Psi, sy, particle, also called a J particle, is a type of subatomic particle. A psi particle consists of a quark and an antiquark. A quark is an elementary particle that combines with other quarks to form such familiar particles as protons and neutrons. An antiquark has the same properties as a quark but carries an opposite electric charge. In a psi particle, the quark is a charm quark, and the antiquark is an anticharm quark. See Quark.