Measurement of the charged-pion polarisability at COMPASS

The electric (${\alpha }_{\pi }$) and the magnetic (${\beta }_{\pi }$) polarisabilities are fundamental properties of the pion characterising the rigidity of its internal structure. They have been precisely measured at the COMPASS experiment at CERN with a ${\pi }^{-}$ beam of 190~GeV/c assuming ${\alpha }_{\pi }+{\beta }_{\pi }=0$. Muons of the same momentum were used for controlling of systematic effects. The obtained result ${\alpha}{\pi}=-{\beta}{\pi}=(2.0\pm 0.6_{stat.}\pm 0.7_{syst.})\times 10^{-4} fm^3$ is in agreement with the prediction of the Chiral Perturbation Theory.