F. Mazzetti - Lattice QCD determination of radiative form factors in leptonic decays

Monday 12 Jun 2023, 16:15 → 17:15 Europe/Berlin

Nußallee 14-16/3.013 (HISKP) - Seminar Room II (HISKP)

Simone Romiti (uni-bonn)

We present a non-perturbative lattice calculation of the radiative $D_s\to \ell\,\nu_\ell\,\gamma$ leptonic decay, where $\ell$ is a charged lepton, based on the new $N_f = 2 + 1 + 1$ Wilson-clover twisted-mass gauge ensembles generated by the Extended Twisted Mass Collaboration with quark masses tuned very close to their physical values. In addition to the trivial, ``point-like" contribution in which the photon is emitted either from the lepton or the meson treated as a point-like particle, two structure-dependent (SD) form factors, $F_V$ and $F_A$, contribute to the amplitude. It is the SD form factors which describe the interaction between the virtual photon and the internal hadronic structure of the decaying meson, and in our procedure we separate the SD and point-like contributions to the amplitudes.

We provide a continuum-extrapolated lattice determination of the SD form factors over the full physical kinematic range, which we employ to compute the differential decay rate and the corresponding branching fraction, as a function of the lower cut on the photon energy, which is in agreement with the experimental upper bound set by the BESIII collaboration.

Having calculated the form factors from first principles, we test the accuracy of model calculations based on vector meson dominance and light-cone sum rules.

Moreover, we present an exploratory lattice calculation of the leptonic $K\to\ell\nu_\ell\,\ell^{\prime\,+}\ell^{\prime\,-}$ decay, which is mediated by the emission of a virtual photon, and requires the knowledge of four SD form factors. We also compare these preliminary lattice results with experimental measures and chiral perturbation theory predictions.abstr