arXiv subjects

J. Rizos

Publications and source records attributed to J. Rizos.

At least 37 records · Page 2Linked to original sources

arXiv2002

Anomalous U(1)s in Type I superstring vacua

We perform a systematic string computation of the masses of anomalous U(1) gauge bosons in four-dimensional orientifold vacua, and we study their localization properties in the internal (compactified) space. We find that N=1 supersymmetric sectors yield four-dimensional contributions, localized in the whole six-dimensional internal space, while N=2 sectors give contributions localized in four internal dimensions. As a result, the U(1) gauge fields can be much lighter than the string scale, so that when the latter is at the TeV, they can mediate new non-universal repulsive forces at submillimeter distances much stronger than gravity. We also point out that even U(1)s which are free of four-dimensional anomalies may acquire non-zero masses as a consequence of six-dimensional anomalies.

hep-th↗

arXiv2002

Non-factorisable metrics and Gauss--Bonnet terms in higher dimensions

An iterative construction of higher order Einstein tensors for a maximally Gauss-Bonnet extended gravitational Lagrangian was introduced in a previous paper. Here the formalism is extended to non-factorisable metrics in arbitrary ($d+1$) dimensions in the presence of superposed Gauss-Bonnet terms. Such a generalisation turns out to be remarkably convenient and elegant. Having thus obtained the variational equations we first construct bulk solutions, with nonzero and zero cosmological constant. It is also pointed out that in the absence of Gauss-Bonnet terms a Schwarzschild type solution can be obtained in the non-factorisable case. Two positive tension branes are then inserted and their tensions are obtained in terms of parameters in the warp factor. Relations to recent studies of several authors are pointed out.

hep-th↗

arXiv2001

A Pati-Salam model from branes

We explore the possibility of embedding the Pati-Salam model in the context of Type I brane models. We study a generic model with U(4)_C x U(2)_L x U(2)_R gauge symmetry and matter fields compatible with a Type I brane configuration. Examining the anomaly cancellation conditions of the surplus abelian symmetries we find an alternative hypercharge embedding that is compatible with a low string/brane scale of the order of 5-7 TeV, when the U(4)_C and U(2)_R brane stack couplings are equal. Proton stability is assured as baryon number is associated to a global symmetry remnant of the broken abelian factors. It is also shown that this scenario can accommodate an extra low energy abelian symmetry that can be associated to lepton number. The issue of fermion and especially neutrino masses is also discussed.

hep-ph↗

arXiv2000

Implications of Anomalous U(1) Symmetry in Unified Models: the Flipped SU(5) x U(1) Paradigm

A generic feature of string-derived models is the appearance of an anomalous Abelian U(1)_A symmetry which, among other properties, constrains the Yukawa couplings and distinguishes the three families from each other. In this paper, we discuss in a model-independent way the general constraints imposed by such a U(1)_A symmetry on fermion masses, R-violating couplings and proton-decay operators in a generic flipped SU(5) x U(1)' model. We construct all possible viable fermion mass textures and give various examples of effective low-energy models which are distinguished from each other by their different predictions for B-, L- and R-violating effects. We pay particular attention to predictions for neutrino masses, in the light of the recent Super-Kamiokande data.

hep-ph↗

arXiv1999

New fermion mass textures from anomalous U(1) symmetries with baryon and lepton number conservation

In this paper, we present solutions to the fermion mass hierarchy problem in the context of the minimal supersymmetric standard theory augmented by an anomalous family-dependent U(1)_X symmetry. The latter is spontaneously broken by non-zero vevs of a pair of singlet fields whose magnitude is determined through the D- and F-flatness conditions of the superpotential. We derive the general solutions to the anomaly cancellation conditions and show that they allow numerous choices for the U(1)_X fermion charges which give several fermion mass textures in agreement with the observed fermion mass hierarchy and mixing. Solutions with U(1)_X fermion charge assignments are found which forbid or substantially suppress the dangerous baryon and lepton number violating operators and the lepton-higgs mixing coupling while a higgs mixing mass (μ-term) can be fixed at the electroweak level. We give a general classification of the fermion mass textures with respect to the sum of the doublet-higgs U(1)_X-charges and show that suppression of dimension-five operators naturally occurs for various charge assignments. We work out cases which retain a quartic term providing the left-handed neutrinos with Majorana masses in the absence of right-handed neutrino components and consistent with the experimental bounds. Although there exist solutions which naturally combine all the above features with rather natural U(1)_X charges, the suppression of the μ-term occurs for particular assignments.

hep-ph↗

arXiv1999

Higgs Mass Textures in Flipped SU(5)

We analyze the Higgs doublet-triplet mass splitting problem in the version of flipped SU(5) derived from string theory. Analyzing non-renormalizable terms up to tenth order in the superpotential, we identify a pattern of field vev's that keeps one pair of electroweak Higgs doublets light, while all other Higgs doublets and all Higgs triplets are kept heavy, with the aid of the economical missing-doublet mechanism found in the field-theoretical version of flipped SU(5). The solution predicts that second-generation charge -1/3 quarks and charged leptons are much lighter than those in the third generation.

hep-ph↗

arXiv1999

Classification of the N=2, Z2 X Z2-symmetric type II orbifolds and their type II asymmetric duals

Using free world-sheet fermions, we construct and classify all the N=2, Z2 X Z2 four-dimensional orbifolds of the type IIA/B strings for which the orbifold projections act symmetrically on the left and right movers. We study the deformations of these models out of the fermionic point, deriving the partition functions at a generic point in the moduli of the internal torus T6=T2 X T2 X T2. We investigate some of their perturbative and non-perturbative dualities and construct new dual pairs of type IIA/type II asymmetric orbifolds, which are related non-perturbatively and allow us to gain insight into some of the non-perturbative properties of the type IIA/B strings in four dimensions. In particular, we consider some of the (non-)perturbative gravitational corrections.

hep-th↗

arXiv1999

N=1 supersymmetric SU(4)xSU(2)LxSU(2)R effective theory from the weakly coupled heterotic superstring

In the context of the free-fermionic formulation of the heterotic superstring, we construct a three generation N=1 supersymmetric SU(4)xSU(2)LxSU(2)R model supplemented by an SU(8) hidden gauge symmetry and five Abelian factors. The symmetry breaking to the standard model is achieved using vacuum expectation values of a Higgs pair in (4bar,2R)+(4,2R) at a high scale. One linear combination of the Abelian symmetries is anomalous and is broken by vacuum expectation values of singlet fields along the flat directions of the superpotential. All consistent string vacua of the model are completely classified by solving the corresponding system of F- and D-flatness equations including non-renormalizable terms up to sixth order. The requirement of existence of electroweak massless doublets further restricts the phenomenologically viable vacua. The third generation fermions receive masses from the tree-level superpotential. Further, a complete calculation of all non-renormalizable fermion mass terms up to fifth order shows that in certain string vacua the hierarchy of the fermion families is naturally obtained in the model as the second and third generation fermions earn their mass from fourth and fifth order terms. Along certain flat directions it is shown that the ratio of the SU(4) breaking scale and the reduced Planck mass is equal to the up quark ratio m_c/m_t at the string scale. An additional prediction of the model, is the existence of a U(1) symmetry carried by the fields of the hidden sector, ensuring thus the stability of the lightest hidden state. It is proposed that the hidden states may account for the invisible matter of the universe.

hep-th↗

arXiv1999

String Threshold corrections in models with spontaneously broken supersymmetry

We analyse a class of four-dimensional heterotic ground states with N=2 space-time supersymmetry. From the ten-dimensional perspective, such models can be viewed as compactifications on a six-dimensional manifold with SU(2) holonomy, which is locally but not globally K3 x T^2. The maximal N=4 supersymmetry is spontaneously broken to N=2. The masses of the two massive gravitinos depend on the (T,U) moduli of T^2. We evaluate the one-loop threshold corrections of gauge and R^2 couplings and we show that they fall in several universality classes, in contrast to what happens in usual K3 x T^2 compactifications, where the N=4 supersymmetry is explicitly broken to N=2, and where a single universality class appears. These universality properties follow from the structure of the elliptic genus. The behaviour of the threshold corrections as functions of the moduli is analysed in detail: it is singular across several rational lines of the T^2 moduli because of the appearance of extra massless states, and suffers only from logarithmic singularities at large radii. These features differ substantially from the ordinary K3 x T^2 compactifications, thereby reflecting the existence of spontaneously-broken N=4 supersymmetry. Although our results are valid in the general framework defined above, we also point out several properties, specific to orbifold constructions, which might be of phenomenological relevance.

hep-th↗

arXiv1998

Singularity-free cosmological solutions in quadratic gravity

We study a general field theory of a scalar field coupled to gravity through a quadratic Gauss-Bonnet term $ξ(ϕ) R^2_{GB}$. The coupling function has the form $ξ(ϕ)=ϕ^n$, where $n$ is a positive integer. In the absence of the Gauss-Bonnet term, the cosmological solutions for an empty universe and a universe dominated by the energy-momentum tensor of a scalar field are always characterized by the occurrence of a true cosmological singularity. By employing analytical and numerical methods, we show that, in the presence of the quadratic Gauss-Bonnet term, for the dual case of even $n$, the set of solutions of the classical equations of motion in a curved FRW background includes singularity-free cosmological solutions. The singular solutions are shown to be confined in a part of the phase space of the theory allowing the non-singular solutions to fill the rest of the space. We conjecture that the same theory with a general coupling function that satisfies certain criteria may lead to non-singular cosmological solutions.

gr-qc↗

arXiv1998

Suppressing dimension-5 operators in general SU(5) models

We discuss dimension-5 operators in supersymmetric models containing extra hypercharge 1/3 color-triplets. We derive a general formula relating dimension-5 operator to the color-triplet mass matrix. We show that certain zeros in the triplet mass-matrix together with some triplet coupling selection rules can lead to elimination of dimension-5 operators. In particular we focus on SU(5) models and show that (a) Dimension-5 operators can be eliminated in the standard SU(5) model by the introduction of an extra pair of 5+5b Higgses with specific couplings (b) Flipped SU(5) models with extra 10+10b Higgses are free of dimension-5 operators (c) Flipped SU(5) models with extra 5+5b and/or extra 10+10b Higgses can be made free of dimension-5 operators for a textured form of the triplet mass-matrix accompanied by constraints on the 5-plet couplings to matter. Our analysis is motivated by the recently put forward M-theory phenomenological framework that requires a strong string coupling and reintroduces the problem of eliminating dimension-5 operators.

hep-ph↗

arXiv1998

String scale unification in an SU(6)xSU(2) GUT

We construct and analyze an $SU(6)\times SU(2)$ GUT. The model is k=1 string embedable in the sense that we employ only chiral representations allowed at the k=1 level of the associated Kač-Moody Algebra. Both cases $SU(6)\times SU(2)_{L}$ and $SU(6)\times SU(2)_{R}$ are realized. The model is characterized by the $SU(6)\times SU(2) \to SU(4)\times SU(2)\times SU(2)$ breaking scale $M_X$, and the $SU(4)\times SU(2)\times SU(2) \to SU(3)_{C}\times SU(2)_{L}\times U(1)_{Y}$ breaking scale $M_{R}$ . The spectrum bellow $M_R$ includes an extra pair of charge-1/3 colour-triplets of mass $M_{I}\leq M_{R}$ that does not couple to matter fields and, possibly, an extra pair of isodoublets. Above $M_{X}$ the SU(6) and SU(2) gauge couplings always unify at a scale which can be taken to be the string unification scale $M_{s}\sim 5\times 10^{17} GeV$. The model has Yukawa coupling unification since quarks and leptons obtain their masses from a single Yukawa coupling. Neutrinos obtain acceptably small masses through a see-saw mechanism. Coloured triplets that couple to matter fields are naturally split from the coexisting isodoublets without the need of any numerical fine tuning.

hep-ph↗

arXiv1997

Dilatonic Black Holes in Higher-Curvature String Gravity II: Linear Stability

We demonstrate linear stability of the dilatonic Black Holes appearing in a string-inspired higher-derivative gravity theory with a Gauss- Bonnet curvature-squared term. The proof is accomplished by mapping the system to a one-dimensional Schrodinger problem which admits no bound states. This result is important in that it constitutes a linearly stable example of a black hole that bypasses the `no-hair conjecture'. However, the dilaton hair is `secondary'in the sense that it is not accompanied by any new quantum number for the black hole solution.

hep-th↗

arXiv1996

Threshold effects and radiative electroweak symmetry breaking in $SU(5)$ extensions of the MSSM

We make a complete analysis of radiative symmetry breaking in the MSSM and its $SU(5)$ extensions including low- and high-energy threshold effects in the framework of the two-loop renormalization group. In particular, we consider minimal $SU(5)$, the ``missing-doublet'' $SU(5)$, a Peccei-Quinn invariant version of $SU(5)$ as well as a version with light adjoint remnants. We derive permitted ranges for the parameters of these models in relation to predicted $α_{s}$ and $M_G$ values within the present experimental accuracy. The parameter regions allowed under the constraints of radiative symmetry breaking, perturbativity and proton stability, include the experimentally designated domain for $α_s$. In the case of the minimal $SU(5)$, the values of $α_s$ obtained are somewhat large in comparison with the experimental average. The ``missing-doublet'' $SU(5)$, generally, predicts smaller values of $α_s$. In both versions of the ``missing-doublet'', the high energy threshold effects on $α_s$ operate in the opposite direction than in the case of the minimal model, leading to small values. In the case of the Peccei-Quinn version however the presence of an extra intermediate scale allows to achieve an excellent agreement with the experimental $α_s$ values. Finally, the last considered version, with light remnants, exhibits unification of couplings at string scale at the expense however of rather large $α_s$ values.

hep-ph↗

arXiv1996

Solving the Decompactification Problem in String Theory

We investigate heterotic ground states in four dimensions in which N=4 supersymmetry is spontaneously broken to N=2. N=4 supersymmetry is restored at a decompactification limit corresponding to $m_{3/2}\to 0$. We calculate the full moduli dependent threshold corrections and confirm that they are supressed in the decompactification limit $m_{3/2}\to 0$ as expected from the restoration of N=4 supersymmetry. This should be contrasted with the behavior of the standard N=2 groundstates where the coupling blow up linearly with the volume of the decompactifying manifold. This mechanism provides a solution to the decompactification problem for the gauge coupling constants. We also discuss how the mechanism can be implemented in ground states with lower supersymmetry.

hep-th↗

arXiv1996

Universal moduli-dependent thresholds in Z(2)xZ(2) orbifolds

In the context of a recently proposed method for computing exactly string loop corrections regularized in the infra-red, we determine and calculate the universal moduli-dependent part of the threshold corrections to the gauge couplings for the symmetric $Z_2\times Z_2$ orbifold model. We show that these corrections decrease the unification scale of the underlying effective field theory. We also comment on the relation between this infra-red regularization scheme and other proposed methods.

hep-th↗

arXiv1996

Universality properties of N=2 and N=1 Heterotic threshold corrections

In the framework of heterotic compactifications, we consider the one-loop corrections to the gauge couplings, which were shown to be free of any infra-red ambiguity. For a class of N=2 models, namely those that are obtained by toroidal compactification to four dimensions of generic six-dimensional N=1 ground states, we give an explicit formula for the gauge-group independent thresholds, and show that these are equal within this class, as a consequence of an anomaly-cancellation constraint in six dimensions. We further use these results to compute the (N=2)-sector contributions to the thresholds of N=1 orbifolds. We then consider the full contribution of N=1 sectors to the gauge couplings which generically are expected to modify the unification picture. We compute such corrections in several models. We finally comment on the effect of such contributions to the issue of string unification.

hep-th↗

arXiv1996

On the Heterotic Effective Action at One-Loop, Gauge Couplings and the Gravitational Sector

We present in detail the procedure for calculating the heterotic one-loop effective action. We focus on gravitational and gauge couplings. We show that the two-derivative couplings of the gravitational sector are not renormalized at one loop when the ground state is supersymmetric. Arguments are presented that this non-renormalization theorem persists to all orders in perturbation theory. We also derive the full one-loop correction to the gauge coupling. For a class of $N=2$ ground states, namely those that are obtained by toroidal compactification to four dimensions of generic six-dimensional $N=1$ models, we give an explicit formula for the gauge-group independent thresholds, and show that these are equal within the whole family.

hep-th↗