The Z2×Z2Z_2\times Z_2 Orbifold and the SUSY Flavor Problem

The characteristic property of the $Z_2\times Z_2$ orbifold is its cyclic permutation symmetry. It is argued that this property may be instrumental in explaining simultaneously the fermion mass hierarchy and the squark mass degeneracy. Detailed studies in free fermionic models that preserve the cyclic permutation symmetry of anomalous U(1) charges of the three generations are discussed.Comment: 9 pages. Standard Late

On the origin of three generation free fermionic superstring models

The three generation superstring models in the free fermionic models have had remarkable success in describing the real--world. The most explored models use the NAHE set to obtain three generations and to separate the hidden and observable sectors. It is of course well known that the full NAHE set is not required in order to construct three generation free fermionic models. I argue that all the semi--realistic free fermionic models that have been constructed to date correspond to $Z_2\times Z_2$ orbifolds. Thus, the successes of the semi--realistic free fermionic models, if taken seriously, suggest that the true string vacuum is a $Z_2\times Z_2$ orbifold with nontrivial background fields and quantized Wilson lines.Comment: 12 pages, phyzzx.tex, tables.te

Duality, Equivalence, Mass and The Quest For The Vacuum

I contemplate the possibility that the mismatch between the maximally symmetric point (the free fermionic point) and the strictly self-dual point in the Narain moduli space plays a role in the string vacuum selection. The role of self-duality in the recent formulation of quantum mechanics from an equivalence postulate, and the new perspective that it offers on the foundations of quantum gravity and the origin of mass, are discussed.Comment: 8 pages. Standard Latex & sprocl.tex. Invited talk presented at PASCOS 99, Lake Tahoe, CA December 10-16 199

Phenomenological aspects of M-theory

The Standard Model data suggests the realization of grand unification structures in nature, in particular that of SO(10). A class of string vacua that preserve the SO(10) embedding, are the three generation free fermion heterotic-string models, that are related to Z2xZ2 orbifold compactification. Attempts to use the M-theory framework to explore further this class of models are discussed. Wilson line breaking of the SO(10) GUT symmetry results in super-heavy meta-stable states, which produce several exotic dark matter and UHECR candidates, with differing phenomenological characteristics. Attempts to develop the tools to study the properties of these states in forthcoming UHECR experiments are discussed. It is proposed that quantum mechanics arises from an equivalence postulate, that may lay the foundations for the rigorous formulation of quantum gravity.Comment: 25 pages. 6 figures. Standard Latex & iopart.cls. To appear in the proceedings of Beyond the Desert 02 - Accelerator and non Accelerator Approaches, Oulu, Finland, 2-7 June 200

Cosmological and phenomenological implications of Wilsonian matter in realistic superstring derived models

Superstring phenomenology aims at achieving two goals. The first is to reproduce the observed physics of the Standard Model. The second is to identify experimental signatures of superstring unification which, if observed, will provide further evidence for the validity of superstring theory. I discuss such potential signatures of superstring unification. I propose that proton lifetime constraints imply that the Standard Model gauge group must be obtained directly at the string level. In this case the unifying gauge group, for example $SO(10)$, is broken to the Standard Model gauge group by ``Wilson lines''. The symmetry breaking by ``Wilson line'' has important implications. It gives rise to exotic massless states which cannot fit into multiplets of the original unifying gauge group. This is an important feature because it results in conservation laws which forbid the interaction of the exotic ``Wilsonian'' states with the Standard Model states. The ``Wilsonian'' matter states then have important phenomenological implications. I discuss two such implications: exotic ``Wilsonian'' states as dark matter candidates and ``Wilsonian'' matter as the messenger sector in gauge mediated dynamical SUSY breaking scenarios.Comment: 22 pages. Standard Latex. 4 figures. epsf.te

Deriving The Standard Model From Superstring Theory

I outline a program to derive the Standard Model directly from superstring theory. I present a class of three generation superstring standard--like models in the free fermionic formulation. I discuss some phenomenological properties of these models. In particular these models suggest an explanation for the top quark mass hierarchy. A numerical estimate yielded $m_t\sim175-180~GeV$. The general texture of fermion mass matrices was obtained from analysis of nonrenormalizable terms up to order $N=8$.I argue that the realistic features of these models are due to the underlying $Z_2\times Z_2$ orbifold, with standard embedding, at the free fermionic point in toroidal compactification space.Comment: IASSNS--HEP--94/21, 11 page

String Phenomenology: Past, Present and Future Perspectives

The observation of a scalar resonance at the LHC, compatible with perturbative electroweak symmetry breaking, reinforces the Standard Model parameterisation of all subatomic data. The logarithmic evolution of the SM gauge and matter parameters suggests that this parameterisation remains viable up to the Planck scale, where gravitational effects are of comparable strength. String theory provides a perturbatively consistent scheme to explore how the parameters of the Standard Model may be determined from a theory of quantum gravity. The free fermionic heterotic string models provide concrete examples of exact string solutions that reproduce the spectrum of the Minimal Supersymmetric Standard Model. Contemporary studies entail the development of methods to classify large classes of models. This led to the discovery of exophobic heterotic-string vacua and the observation of spinor-vector duality, which provides an insight to the global structure of the space of (2,0) heterotic-string vacua. Future directions entail the study of the role of the massive string states in these models and their incorporation in cosmological scenarios. A complementary direction is the formulation of quantum gravity from the principle of manifest phase space duality and the equivalence postulate of quantum mechanics, which suggest that space is compact. The compactness of space, which implies intrinsic regularisation, may be tightly related to the intrinsic finite length scale, implied by string phenomenology.Comment: 35 pages. No figures. To appear in the special volume edited by Gerald Cleaver on "Particle Physics and Quantum Gravity Implications for Cosmology". Based on talk presented at the 2012 CERN Summer Institute on String Phenomenolog