Abstract
The COMPASS Collaboration at CERN has investigated the reaction pi(-)gamma -> pi(-)pi(-)pi(+) embedded in the Primakoff reaction of 190 GeV pions scattering in the Coulomb field of a lead target, pi Pb- -> 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) -> pi gamma) = (358 +/- 6(stat) +/- 42(syst)) keV.
As the first measurement, Gamma(0)(pi(2)(1670) -> pi gamma) = (181 +/- 11(stat) +/- 27(syst)) keV . (BRf2 pi PDG/BRf2 pi) is obtained for the radiative width of the pi(2)(1670), where in this analysis the branching ratio BRf2 pi PDG = 0.56 has been used. We compare these values to previous measurements and theoretical predictions.