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

Abstract.

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.