Measurement of the charged-pion polarisability The COMPASS Collaboration

The COMPASS collaboration at CERN has investigated pion Compton scattering, pi−γ → pi−γ, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction pi−Ni→ pi−γ Ni, which is initiated by 190 GeV pions impinging on a nickel target. The exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, Q2 < 0.0015 (GeV/c)2. From a sample of 63 000 events, the electric pion polarisability is deter-mined to be αpi = (2.0 ± 0.6stat ± 0.7syst) × 10−4 fm3 under the assumption αpi = −βpi that relates the electric and magnetic dipole polarisabilities. This result is in tension with previous ded-icated measurements, while it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambigiously known, was performed for an independent estimate of the systematic uncertainty. (to be submitted to Physical Review Letters) ar X i