Measurement of radiative widths of $a_2(1320)$ and $\ddot{I}{€}_2(1670)$

Abstract

The COMPASS Collaboration at CERN has investigated the reaction $\ddot{I}{€}^{-}{\hat{I}}^3\to \ddot{I}{€}^{-}\ddot{I}{€}^{-}\ddot{I}{€}^{+}$ embedded in the Primakoff reaction of $190\text{GeV}$ pions scattering in the Coulomb field of a lead target, $\ddot{I}{€}^{-}\text{Pb}\to \ddot{I}{€}^{-}\ddot{I}{€}^{-}\ddot{I}{€}^{+}\text{Pb}$. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below $0.001(\text{GeV}/c{)}^2$. Using a partial-wave analysis the amplitudes and relative phases of the $a_2(1320)$ and $\ddot{I}{€}_2(1670)$ 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 $\hat{I}{“}_0(a_2(1320)\to \ddot{I}€{\hat{I}}^3)=(358\pm 6_{\text{stat}}\pm 42_{\text{syst}})\text{keV}$. As the first measurement, $\hat{I}{“}_0(\ddot{I}{€}_2(1670)\to \ddot{I}€{\hat{I}}^3)=(181\pm 11_{\text{stat}}\pm 27_{\text{syst}})\text{keV}\cdot ({\text{BR}}_{f_2\ddot{I}€}^{\text{PDG}}/{\text{BR}}_{f_2\ddot{I}€})$ is obtained for the radiative width of the $\ddot{I}{€}_2(1670)$, where in this analysis the branching ratio ${\text{BR}}_{f_2\ddot{I}€}^{\text{PDG}}=0.56$ has been used. We compare these values to previous measurements and theoretical predictions.

27 pages, 12 figures and 3 tables