Speaker
Description
The quark-level transitions of a beauty quark into a strange quark, such as $b\rightarrow s\ell\ell$, are flavour-changing neutral current (FCNC) processes, highly suppressed in the Standard Model (SM), and provide powerful indirect probes of physics beyond the SM. In recent years, measurements of these transitions have revealed intriguing tensions with SM predictions, including suppressed branching fractions and deviations in angular observables across multiple decay modes. Within an effective field theory framework, such anomalies could arise from modified short-distance interactions, although underestimated hadronic effects may also contribute. Further experimental input across complementary decay channels and kinematic regions is therefore essential to clarify the origin of these discrepancies.
Among the most sensitive probes of such effects is the decay $B\rightarrow K\pi\mu\mu$, whose hadronic system is dominated by the $K^{*0}(892)$ vector resonance at low $K\pi$ invariant mass. Consequently, previous measurements have focused primarily on the region $m(K\pi)<1050$ MeV$/c^2$. However, at higher $K\pi$ masses, where additional and overlapping resonant contributions become accessible, the hadronic structure is significantly less understood. If the observed deviations arise from modified short-distance dynamics, similar effects may also appear in this region. More generally, non-exclusive multi-body FCNC decays, characterised by rich and only partially understood resonant structures, provide an important and complementary laboratory for probing the same underlying dynamics.
In this talk, recent results on $b\rightarrow s\ell\ell$ transitions obtained using the full LHCb Run 1 and Run 2 dataset are presented, alongside updated studies of rare FCNC decays with complex hadronic structure. These include measurements of the differential branching fraction of $B\rightarrow K\pi\mu\mu$ as a function of both the $K\pi$ invariant mass and the dimuon invariant mass squared, $q^2$, across the extended kinematic region $1000\leq m(K\pi)\leq 1850 \, \mathrm{MeV}/c^2$ and $0.1\leq q^2\leq 18$ GeV$^2/c^4$, together with complementary recent studies of rare $b\rightarrow s\ell\ell$ decays.