Measurement of radiative widths of a2(1320) and π2(1670)

Abstract

The COMPASS Collaboration at CERN has investigated the reaction π̄γ → π̄π̄π+ embedded in the Primakoff reaction of 190 GeV pions scattering in the Coulomb field of a lead target, π̄Pb → π̄π̄π+ 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 a2(1320) and π2(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 a2(1320) to be Γ0(a2(1320) → πγ) = (358 ± 6stat ± 42syst) keV. As the first measurement, Γ0(π2(1670) → πγ) = (181 ± 11stat ± 27syst)keV · (BRf2π /BR/f2π) is obtained for the radiative width of the π2(1670), where in this analysis the branching ratio BRff2πPDG = 0.56 has been used. We compare these values to previous measurements and theoretical predictions.