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

Details: Published on 29 April 2014

alt — Intensity of the 2^-+ partial-wave in π^- γ→π^- π^- π^+, interpreted as radiative width of the π₂(1670).

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).

Measurement of radiative widths of α₂(1320) and π₂(1670), C. Adolph et al. (2014), European Physical Journal A 50: 79, DOI 10.1140/epja/i2014-14079-8

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