First Measurement of Chiral Dynamics in \pi^- \gamma -> \pi^- \pi^- \pi^+

The COMPASS Collaboration at CERN has investigated the pi(-)gamma -> pi(-)pi(-)pi(+) reaction at center-of-momentum energy below five pion masses, root s < 5m(pi), 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' < 0.001 GeV2/c(2). Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3 pi-invariant mass m(3 pi) = root s were extracted. The absolute cross section was determined in seven bins of root 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.