The J/ψ (J/psi) meson or psion is a subatomic particle, a flavor-neutral meson consisting of a charm quark and a charm antiquark. Mesons formed by a bound state of a charm quark and a charm anti-quark are generally known as "charmonium". The J/ψ is most common form of charmonium, due to its low rest mass. The J/ψ has a rest mass of 3.0969 GeV/c2, just above that of the ηc, and a mean lifetime of 7.2×10−21 s. This lifetime was about a thousand times longer than expected.
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