Cross sections for low-energy pi- gamma reactions

Abstract

We review the cross sections for low-energy $pi^- gamma$ reactions in the framework of chiral perturbation theory. Charged pion Compton scattering, $pi^- gammato pi^-gamma$, is considered up to one-loop order where the pion_s14s internal structure enters through the difference of the electric and magnetic pion polarizability, $alpha_pi - beta_pi$. The ongoing COMPASS experiment aims at measuring this important structure constant with high statistics using the Primakoff effect. In the same way, the reaction $pi^- gamma to pi^- pi^0$ serves as a test of the QCD chiral anomaly (via the $gamma 3pi$-coupling constant $F_{gamma3pi}$). Furthermore, we calculate the total cross sections for neutral and charged pion-pair production, $pi^- gamma to pi^- pi^0pi^0$ and $pi^- gamma to pi^-pi^_s16pi^-$, which are governed by the chiral $pipi$-interaction. Finally, we investigate the radiative (correction) process $pi^- gamma to pi^- gamma gamma$ and calculate the corresponding two-photon mass spectrum. This information will be useful for analyzing the $pi^-$ Primakoff scattering events with photons in the final state.