String Loop Threshold Corrections for N=1 Generalized Coxeter Orbifolds

We discuss the calculation of threshold corrections to gauge coupling constants for the, only, non-decomposable class of abelian (2, 2) symmetric N=1 four dimensional heterotic orbifold models, where the internal twist is realized as a generalized Coxeter automorphism. The latter orbifold was singled out in earlier work as the only N=1 heterotic $Z_N$ orbifold that satisfy the phenomenological criteria of correct minimal gauge coupling unification and cancellation of target space modular anomalies.Comment: 10 pages, LaTe

SU(5) Unified Theories from Intersecting Branes

We discuss the first string theory examples of three generation non-supersymmetric SU(5) and {\em flipped} SU(5) GUTS, which break to the Standard model at low energy, without extra matter and/or gauge group factors. Our GUT examples are based on IIA $Z_3$ orientifolds with D6-branes intersecting at non-trivial angles. These theories necessarily satisfy RR tadpoles and are free of NSNS tadpoles as the complex structure moduli are frozen (even though a dilaton tadpole remains) to discrete values. We identify appropriately the bifundamental Higgses responsible for electroweak symmetry breaking. In this way, the neutrino see-saw mechanism get nicely realized in these constructions. Moreover, as baryon number is not a gauged symmetry gauge mediated dimension six operators do contribute to proton decay; however proton lifetime may be safely enhanced by appropriately choosing a high GUT scale. An accompanying natural doublet-triplet splitting guarantees the suppression of scalar mediated proton decay modes and the stability of triplet scalar masses against higher dimensional non-renormalizable operators.Comment: 26 pages, 5 figures; no changes, one comment added in the introductio

The open story of the magnetic fluxes

We discuss the effects of oblique internal magnetic fields on the spectrum of type I superstrings compactified on tori. In particular we derive general formulae for the magnetic shifts and multiplicities of open strings connecting D9-branes with arbitrary magnetic fluxes. We discuss the flux induced potential and offer an interpretation of the stabilization of R-R moduli associated to deformations of the complex structure of T^6 in terms of non-derivative mixing with NS-NS moduli. Finally we briefly comment on how to extract other low energy couplings and generalize our results to toroidal orbifolds and other configurations governed by rational conformal field theories on the worldsheet.Comment: 36 pages, LaTeX, References adde

Deformed Intersecting D6-Brane GUTS I

By employing D6-branes intersecting at angles in $D = 4$ type IIA strings, we construct {\em four stack string GUT models} (PS-I class), that contain at low energy {\em exactly the three generation Standard model} with no extra matter and/or extra gauge group factors. These classes of models are based on the Pati-Salam (PS) gauge group $SU(4)_C \times SU(2)_L \times SU(2)_R$. They represent deformations around the quark and lepton basic intersection number structure. The models possess the same phenomenological characteristics of some recently discussed examples (PS-A class) of four stack PS GUTS. Namely, there are no colour triplet couplings to mediate proton decay and proton is stable as baryon number is a gauged symmetry. Neutrinos get masses of the correct sizes. Also the mass relation $m_e = m_d$ at the GUT scale is recovered. Moreover, we clarify the novel role of {\em extra} branes, the latter having non-trivial intersection numbers with quarks and leptons and creating scalar singlets, needed for the satisfaction of RR tadpole cancellation conditions. The presence of N=1 supersymmetry in sectors involving the {\em extra} branes is equivalent to the, model dependent, orthogonality conditions of the U(1)'s surviving massless the generalized Green-Schwarz mechanism. The use of {\em extra} branes creates mass couplings that predict the appearance of light fermion doublets up to the scale of electroweak scale symmetry breaking.Comment: 45 pages, 1 figure, LaTex, The discussion on the hidden orthogonal U(1) D6-branes extende

Constraining the string scale: from Planck to Weak and back again

String and field theory ideas have greatly influenced each other since the so called second string revolution. We review this interrelation paying particular attention to its phenomenological implications. Our guiding principle is the radical shift in the way that we think about the fundamental scale, in particular the way in which string models have been able to accommodate values from the Planck $M_\mathrm{Pl}\sim 10^{18}$ GeV down to the electroweak scale $M_{EW}\sim$ TeV.Comment: Invited review aimed at an experimental audienc

Supersymmetric Intersecting Branes on the Type IIA T^6/Z_4 orientifold

We study supersymmetric intersecting D6-branes wrapping 3-cycles in the Type IIA T^6/Z_4 orientifold background. As a new feature, the 3-cycles in this orbifold space arise both from the untwisted and the Z_2 twisted sectors. We present an integral basis for the homology lattice, H_3(M,Z), in terms of fractional 3-cycles, for which the intersection form involves the Cartan matrix of E8. We show that these fractional D6-branes can be used to construct supersymmetric brane configurations realizing a three generation Pati-Salam model. Via brane recombination processes preserving supersymmetry, this GUT model can be broken down to a standard-like model.Comment: 48 pages, TeX, harvmac, 8 figures, v4: some signs correcte

Seesaw scales and the steps from the Standard Model towards superstring-inspired flipped E_6

Recently in connection with Superstring theory E_8 and E_6 unifications became very promising. In the present paper we have investigated a number of available paths from the Standard Model (SM) to the E_6 unification, considering a chain of flipped models following the extension of the SM: SU(3)_C\times SU(2)_L\times U(1)_Y \to SU(3)_C\times SU(2)_L\times U(1)_X \times U(1)_Z \to SU(5)\times U(1)_X \to SU(5)\times U(1)_{Z1} \times U(1)_{X1} \to SO(10) \times U(1)_{X1} \to SO(10) \times U(1)_{Z2}\times U(1)_{X2} \to E_6\times U(1)_{X2} or E_6, Also we have considered a chain with a left-right symmetry: SU(3)_C\times SU(2)_L\times U(1)_Y \to SU(3)_C\times SU(2)_L \times SU(2)_R\times U(1)_X\times U(1)_Z \to SU(4)_C\times SU(2)_L \times SU(2)_R\times U(1)_Z \to SO(10)\times U(1)_Z \to E_6. We have presented four examples including non-supersymmetric and supersymmetric extensions of the SM and different contents of the Higgs bosons providing the breaking of the flipped SO(10) and SU(5) down to the SM. It was shown that the final unification E_6\times U(1) or E_6 at the (Planck) GUT scale M_{SSG} depends on the number of the Higgs boson representations considered in theory.Comment: 25 pages, 7 figure