Measurement of radiative widths of $a_2(1320)$ and ${\pi }_2(1670)$

The COMPASS Collaboration at CERN has investigated the reaction $\pi^- \gamma \rightarrow \pi^-\pi^-\pi^+$embeddedinthePrimakoffreactionof$190~$\text{GeV}$$pionsscatteringintheCoulombfieldofaleadtarget,$\pi^- $\text{Pb}$ \rightarrow \pi^-\pi^-\pi^+ $\text{Pb}$$. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below $0.001(\text{GeV}\mathrm{/}c{)}^2$. 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 $\Gamma_0(a_2(1320) \rightarrow \pi\gamma) = (358 \pm 6_{$\text{stat}$} \pm 42_{$\text{syst}$})~$\text{keV}$$. As the first measurement, $\Gamma_0(\pi_2(1670) \rightarrow \pi\gamma) = (181 \pm 11_{$\text{stat}$} \pm 27_{$\text{syst}$})~$\text{keV}$ \cdot ($\text{BR}$^{$\text{PDG}$}{f_2 \pi}/$\text{BR}${f_2 \pi})$ is obtained for the radiative width of the ${\pi }_2(1670)$, where in this analysis the branching ratio ${\text{BR}}_{f_2\pi }^{\text{PDG}}=0.56$ has been used. We compare these values to previous measurements and theoretical predictions.