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

TL;DR

This paper reports the first observation and measurement of the M1 transition \\psi(3686)\\to \\\gamma \\\eta_c(2S), providing new data on charmonium states and their radiative transitions.

Contribution

First measurement of the M1 transition between \\\psi(3686)\\ and \\\eta_c(2S), including mass, width, and branching fraction, using BESIII data.

Findings

 \\\eta_c(2S) mass measured at 3637.6 MeV/c^2

 \\\eta_c(2S) width measured at 16.9 MeV

 Branching fraction of the transition \\\psi(3686)\\to \\\gamma \\\eta_c(2S) is (6.8 1.1 4.5) imes 10^{-4}

Abstract

Using a sample of 106 million \psi(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: \psi(3686)\to\gamma\eta_c(2S). Analyses of the processes \psi(2S)\to \gamma\eta_c(2S) with \eta_c(2S)\to \K_S^0 K\pi and K^+K^-\pi^0 gave an \eta_c(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 \eta_c(2S) mass (M(\eta_c(2S))=3637.6\pm 2.9_\mathrm{stat}\pm 1.6_\mathrm{sys} MeV/c^2), width (\Gamma(\eta_c(2S))=16.9\pm 6.4_\mathrm{stat}\pm 4.8_\mathrm{sys} MeV), and the product branching fraction (\BR(\psi(3686)\to \gamma\eta_c(2S))\times \BR(\eta_c(2S)\to K\bar K\pi) = (1.30\pm 0.20_\mathrm{stat}\pm 0.30_\mathrm{sys})\times 10^{-5}). Combining our result with a BaBar measurement of \BR(\eta_c(2S)\to K\bar K \pi), we find the branching fraction of the M1 transition to be \BR(\psi(3686)\to\gamma\eta_c(2S)) = (6.8\pm 1.1_\mathrm{stat}\pm 4.5_\mathrm{sys})\times 10^{-4}.