Measurement of $\pi^{-}\gamma \to \pi^{-}\pi^{-}\pi^{+}$ 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 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. Its measured absolute cross-section at sqrt(s) < 5 m_pi is well in agreement with the prediction from chiral perturbation theory.