Towards a realistic Standard Model from D-brane configurations

Effective low energy models arising in the context of D-brane configurations with Standard Model (SM) gauge symmetry extended by several gauged abelian factors are discussed. The models are classified according to their hypercharge embeddings consistent with the SM spectrum hypercharge assignment. Particular cases are analyzed according to their perspectives and viability as low energy effective field theory candidates. The resulting string scale is determined by means of a two-loop renormalization group calculation. Their implications in Yukawa couplings, neutrinos and flavor changing processes are also presented.Comment: 22 pages, 12 EPS figures, some clarifications/references adde

Gauge Unification and Quark Masses in a Pati-Salam Model from Branes

We investigate the phase space of parameters in the Pati-Salam model derived in the context of D-branes scenarios, requiring low energy string scale. We find that a non-supersymmetric version complies with a string scale as low as 10 TeV, while in the supersymmetric version the string scale raises up to ~2 x 10^7 TeV. The limited energy region for RGE running demands a large tan(beta) in order to have experimentally acceptable masses for the top and bottom quarks.Comment: 11 pages, LaTeX, 7 figures include

String Unification at Intermediate Energies: Phenomenological Viability and Implications

Motivated by the fact that the string scale can be many orders of magnitude lower than the Planck mass, we investigate the required modifications in the MSSM $\beta$--functions in order to achieve intermediate ($10^{10-13}$GeV) scale unification, keeping the traditional logarithmic running of the gauge couplings. We present examples of string unified models with the required extra matter for such a unification while we also check whether other MSSM properties (such as radiative symmetry breaking) are still applicable.Comment: 11 pages, 2 figure included in the ps file, uses psfig.st

Modular Weights, U(1)'s and Mass Matrices

We derive the scalar mass matrices in effective supergravity models augmented by a $U(1)_F$ family symmetry. Simple relations between $U(1)_F$ charges and modular weights of the superfields are derived and used to express the matrices with a minimum number of parameters. The model predicts a branching ratio for the $\mu\to e\gamma$ process close to the present experimental limits.Comment: LaTex file, 11 pages, 1 Figure.Typos corrected and a reference has been added. Accepted in Phys. Lett.

Gauge coupling flux thresholds, exotic matter and the unification scale in F-SU(5) GUT

We explore the gauge coupling relations and the unification scale in F-theory SU(5) GUT broken down to the Standard Model by an internal U(1)Y gauge flux. We consider variants with exotic matter representations which may appear in these constructions and investigate their role in the effective field theory model. We make a detailed investigation on the conditions imposed on the extraneous matter to raise the unification scale and make the color triplets heavy in order to avoid fast proton decay. We also discuss in brief the implications on the gaugino masses.Comment: 20 pages, 3 figures, references and extended comments on KK thresholds effects adde

Duality constraints on supersymmetric unified models and radiative symmetry breaking

Motivated from unified models with string origin, we analyse the constraints from duality invariance on effective supergravity models with intermediate gauge symmetry. Requiring vanishing vacuum energy and invariance of the superpotential couplings, the modular weights are subject to various constraints. Further, the intermediate gauge symmetry breaking scale M(U) is related to the values of modular weights of the matter and Higgs fields. For certain regions of values of the latter, M(U) can be close to the conventional unification scale of the minimal supersymmetric standard model. We also examine particular examples where the intermediate gauge symmetry breaks down to the standard gauge group radiatively.Physics Letters

Predictive Ansatz for Fermion Masses in Susy Guts

We reexamine a succesful fermion mass ansatz proposed by Giudice for a wide range of the ratio tan beta = [h-]/[h] (where h-, h are the two standard Higgs fields), in the context of supersymmetric grand unified theories. We find that the seven predictions of the ansau, V(us), V(cb), V(ub), m(u), m(d), m(s) and m(b) are in good agreement with the experiment when either (i) tan beta congruent-to O(1) or (ii) tan beta greater-than-or-equal-to 30. A correct prediction for the bottom mass gives a lower limit on m(t) greater-than-or-equal-to 125 for case (i), in agreement with the previous analysis, while in case (ii) m(t) greater-than-or-equal-to 148.Physics Letters