Study of GigaTracker performance and a search for Axion-Like Particles at the NA62 experiment

The Standard Model (SM) is the most rigorous theory of particle physics; however, as indicated by various experimental observations, it is far from being complete. For that reason, searches for New Physics effects became the central focus of modern experimental particle physics. The work described in this thesis was carried out using data collected by the fixed-target experiment NA62. The primary goal of NA62 is to test the SM by accurately measuring the branching ratio of the ultra-rare flavour-changing neutral current kaon decay into a pion and a neutrino-antineutrino pair. Given the chosen decay-in-flight technique, such a measurement would be impossible without a high-precision tracking system. One of the projects reported in this thesis is dedicated to the silicon hybrid beam spectrometer GigaTracKer (GTK). This work describes the time calibration procedure, data quality monitoring tools and detector performance in 2017-2018 data taking periods. In addition, since the GTK operates in a harsh radiation environment, a preliminary study of detector ageing, including the bulk current measurement and the fluence estimation, is also provided. Lastly, the thesis presents searches for Axion-like particles (ALPs) produced in b-quark to s-quark transitions and decaying into a pair of muons. The thesis describes the developed Monte Carlo simulation, the selection criteria, and the analysis of data collected by NA62 in 2017 and 2018. The expected sensitivity in the 0-background assumption is discussed, along with the preliminary background evaluations and recommendations for future data taking.