Speaker
Description
We present a Lattice QCD calculation of the radiative decay rates of the kaon $K^- \to \ell^- \bar{\nu}_{\ell} \gamma^{(*)}$, with a real or virtual photon, and $\ell = e, \mu$. The simulations are performed with physical kaon and pion masses, using gauge ensembles generated by the Extended Twisted Mass Collaboration with $N_f = 2+1+1$ flavors of Wilson-Clover twisted mass fermions.
For real photon emission ($K^- \to \ell^- \bar{\nu}_\ell \gamma$), we compute the quark-disconnected diagrams for the first time, improving the precision of the two relevant form factors by a factor of two compared to previous lattice calculations.
We observe a significant tension between our results and experimental data.
The decay with a virtual photon ($K^- \to \ell^- \bar{\nu}_{\ell} \gamma^{*} \to \ell^- \bar{\nu}_{\ell} \ell'^+ \ell'^- $) is a powerful probe of New Physics, as its rate starts at $O(\alpha_{\mathrm{em}}^2)$. We compute the four form factors, parametrizing the hadronic part of the decay, over the full kinematical range. For photon virtualities $\sqrt{k^2} > 2m_\pi$, we overcome the issues related to analytic continuation by applying, for the first time in an exclusive decay channel, spectral density methods.
