First measurement of chiral dynamics in π ^-γ→π^-π^-π⁺

The COMPASS Collaboration at CERN has investigated the π ^-γ→π^-π^-π⁺ reaction at center-of-momentum energy below five pion masses, √s<5m_π, embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t ^′<0.001GeV2/c2. Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3π-invariant mass m_3π=√s were extracted. The absolute cross section was determined in seven bins of √s with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated.