Measurement of $J/\psi\to\gamma\eta_{\rm c}$ decay rate and $\eta_{\rm c}$ parameters at KEDR

TL;DR

This study measures the decay rate and parameters of the $ ext{J/}\psi o ext{gamma} ext{eta}_c$ process using the KEDR detector, providing precise values for the $ ext{eta}_c$ mass, width, and the decay rate.

Contribution

First measurement of the $ ext{J/}\psi o ext{gamma} ext{eta}_c$ decay rate and $ ext{eta}_c$ parameters using the inclusive photon spectrum at KEDR.

Findings

Decay rate $ ext{Gamma}^0_{ ext{gamma} ext{eta}_c} = 2.98 ext{ keV}$ with uncertainties.

$ ext{eta}_c$ mass measured as $2983.5$ MeV/$c^2$ with uncertainties.

$ ext{eta}_c$ width measured as $27.2$ MeV with uncertainties.

Abstract

Using the inclusive photon spectrum based on a data sample collected at the $J/\psi$ peak with the KEDR detector at the VEPP-4M $e^+e^-$ collider, we measured the rate of the radiative decay $J/\psi\to\gamma\eta_{\rm c}$ as well as $\eta_{\rm c}$ mass and width. Taking into account an asymmetric photon lineshape we obtained $\Gamma^0_{\gamma\eta_{\rm c}}=2.98\pm0.18 \phantom{|}^{+0.15}_{-0.33}$ keV, $M_{\eta_{\rm c}} = 2983.5 \pm 1.4 \phantom{|}^{+1.6}_{-3.6}$ MeV/$c^2$, $\Gamma_{\eta_{\rm c}} = 27.2 \pm 3.1 \phantom{|}^{+5.4}_{-2.6}$ MeV.