Damir Becirevic (CNRS et Universite Paris Sud) Damir Becirevic (Unknown)
A long standing problem of theoretical estimate of the $J/\psi \to \eta_c \gamma$ decay amplitude has been solved recently by means of QCD on the latice. We describe and explain the progress and discuss the new results that are also a challenge to experimenters. We also discuss several other decay modes of charmonia, including the very first estimate of $\eta_c(2S)\to J/\psi \gamma$ that was theoretically very challenging. Our result suggests the rate of the latter to be large enough to be measured in experiments. We make contact of our findings with various (semi-)analytic, QCD based, descriptions of the same modes.
Damir Becirevic (CNRS et Universite Paris Sud)
Mr Francesco Sanfilippo (Laboratorie de Physique Théorique, Université Paris Sud, France)