Background In 1961,
Murray Gell-Mann introduced the
Eightfold Way as a pattern to group
baryons and
mesons. In 1964, Gell-Mann and
George Zweig independently proposed that all
hadrons are composed of elementary constituents, which Gell-Mann called "quarks". Initially, only the
up quark, the
down quark, and the
strange quark were proposed. These quarks would produce all of the particles in the Eightfold Way. Gell-Mann and
Kazuhiko Nishijima had established
strangeness, a quantum number, in 1953 to describe processes involving
strange particles such as and .
Theoretical prediction explains the rarity of the decay of a into two
muons by involving the charm quark (c) in the process. In 1964,
James Bjorken and Sheldon Glashow theorized "charm" as a new quantum number. 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. Bjorken and Glashow thus hoped to establish parallels between the leptons and the quarks with their theory. According to Glashow, the conjecture came from "aesthetic arguments". 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 and
electromagnetic 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
neutral currents were likely to exist, a fourth quark was "sorely needed" to explain the rarity of the decays of certain
kaons. 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 .
Discovery Glashow predicted that the down quark of a proton could absorb a 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. It took months for Palmer to be convinced the lambda baryon came from a charmed particle. When the magnet of the bubble chamber failed in October 1974, they did not encounter the same event. The two scientists published their observations in early 1975.
Michael Riordan commented that this event was "ambiguous" and "encouraging but not convincing evidence".
J/psi meson (1974) In 1974,
Samuel C. C. Ting was searching for charmed particles at
Brookhaven National Laboratory (BNL). His team was using an electron-pair detector. By the end of August, they found a peak at and the signal's width was less than . 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. 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.
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. By the time of the Lepton–Photon Symposium in August 1975, eight new heavy particles had been discovered. These particles, however, have zero total charm. Starting from the fourth quarter of that year, physicists began to look for particles with a net, or "naked", charm. 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.
Later and current research In 2002, the SELEX Collaboration at
Fermilab published the first observation of the doubly charmed baryon
("double charmed xi+"). 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. In 2007, the
BaBar and
Belle collaborations each reported evidence for the mixing of two neutral charmed mesons,
and. 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
pions. == Characteristics ==