Evidence for the direct two-photon transition from $\psi(3686)$ to $J/\psi$

Abstract

The two-photon transition $\psi(3686)→\gamma\gamma J/\psi$ is studied in a sample of $1.06×10^8$ $\psi(3686)$ decays collected by the BESIII detector. The branching fraction is measured to be $(3.1±0.6(\text{stat})_{−1.0}^{+0.8}(\text{syst}))×10^{−4}$ using $J/\psi→e^+e^−$ and $J/\psi→μ^+μ^−$ decays, and its upper limit is estimated to be $4.5×10^-4$ at the 90% confidence level. This work represents the first measurement of a two-photon transition among charmonium states. The orientation of the ψ(3686) decay plane and the J/ψ polarization in this decay are also studied. In addition, the product branching fractions of sequential $E$1 transitions $\psi(3686)$→$\gamma\chi_{cJ} $ and $\chi_{cJ} $→$\gamma J/\psi(J=0,1,2)$ are reported.