Evidence for \eta_{c}(2S) in \psi(3686) \to \gamma K_{S}^{0}K^{\pm}\pi^{\mp}\pi^{+}\pi^{-}

TL;DR

This paper reports the first observation of the \\eta_{c}(2S) in \\psi(3686) radiative decay via a specific decay channel, measuring its mass, width, and branching fraction with high statistical significance.

Contribution

It provides the first evidence for \\eta_{c}(2S) in \\psi(3686) radiative decay using the decay mode involving K_{S}^{0}K^{}, with precise measurements of its properties.

Findings

 The \\eta_{c}(2S) mass is measured as 3646.9 MeV/c^2.

 The width of \\eta_{c}(2S) is 9.9 MeV/c^2.

 The product branching fraction is (7.03  2.10(stat)  0.70(syst))  10^{-6}.

Abstract

We search for the M1 radiative transition \psi(3686) \to \gamma\eta_{c}(2S) by reconstructing the exclusive \eta_{c}(2S) \to K_{S}^{0}K^{\pm}\pi^{\mp}\pi^{+}\pi^{-} decay using 1.06 \times 10^8 \psi(3686) events collected with the BESIII detector. The signal is observed with a statistical significance of greater than 4 standard deviations. The measured mass of the \eta_{c}(2S) is 3646.9 \pm 1.6(stat) \pm 3.6(syst) MeV/c^2, and the width is 9.9 \pm 4.8(stat) \pm 2.9(syst) MeV/c^2. The product branching fraction is measured to be B(\psi(3686) \to \gamma\eta_{c}(2S)) \times B(\eta_{c}(2S) \to K_{S}^{0}K^{\pm}\pi^{\mp}\pi^{+}\pi^{-}) = (7.03 \pm 2.10(stat) \pm 0.70(syst)) \times 10^{-6}. This measurement complements a previous BESIII measurement of \psi(3686) \to \gamma\eta_{c}(2S) with \eta_{c}(2S) \to K_{S}^{0} K^{\pm}\pi^{\mp} and K^{+}K^{-}\pi^{0}.