Study of the Rare B-meson Decays with the ATLAS Experiment

The searching for New Physics beyond the Standard Model of particles is one of the main aims of the current particle experiments. Rare b-hadron decays, occurring at the first level through loop Feynman diagrams, are strongly sensitive to the possible existence of new particles. Recent analyses from the LHCb experiment, measuring decay angles distributions in rare B-meson decay into a kaon $K^{0*}$ and two muons, hints a deviation from the Standard Model prediction. The ATLAS experiment works on a similar analysis of the decay $B^0_d \to K* \mu^- \mu^+$ in order to confirm this deviation. The new particles’ existence could be observed in the deviations from the predicted decay’s angular distribution. This work provides input for the future analysis of this rare decay in the data-taking period of Run 2. To be more specific, observing of the fine deviations requires the precise description of the measured data and the Standard Model’s predictions. The improvement to the Run 1 analysis was achieved by an expansion of the fit function in a series of spherical harmonic functions. Next, the effect of the remaining background events in the signal dataset was repressed by adding masses $m_B$ and $m_{K*}$ to the final fit. That required to know functions’ prescriptions of the $m_B$ and $m_{K*}$ distributions for the signal decay and the expected background hence looking for their prescriptions was a part of this work too. The invariant $B$ mass was described by the Gauss distribution with a per-event error. The per-event fit has to take into account the so-called Punzi effect in some cases which was also discussed here.