Pion-photon reactions and chiral dynamics in Primakoff processes at COMPASS

At the COMPASS experiment at CERN, pion-photon reactions are investigated using the Primakoff effect, where high-energetic pions react with the quasi-real photons surrounding the target nuclei.The production of a single hard photon in such a pionscattering, at lowest momentum transfer to the nucleus, is related to pionCompton scattering. Studying the energy distribution of the outgoing photons, the pion polarizability can be extracted. In addition to the measurement with a pion beam, control measurements with a muon beam allow us to estimate the systematics. The COMPASS result is in tension with earlier dedicated measurements and rather in agreement with the theoretical expectation from chiralperturbation theory.Based on the same data set, reactions with neutral and charged pions in the final state are studied. At low invariant mass of the pion-photon system, these reactions are governed by chiral dynamics. Using partial-wave analysis techniques, the absolute cross sections for the production of π−π+π− and π−π0π0 states from π−γ interactions are measured and compared to predictions from chiralperturbation theory. At higher pion-photon masses, the production of 3π resonances is studied with the focus on their radiative couplings.