Searches for Beyond Standard Model effects in rare B decays.

The aim of this dissertation was to analyze a decay of the beauty meson B0 , which is made of a quark-antiquark pair (up quark and b antiquark). It is an unstable particle, which means it decays very rapidly and this decay can proceed in very different ways. In one of the scenarios, B0 decays to K*0 (down quark and strange antiquark pair, decaying to kaon K+ and pion π− ) and a muon-antimuon pair μ−μ+. The angular analysis of the final state particles, which is a study of how particles propagate in a detector, can tell a lot about the B0 decay. Along with the decay angles, the B0 meson decay is described by a number of measurable quantities (observables) that can be compared to the SM predictions. The quantities of those observables could be modified by the presence of new, unknown particles. The analysis is based on the data collected by the LHCb experiment in 2016, 2017 and 2018 (Run2), when protons collided at the center of mass energy of 13 TeV (teraelectronovolts). This dissertation presents an analysis of the B0→K+π−μ+μ− decay and is focused on the higher invariant mass range of the Kπ system (1330-1530 MeV/c2 ), where heavy resonant contributions could be found. The first four chapters are devoted to the introduction, the theoretical description of the Standard Model, and the description of the LHCb detector and the B0→K+π−μ+μ− decay. This is followed by the analysis, whose first stage is proper data selection, because the signal candidates are also accompanied by background events such as those with misidentified particles. The data selection is described in chapter five, which is followed by the proper reweighting of the candidates to ensure the compatibility between the data and Monte Carlo simulated samples. In order to account for a distortion in kinematic distributions caused by the data selection, the angular acceptance was corrected by weighting the candidates with the inverse of the five dimensional efficiency parametrization calculated with the simulated B0→K+π−μ+μ− sample. Chapters eight and nine are devoted to the branching fraction calculations and the angular analysis of the B0 decay. In order to extract the angular observables the method of moments was used. In chapter ten six different sources of systematic uncertainties are described. Chapters eleven and twelve show the results of the analysis where the latter compare them with previous results from the Run1 analysis. The results of the branching fraction measurements and angular moments are compatible with the previous Run1 results and have lower statistical uncertainty. Unfortunately, the level of systematic uncertainties is relatively high. The main reason for that is a low number of simulated samples of the B0→K+π−μ+μ− decay used in the acceptance correction calculations. New results make it possible to estimate a contribution from the resonant K2*(1430) component, which can appear in the decay. Unfortunately, the level of the uncertainty is still high and the result is inconclusive, as more data are needed.