Radiative Left-Right Dirac Neutrino Mass

We consider the conventional left-right gauge extension of the standard model of quarks and leptons without a scalar bidoublet. We study systematically how one-loop radiative Dirac neutrino masses may be obtained. In addition to two well-known cases from almost 30 years ago, we find two new scenarios with verifiable predictions.Comment: 11 pages, 6 figures, text and references adde

Gauged B−3LτB - 3 L_\tau and Baryogenesis

It has recently been shown that by extending the minimal standard model to include a right-handed partner to $\nu_\tau$, it is possible to gauge the $B - 3 L_\tau$ quantum number consistently. If we add two scalar triplets, one trivial ($\xi_1$) and one nontrivial ($\xi_2$) under $B - 3 L_\tau$, it is possible also to have desirable neutrino masses and mixing for neutrino oscillations. At the same time, a lepton asymmetry can be generated in the early universe through the novel mechanism of the decay of the heavier $\xi_1$ into the lighter $\xi_2$ plus a neutral singlet ($\zeta^0$). This lepton asymmetry then gets converted into a baryon asymmetry at the electroweak phase transition.Comment: 16 pages including two postscript figure

Probing the Exotic Particle Content Beyond the Standard Model

We explore the possible exotic particle content beyond the standard model by examining all its scalar bilinear combinations. We categorize these exotic scalar fields and show that without the suppression of (A) their Yukawa couplings with the known quarks and leptons, and (B) the trilinear couplings among themselves, most are already constrained to be very heavy from the nonobservation of proton decay and neutron-antineutron oscillations, the smallness of $K^0 - \bar {K^0}$, $D^0 - \bar {D^0}$ and $B_d^0 - \bar {B_d^0}$ mixing, as well as the requirement of a nonzero baryon asymmetry of the universe. On the other hand, assumption (B) may be naturally violated in many models, especially in supersymmetry, hence certain exotic scalars are allowed to be below a few TeV in mass and would be easily detectable at planned future hadron colliders. In particular, large cross sections for the distinctive processes like $\bar p p \to tt,\bar t c$ and $p p \to t t, b b$ would be expected at the Fermilab Tevatron and CERN LHC, respectively.Comment: Many details and three new figures with cross sections added in Tevatron and LHC collider phenomenology sectio

Leptogenesis Below the Davidson-Ibarra Bound

The observed baryon asymmetry of the Universe is suitably created in thermal leptogenesis through the out-of-equilibrium decay of $N_1$, the lightest of the three heavy singlet neutral fermions which anchor the seesaw mechanism to obtain small Majorana neutrino masses. However, this scenario suffers from the incompatibility of a generic lower bound on the mass of $N_1$ and the upper bound on the reheating temperature of the Universe after inflation. A modest resolution of this conundrum is proposed.Comment: 7 pages, 1 figur

Neutrino Masses in Supersymmetry: R-Parity and Leptogenesis

In the supersymmetric standard model of particle interactions, R-parity nonconservation is often invoked to obtain nonzero neutrino masses. We point out here that such interactions of the supersymmetric particles would erase any pre-existing lepton or baryon asymmetry of the universe before the electroweak phase transition through the $B + L$ violating sphaleron processes. We then show how neutrino masses may be obtained in supersymmetry (assuming R-parity conservation) together with successful leptogenesis and predict the possible existence of new observable particles.Comment: LATEX, 12 page

Gauged B-3L_\tau, low-energy unification and proton decay

We point out that if there is a gauged $B-3L_\tau$ symmetry at low energy, it can prevent fast proton decay. This may help building models with theories with extra dimensions at the TeV scale. For purpose of illustration we present an explicit model with large extra dimensions. The Higgs required for a realistic fermion masses and mixing are included. The problem of neutrino masses are solved with triplet Higgs scalars. The proton remains stable even after the $B-3L_\tau$ symmetry breaking.Comment: 8 pages, Late

Revelations of the E_6/U(1)_N Model: Two-Loop Neutrino Mass and Dark Matter

The E_6/U(1)_N gauge extension of the Supersymmetric Standard Model, first proposed by Ma, is shown to have exactly the requisite ingredients to realize the important new idea that dark matter is the origin of neutrino mass. With the implementation of a discrete Z_2 X Z_2 symmetry, and particle content given by three {27} representations of E_6, neutrino masses are naturally generated in two loops, with candidates of dark matter in the loops. All particles of this model are expected to be at or below the TeV scale, allowing them to be observable at the LHC.Comment: 10 pages, 3 figure

Baryon and Lepton Number Violation with Scalar Bilinears

We consider all possible scalar bilinears, which couple to two fermions of the standard model. The various baryon and lepton number violating couplings allowed by these exotic scalars are studied. We then discuss which ones are constrained by limits on proton decay (to a lepton and a meson as well as to three leptons), neutron-antineutron oscillations, and neutrinoless double beta decay.Comment: 11 pages latex fil

CP violation in the mass matrix of heavy neutrinos

We discuss the question of CP-violation in the effective Hamiltonian approach in models of leptogenesis through heavy right handed neutrino decays. We first formulate the problem in four component notation and then point out that before the heavy neutrinos have decayed away, the universe becomes CP-asymmetric in the heavy neutrinos. However, the lepton asymmetry generated after they completely decay are independent of this asymmetry.Comment: 9 pages latex file, minor change

Scale of SU(2)_R symmetry breaking and leptogenesis

Models of leptogenesis often invoke the out-of-equilibrium decays of heavy right-handed neutrinos in order to create a baryon asymmetry of the universe through the electroweak phase transition. Their presumed existence argues strongly for the presence of an $SU(2)_R$ gauge symmetry. We study the equilibrating effects of the resulting additional right-handed interactions and find that successful leptogenesis requires that $m_N \gtrsim 10^{16}$ GeV if $m_N > m_{W_R}$, and $m_{W_R} \gtrsim 3 \times 10^6 GeV (m_N /10^2 GeV)^{2/3}$ if $m_{N}<m_{W_R}$, where $m_{N}$ is the mass of the lightest right-handed neutrino. However, the $m_N > m_{W_R}$ option is excluded in a supersymmetric theory with gravitinos.Comment: 4 pages (Revtex); accepted for publication with small modifications; conclusions unchange