Realistic neutrinogenesis with radiative vertex correction

We propose a new model for naturally realizing light Dirac neutrinos and explaining the baryon asymmetry of the universe through neutrinogenesis. To achieve these, we present a minimal construction which extends the Standard Model with a real singlet scalar, a heavy singlet Dirac fermion and a heavy doublet scalar besides three right-handed neutrinos, respecting lepton number conservation and a Z₂$Z_2$ symmetry. The neutrinos acquire small Dirac masses due to the suppression of weak scale over a heavy mass scale. As a key feature of our construction, once the heavy Dirac fermion and doublet scalar go out of equilibrium, their decays induce the CP asymmetry from the interference of tree-level processes with the radiative vertex corrections (rather than the self-energy corrections). Although there is no lepton number violation, an equal and opposite amount of CP asymmetry is generated in the left-handed and the right-handed neutrinos. The left-handed lepton asymmetry would then be converted to the baryon asymmetry in the presence of the sphalerons, while the right-handed lepton asymmetry remains unaffected.     

Dark matter and LHC phenomenology of a scale-invariant scotogenic model

We study the phenomenology of a model that addresses the neutrino mass, dark matter, and generation of the electroweak scale in a single framework. Electroweak symmetry breaking is realized via the Coleman-Weinberg mechanism in a classically scale invariant theory, while the neutrino mass is generated radiatively through interactions with dark matter in a typically scotogenic manner. The model introduces a scalar triplet and singlet and a vectorlike fermion doublet that carry an odd parity of Z_2, and an even parity scalar singlet that helps preserve classical scale invariance. We sample over the parameter space by taking into account various experimental constraints from the dark matter relic density and direct detection, direct scalar searches, neutrino mass, and charged lepton flavor violating decays. We then examine by detailed simulations possible signatures at the LHC to find some benchmark points of the free parameters. We find that the future high-luminosity LHC will have a significant potential in detecting new physics signals in the dilepton channel.     

See-saw neutrino masses induced by a triplet of leptons

Neutrinos can acquire mass through the see-saw mechanism by extending the lepton sector of the Standard Model. The most generalSU(2) _L assignments allowed for the exotic leptons are the singlet and triplet representations. A model is constructed involving a triplet of exotic leptons. In this model lepton number is broken. Therefore lepton number violating processes, in particular those involving the lepton triplet, could provide distinctive experimental signatures.     

A 4-neutrino model with a Higgs triplet

We take as a starting point the Gelmini–Roncadelli model enlarged by a term with explicit lepton number violation in the Higgs potential and add a neutrino singlet field that is coupled via a scalar doublet to the usual leptons. This scenario allows us to take into account all three present indications in favor of neutrino oscillations provided by the solar, atmospheric, and LSND neutrino oscillation experiments. Furthermore, it suggests a model which reproduces naturally one of the two 4-neutrino mass spectra favored by the data. In this model, the solar neutrino problem is solved by large mixing MSW transitions, and the atmospheric neutrino problem by transitions of into a sterile neutrino. Received: 11 May 1999 / Published online: 3 February 2000     

Leptogenesis bound on spontaneous symmetry breaking of global lepton number

We propose a new class of leptogenesis bounds on the spontaneous symmetry breaking of global lepton number. These models have a generic feature of inducing new lepton number violating interactions, due to the presence of the Majorons. We analyzed the singlet Majoron model with right-handed neutrinos to find that the lepton number should be broken above 10⁵ GeV to realize a successful leptogenesis because the annihilations of the right-handed neutrinos into the massless Majorons and into the standard model Higgs should go out of equilibrium before the sphaleron process is over. We then argue that this type of leptogenesis constraint should exist in the singlet–triplet Majoron models as well as in a class of R-parity violating supersymmetric Majoron models.     

Neutrino magnetic moment inN=1 minimal supergravity

The complete one-loop supersymmetric (SUSY) correction to the magnetic moment (NMM) of a Dirac neutrino is calculated with allowance for mixing between the scalar leptons and for mixing between theW-gaugino and Higgs fermions. The contribution from the charged Higgs loop is negligible for all practical purpose. We thoroughly study the dependence of NMM on the SUSY parameters. The SUSY contribution to the NMM can be a few times less than or comparable to the value of the standard model (with a right-handed neutrino singlet added). The SUSY correction to the NMM increases with decreasing β value (tan β=〈H ₂〉/〈H ₁〉), and is not very sensitive to the charged scalar lepton mass.     

Lepton flavour symmetry and the neutrino magnetic moment

With the standard model gauge group and the three standard left-handed Weyl neutrinos, two minimal scenarios are investigated where an arbitrary non-Abelian lepton flavour symmetry groupG _H is responsible for a light neutrino with a large magnetic moment. In the first case, with scalar fields carrying lepton flavour, some finetuning is necessary to get a small enough neutrino mass for _v =O(10)^–11 _B. In the second scenario, the introduction of heavy charged gauge singlet fermions with lepton flavour allows for a strictly massless neutrino to one-loop order. In both cases, the interference mechanism for smallm and large _v is unique, independently ofG _H . In explicit realizations of the two scenarios, the horizontal groups are found to be non-Abelian extensions of a Zeldovich-Konopinski-Mahmoud lepton number symmetry. Only a discrete part ofG _H is spontaneously broken leading to a light Dirac neutrino with a large magnetic moment.     

Lepton number violation at the LHC with leptoquark and diquark

We investigate a model in which tiny neutrino masses are generated at the two-loop level by using scalar leptoquark and diquark multiplets. The diquark can be singly produced at the LHC, and it can decay into a pair of leptoquarks through the lepton number violating interaction. Subsequent decays of the two leptoquarks can provide a clear signature of the lepton number violation, namely two QCD jets and a pair of same-signed charged leptons without missing energy. We show that the signal process is not suppressed while neutrino masses are appropriately suppressed.     

Radiative transmission of lepton flavor hierarchies

We discuss a one loop model for neutrino masses which leads to a seesaw-like formula with the difference that the charged lepton masses replace the unknown Dirac mass matrix present in the usual seesaw case. This is a considerable reduction of parameters in the neutrino sector and predicts a strong hierarchical pattern in the right handed neutrino mass matrix that is easily derived from a U(1)_H family symmetry. The model is based on the left–right gauge group with an additional Z₄ discrete symmetry which gives vanishing neutrino Dirac masses and finite Majorana masses arising at the one loop level. Furthermore, it is one of the few models that naturally allow for large (but not necessarily maximal) mixing angles in the lepton sector. A generalization of the model to the quark sector requires three iso-spin singlet vector-like down type quarks, as in E₆. The model predicts an inert doublet type scalar dark matter.     

Four-dimensional heterotic string models with third-integral twists

The massless string states are constructed for a four-dimensional heterotic string model with the U(5) gauge group in the untwisted sector. The complete observable gauge group, quark and lepton generations, Higgs scalar structure, quark and lepton mass matrix, couplings to colour triplet scalars, and gauge symmetry breaking are studied for this model.     

Resonant tau leptogenesis with observable lepton number violation

We consider a minimal extension of the standard model with one singlet neutrino per generation that can realize resonant leptogenesis at the electroweak scale. In particular, the baryon asymmetry in the Universe can be created by lepton-to-baryon conversion of an individual lepton number, for example, that of the tau lepton. The current neutrino data can be explained by a simple CP-violating Yukawa texture. The model has several testable phenomenological implications. It contains heavy Majorana neutrinos at the electroweak scale, which can be probed at e+ e- linear colliders, and predicts e- and mu-lepton-number-violating processes, such as 0nu betabeta decay, mu --> e gamma, and mu-e conversion in nuclei, with rates that are within reach of experimental sensitivity.     

Higgs scalar densities and cosmology

Phase transitions in the early universe are discussed with inclusion of the effects of Higgs scalar chemical potentials as well as fermionic chemical potentials. The conditions for equilibrium, and the critical density to prevent symmetry restoration at high temperatures are studied. It is observed that the decay of pre-existing Higgs scalar asymmetries could greatly reduce baryon number and lepton number to entropy ratios from their initial values.     

The hierarchy problem in the scalar triplet model

We investigate the possibility of maintaining the hierarchy of the gauge and lepton number symmetry breakdown in the context of the Gelmini-Roncadelli model of electroweak interactions. Calculating the effective potential in the one-loop approximation, we find the additional constraints one must impose on the scalar self-couplings to protect the tree level hierarchy from dangerous radiative corrections.     

Bileptons: present limits and future prospects

We define bileptons to be bosons coupling to a pair of leptons and construct the most general dimension four lagrangian involving scalar and vector bileptons. We concentrate on fields with lepton number 2, and derive model independent bounds on their masses and couplings from low-energy data. In addition, we study their signals in high-energy experiments and forecast the discovery potential of future colliders. Received: 24 April 1997 / Published online: 20 February 1998     

Scalar Resonance at 750 GeV as Composite of Heavy Vector-Like Fermions

We study a model of scalars which includes both the SM Higgs and a scalar singlet as composites of heavy vector-like fermions. The vector-like fermions are bounded by the super-strong four-fermion interactions. The scalar singlet decays to SM vector bosons through loop of heavy vector-like fermions. We show that the surprisingly large production cross section of di-photon events at 750 GeV resonance and the odd decay properties can all be explained. This model serves as a good model for both SM Higgs and a scalar resonance at 750 GeV.     

Neutrinos,axions and conformal symmetry

We demonstrate that radiative breaking of conformal symmetry (and simultaneously electroweak symmetry) in the standard model with right-chiral neutrinos and a minimally enlarged scalar sector induces spontaneous breaking of lepton number symmetry, which naturally gives rise to an axion-like particle with some unusual features. The couplings of this ‘axion’ to standard model particles, in particular photons and gluons, are entirely determined (and computable) via the conformal anomaly, and their smallness turns out to be directly related to the smallness of the masses of the light neutrinos.     

Hyperbolic Cardassian universe

We propose a hyperbolic function form of the Cardassian component in the Cardassian model. Using the repartition of this Cardassian component, we can obtain a non-zero gravitational interaction between the time derivative of Ricci scalar curvature and the baryon/lepton number current in the radiation-dominated universe.Furthermore, the other term that acts like a non-zero cosmological constant would give an accelerated expansion of current universe and the features of this model do not violate our desired requirements.     

Search for new physics using high-mass tau pairs from 1.96 TeV pp collisions

We present the results of a search for anomalous resonant production of tau lepton pairs with large invariant mass, the first such search using the CDF II Detector in Run II of the Tevatron pp collider. Such anomalous production could arise from various new physics processes. In a data sample corresponding to 195 pb(-1) of integrated luminosity we predict 2.8+/-0.5 events from standard model background processes and observe 4. We use this result to set limits on the production of heavy scalar and vector particles decaying to tau lepton pairs.     

The seesaw mechanism at TeV scale in the 3-3-1 model with right-handed neutrinos

We implement the seesaw mechanism in the 3-3-1 model with right-handed neutrinos. This will be accomplished by the introduction of a scalar sextet into the model and the spontaneous violation of lepton number. The main result of this work is that the seesaw mechanism can work already at the TeV scale with the consequence that the right-handed neutrino masses lie in the electroweak scale, in the range from MeV to tens of GeV. This window provides a great opportunity to test their appearance at current detectors, though when we contrast our results with some previous analyses concerning the detection sensitivity at LHC, we conclude that further work is needed in order to validate this search.