Amplitude Analysis of the Isospin-Violating Decay $J/\psi\rightarrow\gamma\eta\pi^{0}$

TL;DR

This paper presents the first amplitude analysis of the decay $J/\to\gamma\eta\pi^0$ using a large data sample, revealing dominant intermediate processes and first observations of radiative transitions to isospin-triplet scalar mesons.

Contribution

It provides the first amplitude analysis of this decay and reports the first observation of radiative transitions to specific scalar mesons, improving measurement precision.

Findings

Dominant intermediate processes identified in the decay.

First observation of radiative transitions to isospin-triplet scalar mesons.

Branching fraction measured with improved precision.

Abstract

Using $(10\,087 \pm 44) \times 10^6$ $J/\psi$ events collected with the BESIII detector, we perform the first amplitude analysis of the process $J/\psi\to\gamma\eta\pi^0$. The decay is dominated by the intermediate processes $J/\psi\to\pi^0 b_1(1235)^0 \to\gamma\eta\pi^0$, $J/\psi\to\pi^{0}\rho(1450)^0 \to\gamma\eta \pi^0$ and $J/\psi\to\eta h_1(1170) \to\gamma\eta\pi^0$. Contributions from $J/\psi\to\gamma a_0(980)^0\to\gamma\eta\pi^0$, $J/\psi\to\gamma a_2(1320)^0\to\gamma\eta\pi^0$ and $J/\psi\to\gamma a_2(1700)^0\to\gamma\eta\pi^0$ are observed with a statistical significance exceeding $5\sigma$, constituting the first observation of radiative transitions of $J/\psi$ to isospin-triplet scalar mesons. The total branching fraction of $J/\psi\to\gamma \eta \pi^0$ is measured to be $(25.7\pm0.3\pm1.5)\times 10^{-6}$, where the first uncertainty is statistical and the second systematic. This result is consistent with the previous measurement, with the precision improved by more than a factor of two.