Search for CP Violation in Baryon Decays and Radiation Damage of the LHCb VELO

A search for CP violation in baryonic four-body $Λ^0_b → pπ^−π^+π^−$ decays is performed using the data collected by the LHCb detector during Run 1 (2011-2012) and Run 2 (2016-2017) of the LHC. The collected data corresponds to $7fb^{−1}$ of integrated luminosity and contains approximately 36 thousand signal candidates. The measurement is done using two independent methods - Triple Product Asymmetries and the energy test. The Triple Product Asymmetries method is sensitive to the P-odd type of CP violation and is based on the binned and integrated measurements of P-odd asymmetry variables. The energy test is a model independent technique, that can search for P-even and P-odd CP violation. The technique is based on a comparison of the distribution of the events in the decay phase space. Neither of the methods find a deviation from CP conservation hypothesis with a significance higher than $3σ$. Parity violation is measured with both methods with significance higher than the $5σ$ observation threshold. Studies of the radiation damage of the LHCb VELO silicon-strip detector are presented. The data taken during special regular VELO performance tests, called Charge Collection Efficiency scans, is studied. Multiple VELO performance aspects are investigated, such as the bad strip detection and exclusion algorithms, Charge Collection Efficiency and Cluster Finding Efficiency. An investigation of unforeseen tracking efficiency reduction, caused by the Second Metal Layer effect, is carried out. The time dependence of the radiation damage effects of the VELO subdetector are presented. A novel procedure of beneficially annealing the VELO is performed and presented. The high voltage tests of the VELO are performed during the last runs of the LHC, testing the detector beyond its design limitations. The listed studies allowed the evolution of the radiation damage effects of the VELO sensors to be seen. In particular, approximately 2% of VELO strips are found to be bad, but the number is stable throughout the years. The required bias voltage of the sensors is seen to rise with increasing irradiation, as predicted by the Hamburg model. The Second Metal Layer effect evolution in time is quantified. The effect starts early in Run 1 with initial irradiation, but stabilizes for the rest of the VELO operation until the end of Run 2