\ ˌjā-ˈsī- 
, -ˈpsī- \
, -ˈpsī- \Definition of J/psi particle
: an unstable neutral fundamental particle of the meson group that has a mass about 6000 times the mass of an electron
— called also J particle, J/psi, psi particle
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The first known use of J/psi particle was in 1977
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