Pion polarizabilities measurement at COMPASS

The electromagnetic structure of pions is probed in $\pi^{−} + (A,Z)\rightarrow\pi^{−} + (A,Z) +\gamma$ Compton scattering in inverse kinematics (Primakoff reaction) and described by the electric $(\bar{\alpha_{\pi}})$ and the magnetic $(\bar{\beta_{\pi}})$ 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 differential cross section for scattering of pointlike pions with the measured cross section. The pion polarizability measurement was performed with $a \pi^{−}$ beam of 190 GeV. The high beam intensity, the good spectrometer resolution, the high rate capability, the high acceptance and the possibility to use pion and muon beams, unique to the COMPASS experiment, provide the tools to measure precisely the pion polarizabilities in the Primakoff reaction. The preliminary result for pion polarizabilities under the assumption of $\bar{\alpha_{\pi}} + \bar{\beta_{\pi}} =$ 0 is $\bar{\alpha_{\pi}} = −\bar{\beta_{\pi}} =$ (2.5 $\pm$ 1.7$_{stat.} \pm$ 0.6$_{syst.}$) $×$ 10$^{−4}fm^{3}$.