First observation of $\eta(1405)$ decays into $f_{0}(980)\pi^0$

TL;DR

This paper reports the first observation of the isospin-violating decay of $ ext{eta}(1405)$ into $f_0(980) ext{pi}^0$ using $J/\psi$ decays, revealing a narrower $f_0(980)$ width and precise $ ext{eta'}$ decay measurements.

Contribution

It presents the first observation of $ ext{eta}(1405)$ decaying into $f_0(980) ext{pi}^0$ with large isospin violation, and provides new measurements of decay widths and rates.

Findings

First observation of $ ext{eta}(1405) o f_0(980) ext{pi}^0$ decay.

Narrower than expected $f_0(980)$ width.

Precise decay rates for $ ext{eta'}$ three-pion decays.

Abstract

The decays $J/\psi \rightarrow \gamma \pi^+\pi^-\pi^0$ and $J/\psi \rightarrow \gamma \pi^0\pi^0\pi^0$ are analyzed using a sample of 225 million $\jpsi$ events collected with the BESIII detector. The decay of $\eta(1405)\ar f_{0}(980)\pi^0$ with a large isospin violation is observed for the first time. The width of the $f_{0}(980)$ observed in the dipion mass spectra is anomalously narrower than the world average. Decay rates for three-pion decays of the $\eta'$ are also measured precisely.