An electrically neutral meson having a mass 6,060 times that of the electron and a mean lifetime of approximately 1 × 10-20 seconds. The observation of this unusually long lifetime, in 1974, is what led to the hypothesis that other quarks existed in addition to the up, down, and strange quarks. It was the first firm experimental evidence for the charm quark and is considered to be a charm-anticharm quark pair. Also called J particle, psi particle. See Table at subatomic particle.