Heavy Neutral Leptons (HNLs) are hypothetical particles predicted by many extensions of the Standard Model. These particles can, among other things, explain the origin of neutrino masses, generate the observed matter-antimatter asymmetry in the Universe and provide a dark matter candidate. The SHiP experiment will be able to search for HNLs produced in decays of heavy mesons and travelling distances ranging between O(50 m) and tens of kilometers before decaying. We present the sensitivity of the SHiP experiment to a number of HNL's benchmark models and provide a way to calculate the SHiP's sensitivity to HNLs for arbitrary patterns of flavour mixings. The corresponding tools and data files are also made publicly available.

Recently an upper bound on the rate of the lepton number violating decay K + → µ + µ + π − has been significantly reduced by the E865 experiment at BNL and further improvement is expected in the near future. We study this process as a possible source of information on neutrino masses and mixings. We find that it is insensitive to the light(eV domain) and heavy(GeV domain) neutrinos. However due to the effect of a resonant enhancement this decay is very sensitive to neutrinos ν j in the mass region 245 MeV ≤ m ν j ≤ 389 MeV. At present experimental sensitivity we deduce new stringent limits on the neutrino mixing matrix element U µj for neutrino masses in this region.

We study lepton number violating decays of charged K, D, D s , B and B c mesons of the forminduced by the existence of Majorana neutrinos. These processes provide information complementary to neutrinoless double nuclear beta decays, and are sensitive to neutrino masses and lepton mixing. We explore neutrino mass ranges m N from below 1 eV to several hundred GeV. We find that in many cases the branching ratios are prohibitively small, however in the intermediate range m π < m N < m Bc , in specific channels and for specific neutrino masses, the branching ratios can be at the reach of high luminosity experiments like those at the LHC-b and future Super flavor-factories, and can provide bounds on the lepton mixing parameters.

Some of the outstanding questions of particle physics today concern the neutrino sector, in particular whether there are more neutrinos than those already known and whether they are Dirac or Majorana particles. There are different ways to explore these issues. In this article we describe neutrino-mediated decays of charged pseudoscalar mesons such as π ± , K ± and B ± , in scenarios where extra neutrinos are heavy and can be on their mass shell.We discuss semileptonic and leptonic decays of such kinds. We investigate possible ways of using these decays in order to distinguish between the Dirac and Majorana character of neutrinos. Further, we argue that there are significant possibilities of detecting CP violation in such decays when there are at least two almost degenerate Majorana neutrinos involved. This latter type of scenario fits well into the known neutrino minimal standard model (νMSM) which could simultaneously explain the Dark Matter and Baryon Asymmetry of the Universe.