A theoretical prediction for the existence of this particle was made by Glashow and Bjorken in their work on SU(4) symmetry, 
A theoretical prediction for the existence of this particle was made by Glashow and Bjorken in their work on SU(4) symmetry, as well as by the GIM mechanism postulated to solve the kaon lifetime problem. This particle more often undergoes decay into two kaons rather than three pions even though the former branch is energetically disfavored, because in the latter case, the diagram can be "cut" by snipping only gluon lines, suppressing the three-pion decay and extending this particle's lifetime. Experimental setups used to discover this particle consisted of electron-positron collisions in one instance, and proton collisions with a beryllium target; both experiments showed a 3.1 GeV resonance peak, which was interpreted as the bound state of a particle with a mass of about 1.3 GeV and charge +2/3.  Discovered independently by Richter and Ting, for 10 points, identify this meson, the first known bound state of a charm quark and anti-quark.
A theoretical prediction for the existence of this particle was made by Glashow and Bjorken in their work on SU(4) symmetry, as well as by the GIM mechanism postulated to solve the kaon lifetime problem.
J/psi [accept charm quark before "decay", accept charmonium before "charm" is mentioned]