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The COMPASS collaboration at CERN has investigated the \pi^- \gamma -> \pi^-\pi^- \pi^+ reaction at center-of-momentum energy below five pion masses,sqrt(s) < 5 m(\pi), embedded in the Primakoff reaction of 190 GeV pionsimpinging on a lead target. Exchange of quasi-real photons is selected byisolating the sharp Coulomb peak observed at smallest momentum transfers, t' <0.001 (GeV/c)^2. Using partial-wave analysis techniques, the scatteringintensity of Coulomb production described in terms of chiral dynamics and itsdependence on the 3\pi-invariant mass m(3\pi) = sqrt(s) were extracted. Theabsolute cross section was determined in seven bins of $\sqrt{s}$ with anoverall precision of 20%. At leading order, the result is found to be in goodagreement with the prediction of chiral perturbation theory over the wholeenergy range investigated.

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