Search for the Cabibbo-suppressed decays of $\Lambda_b^0$ baryon at the LHCb experiment

The searches for the Cabibbo-suppressed decays $\Lambda_b^0\to\psi(2S)p\pi^-$ and $\Lambda_b^0\to\chi_{c1}p\pi^-$ are presented in this thesis. Using the data sample of proton-proton collisions, corresponding to an integrated luminosity of 4.9 fb$^{-1}$, collected with the LHCb detector, the decay $\Lambda_b^0\to\psi(2S)p\pi^-$ is observed for the first time. Branching fraction of this decay relative to that of the decay $\Lambda_b^0\to\psi(2S)pK^-$ is measured. In both decay modes $\psi(2S)$ mesons are reconstructed using the $\mu^+\mu^-$ final state. The decay $\Lambda_b^0\to\chi_{c1}p\pi^-$ is observed for the first time using the data sample of proton-proton collisions, corresponding to an integrated luminosity of 6 fb$^{-1}$, collected with the LHCb detector. Branching fraction of this decay relative to that of the decay $\Lambda_b^0\to\chi_{c1}pK^-$ is measured. First evidence of the decay $\Lambda_b^0\to\chi_{c2}p\pi^-$ is found and branching fraction of this decay relative to that of the decay $\Lambda_b^0\to\chi_{c1}p\pi^-$ is measured. Also, branching fraction ratio of the decays $\Lambda_b^0\to\chi_{c2}pK^-$ and $\Lambda_b^0\to\chi_{c1}pK^-$ is measured with better precision in comparison with previous measurement. Both $\chi_{c1}$ and $\chi_{c2}$ mesons are reconstructed via the radiative decays $\chi_{c1,2}\to J/\psi\gamma$, where $J/\psi$ mesons are reconstructed using the $\mu^+\mu^-$ final state.