Observation of (3686)→
M. Ablikim (Institute of High Energy Physics, Beijing, People's Republic of China); M.N. Achasov (G.I. Budker Institute of Nuclear Physics SB RAS (BINP), Novosibirsk, Russia); S. Ahmed (Helmholtz Institute Mainz, Mainz, Germany); M. Albrecht (Bochum Ruhr-University, Bochum, Germany); M. Alekseev (University of Turin, Turin, Italy, INFN, Turin, Italy); et al - Show all 459 authors
Using a data sample of events collected with the BESIII detector at the BEPCII collider, we report the first observation of the electromagnetic Dalitz decay , with significances of 7.0σ and 6.3σ when reconstructing the meson via its decay modes and (), respectively. The weighted average branching fraction is determined to be , where the first uncertainty is statistical and the second systematic.