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The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called such as the J/psi meson and the created in particle accelerator collisions. Several , 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 as measured in 2022, and a charge of + e.
The existence of the charm quark was first predicted by James Bjorken and Sheldon Glashow in 1964, and in 1970, Glashow, John Iliopoulos, and Luciano Maiani showed how its existence would account for experimental and theoretical discrepancies. In 1974, its existence was confirmed through the independent discoveries of the J/psi meson at Brookhaven National Laboratory and the Stanford Linear Accelerator Center. In the next few years, several other charmed particles, including the D meson and the charmed strange mesons, were found.
In the 21st century, a baryon containing two charm quarks has been found. There is recent evidence that 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.
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. They further predicted the existence of "charmed particles" and offered suggestions on how to experimentally produce them. They also suggested the charmed quark could provide a mechanism—the GIM mechanism—to facilitate the unification of the weak interaction and electromagnetism forces.
At the Conference on Experimental Meson Spectroscopy (EMS) in April 1974, Glashow delivered his paper titled "Charm: An Invention Awaits Discovery". Glashow asserted because were likely to exist, a fourth quark was "sorely needed" to explain the rarity of the decays of certain . He also made several predictions on the properties of charm quarks. He wagered that, by the next EMS conference in 1976:
In July 1974, at the 17th International Conference on High Energy Physics (ICHEP), Iliopoulos said:
Applying an argument of naturalness to the kaon mass splitting between the K and K states, the mass of the charm quark was estimated by Mary K. Gaillard and Benjamin W. Lee in 1974 to be less than .Giudice, Gian Francesco. "Naturally speaking: the naturalness criterion and physics at the LHC". Perspectives on LHC physics (2008): 155–178.
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 . They called the particle "psi". On 11 November 1974, Richter met Ting at the SLAC, and they announced their discovery.
Theorists immediately began to analyze the new particle. It was shown to have a lifetime on the scale of 10−20 seconds, suggesting special characteristics. Thomas Appelquist and David Politzer suggested that the particle was composed of a charm quark and a charm antiquark whose spins were aligned in parallel. The two called this configuration "charmonium". Charmonium would have two forms: "orthocharmonium", where the spins of the two quarks are parallel, and "paracharmonium", where the spins align oppositely. Murray Gell-Mann also believed in the idea of charmonium. Some other theorists, such as Richard Feynman, initially thought the new particle consisted of an up quark with a charm antiquark.
On 15 November 1974, Ting and Richter issued a press release about their discovery. On 21 November at the SLAC, SPEAR found a resonance of the J/psi particle at as Martin Breidenbach and Terence Goldman had predicted. This particle was called ψ′ ("psi-prime"). 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.
Eventually, on 2 December 1974, Physical Review Letters (PRL) published the discovery papers of J and psi, by Ting and Richter respectively. The discovery of the psi-prime was published the following week. 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". In 1976, Ting and Richter shared the Nobel Prize in Physics for their discovery "of a heavy elementary particle of the new kind".
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". At the next EMS conference, spectroscopists ate Mexican candy hats supplied by the organizers. 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". No-one paid off their bets to Iliopoulos.
On 3 May 1976 at SLAC, Gerson Goldhaber and François Pierre identified a 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". 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. The charmed strange meson was discovered in 1977.
In 2007, the BaBar experiment and Belle experiment collaborations each reported evidence for the mixing of two neutral charmed mesons, D meson. The evidence confirmed the mixing rate is small, as is predicted by the standard model. Neither studies found evidence for CP violation between the decays of the two charmed particles.
In 2022, the NNPDF Collaboration found evidence for the existence of intrinsic charm quarks in the proton. 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. They have determined that the Higgs–charm coupling is weaker than the Higgs–bottom coupling. On 7 July 2022, the LHCb experiment announced they had found evidence of direct CP violation in the decay of the D0 meson into .
The CKM matrix describes the weak interaction of quarks. As of 2022, the values of the CKM matrix relating to the charm quark are:
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. There are several charmed mesons, including and . Charmed baryons include , , , , with various charges and resonances.
The charm quark can decay into other quarks via weak decays. The charm quark also annihilates with the charm antiquark during the decays of ground-state charmonium mesons.
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