Measurement of the charged pion polarizability at COMPASS

The reaction π-+Z→π-+γ+Z in which a photon is produced by a beam pion scattering off a quasi-real photon of the Coulomb field of the target nucleus is identified experimentally by the tiny magnitude of the momentum transfer to the nucleus. This process gives access to the charged pion polarizabilities απ and βπ whose experimental determination constitutes an important test of Chiral Perturbation Theory. In this work, the pion polarizability is obtained as απ=(1.9±0.7stat.±0.8syst.) x 10-4 fm3 from data taken with 190 GeV/c hadron beam provided by SPS to the COMPASS experiment at CERN in November 2009 and under the assumption of απ+βπ=0.