Study of Rare Decays of $B$ Hadrons and Development of a Data Viewer Application of the Detector Control System in the ATLAS Detector

The first part of the thesis is a study of the sensitivity of the ATLAS detector of the di-onia resonances. This analysis, which is among the first studies of di-onia resonances within ATLAS, is including the feasibility studies of the measurement of the Br($\chi_{b} \rightarrow J/\psi J/\psi$). The second part of this thesis, describes the measurement of the Br($B^0_s \rightarrow \mu^− \mu^+$) using 2.4 fb−1 of the LHC ATLAS data. While the author has extensively contributed to some important steps of the study, the final measurement is a result of a work of many people (indicated in the main document accordingly). The parts of the analysis corresponding to the work of the author are documented in all detail, while all the other main parts of the analysis contributing in the extraction of the limit Br($B^0_s \rightarrow \mu^- \mu^+$) < 2.2 × 10−8 at 95% CL are discussed. The third part of the thesis describes a data viewing application for ATLAS Detector Control System (DCS) data. The tool is called Dcs Data Viewer and it is the official ATLAS software that covers the needs of offline detector data visualization. Currently 150 active users are using the tool from many countries all around the world.