Measurement of the Charged-Pion Polarizability
Résumé
The COMPASS collaboration at CERN has investigated pion Compton scattering, π−γ → π−γ, at centerof-
mass energy below 3.5 pion masses. The process is embedded in the reaction π−Ni → π−γNi, which is
initiated by 190 GeV pions impinging on a nickel target. The exchange of quasireal photons is selected by
isolating the sharp Coulomb peak observed at smallest momentum transfers, Q2 < 0.0015 ðGeV=cÞ2.
From a sample of 63 000 events, the pion electric polarizability is determined to be απ ¼ ð2.0 0.6stat
0.7systÞ × 10−4 fm3 under the assumption απ ¼ −βπ, which relates the electric and magnetic dipole
polarizabilities. It is the most precise measurement of this fundamental low-energy parameter of strong
interaction that has been addressed since long by various methods with conflicting outcomes. While this
result is in tension with previous dedicated measurements, it is found in agreement with the expectation
from chiral perturbation theory. An additional measurement replacing pions by muons, for which the crosssection
behavior is unambiguously known, was performed for an independent estimate of the systematic
uncertainty.