The 29th International Workshop on Weak Interactions and Neutrinos

Charmonium weak decay is allowed in the Standard Model but has never been observed. Using (10087±44)×106 / events collected with the BESIII detector at the BEPCII +− storage ring at the center-of-mass energy of √=3.097 GeV, we present a search for the charmonium rare semi-muonic decay /→−+ and its charge conjugation (c.c.) mode. Since no significant signal above the background is observed, we set an upper limit of the branching fraction to be BF(/→−++..)<5.6×10−7 at a confidence level of 90%. This is the first search for the weak decay of charmonium with a muon in the final state and the measurement is compatible with the SM theoretical predictions.