Measurement of pi- gamma -> pi-pi-pi+ at Low Masses, and Comparison to ChPT Prediction, at COMPASS

Understand

This paper presents an analysis of pi- Pb -> X- Pb -> pi-pi-pi+ Pb events at 190 GeV/c beam momentum and very low four-momentum transfer t'<0.001 GeV^2/c^2.

• Coherent scattering off the nucleus as a whole dominates with contributions from Reggeon, Pomeron and photon exchange.
• The latter originates from Primakoff reactions and is identified by the sharp Coulomb peak of intensities at t' approx 0.
• The partial-wave analysis of these data focusses on new techniques for the extraction of the Primakoff contribution at low masses.