Calculating the ρ radiative decay width with lattice QCD

We present the results of our lattice QCD study of the πγ → ππ process, where the ρ resonance appears as an enhancement in the transition amplitude. We use N_f = 2 + 1 clover fermions on a lattice of L = 3.6 fm and a pion mass of 320 MeV. Using a combination of forward, stochastic, and sequential propagators, we calculate the two-point and three-point functions that allow us to determine the πγ → ππ matrix elements for several values of the invariant mass s and momentum transfer q². To fit the q² and s dependence of the πγ → ππ amplitude, we explore a set of general parametrizations based on a Taylor expansion. By analytic continuation to the complex pole corresponding to the ρ resonance, we determine the resonant form factors and calculate the radiative decay width of the ρ.