title: Resonant π^{+}γ→π^{+}π^{0} Amplitude from Quantum Chromodynamics.
abstract: We present the first ab initio calculation of a radiative transition of a hadronic resonance within quantum chromodynamics (QCD). We compute the amplitude for ππ→πγ^{⋆}, as a function of the energy of the ππ pair and the virtuality of the photon, in the kinematic regime where ππ couples strongly to the unstable ρ resonance. This exploratory calculation is performed using a lattice discretization of QCD with quark masses corresponding to m_{π}≈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 ρ pole and identify from its residue the ρ→πγ^{⋆} form factor.
authors.affiliation: Thomas Jefferson National Accelerator Facility, 12000 Jefferson Avenue, Newport News, Virginia 23606, USA.
authors.forename: Raúl A
authors.initials: RA
authors.lastname: Briceño
authors.affiliation: Thomas Jefferson National Accelerator Facility, 12000 Jefferson Avenue, Newport News, Virginia 23606, USA.|Department of Physics, Old Dominion University, Norfolk, Virginia 23529, USA.
authors.forename: Jozef J
authors.initials: JJ
authors.lastname: Dudek
authors.affiliation: Thomas Jefferson National Accelerator Facility, 12000 Jefferson Avenue, Newport News, Virginia 23606, USA.
authors.forename: Robert G
authors.initials: RG
authors.lastname: Edwards
authors.affiliation: Department of Physics, Old Dominion University, Norfolk, Virginia 23529, USA.
authors.forename: Christian J
authors.initials: CJ
authors.lastname: Shultz
authors.affiliation: Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, United Kingdom.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Thomas
authors.affiliation: Department of Physics, Old Dominion University, Norfolk, Virginia 23529, USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Wilson