Measurement of radiative widths at COMPASS

COMPASS is a multipurpose fixed-target experiment at the CERN SPS, which addresses a wide variety of physic topics, in particular the structure and spectroscopy of hadrons. Diffractive dissociation of pions on nuclear targets allows for clean access to the light meson spectrum. In addition meson production can be studied in pion-photon reactions via the Primakoff effect, where high-energetic pions react with the quasi-real photon field surrounding the target nuclei. At low pion-photon center-of-mass energies, these reactions are governed by chiral dynamics and contain information relevant for chiral perturbation theory. At higher energies, resonances are produced and their radiative coupling is investigated. During a short run using a 190GeV $\pi^-$ beam and a lead target in the year 2004, 3 million exclusive $\pi^-\pi^-\pi^+$ events in the region of small squared four-momentum transfer, i.e. t' < 0:01GeV$^2$=c$^2$, have been recorded. At very low t' < 0:001GeV$^2$=c$^2$, the contribution from electromagnetic interactions becomes visible. A partial-wave analysis of the data was performed, clean signals for the a$_2$(1320) and the $\pi_2$(1670) were observed. The radiative couplings of the a$_2$(1320) and, for the first time, that of the $\pi_2$(1670) were measured by isolating the electromagnetic contribution to the corresponding waves.