The light meson guide to New Physics: Latest searches of rare decays using LHCb data

Abstract

This thesis presents the first-ever study of the decay of a neutral kaon into four muons. Using data collected by the LHCb experiment between 2016 and 2018, no signal was found, and stringent upper limits on the branching fractions of both the short- and long-lived modes were set. They represent the first time either decay was experimentally studied, and to date, they are, respectively, the tightest upper limit and the first long-kaon result for an LHC experiment. Additionally, the study of four decays of eta and eta prime mesons are presented. The thesis introduces the first-ever study of the eta prime decaying into a pair of muons, as well as significant improvements on the precision of the branching fractions of the di-muon decay of the eta. Significant improvements on the branching fraction measurement of the eta prime meson decaying into two muons are presented, as well as a new study of the eta meson decaying into two pions and two muons.