A study of the radiative transition ππ → πγ∗ with lattice QCD

Lattice QCD calculations of radiative transitions between hadrons have in the past been limited to processes of hadrons stable under the strong interaction. Recently developed methods for 1 → 2 transition matrix elements in a finite volume now enable the determination of radiative decay rates of strongly unstable particles. Our lattice QCD study focuses on the process ππ → πγ∗, where the ρ meson is present as an enhancement in the cross-section. We use 2+1 flavors of clover fermions at a pion mass of approximately 320 MeV and a lattice size of approximately 3.6 fm. The required 2-point and 3-point correlation functions are constructed from a set of forward, sequential and stochastic light quark propagators. In addition to determining the ρ meson resonance parameters via the Lüscher method, the scattering phase shift is used in conjunction with the 1 → 2 transition matrix element formalism of Briceño, Hansen and Walker-Loud [1] to compute the ππ → πγ∗ amplitude at several values of the momentum transfer and pp invariant mass.