First observation of the $M1$ transition $\psi(3686) \rightarrow \gamma\eta_c (2S)$

Abstract

Using a sample of 106×106 ψ(3686) events collected with the BESIII detector at the BEPCII storage ring, we have made the first measurement of the M1 transition between the radially excited charmonium S−wave spin-triplet and the radially excited S−wave spin-singlet states: ψ(3686)→γηc(2S). Analyses of the processes ψ(3686)→γηc(2S) with ηc(2S)→KS0K±π∓ and K+K−π0 give an ηc(2S) signal with a statistical significance of greater than 10 standard deviations under a wide range of assumptions about the signal and background properties. The data are used to obtain measurements of the ηc(2S) mass (M(ηc(2S))=3637.6±2.9stat±1.6syst MeV/c2), width (Γ(ηc(2S))=16.9±6.4stat±4.8syst MeV), and the product branching-fraction (B(ψ(3686)→γηc(2S))×B(ηc(2S)→KK―π)=(1.30±0.20stat±0.30syst)×10−5). Combining our result with a BABAR measurement of B(ηc(2S)→KK―π), we find the branching fraction of the M1 transition to be B(ψ(3686)→γηc(2S))=(6.8±1.1stat±4.5syst)×10−4.