Search for New Physics in SHiP and at future colliders

SHiP is a newly proposed fixed-target experiment at the CERN SPS with the aim of searching for hidden particles that interact very weakly with Standard Model particles. The work presented in this document investigates SHiP's physics reach in the parameter space of the Neutrino Minimal Standard Model (νMSM), a theory that could solve most problems unexplained by the Standard Model by incorporating sterile neutrinos. A model introducing an extra U(1) symmetry in the hidden sector, providing a natural candidate for dark matter, is also explored. This work shows that the SHiP experiment can improve the sensitivity to Heavy Neutral Leptons below 2 GeV by several orders of magnitude, scanning a large part of the parameter space below the B meson mass. The remainder of the νMSM parameter space, dominated by right-handed neutrinos with masses above 2 GeV, can be explored at a future e(+)e(−) collider. Similarly, SHiP can greatly improve present constraints on U(1) dark photons.