Measurement of $e^{+} e^{-}\to p\bar{p}$ around $\psi(2S)$ with scan method

Abstract

This paper uses the data corresponding to an integrated luminosity of $495$~pb$^{-1}$ in the center-of-mass energy region from 3.58~GeV to 3.71~GeV to study the relative phase between strong and electromagnetic amplitudes in the $\psi(2S)\to p\bar{p}$ decay. The cross sections and the effective form factors around $\psi(2S)$ of proton-antiproton pairs are presented thereby filling the gap in the experimental results around the $\psi(2S)$ resonance for the first time, and the results are consistent with the previous BESIII and BaBar results. The relative phase is extracted from the cross-section line shape as $\Phi_{g,\gamma}=(105.9\pm9.4)^\circ$ or $\Phi_{g,\gamma}=(-105.3\pm9.1)^\circ$. Correspondingly, two solutions for the $\psi(2S)\to p\bar{p}$ branching fractions are obtained as $\mathcal{B}=(3.21\pm0.13)\times10^{-4}$ and $\mathcal{B}=(3.47\pm0.19)\times10^{-4}$. These results support the universality of the phase and provide new insights into its underlying mechanism.