Resonant π+γ → π+π0 Amplitude from Quantum Chromodynamics

We present the first ab initio calculation of a radiative transition of a hadronic resonance within quantum chromodynamics (QCD). We compute the amplitude for pi pi -> pi gamma*, as a function of the energy of the pi pi pair and the virtuality of the photon, in the kinematic regime where pi pi couples strongly to the unstable rho resonance. This exploratory calculation is performed using a lattice discretization of QCD with quark masses corresponding to m(pi) approximate to 400 MeV. We obtain a description of the energy dependence of the transition amplitude, constrained at 48 kinematic points, that we can analytically continue to the rho pole and identify from its residue the rho -> pi gamma* form factor.