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• James Bjorken, Sheldon Glashow (1964)
• Sheldon Glashow, John Iliopoulos, Luciano Maiani (1970)

{edih} {{plainlist|

• Samuel C. C. Ting et al. (BNL, 1974)
• Burton Richter et al. (SLAC, 1974)

}} }}{{Standard model of particle physics}}The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called hadrons such as the J/psi meson and the charmed baryons created in particle accelerator collisions. Several bosons, including the W and Z bosons and the Higgs boson, can decay into charm quarks. All charm quarks carry charm, a quantum number. This second generation particle is the third-most-massive quark with a mass of {{val|1.27|0.02|ul=GeV/c2}} as measured in 2022 and a charge of +{{sfrac|2|3}} e.In 1964, James Bjorken and Sheldon Glashow theorized on the charm quark, and in 1970, Glashow, John Iliopoulos, also theorized on by others.{{sfn|Amati|Bacry|Nuyts|Prentki|1964}}{{sfn|Maki|Ohnuki|1964}}{{sfn|Hara|1964}} In 1974, it was separately discovered through the J/psi meson at Brookhaven National Laboratory and the Stanford Linear Accelerator Center. In the next few years, several charmed particles, including the D meson and the charmed strange mesons, were found.In the 21st century, a baryon containing two charmed quarks has been found. There is recent evidence intrinsic charm quarks exist in the proton, and the coupling of the charm quark and the Higgs boson has been studied. Recent evidence also indicates CP violation in the decay of the D0 meson, which contains the charm quark.{{TOC limit|3}}

Naming

According to Sheldon Glashow, the charm quark received its name because of the "symmetry it brought to the subnuclear world".{{sfn|Glashow|1976}}{{sfn|Riordan|1987|p=210}} 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.{{sfn|Glashow|1976}} The charm quark is also called the "charmed quark" in both academic and non-academic contexts.{{sfn|Harari|1977|p=6}}{{sfn|Riordan|1992|p=1292}}{{sfn|Levine|2017}} The symbol of the charm quark is "c".{{sfn|Workman et al.|2022|p=32}}

History

Background

{{see also|Quark model}}In 1961, Murray Gell-Mann introduced the Eightfold Way as a pattern to group baryons and mesons.{{sfn|Griffiths|2008|p=35}} In 1964, Gell-Mann and George Zweig independently proposed all hadrons are composed of elementary constituents, which Gell-Mann called "quarks".{{sfn|Griffiths|2008|p=37}} Initially, only the up quark, the down quark, and the strange quark were proposed.{{sfn|Griffiths|2008|p=39}} These quarks would produce all of the particles in the Eightfold Way.{{sfn|Griffiths|2008|p=41}} Gell-Mann and Kazuhiko Nishijima established strangeness, a quantum number, in 1953 to describe processes involving strange particles such as {{subatomic particle|Sigma|link=yes}} and {{subatomic particle|Lambda|link=yes}}.{{sfn|Griffiths|2008|p=34}}

Theoretical prediction

{{See also|GIM mechanism|Scientific wager}}File:K0 decay to muons.svg|thumb|The GIM mechanism explains the rarity of the decay of a {{SubatomicParticle|antiKaon0|link=true}} into two muonmuonIn 1964, James Bjorken and Sheldon Glashow theorized "charm" as a new quantum number.{{sfn|Bjorken|Glashow|1964|p=255}} At the time, there were four known leptons—the electron, the muon, and each of their neutrinos—but Gell-Mann initially proposed only three quarks.{{sfn|Riordan|1987|p=210}} Bjorken and Glashow thus hoped to establish parallels between the leptons and the quarks with their theory.{{sfn|Griffiths|2008|pp=44-45}} According to Glashow, the conjecture came from "aesthetic arguments".{{sfn|Glashow|1976}}In 1970, Glashow, John Iliopoulos, and Luciano Maiani proposed a new quark that differed from the three then-known quarks by the charm quantum number.{{sfn|Glashow|Iliopoulos|Maiani|1970|p=1287}}{{sfn|Appelquist|Barnett|Lane|1978|p=390}} They further predicted the existence of "charmed particles" and offered suggestions on how to experimentally produce them.{{sfn|Glashow|Iliopoulos|Maiani|1970|p=1290–1291}} They also suggested the charmed quark could provide a mechanism—the GIM mechanism—to facilitate the unification of the weak and electromagnetic forces.{{sfn|Close|1976|p=537}}At the Conference on Experimental Meson Spectroscopy (EMS) in April 1974, Glashow delivered his paper titled "Charm: An Invention Awaits Discovery". Glashow asserted because neutral currents were likely to exist, a fourth quark was "sorely needed" to explain the rarity of the decays of certain kaons.{{sfn|Riordan|1987|p=297}} He also made several predictions on the properties of charm quarks.{{sfn|Rosner|1998|p=14}} He wagered, by the next EMS conference in 1976:{{blockquote|text=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,{{efn|According to Riordan, the word "outlanders" means "other kinds of physicists who did neutrino scattering or measured electron–positron collisions in storage rings."{{sfn|Riordan|1987|p=295}} }} and you eat your hats.{{sfn|Rosner|1998|p=14}}

}}In July 1974, at the 17th International Conference on High Energy Physics (ICHEP), Iliopoulos said:{{blockquote|text=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.{{sfn|Iliopoulos|1974|p=100}}}}Applying the principle of naturalness to the kaon mass splitting between the K{{su|p=0|b=L}} and K{{su|p=0|b=S}} states, the mass of the charm quark was estimated by Mary K. Gaillard and Benjamin W. Lee in 1974 to be less than {{val|5|u=GeV/c2}}.Giudice, Gian Francesco. "Naturally speaking: the naturalness criterion and physics at the LHC". Perspectives on LHC physics (2008): 155–178.{{sfn|Gaillard|Lee|1974}}

Discovery

Glashow predicted the down quark of a proton could absorb a {{subatomic particle|W boson+|link=yes}} and become a charm quark. Then, the proton would be transformed into a charmed baryon before it decays into several particles, including a lambda baryon. In late May 1974, Robert Palmer and Nicholas P. Samios found an event generating a lambda baryon from their bubble chamber at Brookhaven National Laboratory.{{sfn|Riordan|1987|pp=295–297}} It took months for Palmer to be convinced the lambda baryon came from a charmed particle.{{sfn|Riordan|1987|pp=296}} The magnet of the bubble chamber failed in October 1974 and they did not encounter the same event.{{sfn|Riordan|1987|p=297}} The two scientists published their observations in early 1975.{{sfn|Cazzoli et al.|1975}}{{sfn|Riordan|1987|p=306}} Michael Riordan commented that this event was "ambiguous" and "encouraging but not convincing evidence".{{sfn|Riordan|1987|loc = p. 306, "It was encouraging, but not convincing, evidence [...] this one was ambiguous"}}

J/psi meson (1974)

In 1974, Samuel C. C. Ting was searching for charmed particles at Brookhaven National Laboratory (BNL).{{sfn|Riordan|1987|pp=297–298}} His team was using an electron-pair detector.{{sfn|Ting|1977|p=239}} By the end of August, they found a peak at {{val|3.1|u=GeV/c2}} and the signal's width was less than {{val|5|u=MeV}}.{{sfn|Ting|1977|p=243}} The team was eventually convinced they had observed a massive particle and named it "J". Ting considered announcing his discovery in October 1974 but he postponed the announcement due to his concern on the μ/Ï€ ratio.{{sfn|Ting|1977|p=244}}At the Stanford Linear Accelerator Center (SLAC), Burton Richter's team performed experiments on 9–10 November 1974. They also found a high probability of interaction at {{val|3.1|u=GeV/c2}}. They called the particle "psi".{{sfn|Southworth|1976|p=385}} On 11 November 1974, Richter met Ting at the SLAC,{{sfn|Southworth|1976|pp=385–386}} and they announced their discovery.{{sfn|Rosner|1998|p=16}}Theorists immediately began to analyze the new particle.{{sfn|Riordan|1987|p=300}} It was shown to have a lifetime on the scale of 10−20 seconds, suggesting special characteristics.{{sfn|Southworth|1976|p=385}}{{sfn|Riordan|1987|p=300}} Thomas Appelquist and David Politzer suggested the particle was composed of a charm quark and a charm antiquark with their spin aligned in parallel. The two called this configuration "charmonium".{{sfn|Riordan|1987|p=300}} Charmonium would have two forms: "orthocharmonium", where the spins of the two quarks are parallel, and "paracharmonium", where the spins align oppositely.{{sfn|Riordan|1987|p=304}} Murray Gell-Mann also believed in the idea of charmonium.{{sfn|Riordan|1987|loc = p. 300, "Murray ... thinks that the charm–anticharm vector meson is more likely"}} Some other theorists, such as Richard Feynman, initially thought the new particle consisted of an up quark with a charm antiquark.{{sfn|Riordan|1987|p=300}}On 15 November 1974, Ting and Richter issued a press release about their discovery.{{sfn|Riordan|1987|p=301}} On 21 November at the SLAC, SPEAR found a resonance of the J/psi particle at {{val|3.7|u=GeV/c2}} as Martin Breidenbach and Terence Goldman had predicted.{{sfn|Riordan|1987|p=301}} This particle was called ψ′ ("psi-prime").{{sfn|Riordan|1987|p=303}} 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 and asymptotic freedom to explain the properties of the J/psi meson.{{sfn|Riordan|1987|p=305}}Eventually, on 2 December 1974, Physical Review Letters (PRL) published the discovery papers of J and psi, by Ting{{sfn|Aubert et al.|1974}} and Richter{{sfn|Augustin et al.|1974}} respectively.{{sfn|Riordan|1987|p=305}} The discovery of the psi-prime was published the following week.{{sfn|Riordan|1987|p=305}} 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".{{sfn|Riordan|1987|p=306}} In 1976, Ting and Richter shared the Nobel Prize in Physics for their discovery "of a heavy elementary particle of the new kind".{{sfn|Southworth|1976|p=383}}In August 1976, in The New York Times, Glashow recalled his wager and commented, "John [Iliopoulos]'s wine and my hat had been saved in the nick of time".{{sfn|Glashow|1976}} At the next EMS conference, spectroscopists ate Mexican candy hats supplied by the organizers.{{sfn|Riordan|1987|p=321}}{{sfn|Rosner|1998|p=18}} Frank Close wrote a Nature article titled "Iliopoulos won his bet" in the same year, saying the 18th ICHEP was "indeed dominated by that very discovery".{{sfn|Close|1976|p=537}} No-one paid off their bets to Iliopoulos.{{sfn|Riordan|1987|pp=319–320}}{{sfn|Rosner|1998|p=16}}

Other charmed particles (1975–1977)

In April 1975, E. G. Cazzoli et al., including Palmer and Samios, published their earlier ambiguous evidence for the charmed baryon.{{sfn|Cazzoli et al.|1975}} By the time of the Lepton–Photon Symposium in August 1975, eight new heavy particles had been discovered.{{sfn|Riordan|1987|pp=310–311}} These particles, however, have zero total charm.{{sfn|Riordan|1987|p=312}} Starting from the fourth quarter of that year, physicists began to look for particles with a net, or "naked", charm.{{sfn|Riordan|1987|p=317}}On 3 May 1976 at SLAC, Gerson Goldhaber and François Pierre identified a {{val|1.87|u=GeV/c2}} peak, which suggested the presence of a neutral charmed D meson according to Glashow's prediction. On 5 May, Goldhaber and Pierre published a joint memorandum about their discovery of the "naked charm".{{sfn|Riordan|1987|p=318}} 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.{{sfn|Riordan|1987|loc=p. 319, "Solid evidence for charm surfaced in session after session. There was no longer any doubt"}}{{sfn|Griffiths|2008|loc=p. 47, "With these discoveries, the interpretation ... was established beyond reasonable doubt. More important, the quark model itself was put back on its feet"}} The charmed strange meson was discovered in 1977.{{sfn|Brandelik et al.|1977}}{{sfn|Griffiths|2008|p=47}}

Later and current research

In 2002, the SELEX Collaboration at Fermilab published the first observation of the doubly charmed baryon {{subatomic particle|Double charmed xi+}} ("double charmed xi+").{{sfn|Mattson et al.|2002}} It is a three-quark particle containing two charm quarks. The team found doubly charmed baryons with an up quark are more massive and has a higher rate of production than those with a down quark.{{sfn|Yap|2002}}In 2007, the BaBar and Belle collaborations each reported evidence for the mixing of two neutral charmed mesons, {{subatomic particle|D0}} and {{subatomic particle|antiD0}}.{{sfn|Aubert et al.|2007}}{{sfn|Starič et al.|2007}}{{sfn|Gersabeck|2014|p=2}} The evidence confirmed the mixing rate is small, as is predicted by the standard model.{{sfn|Aubert et al.|2007|p=4}} Neither studies found evidence for CP violation between the decays of the two charmed particles.{{sfn|Aubert et al.|2007}}{{sfn|Starič et al.|2007}}In 2022, the NNPDF Collaboration found evidence for the existence of intrinsic charm quarks in the proton.{{sfn|The NNPDF Collaboration|2022}}{{sfn|Thompson|Howe|2022}} In the same year, physicists also conducted a direct search for Higgs boson decays into charm quarks using the ATLAS detector of the Large Hadron Collider.{{sfn|Aad et al.|2022}} They have determined the Higgs–charm coupling is weaker than the Higgs–bottom coupling.{{sfn|ATLAS experiment|2022}} On 7 July 2022, the LHCb experiment announced they had found evidence of direct CP violation in the decay of the D0 meson into pions.{{sfn|LHCb experiment|2022|loc = "This is the first evidence of direct CP violation in an individual charm–hadron decay (D0 → π– π+), with a significance of 3.8σ"}}

Characteristics

The charm quark is a second-generation up-type quark.{{sfn|Harari|1977|p=6}}{{sfn|Gersabeck|2014|p=2}} It carries charm, a quantum number.{{sfn|Appelquist|Barnett|Lane|1978|p=388}} According to the 2022 Particle Physics Review, the charmed quark has a mass of {{val|1.27|0.02|ul=GeV/c2}},{{efn|The Particle Physics Review uses the unit GeV instead of GeV/c2.{{sfn|Workman et al.|2022|p=32}} This is because particle physics uses natural units, in which the speed of light is set to be one.{{sfn|Thomson|2013|p=31}} The Review also notes this mass corresponds to the "running" mass in the minimal subtraction scheme (MS scheme).{{sfn|Workman et al.|2022|p=32}}}} a charge of +{{sfrac|2|3}} e, and a charm of +1.{{sfn|Workman et al.|2022|p=32}} The charm quark is more massive than the strange quark: the ratio between the masses of the two is about {{val|11.76|+0.05|-0.10}}.{{sfn|Workman et al.|2022|p=32}}The CKM matrix describes the weak interaction of quarks.{{sfn|Thomson|2013|p=368}} As of 2022, the values of the CKM matrix relating to the charm quark are:{{sfn|Workman et al.|2022|pp=262–263}}begin{align}

& = 0.221 pm 0.004

& = 0.975 pm 0.006

& = (40.8 pm 1.4) times 10^{-3}end{align}File:Baryon Supermultiplet using four-quark models and half spin.svg|thumb|A supermultipletsupermultipletCharm 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.{{sfn|Gersabeck|2014|p=2}} There are several charmed mesons, including {{subatomic particle|D+-}} and {{subatomic particle|D0}}.{{sfn|Workman et al.|2022|pp=43–45}} Charmed baryons include {{subatomic particle|Charmed Lambda|link=yes}}, {{subatomic particle|Charmed Sigma|link=yes}}, {{subatomic particle|Charmed Xi|link=yes}}, {{subatomic particle|Charmed Omega|link=yes}}, with various charges and resonances.{{sfn|Workman et al.|2022|pp=100-4}} Production and decay Particles containing charm quarks can be produced via electron–positron collisions or in hadron collisions.{{sfn|Gersabeck|2014|pp=3–4}} Using different energies, electron–positron colliders can produce psi or upsilon mesons.{{sfn|Gersabeck|2014|p=3}} Hadron colliders produce particles that contain charm quarks at a higher cross section.{{efn|According to Mark Thomson, a cross section in particle physics is a measure of quantum mechanical probability for the interaction.{{sfn|Thomson|2013|p=26}} It is the ratio between the interaction rate per target particle and the incident particle flux.{{sfn|Thomson|2013|p=69}}}}{{sfn|Gersabeck|2014|p=4}} The W boson can also decay into hadrons containing the charm quark or the charm antiquark.{{sfn|Thomson|2013|p=412}} The Z boson can decay into charmonium through charm quark fragmentation.{{sfn|Braaten|Cheung|Yuan|1993}} The Higgs boson can also decay to {{subatomic particle|J/Psi}} or {{subatomic particle|charmed eta}} through the same mechanism. The decay rate of the Higgs boson into charmonium is "governed by the charm-quark Yukawa coupling".{{sfn|Han|Leibovich|Ma|Tan|2022}}The charm quark can decay into other quarks via weak decays.{{sfn|Gersabeck|2014|p=2}} The charm quark also annihilates with the charm antiquark during the decays of ground-state charmonium mesons.{{sfn|Gersabeck|2014|p=2}} References Notes {{notelist|30em}} Citations {{reflist|30em|}} Bibliography News articles NEWS ,weblink , Higgs-boson charm coupling weaker than bottom , 2 May 2022 , 2023-06-03 , CERN Courier , ATLAS experiment , NEWS , Sheldon L., Glashow , The New York Times , July 18, 1976 , en , The hunting of the quark ,weblink , MAGAZINE , Levine, Alaina G. , November 2017 , This Month in Physics History ,weblink , APS News , , American Physical Society , 2023-06-01 , 26 , 10 , NEWS , LHCb experiment , LHCb digs deeper in CP-violating charm decays ,weblink , 7 July 2022 , 2023-07-27 , CERN Courier , MAGAZINE , CERN Courier , 16 , 11 , November 1976 , 1976 Nobel Prize for Physics , Southworth , Brian , CERN , Geneva, Switzerland ,weblink , 383–88 , MAGAZINE , Hunting the Doubly Charmed Baryon , Wired (magazine), Wired , Yap, Diana Michele , 2002-06-10 ,weblink , 2023-06-06 , Journal articles JOURNAL , Aad, G. , etal , ATLAS Collaboration , Direct constraint on the Higgs–charm coupling from a search for Higgs boson decays into charm quarks with the ATLAS detector. , The European Physical Journal C , 82 , 717 , 2022 , 717 , 10.1140/epjc/s10052-022-10588-3 , 2201.11428 , 2022EPJC...82..717A , 251688816 ,weblink , {{harvid, Aad et al., 2022, }} JOURNAL , Amati, D. , Bacry, H. , Nuyts, J. , Prentki, J. , SU 4 and strong interactions , Il Nuovo Cimento , December 1964 , 34, 6, 1732–1750 , 10.1007/BF02750568, 1964NCim...34.1732A , 118803104 ,weblink , JOURNAL , Charm and Beyond , Thomas, Appelquist , R. 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L. , etal , Particle Data Group , Review of Particle Physics , Progress of Theoretical and Experimental Physics , 2022 , 083C01 , 2022 , 8 ,weblink , 10.1093/ptep/ptac097 , {{harvid, Workman et al., 2022, |doi-access = free |hdl = 20.500.11850/571164 |hdl-access = free }} Conferences CONFERENCE , Harari, Haim, Haim Harari , Three Generations of Quarks and Leptons , SLAC-PUB-1974 ,weblink , V International Conference on Experimental Meson Spectroscopy , Boston, Mass. , April 29–30, 1977 , CONFERENCE , Iliopoulos , J. , Ecole Normale Superieure , Progress in Gauge Theories , XVII International Conference on High Energy Physics , London , July 1–10, 1974 , PTENS-74-4 , III , 89–116 ,weblink , CONFERENCE , Introduction to Charm Physics , Gersabeck, Marco , Flavorful Ways to New Physics , Freudenstadt - Lauterbad, Germany , 28–31 October 2014 , 1503.00032 ,weblink , CONFERENCE , Rosner, Jonathan L. , 1998 , The Arrival of Charm,weblink , Heavy Quarks at Fixed Targets , AIP Conference Proceedings , 459, 1 , 9–27 , 10.1063/1.57782 , 1999 , 2023-06-02 , , hep-ph/9811359 , Books BOOK , David, Griffiths, David J. Griffiths , Introduction to Elementary Particles , Wiley-VCH , 2008 , 978-3-527-40601-2 , BOOK , Michael, Riordan, Michael Riordan (physicist) , The Hunting of the Quark: A True Story of Modern Physics , Simon & Schuster , 1987 , 978-0-671-50466-3 ,weblink , BOOK , Mark, Thomson, Mark Thomson (physicist) , Modern Particle Physics , 978-1-107-03426-6 , Cambridge University Press , 2013 , Further reading WEB , R. Nave , Quarks ,weblink , HyperPhysics , Georgia State University, Department of Physics and Astronomy , 2008-06-29 , BOOK , A. Pickering , Constructing Quarks , 114–125 , University of Chicago Press , 1984 , 978-0-226-66799-7 , {{Particles}}{{Authority control}}