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We review the possibility that the Supersymmetric Standard Model arises from orbifold constructions of the E_8 x E_8 Heterotic Superstring, and the phenomenological properties that such a model should have. In particular, trying to solve…
Generation dependent discrete symmetries often appear in models derived from superstring theories. In particular, in the framework of E$_6$ models the presence of such symmetries is required in order to allow for the radiative generation of…
New models have recently been proposed in which a second Higgs doublet couples only to the lepton doublets and right-handed neutrinos, yielding Dirac neutrino masses. The vacuum value of this second "nu-Higgs" doublet is made very small by…
We study a supersymmetric E6 grand unified model in which the SU(5) 5^* components are twisted in the third generation 27. Supplementing the adjoint Higgs field to a model analyzed previously, we calculate the mass matrices for the up and…
Neutrino masses have been extensively studied in the context of discrete symmetries. A family permutation symmetry easily explains all neutrino oscillations and their mixing angles. However it is not obvious the embedding of such symmetry…
We point out that in a large class of superstring--inspired $E(6)$ models, either an e+e- collider operating at a center--of--mass energy $\rs= 1.5$ TeV or higher must detect the pair production of charged or neutral exotic leptons, or an…
A simple model to accomodate light sterile neutrinos naturally with large mixing with the usual neutrinos has been proposed. The standard model gauge group is extended to include an $SU(2)_S$ gauge symmetry. Heavy triplet higgs scalars give…
We study the neutralino mass spectra and production cross sections at an e^+e^- linear collider in superstring-inspired E_6 models. These models are characterized by the existence of new neutral gauge bosons and singlet Higgs fields which…
We explore an extension of the fourth generation model with multi-Higgs doublets and three fermion singlets. The Standard Model neutrinos acquire mass radiatively at one loop level while the fourth generation neutrinos acquire a heavy…
It is well known that the Standard Model of the Electroweak interactions rests on a metastable vacuum. This can only be fixed by means of new physics. Presently neutrino physics provides the most intriguing framework to formulate new…
The leptonic Higgs doublet model of neutrino masses is implemented with an A_4 discrete symmetry (the even permutation of 4 objects or equivalently the symmetry of the tetrahedron) which has 4 irreducible representations: 1, 1', 1'', and 3.…
In the E6 inspired composite Higgs model (E6CHM) the strongly interacting sector possesses an SU(6) global symmetry which is expected to be broken down to its SU(5) subgroup at the scale f > 10 TeV. This breakdown results in a set of…
The vacuum expectation value $v_s$ of a Higgs triplet field $\Delta$ carrying two units of lepton number $L$ induces neutrino masses $\propto v_s$. The neutral component of $\Delta$ gives rise to two Higgs particles, a pseudoscalar $A$ and…
We discuss the possibility that the Higgs triplet can be light (1TeV in the most interesting case) without contradictions with the proton stability %and the gauge coupling unification in the context of higher dimensional theory. The proton…
The neutral fermion sectors of E_6-inspired low energy models, in particular the Alternative Left-Right and Inert models, are considered in detail within the non-supersymmetric scenario. We show that in their simplest form, these models…
Little Higgs models are formulated as effective theories with a cut-off of up to 100 times the electroweak scale. Neutrino masses are then a puzzle, since the usual see-saw mechanism involves a much higher scale that would introduce…
We propose a model with one large submm size extra dimension in which the gravity and right-handed (RH) neutrino propagate, but the three Standard Model (SM) families are confined to fat branes of TeV^(-1) size or smaller. The charged…
Neutrino masses may be generated by the VEV of an $SU(2)_L$ Higgs triplet. We assume that the doubly charged component of such a triplet has a mass in the range of several 100 GeV, such that it is accessible at LHC. Its decay into like-sign…
We study natural lepton mass matrices, obtained assuming the stability of physical flavour observables with respect to the variations of individual matrix elements. We identify all four possible stable neutrino textures from algebraic…
The R-symmetry formalism is applied for the supersymmetric economical SU(3)_C X SU(3)_L X U(1)_X (3-3-1) model. The generalization of the minimal supersymmetric standard model relation among R-parity, spin and matter parity is derived, and…