No abstract

We analyze the scenario of baryogenesis through leptogenesis induced by the out-of-equilibrium decays of heavy Majorana neutrinos and pay special attention to CP violation. Extending a recently proposed resummation formalism for two-fermion mixing to decay amplitudes, we calculate the resonant phenomenon of CP violation due to the mixing of two nearly degenerate heavy Majorana neutrinos. Solving numerically the relevant Boltzmann equations, we find that the isosinglet Majorana mass may range from 1 TeV up to the grand unification scale, depending on the mechanism of CP violation and/or the flavor structure of the neutrino mass matrix assumed. Finite temperature effects and possible constraints from the electric dipole moment of electron and other low-energy experiments are briefly discussed. ͓S0556-2821͑97͒04121-0͔

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

The Higgs sector of the Standard Model (SM) with one right-handed neutrino per family is systematically analyzed. In a model with intergenerational independent mixings between families, we can account for very light neutrinos acquiring Majorana masses radiatively at the first electroweak loop level. We also find that in such a scenario the Higgs coupling to the light-heavy neutrinos and to the heavy-heavy ones may be remarkably enhanced with significant implications for the production of these heavy neutrinos at high energy colliders.

Analytic expressions of lepton-flavour-and lepton-number-violating decays of charged leptons are derived in the context of general SU(2) L ⊗ U(1) Y seesaw scenarios that are motivated by grand unified theories (GUT's) or superstring models, in which left-handed and/or right-handed neutral singlets are present. Possible constraints imposed by cosmology and low-energy data are briefly discussed. The violation of the decoupling theorem in flavourdependent graphs due to the presence of heavy neutral leptons of Dirac or Majorana nature is emphasized. Numerical estimates reveal that the decays τ − → e − e − e + or τ − → e − µ − µ + can be as large as ∼ 10 −6 , which may be observed in LEP experiments or other τ factories.The quest for an understanding of the problem of smallness in mass or masslessness of the light known neutrinos, ν e , ν µ , and ν τ , has relied on interesting solutions in the context of extended gauge structures of the minimal Standard Model (SM), such as grand unified theories, e.g. SO(10) models [1], or superstring models with an E 6 symmetry [2]. Among the various solutions, the most attractive one, known as the seesaw mechanism, has been conceived by the authors in [3] within the framework of SO(10) or left-right symmetric models. In these theories, right-handed neutrinos are introduced with the simultaneous inclusion of Majorana masses that violate the lepton-number (L) by ∆L = 2 operators in the Yukawa sector. The neutrino-mass spectrum of a simple seesaw model with one generation of quarks and leptons consists of two massive Majorana neutrinos, ν and N, having masses m ν ≃ m 2 D /m M and m N ≃ m M . If the Dirac mass term m D is of the order of a typical charged-lepton or quark mass, as dictated by GUT relations [4], and the Majorana-mass scale m M is sufficiently large, one can then obtain a very light neutrino ν. The general situation of an interfamily seesaw-type model with a number n G of weak isodoublets and a number n R of right-handed neutrinos is more involved [5] and will be discussed in Section 2.If nature keeps to the pathway of a seesaw-type solution, then heavy Majorana neutrinos at the mass scale of TeV may manifest themselves in L-violating processes at highenergy ee [6,7], ep [8], and pp colliders [9,10], in possible lepton-flavour-violating decays of the Z [11] and Higgs particles (H) [12] or through universality-breaking effects in leptonic diagonal Z-boson decays [13]. Their existence may also influence [14,15] the size of electroweak oblique parameters [16,17], tri-gauge boson W W Z-and ZZZ-couplings [18], or specific Higgs observables considered recently [19,20]. Finally, there are many other places scanned by exhaustive combined analyses of charged-current-universality effects in leptonic π decays, neutral-current interactions in neutrino-nucleon scatterings, τ -polarization asymmetries, neutrino-counting experiments at the CERN Large Electron Positron Collider (LEP), etc. [21,22], in which Majorana neutrinos could also manifest their presence.Another possible solution o...