Towards Precision Predictions for $B \to K\pi \ell^+\ell^-$: Handling the $K^*$ Resonance with Lattice QCD

by Felix Erben (CERN)

Thursday 4 Dec 2025, 14:00 → 15:00 Europe/London

The study of rare semileptonic $B$ decays, such as $B \to K^* \ell^+ \ell^-$, provides a powerful probe of the Standard Model and a sensitive window on possible new physics. A central theoretical challenge in interpreting these modes is the treatment of the $K^*$, which is not a stable particle but a resonance that decays strongly to $K\pi$. Most phenomenological analyses and existing lattice QCD computations effectively treat the $K^*$ as stable, an approximation whose limitations are increasingly relevant as experimental precision improves. In this seminar, I present recent progress towards a first-principles description of the hadronic final state, focusing on the dynamics of the $K\pi$ system in the $P$-wave channel that gives rise to the $K^*$. Using lattice QCD as a controlled nonperturbative framework, we study $K\pi$ scattering and extract the finite-volume spectrum and associated phase shifts. These results form the foundation for determining the full set of $B \to K\pi$ matrix elements without invoking the narrow-width approximation.