- The title given to this article is incorrect due to technical limitations. The correct title is J/ψ particle.
Spark-chamber trace of a ψ' to J/ψ decay at SLAC
The J/ψ is an elementary particle, namely a flavor-neutral meson consisting of a charm quark and a charm anti-quark. It is also sometimes known as charmonium.
It has a rest mass of 3096.9 MeV/c2 and a half life of 8×10-21 s. This lifetime was about a thousand times longer than naively expected.
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. They accidently discovered they had found the same particle, and both announced their discoveries on November 11, 1974. The importance of this discovery 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 rewarded for their discovery with 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 sixties, 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 three kinds of fractionally charged particles, the "quarks", that come in three different types or "flavors", called up, down, and strange. Despite the impressive ability of quark models to bring order to the "elementary particle zoo", their status was considered something like mathematical fiction at the time, a simple artifact of deeper physical reasons.
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 subprotonic components. To a first approximation, they were indeed the already described quarks.
On the theoretical front, gauge theories with broken symmetry became the first fully viable contenders for explaining the weak interaction after Gerardus 't Hooft discovered in 1971 how to calculate with them. The first experimental evidence for these electroweak unification theories was the discovery of the weak neutral current in 1973. Gauge theories with quarks became a viable contender for the strong interaction in 1973 when the concept of asymptotic freedom was identified.
However, a naive mixture of electroweak theory and the quark model led to calculations about known decay modes that contradicted observation. A 1970 idea of Sheldon Glashow, John Iliopoulos, and Luciano Maiani, known as the GIM mechanism, showed the contradiction in question would not be a problem if there were a fourth quark, charm, that paired with the strange quark. This work led, by the summer of 1974, to theoretical predictions of what a charm/anticharm meson would be like. These predictions were ignored.
The work of Richter and Ting was done for other reasons, mostly to explore new energy regimes.
Because of the simultaneous discovery, the J/ψ is the only elementary particle to have a two letter name. Richter chose the Greek letter psi as a name, in honor of the SPEAR accelerator used at SLAC, since psi vaguely sounded like SPEAR. Coincidentally, later spark chamber pictures, as in the graphic above, often resembled the psi shape. Ting assigned the name "J" to it, which is one letter removed from "K", the name of the already known strange meson. But widespread rumor says "J" was chosen because Ting's Chinese name (丁) looks like it.
Since the scientific community considered it unjust to give one of the two discoverers priority, most subsequent publications hvae referred to the particle as the "J/ψ".
The name charmonium is occasionally used for the J/ψ. This is by analogy with positronium, which also consists of a particle (a positron in the case of positronium) and its antiparticle.
Informally, it is sometimes called the "gypsy".
When referring to excited states of the J/ψ (that is, bound states of a charm quark and a charm antiquark with higher rest mass), the particle is identified as the ψ(mass in MeV). For example, the first excited state (previously called the ψ'), is called the ψ(3686) because its mass is 3686 MeV. The "J" is not used, since Richter's group alone first found excited states.