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Charm quark

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From Wikipedia, the free encyclopedia
(Redirected fromCharm antiquark)
Type of quark
"Charm (physics)" redirects here. For other uses, seeCharm (disambiguation) § Science and technology.

Charm quark
CompositionElementary particle
StatisticsFermion
FamilyQuark
GenerationSecond
Interactionsstrong,electromagnetic,weak,gravity
Symbol
c
AntiparticleCharm antiquark (
c
)
Theorized
Discovered
Mass1.27±0.02 GeV/c2
Electric charge+⁠2/3⁠e
Color chargeYes
Spin⁠1/2⁠ ħ
Weak isospinLH: +⁠1/2⁠,RH: 0
Weak hyperchargeLH: +⁠1/3⁠,RH: +⁠4/3⁠
Standard Model ofparticle physics
Elementary particles of the Standard Model

Thecharm quark,charmed quark, orc quark is anelementary particle found in composite subatomic particles calledhadrons such as theJ/psi meson and thecharmed baryons created in particle accelerator collisions. Severalbosons, including theW and Z bosons and theHiggs boson, can decay into charm quarks. All charm quarks carrycharm, aquantum number. This second-generation particle is the third-most-massivequark, with a mass of1.27±0.02 GeV/c2 as measured in 2022, and a charge of +⁠2/3⁠ e.

The existence of the charm quark was first predicted byJames Bjorken andSheldon Glashow in 1964,[1][2][3] and in 1970, Glashow,John Iliopoulos, andLuciano Maiani showed how its existence would account for experimental and theoretical discrepancies.[4] In 1974, its existence was confirmed through the independent discoveries of theJ/psi meson atBrookhaven National Laboratory and theStanford Linear Accelerator Center. In the next few years, several other charmed particles, including theD meson and the charmed strange mesons, were found.

In the 21st century, abaryon containing two charm quarks has been found. There is recent evidence that intrinsic charm quarks exist in theproton, and the coupling of the charm quark and the Higgs boson has been studied. Recent evidence also indicatesCP violation in the decay of the D0 meson, which contains the charm quark.

Naming

[edit]

According toSheldon Glashow, the charm quark received its name because of the "symmetry it brought to the subnuclear world".[5][6] Glashow also justified the name as "a magical device to avert evil", because adding the charm quark would prohibit unwanted and unseen decays in the three-quark theory at the time.[5] The charm quark is also called the "charmed quark" in both academic and non-academic contexts.[7][8][9] The symbol of the charm quark is "c".[10]

History

[edit]

Background

[edit]
See also:Quark model

In 1961,Murray Gell-Mann introduced theEightfold Way as a pattern to groupbaryons andmesons.[11] In 1964, Gell-Mann andGeorge Zweig independently proposed that allhadrons are composed of elementary constituents, which Gell-Mann called "quarks".[12] Initially, only theup quark, thedown quark, and thestrange quark were proposed.[13] These quarks would produce all of the particles in the Eightfold Way.[14] Gell-Mann andKazuhiko Nishijima had establishedstrangeness, a quantum number, in 1953 to describe processes involvingstrange particles such as
Σ
 and 
Λ
.[15]

Theoretical prediction

[edit]
See also:GIM mechanism andScientific wager
TheGIM mechanism explains the rarity of the decay of a
K0
 into twomuons by involving the charm quark (c) in the process.

In 1964,James Bjorken and Sheldon Glashow theorized "charm" as a new quantum number.[16] At the time, there were four knownleptons—theelectron, themuon, and each of theirneutrinos—but Gell-Mann initially proposed only three quarks.[6] Bjorken and Glashow thus hoped to establish parallels between the leptons and the quarks with their theory.[17] According to Glashow, the conjecture came from "aesthetic arguments".[5]

In 1970, Glashow,John Iliopoulos, andLuciano Maiani proposed a new quark that differed from the three then-known quarks by thecharm quantum number.[4][18] They further predicted the existence of "charmed particles" and offered suggestions on how to experimentally produce them.[19] They also suggested the charmed quark could provide a mechanism—theGIM mechanism—to facilitate the unification of theweak andelectromagnetic forces.[20]

At the Conference on Experimental Meson Spectroscopy (EMS) in April 1974, Glashow delivered his paper titled "Charm: An Invention Awaits Discovery". Glashow asserted becauseneutral currents were likely to exist, a fourth quark was "sorely needed" to explain the rarity of the decays of certainkaons.[21] He also made several predictions on the properties of charm quarks.[22] He wagered that, by the next EMS conference in 1976:

There are just three possibilities:

  1. Charm is not found, and I eat my hat.
  2. Charm is found by hadron spectroscopers, and we celebrate.
  3. Charm is found by outlanders,[a] and you eat your hats.[22]

In July 1974, at the 17thInternational Conference on High Energy Physics (ICHEP), Iliopoulos said:

I have won already several bottles of wine by betting for the neutral currents and I am ready to bet now a whole case that if the weak interaction sessions of this Conference were dominated by the discovery of the neutral currents, the entire next Conference will be dominated by the discovery of the charmed particles.[24]

Applying an argument of naturalness to the kaon mass splitting between the K0
L and K0
S states, the mass of the charm quark was estimated byMary K. Gaillard andBenjamin W. Lee in 1974 to be less than5 GeV/c2.[25][26]

Discovery

[edit]

Glashow predicted that the down quark of a proton could absorb a
W+
 and become a charm quark. Then, the proton would be transformed into a charmed baryon before it decayed into several particles, including alambda baryon. In late May 1974, Robert Palmer andNicholas P. Samios found an event generating alambda baryon from theirbubble chamber atBrookhaven National Laboratory.[27] It took months for Palmer to be convinced the lambda baryon came from a charmed particle.[28] When the magnet of the bubble chamber failed in October 1974, they did not encounter the same event.[21] The two scientists published their observations in early 1975.[29][30]Michael Riordan commented that this event was "ambiguous" and "encouraging but not convincing evidence".[31]

J/psi meson (1974)

[edit]
Main article:J/psi meson

In 1974,Samuel C. C. Ting was searching for charmed particles atBrookhaven National Laboratory (BNL).[32] His team was using an electron-pair detector.[33] By the end of August, they found a peak at3.1 GeV/c2 and the signal's width was less than5 MeV.[34] The team was eventually convinced they had observed a massive particle and named it "J". Ting considered announcing his discovery in October 1974, but postponed the announcement due to his concern about the μ/π ratio.[35]

At theStanford Linear Accelerator Center (SLAC),Burton Richter's team performed experiments on 9–10 November 1974. They also found a high probability of interaction at3.1 GeV/c2. They called the particle "psi".[36] On 11 November 1974, Richter met Ting at the SLAC,[37] and they announced their discovery.[38]

Theorists immediately began to analyze the new particle.[39] It was shown to have a lifetime on the scale of 10−20 seconds, suggesting special characteristics.[36][40]Thomas Appelquist andDavid Politzer suggested that the particle was composed of a charm quark and a charm antiquark whosespins were aligned in parallel. The two called this configuration "charmonium".[39] Charmonium would have two forms: "orthocharmonium", where the spins of the two quarks are parallel, and "paracharmonium", where the spins align oppositely.[41] Murray Gell-Mann also believed in the idea of charmonium.[42] Some other theorists, such asRichard Feynman, initially thought the new particle consisted of anup quark with a charm antiquark.[39]

On 15 November 1974, Ting and Richter issued a press release about their discovery.[43] On 21 November at the SLAC,SPEAR found a resonance of the J/psi particle at3.7 GeV/c2 asMartin Breidenbach and Terence Goldman had predicted.[43] This particle was called ψ′ ("psi-prime").[44] In late November, Appelquist and Politzer published their paper theorizing charmonium. Glashow and Alvaro De Rujula also published a paper called "Is Bound Charm Found?", in which they used the charm quark andasymptotic freedom to explain the properties of the J/psi meson.[45]

Eventually, on 2 December 1974,Physical Review Letters (PRL) published the discovery papers of J and psi, by Ting[46] and Richter[47] respectively.[45] The discovery of the psi-prime was published the following week.[45] Then, on 6 January 1975,PRL published nine theoretical papers on the J/psi particle; according to Michael Riordan, five of them "promoted the charm hypothesis and its variations".[30] In 1976, Ting and Richter shared theNobel Prize in Physics for their discovery "of a heavy elementary particle of the new kind".[48]

In August 1976, inThe New York Times, Glashow recalled his wager and commented, "John [Iliopoulos]'s wine and my hat had been saved in the nick of time".[5] At the next EMS conference, spectroscopists ate Mexican candy hats supplied by the organizers.[49][50]Frank Close wrote aNature article titled "Iliopoulos won his bet" in the same year, saying the 18th ICHEP was "indeed dominated by that very discovery".[20] No-one paid off their bets to Iliopoulos.[51][38]

Other charmed particles (1975–1977)

[edit]

In April 1975, E. G. Cazzoli et al., including Palmer and Samios, published their earlier ambiguous evidence for the charmed baryon.[29] By the time of the Lepton–Photon Symposium in August 1975, eight new heavy particles had been discovered.[52] These particles, however, have zero total charm.[53] Starting from the fourth quarter of that year, physicists began to look for particles with a net, or "naked", charm.[54]

On 3 May 1976 at SLAC,Gerson Goldhaber and François Pierre identified a1.87 GeV/c2 peak, which suggested the presence of a neutral charmedD meson according to Glashow's prediction. On 5 May, Goldhaber and Pierre published a joint memorandum about their discovery of the "naked charm".[55] By the time of the 18th International Conference on High Energy Physics, more charmed particles had been discovered. Riordan said "solid evidence for charm surfaced in session after session" at the conference, confirming the existence of the charm quark.[56][57] The charmed strange meson was discovered in 1977.[58][59]

Later and current research

[edit]

In 2002, the SELEX Collaboration atFermilab published the first observation of the doubly charmed baryon
Ξ+
cc ("double charmed xi+").[60] It is a three-quark particle containing two charm quarks. The team found doubly charmed baryons with an up quark are more massive and have a higher rate of production than those with a down quark.[61]

In 2007, theBaBar andBelle collaborations each reported evidence for the mixing of two neutral charmed mesons,
D0
 and
D0
.[62][63][64] The evidence confirmed the mixing rate is small, as is predicted by thestandard model.[65] Neither studies found evidence forCP violation between the decays of the two charmed particles.[62][63]

In 2022, theNNPDF Collaboration found evidence for the existence of intrinsic charm quarks in the proton.[66][67] In the same year, physicists also conducted a direct search forHiggs boson decays into charm quarks using theATLAS detector of theLarge Hadron Collider.[68] They have determined that the Higgs–charm coupling is weaker than the Higgs–bottom coupling.[69] On 7 July 2022, theLHCb experiment announced they had found evidence of direct CP violation in the decay of the D0 meson intopions.[70]

Characteristics

[edit]

The charm quark is asecond-generation up-type quark.[7][64] It carries charm, aquantum number.[71] According to the 2022Particle Physics Review, the charmed quark has a mass of1.27±0.02 GeV/c2,[b] a charge of +⁠2/3⁠ e, and a charm of +1.[10] The charm quark is more massive than the strange quark: the ratio between the masses of the two is about11.76+0.05
−0.10.[10]

TheCKM matrix describes the weak interaction of quarks.[73] As of 2022, the values of the CKM matrix relating to the charm quark are:[74]|Vcd|=0.221±0.004|Vcs|=0.975±0.006|Vcb|=(40.8±1.4)×10−3{\displaystyle {\begin{aligned}|V_{\text{cd}}|&=0.221\pm 0.004\\|V_{\text{cs}}|&=0.975\pm 0.006\\|V_{\text{cb}}|&=(40.8\pm 1.4)\times 10^{-3}\end{aligned}}}

Asupermultiplet of baryons that contain the up, down, strange and charm quarks with half-spin

Charm quarks can exist in either "open charm particles", which contain one or several charm quarks, or as charmonium states, which are bound states of a charm quark and a charm antiquark.[64] There are several charmed mesons, including
D±
 and
D0
.[75] Charmed baryons include
Λ
c,
Σ
c,
Ξ
c,
Ω
c, with various charges andresonances.[76]

Production and decay

[edit]

Particles containing charm quarks can be produced via electron–positron collisions or in hadron collisions.[77] Using different energies, electron–positron colliders can produce psi orupsilon mesons.[78] Hadron colliders produce particles that contain charm quarks at a highercross section.[c][81] TheW boson can also decay into hadrons containing the charm quark or the charm antiquark.[82] The Z boson can decay intocharmonium through charm quark fragmentation.[83] The Higgs boson can also decay to
J/ψ
 or
η
c through the same mechanism. The decay rate of the Higgs boson into charmonium is "governed by the charm-quarkYukawa coupling".[84]

The charm quark can decay into other quarks via weak decays.[64] The charm quark also annihilates with the charm antiquark during the decays of ground-state charmonium mesons.[64]

References

[edit]

Notes

[edit]
  1. ^According to Riordan, the word "outlanders" means "other kinds of physicists who did neutrino scattering or measured electron–positron collisions in storage rings."[23]
  2. ^TheParticle Physics Review uses the unit GeV instead of GeV/c2.[10] This is because particle physics usesnatural units, in which thespeed of light is set to be one.[72] TheReview also notes this mass corresponds to the "running" mass in theminimal subtraction scheme (MS scheme).[10]
  3. ^According toMark Thomson, across section in particle physics is a measure of quantum mechanical probability for the interaction.[79] It is the ratio between the interaction rate per target particle and the incident particle flux.[80]

Citations

[edit]
  1. ^Amati et al. 1964.
  2. ^Maki & Ohnuki 1964.
  3. ^Hara 1964.
  4. ^abGlashow, Iliopoulos & Maiani 1970, p. 1287.
  5. ^abcdGlashow 1976.
  6. ^abRiordan 1987, p. 210.
  7. ^abHarari 1977, p. 6.
  8. ^Riordan 1992, p. 1292.
  9. ^Levine 2017.
  10. ^abcdeWorkman et al. 2022, p. 32.
  11. ^Griffiths 2008, p. 35.
  12. ^Griffiths 2008, p. 37.
  13. ^Griffiths 2008, p. 39.
  14. ^Griffiths 2008, p. 41.
  15. ^Griffiths 2008, p. 34.
  16. ^Bjorken & Glashow 1964, p. 255.
  17. ^Griffiths 2008, pp. 44–45.
  18. ^Appelquist, Barnett & Lane 1978, p. 390.
  19. ^Glashow, Iliopoulos & Maiani 1970, p. 1290–1291.
  20. ^abClose 1976, p. 537.
  21. ^abRiordan 1987, p. 297.
  22. ^abRosner 1998, p. 14.
  23. ^Riordan 1987, p. 295.
  24. ^Iliopoulos 1974, p. 100.
  25. ^Giudice, Gian Francesco. "Naturally speaking: the naturalness criterion and physics at the LHC". Perspectives on LHC physics (2008): 155–178.
  26. ^Gaillard & Lee 1974.
  27. ^Riordan 1987, pp. 295–297.
  28. ^Riordan 1987, pp. 296.
  29. ^abCazzoli et al. 1975.
  30. ^abRiordan 1987, p. 306.
  31. ^Riordan 1987, p. 306, "It was encouraging, but not convincing, evidence [...] this one was ambiguous".
  32. ^Riordan 1987, pp. 297–298.
  33. ^Ting 1977, p. 239.
  34. ^Ting 1977, p. 243.
  35. ^Ting 1977, p. 244.
  36. ^abSouthworth 1976, p. 385.
  37. ^Southworth 1976, pp. 385–386.
  38. ^abRosner 1998, p. 16.
  39. ^abcRiordan 1987, p. 300.
  40. ^Riordan 1987, p. 300.
  41. ^Riordan 1987, p. 304.
  42. ^Riordan 1987, p. 300, "Murray ... thinks that the charm–anticharm vector meson is more likely".
  43. ^abRiordan 1987, p. 301.
  44. ^Riordan 1987, p. 303.
  45. ^abcRiordan 1987, p. 305.
  46. ^Aubert et al. 1974.
  47. ^Augustin et al. 1974.
  48. ^Southworth 1976, p. 383.
  49. ^Riordan 1987, p. 321.
  50. ^Rosner 1998, p. 18.
  51. ^Riordan 1987, pp. 319–320.
  52. ^Riordan 1987, pp. 310–311.
  53. ^Riordan 1987, p. 312.
  54. ^Riordan 1987, p. 317.
  55. ^Riordan 1987, p. 318.
  56. ^Riordan 1987, p. 319, "Solid evidence for charm surfaced in session after session. There was no longer any doubt".
  57. ^Griffiths 2008, p. 47, "With these discoveries, the interpretation ... was established beyond reasonable doubt. More important, the quark model itself was put back on its feet".
  58. ^Brandelik et al. 1977.
  59. ^Griffiths 2008, p. 47.
  60. ^Mattson et al. 2002.
  61. ^Yap 2002.
  62. ^abAubert et al. 2007.
  63. ^abStarič et al. 2007.
  64. ^abcdeGersabeck 2014, p. 2.
  65. ^Aubert et al. 2007, p. 4.
  66. ^The NNPDF Collaboration 2022.
  67. ^Thompson & Howe 2022.
  68. ^Aad et al. 2022.
  69. ^ATLAS experiment 2022.
  70. ^LHCb experiment 2022, "This is the first evidence of direct CP violation in an individual charm–hadron decay (D0 → π– π+), with a significance of 3.8σ".
  71. ^Appelquist, Barnett & Lane 1978, p. 388.
  72. ^Thomson 2013, p. 31.
  73. ^Thomson 2013, p. 368.
  74. ^Workman et al. 2022, pp. 262–263.
  75. ^Workman et al. 2022, pp. 43–45.
  76. ^Workman et al. 2022, pp. 100–4.
  77. ^Gersabeck 2014, pp. 3–4.
  78. ^Gersabeck 2014, p. 3.
  79. ^Thomson 2013, p. 26.
  80. ^Thomson 2013, p. 69.
  81. ^Gersabeck 2014, p. 4.
  82. ^Thomson 2013, p. 412.
  83. ^Braaten, Cheung & Yuan 1993.
  84. ^Han et al. 2022.

Bibliography

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Books

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