A simple connection between neutrino oscillation and leptogenesis

The usual see-saw formula is modified by the presence of two Higgs triplets in left-right symmetric theories. The contribution from the left-handed Higgs triplet to the see-saw formula can dominate over the conventional one when the neutrino Dirac mass matrix is identified with the charged lepton or down quark mass matrix. In this case an analytic calculation of the lepton asymmetry, generated by the decay of the lightest right-handed Majorana neutrino, is possible. For typical parameters, the out-of-equilibrium condition for the decay is automatically fulfilled. The baryon asymmetry has the correct order of magnitude, as long as the lightest mass eigenstate is not much lighter then 10^{-6} to 10^{-8} eV, depending on the solution of the solar neutrino problem. A sizable signal in neutrinoless double beta decay can be expected, as long as the smallest mass eigenstate is not much lighter than 10^{-3} eV and the Dirac mass matrix is identified with the charged lepton mass matrix.Comment: 16 pages, 3 figures. One paragraph and some references added, typos correcte

Breaking of B-L in superstring inspired E6 model

In the framework of the superstring inspired E6 model, low-energy extensions of the standard model compatible with leptogenesis are considered and masses of right-handed neutrinos in two scenarios allowed by long-lived protons are discussed. The presence of two additional generations allows breaking of B-L without generating nonzero vacuum expectation values of right-handed sneutrinos of the three known generations. After the symmetry breaking, right-handed neutrinos acquire Majorana masses of order of 10^11 GeV. Within the framework of a simple discrete symmetry, assumptions made to provide a large mass of right-handed neutrinos are shown to be self-consistent. Supersymmetric structure of the theory ensures that large corrections, associated with the presence of a (super)heavy gauge field, cancel out.Comment: 18 pages, 6 tables, axodraw use

Leptogenesis

I present the theoretical basis for Leptogenesis and its implications for the structure of the universe. It is suggested that density fluctuations grow during the transition period and remnants of this effect should be sought in the universe. The relation between theories with Majorana neutrinos and low energy phenomena, including oscillations, advanced considerably during the past two years with a consistent picture developed in several models.Comment: 9 pages, 3 figures.To appear in the proceedings of The IXth International Symposium on Particles, Strings and Cosmology at the Tata Institute of Fundamental Research, Mumbai (Bombay), India, during 3-8 January 200

Baryon and Lepton Number Assignment in E6E_6 Models

In $E_6$ models there are new particles whose baryon number is not uniquely assigned. We point out that the baryon and lepton number assignment to these particles can change the baryogenesis scenario significantly. We consider left-right symmetric extension of the standard model in which $(B-L)$ quantum number is gauged. The identification of $(B-L)$ with a generator of $E_6$ is used to define the baryon and lepton numbers for the exotic particles in a way that the electroweak baryon and lepton number anomaly corresponding to the $SU(2)_L$ group vanishes, {\it i.e.}, there is no non-perturbative baryon or lepton number violation during the electroweak phase transition. We study some consequences of the new assignment.Comment: 14 pages, LaTeX file, 1 submitted Figure file(.eps

Large Neutrino Mixing from Renormalization Group Evolution

The renormalization group evolution equation for two neutrino mixing is known to exhibit nontrivial fixed point structure corresponding to maximal mixing at the weak scale. The presence of the fixed point provides a natural explanation of the observed maximal mixing of $\nu_{\mu}-\nu_{\tau}$ if the $\nu_{\mu}$ and $\nu_{\tau}$ are assumed to be quasi-degenerate at the seesaw scale without constraining on the mixing angles at that scale. In particular, it allows them to be similar to the quark mixings as in generic grand unified theories. We discuss implementation of this program in the case of MSSM and find that the predicted mixing remains stable and close to its maximal value, for all energies below the $O$(TeV) SUSY scale. We also discuss how a particular realization of this idea can be tested in neutrinoless double beta decay experiments.Comment: Latex file, 21 pages and 4 ps figures include

Higgs-mediated FCNCs: Natural Flavour Conservation vs. Minimal Flavour Violation

We compare the effectiveness of two hypotheses, Natural Flavour Conservation (NFC) and Minimal Flavour Violation (MFV), in suppressing the strength of flavour-changing neutral-currents (FCNCs) in models with more than one Higgs doublet. We show that the MFV hypothesis, in its general formulation, is more stable in suppressing FCNCs than the hypothesis of NFC alone when quantum corrections are taken into account. The phenomenological implications of the two scenarios are discussed analysing meson-antimeson mixing observables and the rare decays B -> mu+ mu-. We demonstrate that, introducing flavour-blind CP phases, two-Higgs doublet models respecting the MFV hypothesis can accommodate a large CP-violating phase in Bs mixing, as hinted by CDF and D0 data and, without extra free parameters, soften significantly in a correlated manner the observed anomaly in the relation between epsilon_K and S_psi_K.Comment: 27 pages, 4 figures. v3: minor modifications (typos corrected and few refs. added), conclusions unchanged; journal versio