Pion Polarizability at CERN COMPASS

The electric $\alpha_{\pi}$ and magnetic $\beta_{\pi}$ charged pion Compton polarizabilities provide stringent tests of Chiral Perturbation Theory. The combination ($\alpha_{\pi}$-$\beta _{\pi})$ was measured at CERN COMPASS via radiative pion Primakoff scattering (Bremsstrahlung of 190 GeV/c $\pi^{-}$s) in the nuclear Coulomb field: $\pi +$ Z $\to \pi + \gamma+$ Z. This reaction is identified experimentally by virtue of the very small momentum transfer to the target nucleus; and is equivalent to $\gamma +$ $\pi \to \gamma + \pi $ Compton scattering for laboratory $\gamma$'s of order 1 GeV/c incident on a target pion at rest. COMPASS data analysis (assuming $\alpha_{\pi}+\beta_{\pi}=$0 based on theory) gives a preliminary value of $\alpha_{\pi} =-\beta_{\pi} =$ (1.9 $\pm$ 0.7$_{\mathrm{stat.}}$ $\pm$ 0.8$_{\mathrm{syst.}})$ $\times$ 10$^{-4}$ fm$^{3}$.