Measurement of the Branching Fraction of \boldmath{$\psi(2S) \to \gamma \pi^0$}

TL;DR

This paper measures the branching fraction of the decay $oldsymbol{ ext{psi(2S)} o ext{gamma} ext{pi}^0}$ using large data samples, and investigates the form factor and interference effects in $e^+e^-$ collisions at high momentum transfer.

Contribution

First measurement of the $ ext{psi(2S)} o ext{gamma} ext{pi}^0$ branching fraction with BESIII data, including interference analysis and form factor determination at high $Q^2$.

Findings

Branching fraction range: $[2.7, 9.7] imes 10^{-7}$.

Two solutions for the cross section with different phase angles.

Form factor measured at $Q^2 o 13$ GeV$^2$.

Abstract

Based on $(2712.4\pm14.1)\times10^{6}~\psi(2S)$ events, 7.9 fb$^{-1}$ $\psi(3773)$ data, and 0.8 fb$^{-1}$ off-resonance data samples collected with the BESIII detector, we measure the branching fraction of $\psi(2S)\rightarrow\gamma\pi^{0}$ and $e^{+}e^{-}\rightarrow\gamma\pi^{0}$ form factor at momentum transfers $Q^{2}\sim13$ GeV$^{2}$. The $e^{+}e^{-}\rightarrow\gamma\pi^{0}$ cross section is fitted with considering the interference between the $\psi(2S)$ and continuum amplitudes and two solutions are found, ${\cal B}=3.74\times10^{-7}$ with $\phi=3.93$ rad and ${\cal B}=7.87\times10^{-7}$ with $\phi=2.08$ rad. Here, ${\cal B}$ is the branching fraction of $\psi(2S)\rightarrow\gamma\pi^{0}$ and $\phi$ is the relative phase angle between the $\psi(2S)$ and continuum amplitudes. Due to insufficient off-resonance data, the branching fraction ${\cal B}(\psi(2S)\rightarrow\gamma\pi^{0})$ is determined to be in the range $[2.7, 9.7]\times10^{-7}$ within one standard deviation of the contour region.