J/psi particle

n.
An electrically neutral meson having a mass 7,213 times that of the electron and a mean lifetime of approximately 1 × 10 ^-20 seconds. Also called J particle, psi particle.

An elementary particle with an unusually long lifetime or, from the Heisenberg uncertainty principle, with an extremely narrow width Γ = 91.0 ± 3.2 keV, and a large mass m = 3096.916 ± 0.011 MeV. It is a bound state containing a charm quark and an anticharm quark. The discovery of the J/psi particle is one of the cornerstones of the standard model.

Since its discovery in 1974, more than 10⁹ J/psi particles have been produced. More than 100 different decay modes and new particles radiating from the J/psi particle have been observed. The J/psi particle has been shown to be a bound state of charm quarks. The long lifetime of the J/psi results from its mass being less than the masses of particles which separately contain a charm and an anticharm quark. This situation permits the J/psi to decay only into noncharm quarks, and empirically this restriction was found to lead to a suppression of the decay rate resulting in a long lifetime and narrow width. The subsequent discovery of the b quark and the intermediate vector bosons Z⁰ and W^±, and studies of Z⁰ decays into charm, b, and other quarks, show that the theory of the standard model is in complete agreement with experimental data to an accuracy of better than 1%. See also Charm; Elementary particle; Intermediate vector boson; Quarks; Standard model; Upsilon particles.

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The noun has one meaning:

Meaning #1: a neutral meson with a large mass
  Synonym: psi particle

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