The K* resonance in Kγ* → Kπ from QCD

We present the first calculation of the Kγ*→ Kπ transition with a resonant K* from lattice quantum chromodynamics. In this process, the kaon interacts with a photon and scatters strongly to a Kπ. The Kπ state is in a lowest relative S & P-wave, with the K* resonance appearing in the P-wave. To map the finite volume matrix elements from lattice QCD to the infinite volume transition amplitudes measured in experiments, we apply the Lellouch-Lüscher formalism [1] using information from the Kπ scattering in S & P-wave [2]. This is the first test for this formalism in the presence of multiple partial waves.

We determine the transition amplitude as a function of the Kπ energy and the photon virtuality and observe the expected enhancement due to the K* resonance. From the energy dependence, we extract the K* → Kπ transition form factor for the unstable K* by analytically continuing into the complex energy plane and from the residue at the K* pole.