Search for Heavy Neutral Leptons at the SPS

A minimal extension of the Standard Model with three right-handed singlet Majorana leptons with masses below the Fermi scale (vMSM) allows incorporating a mechanism to generate the baryon asymmetry of the Universe, account for the pattern of small neutrino masses and oscillations, and potentially provide a Dark Matter candidate. This avoids introducing new energy scales between the Fermi scale and the Planck scale, the lack of which has also been shown to be compatible with the mass of the recently discovered Higgs boson. Considerations based on cosmological constraints and based on the data on neutrino oscillations favour a range of masses and couplings for these new heavy neutral leptons (HNL) which is largely accessible to experimental verification or falsification. This paper outlines a proposal for a new fixed-target experiment at the CERN SPS to search for the HNLs, together with an introduction to the theoretical motivation, a description of the experimental setup with the associated beam line and detector, and a discussion of the background sources and the expected sensitivity. With an integrated total of 2x10$^{20}$ protons on target at 400 GeV, the experiment is able to achieve a sensitivity which is four orders of magnitude better than previous searches. In addition to HNLs, the experiment will be sensitive to many other types of physics models that predict weakly interacting long-lived exotic particles.