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

The COMPASS collaboration at CERN has investigated the $\pi^-\gamma \to \pi^-\pi^-\pi^+$ reaction at center-of-momentum energy below five pion masses, $\sqrt{s} \lt 5m_\pi$ , embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $t' \lt 0.001 GeV^2/c^2$. Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3pi-invariant mass $m_{3\pi} = \sqrt{s}$ were extracted. The absolute cross section was determined in seven bins of $\sqrt{s}$ 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.