A New Parametrization of the Neutrino Mixing Matrix

The neutrino mixing matrix is expanded in powers of a small parameter $\lambda$, which approximately equals to 0.1. The meaning of every order of the expansion is discussed respectively, and the range of $\lambda$ is carefully calculated. We also present some applications of this new parametrization, such as to the expression of the Jarlskog parameter $J$, in which the simplicities and advantages of this parametrization are shown.Comment: 11 pages, 4 figures, version published in PL

Verifiable Origin of Neutrino Mass at TeV Scale

The physics responsible for neutrino mass may reside at or below the TeV energy scale. The neutrino mass matrix in the $(\nu_e, \nu_\mu, \nu_\tau)$ basis may then be deduced from future high-energy accelerator experiments. The newly observed excess in the muon anomalous magnetic moment may also be related.Comment: 9 pages, 1 figure, talk at TAUP 200

Double Threefold Degeneracies for Active and Sterile Neutrinos

We explore the possibility that the 3 active (doublet) neutrinos have nearly degenerate masses which are split only by the usual seesaw mechanism from 3 sterile (singlet) neutrinos in the presence of a softly broken $A_4$ symmetry. We take the unconventional view that the sterile neutrinos may be light, i.e. less than 1 keV, and discuss some very interesting and novel phenomenology, including a connection between the LSND neutrino data and solar neutrino oscillations.Comment: 8 pages, no figur

Neutrino Mass from Triplet and Doublet Scalars at the TeV Scale

If the minimal standard model of particle interactions is extended to include a scalar triplet with lepton number $L=-2$ and a scalar doublet with $L=-1$, neutrino masses $m_\nu \sim \mu_{12}^4 v^2/M^5 \sim 10^{-2}$ eV is possible, where $v \sim 10^2$ GeV is the electroweak symmetry breaking scale, $M \sim 1$ TeV is the typical mass of the new scalars, and $\mu_{12} \sim 1$ GeV is a soft lepton-number-violating parameter.Comment: 6 pages, no figur

Connecting Dark Energy to Neutrinos with an Observable Higgs Triplet

To connect the scalar field (acceleron) responsible for dark energy to neutrinos, the usual strategy is to add unnaturally light neutral singlet fermions (right-handed neutrinos) to the Standard Model. A better choice is actually a Higgs triplet, through the coupling of the acceleron to the trilinear Higgs triplet-double-doublet interaction. This hypothesis predicts an easily observable doubly-charged Higgs boson at the forthcoming Large Hadron Collider (LHC).Comment: 9 page

Inverse seesaw in supersymmetry

We study a mechanism where tiny neutrino masses arise only from radiative contribution in a supersymmetric model. In each generation, the tree-level light neutrino mass is rotated away by introducing a second SM singlet $s_{L}$ that forms a Dirac mass term with the right-handed neutrino $n_{R}$. Even with non-zero Majorana neutrino mass for the right-handed neutrinos $M_{R} \bar{n^{c}_{R}} n_{R}$, the lightest neutrino remains massless at tree level due to an approximate symmetry as $U(1)_{\nu-s}$. Holomorphic feature of superpotential ensures that the Majorana neutrino masses $M_{R} \bar{n^{c}_{R}} n_{R}$ and $M^{*}_{R} \bar{s^{c}_{L}} s_{L}$ are not generated simultaneously. However, the $U(1)_{\nu-s}$ is not respected by the SM gauge interactions or interaction with Higgs. Consequently, tiny neutrino masses arise from radiative contributions. It is also shown that the right-handed neutrino Majorana mass $M_{R}$ can be at $\cal O$(KeV) to obtain the proper light neutrino mass.Comment: 10 page

Novel TeV-scale seesaw mechanism with Dirac mediators

We propose novel tree level seesaw mechanism with TeV-scale vectorlike Dirac mediators that produce Majorana masses of the known neutrinos. The gauge quantum number assignment to the Dirac mediators allows them to belong to a weak triplet and a five-plet of nonzero hypercharge. The latter leads to new seesaw formula m_\nu ~ v^6/M^5, so that the empirical masses m_\nu ~ 10^{-1} eV can be achieved by M ~ TeV new states. There is a limited range of the parameter space with M < a few 100 GeV where the tree level contribution dominates over the respective loop contributions and the proposed mechanism is testable at the LHC. We discuss specific signatures for Dirac type heavy leptons produced by Drell-Yan fusion at the LHC.Comment: 10 pages, 1 figure, version corresponding to PL

Leptogenesis, Dark Matter and Higgs Phenomenology at TeV

We propose an interesting model of neutrino masses to realize leptogenesis and dark matter at the TeV scale. A real scalar is introduced to naturally realize the Majorana masses of the right-handed neutrinos. We also include a new Higgs doublet that contributes to the dark matter of the universe. The neutrino masses come from the vacuum expectation value of the triplet Higgs scalar. The right-handed neutrinos are not constrained by the neutrino masses and hence they could generate leptogenesis at the TeV scale without subscribing to resonant leptogenesis. In our model, all new particles could be observable at the forthcoming Large Hardon Collider or the proposed future International Linear Collider.Comment: 7 pages, 3 figures. References added. Accepted by NP

Light Unstable Sterile Neutrino

The three massless active (doublet) neutrinos may mix with two heavy and one \underline {light} sterile (singlet) neutrinos so that the induced masses and mixings among the former are able to explain the present data on atmospheric and solar neutrino oscillations. If the LSND result is also to be explained, one active neutrino mass eigenstate must mix with the light sterile neutrino. A specific model is proposed with the spontaneous and soft explicit breaking of a new global $U(1)_S$ symmetry so that a sterile neutrino will decay into an active antineutrino and a nearly massless pseudo-Majoron.Comment: Discussion and references adde