The J/ψ (J/psi) meson/ˈdʒeɪˈsaɪˈmiːzɒn/ is a subatomic particle, a flavor-neutral meson consisting of a charm quark and a charm antiquark. Mesons formed by a bound state of a charm quark and a charm anti-quark are generally known as "charmonium" or psions.[1] The J/ψ is the most common form of charmonium, due to its spin of 1 and its low rest mass. The J/ψ has a rest mass of 3.0969 GeV/c2, just above that of the η c (2.9836 GeV/c2), and a mean lifetime of 7.2×10−21s. This lifetime was about a thousand times longer than expected.[2]
Its discovery was made independently by two research groups, one at the Stanford Linear Accelerator Center, headed by Burton Richter, and one at the Brookhaven National Laboratory, headed by Samuel Ting of MIT. They discovered that they had found the same particle, and both announced their discoveries on 11 November 1974. The importance of this discovery[citation needed] is highlighted by the fact that the subsequent, rapid changes in high-energy physics at the time have become collectively known as the "November Revolution". Richter and Ting were awarded the 1976 Nobel Prize in Physics.
Background to discovery
The background to the discovery of the J/ψ was both theoretical and experimental. In the 1960s, the first quark models of elementary particle physics were proposed, which said that protons, neutrons, and all other baryons, and also all mesons, are made from fractionally charged particles, the "quarks", which come in six types or "flavors", called up, down, top, bottom, strange and charm. Despite the ability of quark models to bring order to the "elementary particle zoo", they were considered something like mathematical fiction at the time, a simple artifact of deeper physical reasons.[3]
Starting in 1969, deep inelastic scattering experiments at SLAC revealed surprising experimental evidence for particles inside of protons. Whether these were quarks or something else was not known at first. Many experiments were needed to fully identify the properties of the sub-protonic components. To a first approximation, they indeed were a match for the previously described quarks.
However, a naive mixture of electroweak theory and the quark model led to calculations about known decay modes that contradicted observation: In particular, it predicted Z boson-mediated flavor-changing decays of a strange quark into a down quark, which were not observed. A 1970 idea of Sheldon Glashow, John Iliopoulos, and Luciano Maiani, known as the GIM mechanism, showed that the flavor-changing decays would be strongly suppressed if there were a fourth quark (now called the charm quark) that was a complementary counterpart to the strange quark. By summer 1974 this work had led to theoretical predictions of what a charm + anticharm meson would be like.
The predictions were ignored.[citation needed] The work of Richter and Ting was done mostly to explore new energy regimes, not to test the theoretical predictions.[citation needed]
The group at Brookhaven,[a] were the first to discern a peak at 3.1 GeV in plots of production rates, first recognizing the ψ meson – that Ting named the "J" meson (after himself – his last-name written in Chinese is 丁 – or maybe not after himself[4]).
Decay modes
Hadronic decay modes of J/ψ are strongly suppressed because of the OZI rule. This effect strongly increases the lifetime of the particle and thereby gives it its very narrow decay width of just 93.2±2.1 keV. Because of this strong suppression, electromagnetic decays begin to compete with hadronic decays. This is why the J/ψ has a significant branching fraction to leptons.
In a hot QCDmedium, when the temperature is raised well beyond the Hagedorn temperature, the J/ψ and its excitations are expected to melt.[6] This is one of the predicted signals of the formation of the quark–gluon plasma. Heavy-ion experiments at CERN's Super Proton Synchrotron and at BNL's Relativistic Heavy Ion Collider have studied this phenomenon without a conclusive outcome as of 2009. This is due to the requirement that the disappearance of J/ψ mesons is evaluated with respect to the baseline provided by the total production of all charm quark-containing subatomic particles, and because it is widely expected that some J/ψ are produced and/or destroyed at time of QGPhadronization. Thus, there is uncertainty in the prevailing conditions at the initial collisions.
In fact, instead of suppression, enhanced production of J/ψ is expected[7] in heavy ion experiments at LHC where the quark-combinant production mechanism should be dominant given the large abundance of charm quarks in the QGP. Aside of J/ψ , charmed B mesons ( B c), offer a signature that indicates that quarks move freely and bind at-will when combining to form hadrons.[8][9]
Name
Chinese character for the surname Ting, which resembles the Latin letter J.
Because of the nearly simultaneous discovery, the J/ψ is the only particle to have a two-letter name. Richter named it "SP", after the SPEAR accelerator used at SLAC; however, none of his coworkers liked that name. After consulting with Greek-born Leo Resvanis to see which Greek letters were still available, and rejecting "iota" because its name implies insignificance, Richter chose "psi" – a name which, as Gerson Goldhaber pointed out, contains the original name "SP", but in reverse order.[10] Coincidentally, later spark chamber pictures often resembled the psi shape. Ting assigned the name "J" to it, which is one letter away from "K", the name of the already-known strange meson; another reason is that "j" is the symbol for electromagnetic current.[4] Possibly by coincidence, "J" strongly resembles the Chinese character for Ting's name (丁 Dīng). J is also the first letter of Ting's eldest daughter's name, Jeanne.
Much of the scientific community considered it unjust to give one of the two discoverers priority, so most subsequent publications have referred to the particle as the " J/ψ ".
The first excited state of the J/ψ was called the ψ′; it is now called the ψ(2S), indicating its quantum state. The next excited state was called the ψ″; it is now called ψ(3770), indicating mass in MeV/c2. Other vector charm–anticharm states are denoted similarly with ψ and the quantum state (if known) or the mass.[11] The "J" is not used, since Richter's group alone first found excited states.
The name charmonium is used for the J/ψ and other charm–anticharm bound states.[b] This is by analogy with positronium, which also consists of a particle and its antiparticle (an electron and positron in the case of positronium).
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Glenn Everhart, Terry Rhoades, Min Chen, and Ulrich Becker, at Brookhaven first to discerned the 3.1 GeV peak in pair-production rates.
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There are two different regimes of flavorless, neutral mesons: Low mass and high mass.
Lighter mesons, such as the neutral pion ( π0 , the lightest of all mesons), the η and η′ , ρ0 , ω0 , and so-on. Whether high or low mass, since all of the flavorless mesons’ quantum numbers are zero they can only be distinguished by their masses. Generally their quark content is invisible, especially the low-mass flavorless mesons, not only because their very similar small masses can be easily confused, but also because the low-mass particles themselves do actually exist as mixtures. For example the lowest mass of all mesons is the neutral pion; it is approximately an equal mix of dd and uu matching quark–antiquark pairs.
However, the heavy c and b quarks are sufficiently distinct in mass to tell them apart:
^ abWe discussed the name of the new particle for some time. Someone pointed out to me that the really exciting stable particles are designated by Roman characters – like the postulated W0, the intermediate vector boson, the Z0, etc. – whereas the “classical” particles have Greek designations like ρ, ω etc. This, combined with the fact that our work in the last decade had been concentrated on the electromagnetic current gave us the idea to call this particle the J particle. Samuel Ting, The Discovery of the J Particle Nobel prize lecture, 11. December 1976 [1]
