Fundamental particles and their interactions are currently best described by the Standard Model of particle physics. One of the predictions of this theory is the fact that charged leptons (electron, muon, and tau) interact in the same way with other particles, notwithstanding the Higgs interactions that give them different masses.
This principle is known as lepton flavour universality, and has been tested through a host of various measurements. One of these is the ratio between the rates at which two beauty-meson decays, B+ --> K+ mu+ mu- and B+ --> K+ e+ e-, occur. This ratio is known as RK, and its most precise measurement to date is the subject of this thesis. The result is obtained using 9 fb^-1 of proton-proton collision data, recorded between the years 2011 and 2018 by the LHCb detector at CERN’s Large Hadron Collider. The result is RK = 0.846 (+ 0.042 −0.039) (+ 0.013 −0.012), where the first uncertainty is statistical, and the second systematic. This measurement is in tension with the Standard Model prediction at the level of 3.1 sigma. It therefore constitutes evidence for the violation of lepton flavour universality in B+ --> K+ mu+ mu- and B+ --> K+ e+ e- decays. Subsequent measurements of RK and related observables are expected to improve the global picture, potentially leading to the discovery of physics currently beyond the Standard Model.