The ρ radiative decay width from lattice QCD

Pion photoproduction constitutes a prototype process, which allows for the study of the electro-weak decay of a QCD-unstable state, the ρ resonance. It is also the most straightforward process to investigate such (resonant) transition matrix elements in lattice QCD. We performed a lattice calculation of the process ππ → ρ → πγ, at presently the lightest pion mass of 317 MeV and in a large box of size (3.6 fm)³. In addition to outlining the method for our calculation, we give account of our analysis of systematic uncertainties, with focus on parametrization dependence of the P−wave phase shift and the analytic continuation of our lattice data to the ρ resonance pole to extract the coupling |G_ρπγ | = 0.0802 (38).