Higher-order multipole amplitude measurement in $\psi(2S)\to\gamma\chi_{c2}$

TL;DR

This paper measures higher-order multipole amplitudes in the radiative decay of $oldsymbol{ ext{ψ(2S)}}$ to $oldsymbol{ ext{χ}_{c2}}$, providing evidence for the magnetic quadrupole component and testing charm quark properties.

Contribution

It presents the first measurement of the magnetic quadrupole amplitude in $ ext{ψ(2S)}$ radiative decay with high statistical significance, confirming theoretical predictions.

Findings

Evidence for M2 amplitude with 4.4σ significance

Measured M2 = 0.046 ± 0.010 ± 0.013

Measured E3 = 0.015 ± 0.008 ± 0.018

Abstract

Using $106\times10^6$ $\psi(2S)$ events collected with the BESIII detector at the BEPCII storage ring, the higher-order multipole amplitudes in the radiative transition $\psi(2S)\to\gamma\chi_{c2}\to\gamma\pi\pi/\gamma KK$ are measured. A fit to the $\chi_{c2}$ production and decay angular distributions yields $M2=0.046\pm0.010\pm0.013$ and $E3=0.015\pm0.008\pm0.018$, where the first errors are statistical and the second systematic. Here $M2$ denotes the normalized magnetic quadrupole amplitude and $E3$ the normalized electric octupole amplitude. This measurement shows evidence for the existence of the $M2$ signal with $4.4\sigma$ statistical significance and is consistent with the charm quark having no anomalous magnetic moment.