This thesis presents two analyses of semileptonic b sl+l decays using Flavour Changing Neutral Currents (FCNCs) to test for the presence of new physics and lepton avour universality, and the equality of couplings for different leptons, which on the basis of experimental evidence is assumed to hold in the Standard Model, free from uncertainties as a result of knowledge of the hadronic matrix elements. It also includes the angular analysis of Lambda_b->Lambda mumu decay and the RK* measurement, both of which are first measurements, not yet performed by any other experiment.

Luca Pescatore's main interest is fundamental physics including both particle physics and astrophysics but also technology: programming, data analysis, machine learning and electronics. He obtained his master's degree at the University of Bologna, Italy, looking for Quark Gluon plasma signature with the CMS experiment. He then joined the LHCb experiment and continued his studies at the University of Birmingham, UK. He was the main author for two papers on rare decays of b hadrons, where new physics can contribute at the level of quantum corrections. Currently he is a researcher at the École Polytechnique Fédérale de Lausanne, Switzerland.

Introduction.- The LHCb Detector at the Large Hadron Collider.- Differential Branching Fraction of b0 µ+µ.- Angular Analysis of b0 µ+µ Decays.- Testing Lepton Favour Universality with RK0.- Conclusions.