The Name
Because of the nearly simultaneous discovery, the J/ψ is the only elementary 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. Coincidentally, later spark chamber pictures 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; possibly by coincidence, "J" strongly resembles the Chinese character for Ting's name (丁). (Cf. the naming of Gallium.) J is also the first letter of Ting's oldest daughter's name, Jeanne.
Since the scientific community considered it unjust to give one of the two discoverers priority, 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 termed the ψ(2S) or occasionally ψ(3686), indicating respectively its quantum state or mass in MeV. Other vector charm-anticharm states are denoted similarly with ψ and the quantum state (if known) or the mass. 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. This is by analogy with positronium, which also consists of a particle and its antiparticle (an electron and positron in the case of positronium).
Read more about this topic: J/psi Meson
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