Measurement of radiative widths of a2(1320) and $ \pi_{2}$(1670)

The COMPASS Collaboration at CERN has investigated the reaction \( \pi^{-}\gamma\rightarrow\pi^{-}\pi^{-}\pi^{+}\) embedded in the Primakoff reaction of 190 GeV pions scattering in the Coulomb field of a lead target, \( \pi^{-} Pb \rightarrow\pi^{-}\pi^{-}\pi^{+} Pb\). Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below 0.001 (GeV/c)2. Using a partial-wave analysis the amplitudes and relative phases of the \( a_{2}(1320)\) and \( \pi_{2}(1670)\) mesons have been extracted, and the Coulomb and the diffractive contributions have been disentangled. Measuring absolute production cross sections we have determined the radiative width of the \( a_{2}(1320)\) to be \( \Gamma_{0}(a_{2}(1320)\rightarrow\pi\gamma)=(358\pm 6_{stat} \pm 42_{syst})\) keV. As the first measurement, \(\Gamma_{0}(\pi_{2}(1670)\rightarrow\pi\gamma)=(181\pm 11_{stat}\pm 27_{syst}) keV \cdot (BR^{PDG}_{f_{2}\pi}/BR_{f_{2}\pi})\) is obtained for the radiative width of the \( \pi_{2}(1670)\), where in this analysis the branching ratio \(BR^{PDG}_{f_{2}\pi}=0.56\) has been used. We compare these values to previous measurements and theoretical predictions.