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 only 8×10^{-21 s.}

^{It was discovered in 1974 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. These two scientists were both rewarded for their discovery with the 1976 Nobel Prize in Physics.}

^{The significance of the discovery of the J/Ψ was that it confirmed the quark model of elementary particle physics, which says that protons, neutrons and all the other baryons are made up from a fundamental substance, namely the quark fields, which comes in 3 "colors" and 6 "flavors". At that time, 3 flavors were known (called up, down and strange) and the important point of the model was the claim that the flavors come in pairs: Up and down form the pair of the "first generation", and the "second generation" is a heavier and instable copy of the first. The "strange quark", was suspected to be behind the strange behavior of the kaons and took in the second generation the place corresponding to the down quark in first generation. For this to be true, one should be able to find particles, which indicate by the products into which they decay, that they contain a second generation quark, that corresponded to the up quark, and was given the name "charm quark" in the theory.}

^{This all sounded nice, but in order to make it also convincing, the high energy theorists awaited with suspense whether their experimental collegues would find such a particle. The discovery of the J/Ψ was hence the triumph of the quark model as suggested before by Murray Gell-Mann and others.}

^{The J/Ψ is the only elementary particle to have two letters assigned. The story behind is this: Richter and his coworkers observed the decay of an excited J/Ψ (just created in the SLAC particle accelerator) into the ground state J/Ψ, emitting two low-energy pions in the process, which left curved traces in their spark-chamber detector. The J/Ψ immediately went on to decay into an electron and a positron, which spread away rapidly, leaving a straight trace in the detector readout. As you can see in the picture above, the whole picture formed the Greek letter Psi, and hence Richter named it so.}

^{Ting, at about the same time, assigned the name J to it (without as obvious reasons, though rumor has it, because his Chinese name Ting (丁) looks like a letter J in Chinese writing.)}

^{As the discoveries were about simultaenous, and the scientific community considered it unjust to give one of the two discoverors priority, most subsequent publications 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.}

^{When referring to excited states of the J/Ψ (i.e. 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.}