Exclusive Measurement of the eta --> pi+ pi- gamma Decay
  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. 
P. Adlarson, C. Adolph, W. Augustyniak, W. Bardan, M. Bashkanov, T.   Bednarski, F.S. Bergmann, M. Ber{\l}owski, H. Bhatt, K...
nucl-ex hep-ex