Searches for New Physics effects in b →sl-sl+ transitions

Abstract

The dissertation aims at presenting the current situation in the measurements of electroweak penguin diagrams dominated decays: b → sl−l+1 . These decays have been a smoking gun for hunting for New Physics effects over many years, but in the last three years the research on these phenomena has intensified due to new measurements. Enormous progress has been made both on the theoretical and the experimental sides to understand the measured deviations from the current Standard Model predictions, referred to in what follows as “anomalies”. The author of this dissertation has been one of the main authors of the angular analysis of B0→ K∗ 0µ+µ− decay in the LHCb experiment, which has been widely regarded as one of the most important results of the flavour physics sector in recent years. He has proposed a method called “the method of moments” to measure the angular terms of this decay, which he has later successfully applied in the measurement itself. Moreover, he has been the driving force behind the two other important analyses in LHCb: the measurement of the angular distribution and branching ratio of the B0→ K∗ 0 (1430)µ+µ− decay, where again the method of moments has been used to obtain the angular coefficients, and the search for the light scalar particle that can be produced in the b → s transitions and that decays to a dimuon pair. In this case no signal has been observed and the upper limits on the branching fraction have been set, later to be used for constraining the inflaton model. The dissertation is organized as follows: the brief introduction is followed by, the second chapter devoted to a theoretical description of rare B decays, where the effective field theory formalism is introduced. Furthermore, the author discusses the current theoretical problems in calculating the Standard Model predictions for the b → sl−l+ processes. Last but not least, the optimised angular observables that are less dependent on the form factors uncertainness are derived. The third chapter describes the experimental apparatus used in the b → sl−l+ measurements. Special focus is put on the sub-detectors that play an important role in the studies of b → sl−l+ transitions. Chapters 4, 5, 6 are devoted to describing the data analyses performed by the author in the LHCb experiment. In Chapter 7 the global analysis of electroweak penguin decays is presented. This kind of global analysis has become extremely popular in the past few years as it helps to constrain and pin down those New Physics models that are likely to be responsible for the observed anomalies. The author of this monograph is involved in one of the biggest collaborations performing New Physics fits, where he is the convenor of the Flavour Working group. Furthermore, the author presents his own study on separating the long distance effects in the B0→ K∗ 0µ+µ−decay. This is the state of the art way of determining those contributions. The chapter ends with a description of possible New Physics models that can explain the observed discrepancies.