Search for a very rare B meson decay with multiple muons in the final state at the LHCb experiment

Indirect searches for new physics beyond the Standard Model rely on comparing the properties of various processes to their Standard Model predictions. Leptonic and semileptonic decays of heavy-quark hadrons offer an attractive laboratory for indirect searches for potential new particles or interactions. The leptonic decay modes of $B$ mesons, such as ${B^+ \to \mu^+ \nu_\mu}$, are challenging to study in hadron-collider experiments owing to their single-charged-track topology. To tackle this challenge, the emission of a virtual photon, decaying into a dilepton, allows to turn the one-track topology into a three-track one, allowing to determine the $B$-meson decay vertex position. This leads to a much simpler experimental measurement at hadron colliders. Since these processes have a neutrino in the final state, it is beneficial to validate our experimental understanding of the virtual photon emission, using fully reconstructed decay modes of $B$ mesons.

This thesis presents the first study of one such process: ${B^+ \to K^+ J/\psi \mu^+\mu^-}$, which is formed by an emission of a virtual photon from the abundant tree-level decay ${B^+ \to J/\psi K^+}$. It is shown that, beyond the virtual-photon contribution, other processes need to be taken into account, that come from light vector mesons decaying to dimuons, but also excited charmonium states decaying to ${J/\psi \mu\mu}$. The total rate of the ${B^+ \to K^+ J/\psi \mu^+\mu^-}$ process, including resonant contributions, is measured, using the data collected by the LHCb experiment in 2016--2018. It is compatible, within uncertainty, with the expectations, but a larger dataset is needed to achieve a firm conclusion.

This measurement serves as a proof-of-concept for the rich multilepton physics programme that will become accessible with the large dataset to be collected by upgraded LHCb detector in the coming years (Run 3 and after).