Measurement of pi−γ → pi−pi−pi+ at Low Masses, and Comparison to ChPT Prediction, at COMPASS

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 GeV2/c2. 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 ′ ≈ 0. The partial-wave analysis of these data focusses on new techniques for the extraction of the Primakoff contribution at low masses. Its measured absolute cross-section at s < 5mpi is well in agreement with the prediction from chiral perturbation theory