Measurement of the Charged Pion Polarizability at COMPASS

The reaction pi- + Z --> pi- + gamma + 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 may be identified experimentally by the tiny magnitude of the momentum transfer to the nucleus. This process gives access to the charged pion polarizabilities alpha_pi and beta_pi whose experimental determination constitutes an important test of Chiral Perturbation Theory. Under the assumption of alpha_pi + beta_pi = 0 the pion polarizability is obtained in this work as alpha_pi = 1.9 x 10^-4 fm^3 with random error 0.7 and systematic uncertainty 0.8 from data taken with 190 GeV/c hadron beam provided by SPS to the COMPASS experiment at CERN in November 2009. The analysis has passed internal peer-review and its outcome has been released as preliminary result by the COMPASS collaboration.