arXiv subjects

John Rizos

Publications and source records attributed to John Rizos.

29 records · Page 2Linked to original sources

arXiv2012

String Derived Exophobic SU(6)xSU(2) GUTs

With the apparent discovery of the Higgs boson, the Standard Model has been confirmed as the theory accounting for all sub-atomic phenomena. This observation lends further credence to the perturbative unification in Grand Unified Theories (GUTs) and string theories. The free fermionic formalism yielded fertile ground for the construction of quasi--realistic heterotic--string models, which correspond to toroidal Z2xZ2 orbifold compactifications. In this paper we study a new class of heterotic-string models in which the GUT group is SU(6)xSU(2) at the string level. We use our recently developed fishing algorithm to extract an example of a three generation SU(6)xSU(2) GUT model. We explore the phenomenology of the model and show that it contains the required symmetry breaking Higgs representations. We show that the model admits flat directions that produce a Yukawa coupling for a single family. The novel feature of the SU(6)xSU(2) string GUT models is that they produce an additional family universal anomaly free U(1)symmetry that may remain unbroken below the string scale. The massless spectrum of the model is free of exotic states.

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arXiv2011

Top Quark Mass in Exophobic Pati--Salam Heterotic String Model

We analyse the phenomenology of an exemplary exophobic Pati-Salam heterotic string vacuum, in which no exotic fractionally charged states exist in the massless string spectrum. Our model also contains the Higgs representations that are needed to break the gauge symmetry to that of the Standard Model and to generate fermion masses at the electroweak scale. We show that the requirement of a leading mass term for the heavy generation, which is not degenerate with the mass terms of the lighter generations, places an additional strong constraint on the viability of the models. In many models a top quark Yukawa may not exist at all, whereas in others two or more generations may obtain a mass term at leading order. In our exemplary model a mass term at leading order exist only for one family. Additionally, we demonstrate the existence of supersymmetric F- and D-flat directions that give heavy mass to all the colour triplets beyond those of the Standard Model and leave one pair of electroweak Higgs doublets light. Hence, below the Pati-Salam breaking scale, the matter states in our model that are charged under the observable gauge symmetries, consist solely of those of the Minimal Supersymmetric Standard Model.

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arXiv2011

Classification of Heterotic Pati-Salam Models

We extend the classification of free fermionic heterotic-string models to vacua in which the SO(10) GUT symmetry is broken at the string level to the Pati-Salam subgroup. Using our classification method we recently presented the first example of a quasi--realistic heterotic-string vacuum that is free of massless exotic states. Within this method we are able to derive algebraic expressions for the generalised GSO projections for all sectors that appear in the models. This facilitates the programming of the entire spectrum analysis in a computer code. The total number of vacua in the class of models that we classify is 2^{51} ~ 10^{15}. We perform a statistical sampling in this space of models and extract 10^{11} distinct configurations with Pati-Salam gauge group. Our results demonstrate that one in every 10^{6} vacua correspond to a three generation exophobic model with the required Higgs states, needed to induce spontaneous breaking to the Standard Model.

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arXiv2009

Exophobic Quasi-Realistic Heterotic String Vacua

We demonstrate the existence of heterotic-string vacua that are free of massless exotic fields. The need to break the non-Abelian GUT symmetries in k=1 heterotic-string models by Wilson lines, while preserving the GUT embedding of the weak-hypercharge and the GUT prediction sin^2θ_w(M(GUT))=3/8, necessarily implies that the models contain states with fractional electric charge. Such states are severely restricted by observations, and must be confined or sufficiently massive and diluted. We construct the first quasi-realistic heterotic-string models in which the exotic states do not appear in the massless spectrum, and only exist, as they must, in the massive spectrum. The SO(10) GUT symmetry is broken to the Pati-Salam subgroup. Our PS heterotic-string models contain adequate Higgs representations to break the GUT and electroweak symmetry, as well as colour Higgs triplets that can be used for the missing partner mechanism. By statistically sampling the space of Pati-Salam vacua we demonstrate the abundance of quasi--realistic three generation models that are completely free of massless exotics, rendering it plausible that obtaining realistic Yukawa couplings may be possible in this space of models.

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arXiv2008

Spinor-Vector Duality in Heterotic SUSY Vacua

We elaborate on the recently discovered spinor-vector duality in realistic free fermionic heterotic vacua. We emphasize the interpretation of the freely-acting orbifolds carried out on the six internal dimensions as coordinate-dependent compactifications; they play a central role in the duality, especially because of their ability to break the right-moving superconformal algebra of the space-time supersymmetric heterotic vacua. These considerations lead to a simple and intuitive proof of the spinor-vector duality, and to the formulation of explicit rules to find the dual of a given model. We discuss the interest of such a duality, notably concerning the structure of the space of vacua of superstring theory.

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arXiv2007

Spinor-Vector Duality in N=2 Heterotic String Vacua

Classification of the N=1 space-time supersymmetric fermionic Z2XZ2 heterotic-string vacua with symmetric internal shifts, revealed a novel spinor-vector duality symmetry over the entire space of vacua, where the S_t <-> V duality interchanges the spinor plus anti-spinor representations with vector representations. In this paper we demonstrate that the spinor--vector duality exists also in fermionic Z2 heterotic string models, which preserve N=2 space-time supersymmetry. In this case the interchange is between spinorial and vectorial representations of the unbroken SO(12) GUT symmetry. We provide a general algebraic proof for the existence of the S_t <-> V duality map. We present a novel basis to generate the free fermionic models in which the ten dimensional gauge degrees of freedom are grouped into four groups of four, each generating an SO(8) modular block. In the new basis the GUT symmetries are produced by generators arising from the trivial and non--trivial sectors, and due to the triality property of the SO(8) representations. Thus, while in the new basis the appearance of GUT symmetries is more cumbersome, it may be more instrumental in revealing the duality symmetries that underly the string vacua.

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arXiv2007

Chiral family classification of fermionic Z2xZ2 heterotic orbifold models

Free fermionic construction of four dimensional string vacua, are related to the Z2XZ2 orbifolds at special points in the moduli space, and yielded the most realistic three family string models to date. Using free fermionic construction techniques we are able to classify more than 10^10 string vacua by the net family and anti-family number. Using a montecarlo technique we find that a bell shaped distribution that peaks at vanishing net number of chiral families. We also observe that ~15% of the models have three net chiral families. We find that in addition to mirror symmetry that the distribution exhibits a symmetry under the exchange of (spinor plus anti-spinor) representations with vectorial representations.

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arXiv2006

Spinor-Vector Duality in fermionic Z2XZ2 heterotic orbifold models

We continue the classification of the fermionic Z2XZ2 heterotic string vacua with symmetric internal shifts. The space of models is spanned by working with a fixed set of boundary condition basis vectors and by varying the sets of independent Generalized GSO (GGSO) projection coefficients (discrete torsion). This includes the Calabi-Yau like compactifications with (2,2) world-sheet superconformal symmetry, as well as more general vacua with only (2,0) superconformal symmetry. In contrast to our earlier classification that utilized a montecarlo technique to generate random sets of GGSO phases, in this paper we present the results of a complete classification of the subclass of the models in which the four dimensional gauge group arises solely from the null sector. In line with the results of the statistical classification we find a bell shaped distribution that peaks at vanishing net number of generations and with ~15% of the models having three net chiral families. The complete classification reveals a novel spinor-vector duality symmetry over the entire space of vacua. The S <-> V duality interchanges the spinor plus anti-spinor representations with vector representations. We present the data that demonstrates the spinor-vector duality. We illustrate the existence of a duality map in a concrete example. We provide a general algebraic proof for the existence of the S <-> V duality map. We discuss the case of self-dual solutions with an equal number of vectors and spinors, in the presence and absence of E6 gauge symmetry, and present a couple of concrete examples of self-dual models without E6 symmetry.

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arXiv2004

Fermion masses and proton decay in string-inspired SU(4)SU(2)^2U(1)_X

We present a supersymmetric model of fermion masses with SU(4)*SU(2)^2*U(1)_X gauge group with matter in fundamental and antisymmetric tensor representations only. The up, down, charged lepton and neutrino Yukawa matrices are distinguished by different Clebsch-Gordan coefficients due to contracting over SU(4) and SU(2)_R indices. We obtain a hierarchical light neutrino mass spectrum with bi-large mixing. The condition that anomalies be cancelled by a Green-Schwarz mechanism leads to fractional U(1)_X charges which exclude $B$ violation through dimension-4 and -5 operators.

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arXiv2002

Exact Solutions and the Cosmological Constant Problem in Dilatonic-Domain-Wall Higher-Curvature String Gravity

In this article we extend previous work by the authors, and elaborate further on the structure of the general solution to the graviton and dilaton equations of motion in brane world scenaria, in the context of five-dimensional effective actions with O(α')$ higher-curvature corrections, compatible with bulk string-amplitude calculations. We consider (multi)brane scenaria, dividing the bulk space into regions, in which one matches two classes of general solutions, a linear Randall-Sundrum solution and a (logarithmic) dilatonic domain wall (bulk naked singularity).We pay particular attention to examining the possibility of resolving the mass hierarchy problem together with the vanishing of the vacuum energy on the observable world, which is taken to be a positive tension brane. The appearance of naked dilatonic domain walls provides a dynamical restriction of the bulk space time. Of particular interest is a dilatonic-wall solution, which after appropriate coordinate transformation, results in a linear dilaton conformal field theory. The latter may provide a holographic resolution of the naked singularity problem. All the string-inspired models involved have the generic feature that the brane tensions are proportional to the string coupling $g_s$; it remains a challenge for string theory, therefore, to show whether microscopic models respecting this feature can be constructed.

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arXiv2000

String-Inspired Higher-Curvature Terms and the Randall-Sundrum Scenario

We consider the O(a') string effective action, with Gauss-Bonnet curvature-squared and fourth-order dilaton-derivative terms, which is derived by a matching procedure with string amplitudes in five space-time dimensions. We show that a non-factorizable metric of the Randall-Sundrum (RS) type, with four-dimensional conformal factor Exp(-2 k|z|), can be a solution of the pertinent equations of motion. The parameter k is found proportional to the string coupling g_s and thus the solution appears to be non-perturbative. It is crucial that the Gauss-Bonnet combination has the right (positive in our conventions) sign, relative to the Einstein term, which is the case necessitated by compatibility with string (tree) amplitude computations. We study the general solution for the dilaton and metric functions, and thus construct the appropriate phase-space diagram in the solution space. In the case of an anti-de-Sitter bulk, we demonstrate that there exists a continuous interpolation between (part of) the RS solution at z=infinity and an (integrable) naked singularity at z=0. This implies the dynamical formation of domain walls (separated by an infinite distance), thus restricting the physical bulk space time to the positive z axis. Some brief comments on the possibility of fine-tuning the four-dimensional cosmological constant to zero are also presented.

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