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Measurement of the charged-pion polarisability at COMPASS

The electric ($α_π$) and the magnetic ($β_π$) 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 $π^{-}$ beam of 190~GeV/c assuming $α_π+β_π=0$. Muons of the same momentum were used for controlling of systematic effects. The obtained result $α_π=-β_π=(2.0\pm 0.6_{stat.}\pm 0.7_{syst.})\times 10^{-4} fm^3$ is in agreement with the prediction of the Chiral Perturbation Theory.