Measurement of the pion polarizabilities at COMPASS

The electromagnetic structure of pions is probed in $\pi\gamma$ Compton scattering in inverse kinematics (Primakoff effect) and described by the electric ($\alpha_{\pi}$) and magnetic ($\beta_{\pi}$) polarizabilities, that depend on the rigidity of pion’s internal structure as a composite particle. Values for pion polarizabilities can be extracted from the comparison of the theoretically predicted (under approximation of unstructured pion) cross section of Primakoff scattering and the measured cross section. The high beam intensity, good spectrometer resolution, the high rate capability, the high acceptance and possibility to use pion and muon beams, that are unique to the COMPASS experiment provide the tools to measure precisely the pion polarizabilities in the $\pi^{-} + (A,Z)\rightarrow\pi^{-} + (A,Z) + \gamma$ Primakoff reaction. This cross section is related to the cross section of Compton scattering on pion. A precise tracking system, electromagnetic and hadron calorimeters provide good conditions for Primakoff events selection and background suppression. The pion polarizability measurement was performed with $a \pi^{-}$ beam of 190 GeV . Most of the statistics was collected with a 3 mm thick lead target, along with smaller samples using carbon, copper, 1.6 mm lead and empty target. Total statistics collected with lead target is about 60 000 events.