Editing Charm quark (section)

== History ==
=== Background ===
{{see also|Quark model}}
In 1961, [[Murray Gell-Mann]] introduced the [[Eightfold way (physics)|Eightfold Way]] as a pattern to group [[baryon]]s and [[meson]]s.{{sfn|Griffiths|2008|p=35}} In 1964, Gell-Mann and [[George Zweig]] independently proposed that all [[hadron]]s 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]] had established [[strangeness]], a quantum number, in 1953 to describe processes involving [[strange particle]]s such as {{subatomic particle|Sigma|link=yes}} and&nbsp;{{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 [[muon]]s by involving the charm quark (c) in the process.]]

In 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 [[lepton]]s—the [[electron]], the [[muon]], and each of their [[neutrino]]s—but Gell-Mann initially proposed only three quarks.{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/210 210]}} 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)|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 interaction|weak]] and [[electromagnetism|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 current]]s were likely to exist, a fourth quark was "sorely needed" to explain the rarity of the decays of certain [[kaon]]s.{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/297 297]}} He also made several predictions on the properties of charm quarks.{{sfn|Rosner|1998|p=14}} He wagered that, by the next EMS conference in 1976:
{{blockquote|text=
There are just three possibilities:
# Charm is not found, and I eat my hat.
# Charm is found by hadron spectroscopers, and we celebrate.
# 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=[https://archive.org/details/huntingofquarktr00mich/page/295 295]}} }} 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 an argument 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}}.<ref>Giudice, Gian Francesco. "Naturally speaking: the naturalness criterion and physics at the LHC". Perspectives on LHC physics (2008): 155–178.</ref>{{sfn|Gaillard|Lee|1974}}

=== Discovery ===
Glashow predicted that 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 decayed 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=[https://archive.org/details/huntingofquarktr00mich/page/295 295–297]}} It took months for Palmer to be convinced the lambda baryon came from a charmed particle.{{sfn|Riordan|1987|pp=296}} When the magnet of the bubble chamber failed in October 1974, they did not encounter the same event.{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/297 297]}} The two scientists published their observations in early 1975.{{sfn|Cazzoli et al.|1975}}{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/306 306]}} [[Michael Riordan (physicist)|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) ====
{{Main|J/psi meson}}

In 1974, [[Samuel C. C. Ting]] was searching for charmed particles at [[Brookhaven National Laboratory]] (BNL).{{sfn|Riordan|1987|pp=[https://archive.org/details/huntingofquarktr00mich/page/297 297–298]}} 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 postponed the announcement due to his concern about the μ/π 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=[https://archive.org/details/huntingofquarktr00mich/page/300 300]}} It was shown to have a lifetime on the scale of 10<sup>−20</sup> seconds, suggesting special characteristics.{{sfn|Southworth|1976|p=385}}{{sfn|Riordan|1987|p=300}} [[Thomas Appelquist]] and [[David Politzer]] suggested that the particle was composed of a charm quark and a charm antiquark whose [[Spin (particle physics)|spins]] were aligned in parallel. The two called this configuration "charmonium".{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/300 300]}} 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=[https://archive.org/details/huntingofquarktr00mich/page/304 304]}} Murray Gell-Mann also believed in the idea of charmonium.{{sfn|Riordan|1987|loc = p. 300, "Murray&nbsp;... 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=[https://archive.org/details/huntingofquarktr00mich/page/300 300]}}

On 15 November 1974, Ting and Richter issued a press release about their discovery.{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/301 301]}} 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=[https://archive.org/details/huntingofquarktr00mich/page/301 301]}} This particle was called ψ′ ("psi-prime").{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/303 303]}} 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=[https://archive.org/details/huntingofquarktr00mich/page/305 305]}}

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=[https://archive.org/details/huntingofquarktr00mich/page/305 305]}} The discovery of the psi-prime was published the following week.{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/305 305]}} 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=[https://archive.org/details/huntingofquarktr00mich/page/306 306]}} 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=[https://archive.org/details/huntingofquarktr00mich/page/321 321]}}{{sfn|Rosner|1998|p=18}} [[Frank Close]] wrote a ''[[Nature (journal)|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=[https://archive.org/details/huntingofquarktr00mich/page/319 319–320]}}{{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=[https://archive.org/details/huntingofquarktr00mich/page/310 310–311]}} These particles, however, have zero total charm.{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/312 312]}} Starting from the fourth quarter of that year, physicists began to look for particles with a net, or "naked", charm.{{sfn|Riordan|1987|p=[https://archive.org/details/huntingofquarktr00mich/page/317 317]}}

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=[https://archive.org/details/huntingofquarktr00mich/page/318 318]}} 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&nbsp;... 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 [[Ξcc++|{{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 have a higher rate of production than those with a down quark.{{sfn|Yap|2002}}

In 2007, the [[BaBar experiment|BaBar]] and [[Belle experiment|Belle]] collaborations each reported evidence for the mixing of two neutral charmed mesons, [[D meson|{{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 that 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 D<sup>0</sup> meson into [[pion]]s.{{sfn|LHCb experiment|2022|loc = "This is the first evidence of direct CP violation in an individual charm–hadron decay (D<sup>0</sup> → π<sup>–</sup> π<sup>+</sup>), with a significance of 3.8''σ''"}}