Charm quark

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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 Template:Val as measured in 2022, and a charge of +Template:Sfrac e.

The existence of the charm quark was first predicted by James Bjorken and Sheldon Glashow in 1964,Template:Sfn Template:Sfn Template:Sfn and in 1970, Glashow, John Iliopoulos, and Luciano Maiani showed how its existence would account for experimental and theoretical discrepancies.Template:Sfn 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.

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NamingEdit

According to Sheldon Glashow, the charm quark received its name because of the "symmetry it brought to the subnuclear world".Template:Sfn Template:Sfn 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.Template:Sfn The charm quark is also called the "charmed quark" in both academic and non-academic contexts.Template:Sfn Template:Sfn Template:Sfn The symbol of the charm quark is "c".Template:Sfn

HistoryEdit

BackgroundEdit

Template:See also In 1961, Murray Gell-Mann introduced the Eightfold Way as a pattern to group baryons and mesons.Template:Sfn In 1964, Gell-Mann and George Zweig independently proposed that all hadrons are composed of elementary constituents, which Gell-Mann called "quarks".Template:Sfn Initially, only the up quark, the down quark, and the strange quark were proposed.Template:Sfn These quarks would produce all of the particles in the Eightfold Way.Template:Sfn Gell-Mann and Kazuhiko Nishijima had established strangeness, a quantum number, in 1953 to describe processes involving strange particles such as Template:Subatomic particle and Template:Subatomic particle.Template:Sfn

Theoretical predictionEdit

Template:See also

File:K0 decay to muons.svg

The GIM mechanism explains the rarity of the decay of a Template:SubatomicParticle 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.Template:Sfn 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.Template:Sfn Bjorken and Glashow thus hoped to establish parallels between the leptons and the quarks with their theory.Template:Sfn According to Glashow, the conjecture came from "aesthetic arguments".Template:Sfn

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.Template:Sfn Template:Sfn They further predicted the existence of "charmed particles" and offered suggestions on how to experimentally produce them.Template:Sfn They also suggested the charmed quark could provide a mechanism—the GIM mechanism—to facilitate the unification of the weak and electromagnetic forces.Template:Sfn

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.Template:Sfn He also made several predictions on the properties of charm quarks.Template:Sfn He wagered that, by the next EMS conference in 1976:

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,Template:Efn and you eat your hats.Template:Sfn{{#if:|{{#if:|}}

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In July 1974, at the 17th International 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.Template:Sfn{{#if:|{{#if:|}}
— {{#if:|, in }}Template:Comma separated entries
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Applying an argument of naturalness to the kaon mass splitting between the KTemplate:Su and KTemplate:Su states, the mass of the charm quark was estimated by Mary K. Gaillard and Benjamin W. Lee in 1974 to be less than Template:Val.<ref>Giudice, Gian Francesco. "Naturally speaking: the naturalness criterion and physics at the LHC". Perspectives on LHC physics (2008): 155–178.</ref>Template:Sfn

DiscoveryEdit

Glashow predicted that the down quark of a proton could absorb a Template:Subatomic particle 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.Template:Sfn It took months for Palmer to be convinced the lambda baryon came from a charmed particle.Template:Sfn When the magnet of the bubble chamber failed in October 1974, they did not encounter the same event.Template:Sfn The two scientists published their observations in early 1975.Template:Sfn Template:Sfn Michael Riordan commented that this event was "ambiguous" and "encouraging but not convincing evidence".Template:Sfn

J/psi meson (1974)Edit

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In 1974, Samuel C. C. Ting was searching for charmed particles at Brookhaven National Laboratory (BNL).Template:Sfn His team was using an electron-pair detector.Template:Sfn By the end of August, they found a peak at Template:Val and the signal's width was less than Template:Val.Template:Sfn 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.Template:Sfn

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 Template:Val. They called the particle "psi".Template:Sfn On 11 November 1974, Richter met Ting at the SLAC,Template:Sfn and they announced their discovery.Template:Sfn

Theorists immediately began to analyze the new particle.Template:Sfn It was shown to have a lifetime on the scale of 10⁻²⁰ seconds, suggesting special characteristics.Template:Sfn Template:Sfn 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".Template:Sfn Charmonium would have two forms: "orthocharmonium", where the spins of the two quarks are parallel, and "paracharmonium", where the spins align oppositely.Template:Sfn Murray Gell-Mann also believed in the idea of charmonium.Template:Sfn Some other theorists, such as Richard Feynman, initially thought the new particle consisted of an up quark with a charm antiquark.Template:Sfn

On 15 November 1974, Ting and Richter issued a press release about their discovery.Template:Sfn On 21 November at the SLAC, SPEAR found a resonance of the J/psi particle at Template:Val as Martin Breidenbach and Terence Goldman had predicted.Template:Sfn This particle was called ψ′ ("psi-prime").Template:Sfn 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.Template:Sfn

Eventually, on 2 December 1974, Physical Review Letters (PRL) published the discovery papers of J and psi, by TingTemplate:Sfn and RichterTemplate:Sfn respectively.Template:Sfn The discovery of the psi-prime was published the following week.Template:Sfn 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".Template:Sfn In 1976, Ting and Richter shared the Nobel Prize in Physics for their discovery "of a heavy elementary particle of the new kind".Template:Sfn

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".Template:Sfn At the next EMS conference, spectroscopists ate Mexican candy hats supplied by the organizers.Template:Sfn Template:Sfn 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".Template:Sfn No-one paid off their bets to Iliopoulos.Template:Sfn Template:Sfn

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.Template:Sfn By the time of the Lepton–Photon Symposium in August 1975, eight new heavy particles had been discovered.Template:Sfn These particles, however, have zero total charm.Template:Sfn Starting from the fourth quarter of that year, physicists began to look for particles with a net, or "naked", charm.Template:Sfn

On 3 May 1976 at SLAC, Gerson Goldhaber and François Pierre identified a Template:Val 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".Template:Sfn 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.Template:Sfn Template:Sfn The charmed strange meson was discovered in 1977.Template:Sfn Template:Sfn

Later and current researchEdit

In 2002, the SELEX Collaboration at Fermilab published the first observation of the doubly charmed baryon [[Ξcc++|Template:Subatomic particle ("double charmed xi+")]].Template:Sfn 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.Template:Sfn

In 2007, the BaBar and Belle collaborations each reported evidence for the mixing of two neutral charmed mesons, [[D meson|Template:Subatomic particle and Template:Subatomic particle]].Template:Sfn Template:Sfn Template:Sfn The evidence confirmed the mixing rate is small, as is predicted by the standard model.Template:Sfn Neither studies found evidence for CP violation between the decays of the two charmed particles.Template:Sfn Template:Sfn

In 2022, the NNPDF Collaboration found evidence for the existence of intrinsic charm quarks in the proton.Template:Sfn Template:Sfn 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.Template:Sfn They have determined that the Higgs–charm coupling is weaker than the Higgs–bottom coupling.Template:Sfn 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.Template:Sfn

CharacteristicsEdit

The charm quark is a second-generation up-type quark.Template:Sfn Template:Sfn It carries charm, a quantum number.Template:Sfn According to the 2022 Particle Physics Review, the charmed quark has a mass of Template:Val,Template:Efn a charge of +Template:Sfrac e, and a charm of +1.Template:Sfn The charm quark is more massive than the strange quark: the ratio between the masses of the two is about Template:Val.Template:Sfn

The CKM matrix describes the weak interaction of quarks.Template:Sfn As of 2022, the values of the CKM matrix relating to the charm quark are:Template:Sfn <math display="block"> \begin{align} |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{align} </math>

File:Baryon Supermultiplet using four-quark models and half spin.svg

A supermultiplet 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.Template:Sfn There are several charmed mesons, including Template:Subatomic particle and Template:Subatomic particle.Template:Sfn Charmed baryons include Template:Subatomic particle, Template:Subatomic particle, Template:Subatomic particle, Template:Subatomic particle, with various charges and resonances.Template:Sfn

Production and decayEdit

Particles containing charm quarks can be produced via electron–positron collisions or in hadron collisions.Template:Sfn Using different energies, electron–positron colliders can produce psi or upsilon mesons.Template:Sfn Hadron colliders produce particles that contain charm quarks at a higher cross section.Template:Efn Template:Sfn The W boson can also decay into hadrons containing the charm quark or the charm antiquark.Template:Sfn The Z boson can decay into charmonium through charm quark fragmentation.Template:Sfn The Higgs boson can also decay to Template:Subatomic particle or Template:Subatomic particle through the same mechanism. The decay rate of the Higgs boson into charmonium is "governed by the charm-quark Yukawa coupling".Template:Sfn

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

ReferencesEdit

NotesEdit

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CitationsEdit

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BibliographyEdit