Study of the $\Lambda_b^0 \to D_s^- p$ decays at the LHCb experiment

In this Master’s thesis, the operation principles of LHC accelerator and LHCb detector are referred to. They are followed by the properties description of the LHCb tracking and particle identification systems. One of the beauty to charm flavour decays, $\Lambda_b^0 \to D_s^- p$, has never been observed. Its branching fraction is proportional to the CKM matrix element $V_{ub}$, which precise determination is a crucial test of the CKM unitarity. To minimise any systematic effects, this branching fraction can be measured with respect to the $\bar{\Lambda}_b^0 \to \bar{\Lambda}_c^- \pi^+$ normalisation channel. The main goal of this thesis focuses on the data-driven approach used for the efficient estimation of background contributions in the $\Lambda_b^0 \to D_s^- p$ sample. Two statistically independent control samples of the $B_s^0 \to D_s^- \pi^+$ and $B_s^0 \to D_s^\mp K^\pm$ decays are employed. Together with the signal ($\Lambda_b^0 \to D_s^- p$) and normalisation ($\bar{\Lambda}_b^0 \to \bar{\Lambda}_c^- \pi^+$) modes they are used for the unbiased yields determination. For each channel, the beauty hadron invariant mass distribution is fitted taking into account many possible background contributions. The procedure results in the determination of (102 $\pm$ 14) and (835 $\pm$ 35) $\Lambda_b^0 \to D_s^- p$ candidates for LHC Run1 (2011-2012) and Run2 (2015-2018) data taking periods, respectively. In addition, a high statistic normalisation mode $\bar{\Lambda}_b^0 \to \bar{\Lambda}_c^- \pi^+$ is examined, finding (64050 $\pm$ 260) and (367820 $\pm$ 660) candidates in Run1 and Run2 data samples. Provided in this dissertation studies about these decays are part of the ongoing LHCb measurement performed in the collaboration with NIKHEF group from the Netherlands.