Two-photon widths of the $\chi_{c0, 2}$ states and helicity analysis for $\chi_{c2}\ar\gamma\gamma$}

Abstract

Based on a data sample of 106 M $\psi^{\prime}$ events collected with the
BESIII detector, the decays $\psi^{\prime}\ar\gamma\chi_{c0, 2}$,$\chi_{c0,
2}\ar\gamma\gamma$ are studied to determine the two-photon widths of the
$\chi_{c0, 2}$ states. The two-photon decay branching fractions are determined
to be ${\cal B}(\chi_{c0}\ar\gamma\gamma) = (2.24\pm 0.19\pm 0.12\pm
0.08)\times 10^{-4}$ and ${\cal B}(\chi_{c2}\ar\gamma\gamma) = (3.21\pm 0.18\pm
0.17\pm 0.13)\times 10^{-4}$. From these, the two-photon widths are determined
to be $\Gamma_{\gamma \gamma}(\chi_{c0}) = (2.33\pm0.20\pm0.13\pm0.17)$ keV,
$\Gamma_{\gamma \gamma}(\chi_{c2}) = (0.63\pm0.04\pm0.04\pm0.04)$ keV, and
$\cal R$ $=\Gamma_{\gamma \gamma}(\chi_{c2})/\Gamma_{\gamma
\gamma}(\chi_{c0})=0.271\pm 0.029\pm 0.013\pm 0.027$, where the uncertainties
are statistical, systematic, and those from the PDG ${\cal
B}(\psi^{\prime}\ar\gamma\chi_{c0,2})$ and $\Gamma(\chi_{c0,2})$ errors,
respectively. The ratio of the two-photon widths for helicity $\lambda=0$ and
helicity $\lambda=2$ components in the decay $\chi_{c2}\ar\gamma\gamma$ is
measured for the first time to be $f_{0/2}
=\Gamma^{\lambda=0}_{\gamma\gamma}(\chi_{c2})/\Gamma^{\lambda=2}_{\gamma\gamma}(\chi_{c2})
= 0.00\pm0.02\pm0.02$.