Cabibbo--Kobayashi--Maskawa Mixing in Superstring Derived Standard--like Models

We examine the problem of three generation quark flavor mixing in realistic, superstring derived standard--like models, constructed in the free fermionic formulation. We study the sources of family mixing in these models and discuss the necessary conditions to obtain a realistic Cabibbo--Kobayashi--Maskawa (CKM) mixing matrix. In a specific model, we estimate the mixing angles and discuss the weak CP violating phase. We argue that the superstring standard--like models can produce a realistic CKM mixing matrix. We discuss the possible textures of quark mass matrices that may be obtained in these models.Comment: WIS--93/35/APR--PH, 29 page

Moduli Fixing in Realistic String Vacua

I illustrate the existence of quasi-realistic heterotic-string models in which all the untwisted Kaehler and complex structure moduli, as well as all of the twisted sectors moduli, are projected out by the generalized GSO projections. I discuss the conditions and characteristics of the models that produce this result. The existence of such models offers a novel perspective on the realization of extra dimensions in string theory. In this view, while the geometrical picture provides a useful mean to classify string vacua, in the phenomenologically viable cases there is no physical realization of extra dimensions. The models under consideration correspond to Z2 X Z2 orbifolds of six dimensional tori, plus additional identifications by internal shifts and twists. The special property of the Z2 X Z2 orbifold is that it may act on the compactified dimensions as real, rather than complex, dimensions. This property enables an asymmetric projection on all six internal coordinates, which enables the projection of all the untwisted Kaehler and complex structure moduli.Comment: 31 pages. Standard LaTeX. Additional section on twisted moduli. typos corrected. Revised version to appear in NP

NAHE-based string models with SU(4) X SU(2) X U(1) SO(10) Subgroup

The orbifold GUT doublet-triplet splitting mechanism was discussed in 1994 in the framework of the NAHE-based free fermionic models in which the SO(10) GUT symmetry is broken to SO(6) X SO(4), SU(3) X SU(2) X U(1)^2, or SU(3) X U(1) X SU(2)^2. In this paper we study NAHE-based free fermionic models in which the SO(10) symmetry is broken at the string level to SU(4) X SU(2) X U(1). In addition to the doublet-triplet splitting this case also has the advantage of inducing the doublet-doublet splitting already at the string level. We demonstrate, however, that NAHE-based models with SU(4) X SU(2) X U(1) SO(10) subgroup are not viable. We show that, similarly to the LRS models, and in contrast to the FSU5, PS and SLM models, the SU421 case gives rise to models without an anomalous U(1) symmetry, and discuss the different cases in terms of their N=4 origins.Comment: 25 pages. Standard Latex. Revised version to appear in NP

String Inspired Z' Model With Stable Proton and Light Neutrino Masses

Grand unification, and its incarnation in the form of heterotic-string unification, are the only extensions of the Standard Model that are rooted in the structure of the Standard Model itself. In this context, it was proposed that the exclusiveness of proton stability and suppression of neutrino masses necessitates the existence of an additional U(1)_Z' symmetry, which is of non-GUT origin and remains unbroken down to intermediate, or low, energies. Realistic string models frequently give rise to non-GUT U(1) symmetries, which arise from the flavor symmetries in the models. In this paper we demonstrate in a string-inspired toy model that such a stringy Z' can indeed guarantee proton longevity and viable phenomenology in the neutrino sector, as well as in the quark and charged lepton sectors.Comment: 20 pages. Standard LaTex. Version to appear in NP

Construction of Realistic superstring Standard--like Models

I discuss the construction of realistic superstring standard--like models in the four dimensional free fermionic formulation. I discuss proton lifetime constraints on superstring models. I discuss the massless spectrum of the superstring standard--like models, the texture of fermion mass matrices in these models and argue that the realistic features of these models are due to the underlying $Z_2\times Z_2$ orbifold compactification. (talk presented at the TEXAS\\PASCHOS conference, Berkeley CA, December 13--18 1992)Comment: WIS--92/103/DEC--PH, 8 page

Supersymmetry and Light Quark Masses in a Realistic Superstring Model

We examine the light quark masses in a standard--like superstring model in the four dimensional free fermionic formulation. We find that the supersymmetry constraints in the observable and hidden sectors eliminate all large contributions to $m_u$ and $m_d$ and force them to be much smaller than the other quark masses. The requirement for an acceptable Higgs doublet spectrum results in $m_u<<m_d$. In these models a realistic $m_d$ can always be obtained whereas $m_u$ is at most $10^{-5}~MeV$. For particular choices of flat directions or vacua $m_u$ can be as small as $10^{-7}~MeV$ but cannot vanish.Comment: WIS-93/98/OCT-PH, 22 pages and 2 tables, uses phyzzx.tex and tables.te

A Minimal Superstring Standard Model II: A Phenomenological Study

Recently, we demonstrated the existence of heterotic--string solutions in which the observable sector effective field theory just below the string scale reduces to that of the MSSM, with the standard observable gauge group being just SU(3)_C x SU(2)_L x U(1)_Y and the SU(3)_C x SU(2)_L x U(1)_Y-charged spectrum of the observable sector consisting solely of the MSSM spectrum. Associated with this model is a set of distinct flat directions of vacuum expectation values (VEVs) of non-Abelian singlet fields that all produce solely the MSSM spectrum. In this paper, we study the effective superpotential induced by these choices of flat directions. We investigate whether sufficient degrees of freedom exist in these singlet flat directions to satisfy various phenomenological constraints imposed by the observed Standard Model data. For each flat direction, the effective superpotential is given to sixth order. The variations in the singlet and hidden sector low energy spectrums are analyzed. We then determine the mass matrices (to all finite orders) for the three generations of MSSM quarks and leptons. Possible Higgs mu-terms are investigated. We conclude by considering generalizations of our flat directions involving VEVs of non-Abelian fields.Comment: 41 pages. Standard Late

CP violation in realistic String Models with family universal anomalous U(1)

The characteristic property of the Z2XZ2 orbifold compactification is the cyclic permutation symmetry between the three twisted sectors. We discuss how this property, which is retained in a class of realistic free fermionic string models, may be instrumental in enabling fermion mass hierarchy while ensuring sfermion mass degeneracy, irrespective of the dominant source of supersymmetry breaking. The cyclic symmetry is reflected in some models in the existence of a family universal anomalous U(1)_A. We analyze the FCNC and CP violation effects in a model with a dominant U(1)_A SUSY breaking. In this theories, new sources of FC are always suppressed with respect to the average sfermion masses. We make a phenomenological analysis of these effects and find that in most of the cases they are in qualitative agreement with the phenomenological limits. The most sensitive low-energy observables to these new FC sources are the $\epsilon_K$ parameter, measuring indirect CP violation in neutral kaon mixing and EDMs. These observables set important constraints on the structure of the sfermion mass matrices, but do not require a large fine-tuning of the initial parameters and can be satisfied in most realistic constructions.Comment: 20 pages. Standard LaTeX. Version to appear in NP

Phenomenology of Non-Abelian Flat Directions in a Minimal Superstring Standard Model

Recently, we presented the first non-Abelian flat directions that produce from a heterotic string model solely the three-generation MSSM states as the massless spectrum in the observable sector of the low energy effective field theory. In this paper we continue to develop the systematic techniques for the analysis of nonrenormalizable superpotential terms and non-Abelian flat direction in realistic string models. Some of our non-Abelian directions were F-flat to all finite orders in the superpotential. We study for the same string model the varying phenomenologies resulting from a large set of such all-order flat directions. We focus on the quark, charged lepton, and Higgs doublet mass matrices resulting for our phenomenologically superior non-Abelian flat direction. We review and apply a string-related method for generating large mass hierarchies between MSSM generations, first discussed in string-derived flipped SU(5) models, when all generational mass terms are of renormalizable or very low non-renormalizable order

Classification of the chiral Z2XZ2 fermionic models in the heterotic superstring

The first particle physics observable whose origin may be sought in string theory is the triple replication of the matter generations. The class of Z2XZ2 orbifolds of six dimensional compactified tori, that have been most widely studied in the free fermionic formulation, correlate the family triplication with the existence of three twisted sectors in this class. In this work we seek an improved understanding of the geometrical origin of the three generation free fermionic models. Using fermionic and orbifold techniques we classify the Z2XZ2 orbifold with symmetric shifts on six dimensional compactified internal manifolds. We show that perturbative three generation models are not obtained in the case of Z2XZ2 orbifolds with symmetric shifts on complex tori, and that the perturbative three generation models in this class necessarily employ an asymmetric shift. We present a class of three generation models in which the SO(10) gauge symmetry cannot be broken perturbatively, while preserving the Standard Model matter content. We discuss the potential implications of the asymmetric shift for strong-weak coupling duality and moduli stabilization. We show that the freedom in the modular invariant phases in the N=1 vacua that control the chiral content, can be interpreted as vacuum expectation values of background fields of the underlying N=4 theory, whose dynamical components are projected out by the Z2-fermionic projections. In this class of vacua the chiral content of the models is determined by the underlying N=4 mother theory.Comment: 36 pages. Standard LaTe