Search

A sterile neutrino in the $3+1$ scheme, where the sterile accounts for neutrino anomalies not explained solely by the weak active neutrinos, arises as a natural source for the breaking of the $\mu-\tau$ symmetry suggested by oscillation neutrino data. We explore the predictions for the Dirac CP phases in this scenario, with and without sterile neutrino decay, and show that current limits on $\delta_{CP}$ suggest a normal hierarchy and a lightest neutrino scale below 0.1~eV as the most plausible explanation for that, when Majorana phases are null. Other Dirac phases turn out to be non zero as well., Comment: 29 pages, 10 figures

Nonstandard self-interactions can alter the evolution of cosmological neutrinos, mainly by damping free streaming, which should leave traces in cosmological observables. Although overall effects are opposite to those produced by neutrino mass and a larger $N_{\rm eff}$, they cannot be totally canceled by these last. We harness cosmological data that includes Cosmic Microwave Background from Plank 2018, BAO measurements, local $H_0$, Ly-$\alpha$ and SNIa, to constrain massive neutrino self-interactions with a very light scalar mediator. We find that the effective coupling constant, at the 95\% C.L., should be $g_{\rm eff}< 1.94 \times 10^{-7}$ for only Planck 2018 data and $1.97\times10^{-7}$ when Planck + BAO are considered. This bound relaxes to $2.27\times 10^{-7}$ ($2.3\times 10^{-7}$) for $H_0$ ($H_0$+SNe+Ly-$\alpha$) data. Using the Planck + BAO dataset, the $H_0$ tension lowers from 4.3$\sigma$ (for $\Lambda$CDM) to 3.2$\sigma$. The Akaike Information Criterion penalizes the self-interacting model due to its larger parameter space for Plank or Planck + BAO data, but favors the interacting model when we use local $H_0$ measurements. A somewhat larger value for $H_0$ is preferred when we include the whole data pool, which comes accompanied with a larger value of $N_{\rm eff}$ and a more constricted bound on $\Sigma m_\nu$., Comment: Matches published version, improved discussion

Exchange [Formula: see text] symmetry in the effective Majorana neutrino mass matrix does predict a maximal mixing for atmospheric neutrino oscillations asides to a null mixing that cannot be straightforwardly identified with reactor neutrino oscillation mixing, [Formula: see text], unless a specific ordering is assumed for the mass eigenstates. Otherwise, a nonzero value for [Formula: see text] is predicted already at the level of an exact symmetry. In this case, solar neutrino mixing and scale, as well as the correct atmospheric mixing arise from the breaking of the symmetry. I present a mass matrix proposal for normal hierarchy that realizes this scenario, where the smallness of [Formula: see text] is naturally given by the parameter [Formula: see text] and the solar mixing is linked to the smallness of [Formula: see text]. The proposed matrix remains stable under renormalization effects and it also allows to account for CP violation within the expected region without further constrains.

Coherent forward scattering processes by neutrino-scalar nonstandard interactions (SNSI) induce an effective neutrino mass. In the Early Universe, a large neutrino effective mass restricts the production of neutrinos. The SNSI effect is modulated by two effective couplings, these account for the coupling between neutrinos and electrons/positrons, $G_{\rm eff}$, and the neutrino self-interaction, $G_{\rm S}$. These parameters are directly related to the effective number of relativistic species and non-zero values imply a smaller than expected $N_{\rm eff}$. We employ big bang nucleosynthesis to constraint the SNSI effect. We find that $ G_{\rm eff }< 1.2$ MeV$^{-2}$ and $ G_{\rm S }< 2.0 \times 10^{7}$ MeV$^{-2}$ at 68\% CL. For a scalar mass in the range $10^{-15} {\rm eV}\lesssim m_��\lesssim 10^{-5}{\rm eV}$, our neutrino-scalar coupling constraint is more restrictive than any previous result., Figure added. Improved discussion and computation of the neutrino decoupling due to SNSI. Accepted for publication in PRD

We suggest that quintessential vacuum energy could be the source of right handed neutrino masses that feed the seesaw mechanism, which may provide observed small masses to light standard neutrinos. This idea is naturally implemented in the Cosmological Unification model based on the global $SO(1,1)$ symmetry, where early inflation and late accelerated expansion of the Universe are driven by the degrees of freedom of a doublet scalar field. In this model, the $SO(1,1)$ custodial symmetry naturally provides the quintessence to standard model singlet fermion couplings that source neutrino masses. We also show that the model predicts a highly suppressed contribution to relativistic degrees of freedom from quintessential quanta at any late Universe epoch, ensuring the consistency of the model., Added references, new section added extending the theory, new figure, results unchanged/improved

The observational value of the velocity dispersion, $\Delta\upsilon$, is missing in the Dark Matter (DM) puzzle. Non-zero or non-thermal DM velocities can drastically influence Large Scale Structure and the 21-cm temperature at the epoch of the Cosmic Dawn, as well as the estimation of DM physical parameters, such as the mass and the interaction couplings. To study the phenomenology of $\Delta\upsilon$ we model the evolution of DM in terms of a simplistic and generic Boltzmann-like momentum distribution. Using cosmological data from the Cosmic Microwave Background, Baryonic Acoustic Oscillations, and Red Luminous Galaxies, we constrain the DM velocity dispersion for a broad range of masses $10^{-3} eV < m_\chi < 10^9 eV$, finding $\Delta\upsilon_0 \lesssim$ 0.33 km/s (99% CL). Including the EDGES $T_{21}$-measurements, we extend our study to constrain the baryon-DM interaction in the range of DM velocities allowed by our analysis. As a consequence, we present new bounds on two electromagnetic models of DM, namely minicharged particles (MCPs) and electric dipole moment (EDM). For MCPs, the parameter region that is consistent with EDGES and independent bounds on cosmological and stellar physics is very small, pointing to the sub-eV mass regime of DM. A window in the MeV-GeV may still be compatible with these bounds for MCP models without a hidden photon. But the EDM parameter region consistent with EDGES is excluded by Big-Bang Nucleosynthesis and Collider Physics., Comment: To be published in the Astrophysical Journal. 11 pages, 5 figures

We explore masses and mixings for Dirac neutrinos in models where lepton number is conserved, under the guidance of a hidden, but broken, $\mu-\tau$ exchange symmetry, that makes itself evident in the squared hermitian mass matrix. We study the parameter space in the most general theory as allowed by current neutrino oscillation experiment data. By using a general parameterization of the mass matrix which contains only observable parameters we stablish that the amount of breaking of the symmetry is in the range of the atmospheric mass scale, without regard to the neutrino hierarchy, the absolute neutrino mass and the Dirac CP phase. An estimate of the invisible branching ratio for a Higgs boson decaying into Dirac neutrinos, $H\rightarrow\nu\overline{\nu}$ , is given and compared to recent measurements in this context., Comment: Some references added

The neutrino masses and mixings indicated by current neutrino oscillation experiments suggest that the neutrino mass matrix possesses an approximate $\mu-\tau$ exchange symmetry. In this study, we explore the neutrino parameter space and show that if a small $\mu-\tau$ symmetry breaking is considered, the Majorana $CP$ phases must be unequal and non-zero independently of the neutrino mass scale. Moreover, a small $\mu-\tau$ symmetry breaking favors quasi-degenerate masses. We also show that Majorana phases are strongly correlated with the Dirac $CP$ violating phase. Within this framework, we obtain robust predictions for the values of the Majorana phases when the experimental indications for the Dirac $CP$ phase are used., Comment: 13 pages, 3 figures. Version accepted for publication in PLB

In this work, we use Lorentz invariance violation (LIV) introduced as a generic modification to particle dispersion relations to study some consequences of single photon emission, known as vacuum Cherenkov radiation, and photon decay processes in cosmic and gamma rays. These processes are forbidden in a Lorentz invariant theory but allowed under the hypothesis of LIV. We show that the emission rate have a dependency on the cosmic ray primary mass and the electric charge that could modify the UHECR spectrum. Furthermore, LIV dramatically enhances photon decay into an electro-positron pair above certain energy threshold. This last effect can then be used to set limits to the LIV energy scale from the direct observation of very high energy cosmic photon events by telescopes of gamma-rays., Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC 2017), Busan, Korea

Lorentz Invariance Violation introduced as a generic modification to particle dispersion relations is used to study high energy cosmic ray attenuation processes. It is shown to reproduce the same physical effects for vacuum Cherenkov radiation, as in some particular models with spontaneous breaking of Lorentz symmetry. This approximation is also implemented for the study of photon decay in vacuum, where stringent limits to the violation scale are derived from the direct observation of very high energy cosmic ray photon events on gamma telescopes. Photo production processes by cosmic ray primaries on photon background are also addressed, to show that Lorentz violation may turn off this attenuation process at energies above a well defined secondary threshold., References added and typos corrected

The smallness of the ${\ensuremath{\theta}}_{13}$ mixing angle as observed in neutrino oscillation experiments can be understood through an approximated $\ensuremath{\mu}\ensuremath{-}\ensuremath{\tau}$ exchange symmetry in the neutrino mass matrix. Using recent oscillation neutrino data, but assuming no $CP$ violation, we study $\ensuremath{\mu}\ensuremath{-}\ensuremath{\tau}$ breaking parameter space to establish the conditions under which such a breaking could have a perturbative origin. According to the so-obtained conditions, we suggest that a sterile neutrino, matching LSND/MiniBooNE neutrino oscillation results, could provide the necessary ingredients to properly fix atmospheric and ${\ensuremath{\theta}}_{13}$ mixing angles to observable values, without exceeding the sterile neutrino fraction bound in solar oscillations. In such a scenario, we analyze the general effect of a fourth neutrino on the prediction for the effective ${m}_{ee}$ majorana mass parameter.

We present a new way to break parity symmetry in left-right symmetric models using boundary conditions on the fields residing in the fifth dimension. We also discuss the connection between the limits on the size of extra dimensions and the scale of right handed symmetry breaking obtained from the analysis of neutrinoless double beta decay in the case where the righthanded gauge symmetry is in the bulk., Comment: 9 pages, one figure. Minor changes. Two comments added in Sec. III. (Version to appear at Phys. Lett. B.)

We find a relationship between unitary transformations of the dynamics of quantum systems with time-dependent Hamiltonians and gauge theories. In particular, we show that the nonrelativistic dynamics of spin-$\frac12$ particles in a magnetic field $B^i (t)$ can be formulated in a natural way as an SU(2) gauge theory, with the magnetic field $B^i(t)$ playing the role of the gauge potential A^i. The present approach can also be applied to systems of n levels with time-dependent potentials, U(n) being the gauge group. This geometric interpretation provides a powerful method to find exact solutions of the Schr\"odinger equation. The root of the present approach rests in the Hermiticity property of the Hamiltonian operators involved. In addition, the relationship with true gauge symmetries of n-level quantum systems is discussed., Comment: LaTeX file, 5 pages, published version

The non quantum relativistic version of theproof of Feynman for the Maxwell equations is discussedin a framework with a minimum number of hypothesesrequired. From the present point of view it is clear that the classical equations of motioncorresponding to the gauge field interactions can bededuced from the minimal coupling rule, and we claimhere resides the essence of the proof ofFeynman.

We show that it is possible to achieve one step gauge coupling unification in a general class of non supersymmetric models which at low energies have only the standard particle content and extra Higgs fields doublets. The constraints are the experimental values of $\alpha_{em}$, $\alpha_s$ and $\sin^2\theta_W$ at $10^2 GeVs$, and the lower bounds for FCNC and proton decay rates. Specific example are pointed out., Comment: 10 pages, Latex file,, uses epsf style, Two Postscript figures included. To appear in Europhysics Letters

The Standard Model of particle physics represents a remarkable achievement on the understanding of the fundamental interactions and constituents of matter. Nevertheless, it has a limited capability to answer some of our more fundamental questions about the nature of physics at shorter distances. In particular, it fails short on providing an explanation to the origin of masses and mixings for neutrinos, and the search for a deeper understanding of these issues is driving theorists to explore possible physics beyond the Standard Model. Here we provide a general and broad overview of the issues, and comment on some of the trends that theoretical physics is following on this quest.

We analyze the leptonic sector in the left-right symmetric model dressed with a (Z{sub 2}){sup 3} discrete symmetry which realizes, after weak spontaneous breaking, a small broken {mu}{r_reversible}{tau} symmetry that is suggested to explain observable neutrino oscillation data. {mu}{r_reversible}{tau} symmetry is broken at tree level in the effective neutrino mass matrix due to the mass difference m(tilde sign){sub {tau}{ne}m}(tilde sign){sub {mu}} in the diagonal Dirac mass terms, whereas all lepton mixings arise from a Majorana mass matrix. In the limit of a small breaking we determined {theta}{sub 13}, and the deviation from the maximal value of {theta}{sub 23}, in terms of the light neutrino hierarchy scale, m{sub 3}, and a single free parameter h{sub s} of the model.

Moduli fields are a known ingredient of models that involve extra compact dimensions, as the Kaluza‐Klein theories, String theory, and models with compact extra dimensions. They are scalar fields that emerge when the configuration of the compact space is perturbed. The radion is a particular example of this type of fields, which is associated to the variations of the total volume of compact space. Radions usually couple to all other fields, affecting the definition of coupling constants and gravity strength. They also modify gravitational potentials in a way that may be tested in table top experiments. Usually, these fields are run away modes which manifest the difficulties to stabilize the shape of the compact manifold of extra space. This is feature that can be a threat for cosmology on the early Universe. Here we provide a brief discussion of these general aspects of the radion physics.

In this paper, we consider the hypothesis in which a species of ultra light bosonic dark matter (ULBDM) with mass $m_{B}\sim 10^{-22}$ eV could be the dominant dark matter (DM) in the Universe. As a first approach we work in the context of kinetic theory, where ULBDM is described by the phase space distribution function whose dynamics is dictated by the Boltzmann-Einstein equations. We investigate the effects that this kind of dark matter imprints in the acoustic peaks of the cosmic microwave background. We find that the effect of the Bose-Einstein statistics is small, albeit perceptible, and is equivalent to an increase of non-relativistic matter. It is stressed that in this approach, the mass-to-temperature ratio necessary for ULBDM to be a plausible DM candidate is about five orders of magnitude. We show that reionization is also necessary and we address a range of consistent values for this model. We find that the temperature of ULBDM is below the critical value, impliying that Bose-Einstein condensation is inherent to the ULBDM paradigm., Comment: 6 pages, 5 figures, two authors added, improved, extended in section 2, more discussions to clarify some issues, references added, typos corrected, published version in ApJ

We present a Left‐right symmetric model with a (Z2)3 discrete symmetry which realizes softly broken μ↔τ symmetry, which is broken at tree level in the effective neutrino mass matrix by mass difference in the diagonal Dirac mass terms. Lepton mixings arise from Majorana mass matrix. We determined θ13, and the deviation from maximal value of θATM in terms of the hierarchy scale, m3, and a single free parameter, hs.

When translational invariance is spontaneously broken the volume stabilization in models with flat extra dimension could follow from vacuum energy residing in the extra dimensional space. We study a simple toy model that exemplifies this mechanism which considers a massive scalar field with non trivial boundary conditions at the end points of the compact space, and includes contributions from brane and bulk cosmological constants. We perform our analysis in the conformal frame where the radion field, associated to volume variations, is defined, and present a general strategy for building stabilization potentials out of those ingredients. We also provide working examples for the interval and the Tn/Z2 orbifold configuration.

Neutrino oscillations data indicates that neutrino mixings are consistent with an apparent {nu}{sub {mu}}-{nu}{sub {tau}} exchange symmetry in neutrino mass matrix. We observe that in the minimally extended standard model with the seesaw mechanism, one can impose {mu}{r_reversible}{tau} symmetry at the tree level on all Lagrangian terms, but for the mass difference among {mu} and {tau} leptons. In the absence of any new extra physics, this mass difference becomes the only source for the breaking of such a symmetry, which induces, via quantum corrections, small but predictable values for {theta}{sub 13}, and for the deviation of {theta}{sub ATM} from maximality. In the CP conserving case, the predictions only depend on neutrino mass hierarchy and may provide a unique way to test for new physics with neutrino experiments.

Volume stabilization in models with flat extra dimension could follow from vacuum energy residing in the bulk when translational invariance is spontaneously broken. We study a simple toy model that exemplifies this mechanism which considers a massive scalar field with non trivial boundary conditions at the end points of the compact space, and includes contributions from brane and bulk cosmological constants. We perform our analysis in the conformal frame where the radion field, associated with volume variations, is defined, and present a general strategy for building stabilization potentials out of those ingredients. We also provide working examples for the interval and the $T^n/Z_2$ orbifold configuration., Comments and clarifications added throughout the text. Typos corrected and references added. Final version, 27 pages, five figures included

We present a model where early inflation and late accelerating expansion of the Universe are driven by the real and imaginary parts of a single complex scalar field, which we identified as inflaton and phantom field, respectively. This inflaton-phantom unification is protected by an internal SO(1,1) symmetry, with the two cosmological scalars appearing as the degrees of freedom of a sole fundamental representation. The unification symmetry allows to build successful potentials. We observe that our theory provides a matter-phantom duality, which transforms scalar matter cosmological solutions into phantom solutions and vice versa. We also suggest that a complete unification of all scalar fields of cosmological interest is yet possible under a similar footing., Comment: Title slightly changed to match published version. Typos corrected and references added. Five pages, no figures

Neutrino mixing matrix appears to be close to bimaximal mixing, but for the solar mixing angle which is definitively smaller than forty five degrees. Whereas it seems quite easy to understand bimaximal mixing with the use of new global symmetries, as in models using $L_e - L_\mu - L_\tau$, understanding the about to eleven degrees of deviation in the observed solar angle seems less simple. We suggest that such a deviation could be due to a light sterile neutrino that mixes with the active sector. The mass scale needed to produce the effect has to be smaller than atmospheric scale, and it would introduce a new mass squared difference which should be smaller than the solar scale. We present a toy model that exemplifies these features., Comment: 19 pages, two figures. Discussion extended. References added

Recent years have seen an extraordinary breakthrough in neutrino physics. Compelling experimental evidence indicates that, contrary to earlier believes, this fundamental particles have non zero masses and mix. Such properties are manifested in the oscillation phenomena in neutrino fluxes produced within the Sun, by cosmic rays, and in nuclear plants on the Earth, among other astrophysical and terrestrial sources. In these lecture we provide a short introduction to neutrino properties, mainly intended to give some basic elements of the physics of neutrino oscillations for beginners. We also discuss some of the theoretical questions raised on particle physics by the discovery of neutrino masses and mixings.

Models that involve extra dimensions have introduced completely new ways of looking up on old problems in theoretical physics. The aim of the present notes is to provide a brief introduction to the many uses that extra dimensions have found over the last few years, mainly following an effective field theory point of view. Most parts of the discussion are devoted to models with flat extra dimensions, covering both theoretical and phenomenological aspects. We also discuss some of the new ideas for model building where extra dimensions may play a role, including symmetry breaking by diverse new and old mechanisms. Some interesting applications of these ideas are discussed over the notes, including models for neutrino masses and proton stability. The last part of this review addresses some aspects of warped extra dimensions, and graviton localization., 39 pages. Two figures. Comments and references added. Lectures given at the XI Mexican School of Particles and Fields. Xalapa, Mexico, August 1-13, 2004

Moduli problem is a known common feature of models that involve extra compact dimensions, as the Kaluza‐Klein theories, the String theory, and the large extra dimension models. Moduli are scalar fields that emerge when the configuration of the compact space is perturbed. Usually, these fields feel no potential and represent run away modes which makes it difficult to stabilize the shape of the compact manifold. The radion is a particular example of this type of fields, which is associated to the variations of the total volume of the compact space. Radions usually couple to all other fields, affecting the definition of coupling constants and gravity strength. Thus, they are a potential threat for cosmology of the early Universe. Here we discuss some aspects of radion dynamics during inflation, and show that a heavy radion (m ≫ H) seems to be required for a successful baryogenesis.

We bring together some known ingredients beyond the Standard Model physics which can explain the hot Big Bang model with the observed baryon asymmetry and also the fluctuations in the cosmic microwave background radiation with a minimal set of assumptions. We propose an interesting scenario where the inflaton energy density is dumped into an infinitely large extra dimension. Instead of the inflaton it is the right handed sneutrino condensate, which is acquiring non-zero vacuum expectation value during inflation, whose fluctuations are responsible for the density perturbations seen in the cosmic microwave background radiation with a spectral index $n_s\approx 1$. The decay of the condensate is explaining the reheating of the Universe with a temperature, $T_{rh}\leq 10^{9}$ GeV, and the baryon asymmetry of order one part in $10^{10}$ with no baryon-isocurvature fluctuations., Comment: 4 pages, Title has been modified, trivial changes to match accepted version in Phys. Rev. Lett

We argue that a brane world with a warped, infinite extra dimension allows for the inflaton to decay into the bulk so that after inflation, the effective dark energy disappears from our brane. This is achieved by the redshifting of the decay products into infinity of the 5th dimension. As a consequence, all matter and CMB density perturbations could have their origin in the decay of a MSSM flat direction rather than the inflaton. We also discuss a string theoretical model where reheating after inflation may not affect the observable brane., Comment: 16 pages

We suggest the possibility that in ADD type brane-world scenarios, the higher KK excitations of the graviton may decay to lower ones owing to a breakdown of the conservation of extra dimensional “momenta” and study its implications for astrophysics and cosmology. We give an explicit realization of this idea with a bulk scalar field �, whose nonzero KK modes acquire vacuum expectation values. This scenario helps to avoid constraints on large extra dimensions that come from gamma ray flux bounds in the direction of nearby supernovae as well as those coming from diffuse cosmological gamma ray background. It also relaxes the very stringent limits on reheat temperature of the universe in ADD models.

We discuss cosmology of models with universal extra dimensions, where the Standard Model degrees of freedom live in a $4+n$ dimensional brane, with $n$ compact and small extra spatial dimensions. In these models, the simplest way to obtain the conventional 4-dimensional Planck scale starting with a low string scale is to have also some larger extra dimensions, where only gravity propagates. In such theories, dimensional reduction generically leads to at least two radion fields, one associated with the total volume of the extra spatial dimensions, and the other with the ratio of the sizes of small and large extra dimensions. In this paper, we discuss the impact of the radion fields on cosmology. We emphasize various aspects of radion physics such as radion coupling to the Standard Model fields, bare and dressed radion masses during inflation, dynamical stabilization of radions during and after inflation, radion decay life time and its late dominance in thermal history of the Universe as well as its quantum fluctuations during inflation. We argue that models where the radion plays the role of an inflaton or the inflaton is a brane scalar field, run into problems. We then present a successful inflation model with bulk scalar fields that seems to have all the desired properties. We also briefly discuss the possibility of radion as a cold dark matter candidate., Comment: 37 pages + 3 figures

We show how the problem of small neutrino masses and suppressed proton decay can be simultaneously resolved in 6-D universal extra dimension models (UED) with a low fundamental scale using extended gauge groups that contain the local $B-L$ symmetry. The extra space dimensions are compactified either on a $T^2/Z_2$ or $T^2/Z_2\times Z'_2$ orbifold depending on whether the full gauge group is $SU(2)_L\times U(1)_{I_{3R}}\times U(1)_{B-L}$ or $SU(2)_L\times SU(2)_{R}\times U(1)_{B-L}$. In both cases, neutrino masses are suppressed by an appropriate orbifold parity assignment for the standard model singlet neutrinos and the proton decay rate is suppressed due to a residual discrete symmetry left over from compactification. For lower values of the fundamental scale, a dominant decay mode of the neutron is $n\to 3 \nu$. An interesting consequence of the model is a possible two component picture for dark matter of the universe., Comment: 25 pages, two minor typos corrected

We construct a left-right symmetric model based on the gauge group $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ in five dimensions where both the gauge bosons and fermions reside in all five dimensions. The orbifold boundary conditions are used not only to break the gauge symmetry down to $SU(2)_L\times U(1)_Y\times U(1)_{Y'}$ but also to ``project'' the right handed neutrino out of the zero mode part of the spectrum, providing a new way to understand the small neutrino masses without adding (singlet) bulk neutrinos. This formulation of the left-right model has also two new features: (i) it avoids most existing phenomenological bounds on the scale of the right handed $W_R$ boson allowing for the possibility that the right handed gauge bosons could have masses under a TeV, and (ii) it predicts a stable lepton with mass of order of the inverse radius of the fifth dimension., 20 pages; some new materials and references added

We suggest that many of the free parameters in the supersymmetric extensions of the standard model can be all linked together to the existence of a nonuniversal $U(1)$ gauge symmetry, which has been spontaneously broken at very high scale. Such a symmetry can easily generate, via nonrenormalizable operators, appropriate tree level fermion mass textures as well as the $\ensuremath{\mu}$ term of the Higgs potential. We give a general parametrization of those terms. As output, R parity breaking terms only appear at a nonrenormalizable level and are linked to the Yukawa couplings, giving rise to the possibility of having effective (exact) R parity conservation. As an interesting application of this idea we explore the case where the neutrinos are Dirac particles. The scenario can be embedded in an extended SU(5) unification theory where the extra right handed neutrinos are introduced as singlets. Such a theory has exact R parity conservation and a sneutrino as the lightest supersymmetric particle.

In models with flat extra dimensions tiny Dirac neutrino masses can be generated via the coupling of four dimensional Standard Model fields to a higher dimensional fermion. Here we argue that, in spite of the Dirac nature of the neutrino, quantization of the electric charge can still be understood as a result of anomaly cancellation, charge conservation and naturalness requirements., Comment: 9 pages. Several changes: Discusion on RS models removed, Comments added

In theories with an infinite extra dimension, free particles localized on the brane can leak out to the extra space. We argue that if there were color confinement in the bulk, electrons would be more able to escape than quarks and than protons (which are composed states). Thus, this process generates an electric charge asymmetry on brane matter densities. A primordial charge asymmetry during Big Bang Nucleosynthesis era is predicted. We use current bounds on this and on electron disappearance to constrain the parameter space of these models. Although the generated asymmetry is generically small, it could be particularly enhanced on large densities as in astrophysical objects, like massive stars. We suggest the possibility that such accumulation of charge may be linked, upon supernova collapse, to the formation of a charged Black Hole and the generation of Gamma-Ray Bursts., Comment: Four pages, one figure. Minor changes, conclusions remain

We study supergravity inflationary models where inflation is produced along the angular direction. For this we express the scalar component of a chiral superfield in terms of the radial and the angular components. We then express the supergravity potential in a form particularly simple for calculations involving polynomial expressions for the superpotential and Kahler potential. We show for a simple Polonyi model the angular direction may give rise to a stage of inflation when the radial field is fixed to its minimum. We obtain analytical expressions for all the relevant inflationary quantities and discuss the possibility of supersymmetry breaking in the radial direction while inflating by the angular component., 7 pages, one figure. Final version. Title changed, two figures dropped

The onset of inflation in hybrid models require fine tuning in the initial conditions. The inflaton field should have an initial value close to the Planck scale $M_{P}$, whereas the auxiliary "orthogonal" field must be close to zero with an extreme accuracy. This problem can be alleviated if the orthogonal field fastly decays into some states not coupled to the inflaton. Natural candidates for such states can be the right-handed neutrinos. We show that a non-trivial evolution of the classic sneutrino fields after inflation offers an interesting mechanism for generating a correct amount of lepton asymmetry, which being reprocessed by sphalerons can explain the observed baryon asymmetry of the Universe. Our scenario implies interesting bounds for the neutrino masses in the context of seesaw mechanism., 9 pages, no figures

We make a generic remark on thermal history of a single brane cosmology in models with an infinitely large single extra dimension. We point out that the reheat temperature of the Universe is bounded by an excess production of gravitons from the thermal bath. The actual bound is given by the brane tension. If the initial temperature of the Universe is larger than this bound, then an efficient graviton production shall prevail. However, the brane cools down gradually as the Kaluza-Klein gravitons take the energy in excess away from the brane. The cooling continues until the radiation dominated phase is restored, which occurs before the Big Bang Nucleosynthesis. We argue whatsoever be the early evolution of the Universe, the final radiation dominated phase always starts after the Universe transits from non-conventional era to the standard cosmological era., 11 pages, no figures. Major changes. Bounds on brane tension removed

Baryogenesis in the models where the fundamental scale is as low as TeV in the context of large extra dimensions is a challenging problem. The requirement for the departure from thermal equilibrium necessarily ties any low scale baryogenesis with that of a successful inflationary model which automatically provides the out of equilibrium condition after the end of inflation. However, it is also noticeable that in these models the reheat temperature of the Universe is strongly constrained from the overproduction of Kaluza-Klein modes, which enforces a very low reheat temperature. In this paper we describe a possible scenario for baryogenesis which has a similar characteristics of an Affleck-Dine field. We notice that in order to have an adequate baryon to entropy ratio one requires to promote this Affleck-Dine field to reside in the bulk., 4 pages, LaTeX file, no figures. Important improvements on the paper. Conclusions unchanged. References added

Large extra dimensional theories attempt to solve the hierarchy problem by assuming that the fundamental scale of the theory is at the electroweak scale. This requires the size of the extra dimensions to be stabilized at a scale which is determined by the effective four dimensional Planck mass and the number of extra dimensions. In this paper we concentrate upon the dynamical reasons to stabilize them by providing a running mass to the radion field. We show that it is possible to maintain the size of the extra dimensions once it is stabilized throughout the dynamics of inflation., 5 pages. LaTeX file, no figures. Minor changes, typos corrected

In this paper we propose a new mechanism of inflating the Universe with non-BPS $D4$ branes which decay into stable $D3$ branes via tachyon condensation. In a single brane scenario the tachyon potential is very steep and unable to support inflation. However if the universe lives in a stack of branes produced by a set of non-interacting unstable $\tilde {D4}$ branes, then the associated set of tachyons may drive inflation along our 3 spatial dimensions. After tachyon condensation the Universe is imagined to be filled with a set of parallel stable $D3$ branes. We study the scalar density perturbations and reheating within this setup., Comment: LaTeX file, 10 pages, three figures included. Minor changes, small clarification added in section II. Final version

We describe a simple and predictive scenario for baryogenesis in theories with large extra dimensions which resembles Affleck-Dine baryogenesis. The Affleck-Dine field is a complex scalar field carrying a $U(1)_{\chi}$ charge which is dynamically broken after the end of inflation. This generates an excess of $\chi$ over $\bar \chi$, which then decays into Standard Model fermions to produce an excess of baryons over anti-baryons. Our model is very constrained because the Affleck-Dine field has to be sufficiently flat during inflation. It is also a source for density fluctuations which can be tested in the coming satellite and balloon experiments., Comment: LaTeX file, 24 pages, no figures

We assume that the mass matrix of active neutrinos has hierarchical form with small mixings, similar to quark mass matrix. We show that the large mixings between $\nu_\mu$ and $\nu_\tau$ as well as between $\nu_e$ and certain combination of $\nu_\mu$ and $\nu_\tau$ required by the present data can appear due to the presence of a sterile neutrino state. Two realizations of this possibility are considered: (i) Large flavor mixing appears as a result of sterile neutrino ``decoupling'' ($m_{ss} \gg m_a$), so that the active-sterile mixings are negligible. (ii) Sterile neutrino has a mass of O(1) eV and its mixing with active neutrinos can be observable. In the second case, the (3+1) scheme of neutrino mass, which also accommodates the LSND result, can be reproduced, provided the hierarchy of the mass matrix of active neutrinos is not strong (the ratio of largest to smallest elements is about two orders of magnitude). The enhancement of lepton mixing via coupling with sterile neutrino can be realized in Grand Unified theories with the see-saw mechanism of neutrino mass generation., Comment: 14 pages, latex, no figures, discussion extended, numerical example in Eq. 35 changed, references added

A new mechanism for understanding small neutrino masses using only simple new physics at the TeV scale is proposed. As an application, it is shown how it can naturally lead to the mass hierarchy of the so called bimaximal mixing in the case of three active neutrinos, or the (3+1) scenarios for sterile neutrinos, using only the $SU(2)_L$ quantum numbers of the particles., Comment: 10 pages. LaTeX file, one table and one figure included