EPJ A Highlight – COMPASS: Radiative widths of the a2(1320) and π2(1670) mesons

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Published on 29 April 2014

Radiative transitions are among the most important and insightful processes for the investigation of atomic, nuclear and hadronic systems. They reveal the electromagnetic substructure of the involved particles. The a₂(1320) meson is known since the 1980s to decay radiatively with a branching of about 0.3% into a pion and a photon. Theoretically this can be linked, for example through the vector meson dominance model, to the main hadronic decay channels.

The COMPASS experiment at CERN has taken high-statistics data of pion-nucleus collisions π^- Pb→ π^- π^- π⁺ Pb of which the photon-exchange contribution, so-called Primakoff scattering, was singled out. The radiative coupling of the a₂(1320) resonance was determined with unprecedented precision, employing partial-wave analysis of the three-pion final state. The latter proved the radiative coupling of the 10-times less intensive π₂(1670) resonance, π₂(1670)→πγ, and made possible the first measurement of its radiative width. This challenges theoretical descriptions, that aim at linking this transition intensity to the inner structure of the π₂(1670).