Hidden sector searches with fixed-target experiments

Despite its immense predictive power, there are inconsistencies in the Standard Model of particle physics, the most prominent of which are the neutrino masses. The Standard Model also does not address certain phenomena, notably the amount of observed matter and dark matter and the baryon asymmetry of the universe. Still, these phenomena might be explained by Standard Model extensions. Free parameters of the Standard Model might also be derived in these extensions, often predicting new yet unobserved particles almost decoupled from the Standard Model, which can be a component of the dark matter, the so-called hidden sector of particle physics. Fixed-target experiments are among the most promising probes of the hidden sector, thanks to high-intensity beams. This thesis classifies the hidden sector portals and the various searches for the corresponding processes in the MeV-GeV mass range of the hidden sector mediators. A public software framework for interpreting experimental results in terms of various hidden sector scenarios has been developed during the project. Part of the work is dedicated to the hidden sector searches with the NA62 experiment and the recent results obtained with the data sample collected in the beam-dump mode in 2021. The data analyses performed on this sample are discussed in detail.