Exclusive Measurement of the eta --> pi+ pi- gamma Decay

TL;DR

This study provides an exclusive measurement of the eta --> pi+ pi- gamma decay, revealing detailed differential distributions and challenging simple theoretical models with empirical data.

Contribution

First precise differential measurement of eta --> pi+ pi- gamma decay at COSY, highlighting the need for more complex theoretical descriptions.

Findings

P-wave dominance in pion angular distribution

Photon energy spectrum inconsistent with simple gauge models

Data can be parameterized with empirical pion form factor

Abstract

An exclusive measurement of the decay eta --> pi+ pi- gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd --> 3He X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13340\pm140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta --> pi+ pi- gamma. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi+ pi- system.