihilation".Physical Review Letters.35 (22): 1489.Bibcode:1975PhRvL..35.1489P.doi:10.1103/PhysRevLett.35.1489.
  14. ^"Discovery of the bottom quark" (Press release). Discoveries at Fermilab.Fermilab. 7 August 1977. Retrieved24 July 2009.
  15. ^Lederman, L.M. (2005)."Logbook: Bottom quark".Symmetry Magazine. Vol. 2, no. 8. Archived fromthe original on 4 October 2006.
  16. ^Herb, S.W.; et al. (1977). "Observation of a di‑muon resonance at 9.5 GeV in 400 GeV proton–nucleus collisions".Physical Review Letters.39 (5): 252.Bibcode:1977PhRvL..39..252H.doi:10.1103/PhysRevLett.39.252.OSTI 1155396.
  17. ^abcdLiss, T.M.; Tipton, P.L. (1997)."The discovery of the top quark"(PDF).Scientific American. Vol. 277, no. 3. pp. 54–59.doi:10.1038/scientificamerican0997-54.
  18. ^"The Nobel Prize in Physics 1999" (Press release).The Nobel Foundation. Retrieved10 September 2009.
  19. ^"The Nobel Prize in Physics 1999" (Press release).The Nobel Foundation. 12 October 1999. Retrieved10 September 2009.
  20. ^Chakraborty, D.; et al. (DØ collaboration &CDF collaboration) (2002).Top quark and W/Z results from the Tevatron(PDF). Rencontres de Moriond. p. 26.arXiv:hep-ex/0212027.
  21. ^Abazov, V.M.; et al. (DØ Collaboration) (2007). "Experimental discrimination between charge 2e/3 top quark and charge 4e/3 exotic quark production scenarios".Physical Review Letters.98 (4) 041801.arXiv:hep-ex/0608044.Bibcode:2007PhRvL..98d1801A.doi:10.1103/PhysRevLett.98.041801.hdl:10211.3/194390.PMID 17358756.S2CID 1147194.
  22. ^abAbazov, V.M.; et al. (DØ Collaboration) (2009). "Observation of Single Top Quark Production".Physical Review Letters.103 (9) 092001.arXiv:0903.0850.Bibcode:2009PhRvL.103i2001A.doi:10.1103/PhysRevLett.103.092001.hdl:10211.3/194327.PMID 19792787.S2CID 14919683.
  23. ^Abazov, V.M.; et al. (DØ Collaboration) (2007). "Evidence for production of single top quarks and first direct measurement of |Vtb|".Physical Review Letters.98 (18) 181802.arXiv:hep-ex/0612052.Bibcode:2007PhRvL..98r1802A.doi:10.1103/PhysRevLett.98.181802.hdl:10211.3/194387.PMID 17501561.S2CID 14937909.
  24. ^Aaltonen, T.; et al. (CDF Collaboration) (2009). "First observation of electroweak single top quark production".Physical Review Letters.103 (9) 092002.arXiv:0903.0885.Bibcode:2009PhRvL.103i2002A.doi:10.1103/PhysRevLett.103.092002.hdl:1721.1/52314.PMID 19792788.S2CID 8029289.
  25. ^abZyla, P.A.; et al. (Particle Data Group) (2020)."QUARKS"(PDF).Progress of Theoretical and Experimental Physics: 083C01. Retrieved22 May 2022.
  26. ^Abazov, V.M.; et al. (DØ Collaboration) (2008). "Simultaneous measurement of the ratioB(t → Wb)/B(t → Wq) and the top-quark pair production cross section with the DØ detector at√s = 1.96 TeV".Physical Review Letters.100 (19): 192003.arXiv:0801.1326.Bibcode:2008PhRvL.100s2003A.doi:10.1103/PhysRevLett.100.192003.hdl:10211.3/194369.PMID 18518440.S2CID 2638258.{{cite journal}}: CS1 maint: article number as page number (link)
  27. ^Chekanov, S.; et al. (ZEUS Collaboration) (2003). "Search for single-top production in e-p collisions at HERA".Physics Letters B.559 (3–4):153–170.arXiv:hep-ex/0302010.Bibcode:2003PhLB..559..153Z.doi:10.1016/S0370-2693(03)00333-2.S2CID 119494760.
  28. ^Pendleton, Brian; Ross, Graham (1981). "Mass and mixing angle predictions from infrared fixed points".Physics Letters.98B (4):291–294.Bibcode:1981PhLB...98..291P.doi:10.1016/0370-2693(81)90017-4.
  29. ^Hill, Christopher T. (1981). "Quark and lepton masses from renormalization group fixed points".Physical Review D.24 (3):691–703.Bibcode:1981PhRvD..24..691H.doi:10.1103/PhysRevD.24.691.
  30. ^Hill, Christopher T.; Machado, Pedro; Thomsen, Anders; Turner, Jessica (2019). "Where are the next Higgs bosons?".Physical Review D.100 (1) 015051.arXiv:1904.04257.Bibcode:2019PhRvD.100a5051H.doi:10.1103/PhysRevD.100.015051.S2CID 104291827.
  31. ^Hill, Christopher T.; Machado, Pedro; Thomsen, Anders; Turner, Jessica (2019). "Scalar democracy".Physical Review D.100 (1) 015015.arXiv:1902.07214.Bibcode:2019PhRvD.100a5015H.doi:10.1103/PhysRevD.100.015015.S2CID 119193325.

Further reading

[edit]

External links

[edit]
Elementary
Fermions
Quarks
Leptons
Bosons
Gauge
Scalar
Ghost fields
Hypothetical
Superpartners
Gauginos
Others
Others
Composite
Hadrons
Baryons
Mesons
Exotic hadrons
Others
Hypothetical
Baryons
Mesons
Others
Quasiparticles
Lists
Related
Authority control databasesEdit this at Wikidata
Retrieved from "https://en.wikipedia.org/w/index.php?title=Top_quark&oldid=1316957944"
Categories:
Hidden categories:
[8]ページ先頭
©2009-2025 Movatter.jp