  The value of the pion polarizability is predicted with high precision by
Chiral Perturbation Theory. However, the existing experimental values are at
tension with this prediction as well as among themselves. The COMPASS
experiment at the CERN SPS accesses pion-photon reactions via the Primakoff
effect, where high-energetic pions react with the quasi-real photon field
surrounding the target nuclei. Flagship channel is the Primakoff reaction in
which a single real photon is produced, giving access to pion Compton
scattering. Using this process the pion polarizability is extracted from the
measured cross-section shape. End of 2009 COMPASS performed a measurement of
the pion polarizability using a nickel target. The large amount of data
collected in combination with the possibility to study systematic effects using
the analogous reaction with a muon beam, the most precise experimental value to
date was determined.
