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

Understand

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.