Thesis defense of Alex Seuthe

Recent measurements of the lepton flavour universality show evidence of a violation of the Standard Model of particle physics. However, further investigation with higher sensitivity is needed to either falsify or confirm the anomalies. This thesis describes the first simultaneous measurement of the lepton flavour universality ratios RK with Bp->Kpl+l- decays and RK*0 with B0->K*0l+l decays using the entire current available data set of the LHCb experiment recorded during the years 2011 to 2018. Both ratios are evaluated in two regions of q2, which is the squared invariant mass of the dilepton system. Even though the central values of the results are kept blind and will only be published once the analysis has passed the collaboration-internal review process, the presented sensitivities on RK and RK*0 already show that this will be the most precise test of lepton flavour universality with these decays to date. The analysis work described in this thesis comprises the first measurement of RK at low q2 at LHCb and an updated measurement of RK*0 with an enlarged dataset. Statistical and systematic uncertainties as well as correlations between the four measurements are thus determined coherently for the first time and allow for improved interpretations. The calibration of the simulation used to determine the selection efficiency is a crucial component of the analysis to ensure that deviations of the measurements from the Standard Model expectation are not an effect due to remaining imperfections of the simulation. Therefore, a complex multi-stage calibration procedure has been developed together with an optimised selection strategy. Simulation of the signal decays is calibrated with the help of high-yield control samples chosen to minimise correlations between calibration and signal data sets. The experimental procedure of the analysis is validated by multiple stringent cross-checks.