Note on Mediation of Supersymmetry Breaking from Closed to Open Strings

We discuss the mediation of supersymmetry breaking from closed to open strings, extending and improving previous analysis of the authors in Nucl. Phys. B 695 (2004) 103 [hep-th/0403293]. In the general case, we find the absence of anomaly mediation around any perturbative string vacuum. When supersymmetry is broken by Scherk-Schwarz boundary conditions along a compactification interval perpendicular to a stack of D-branes, the gaugino acquires a mass at two loops that behaves as $m_{1/2}\sim g^4 m_{3/2}^3$ in string units, where $m_{3/2}$ is the gravitino mass and $g$ is the gauge coupling.Comment: 8 pages, 1 figur

Direct collider signatures of large extra dimensions

The realization of low (TeV) scale strings usually requires the existence of large (TeV) extra dimensions where gauge bosons live. The direct production of Kaluza-Klein excitations of the photon and Z-boson at present and future colliders is studied in this work. At the LEPII, NLC and Tevatron colliders, these Kaluza-Klein modes lead to deviations from the standard model cross-sections, which provide lower bounds on their mass. At the LHC the corresponding resonances can be produced and decay on-shell, triggering a characteristic pattern in the distribution of the dilepton invariant mass.Comment: 14 pages, LateX, 5 figure

Radiative Symmetry Breaking on D-branes at Non-supersymmetric Singularities

The possibility of radiative gauge symmetry breaking on D3-branes at non-supersymmetric orbifold singularities is examined. As an example, a simple model of D3-branes at non-supersymmetric C^3/Z_6 singularity with some D7-branes for the cancellations of R-R tadpoles in twisted sectors is analyzed in detail. We find that there are no tachyon modes in twisted sectors, and NS-NS tadpoles in twisted sectors are canceled out, though uncanceled tadpoles and tachyon modes exist in untwisted sectors. This means that this singularity background is a stable solution of string theory at tree level, though some specific compactification of six-dimensional space should be considered for a consistent untwisted sector. On D3-brane three massless "Higgs doublet fields" and three family "up-type quarks" are realized at tree level. Other fermion fields, "down-type quarks" and "leptons", can be realized as massless modes of the open strings stretching between D3-branes and D7-branes. The Higgs doublet fields have Yukawa couplings with up-type quarks, and they also have self-couplings which give a scalar potential without flat directions. Since there is no supersymmetry, the radiative corrections may naturally develop negative Higgs mass squared and "electroweak symmetry breaking". We explicitly calculate the open string one-loop correction to the Higgs mass squared from twisted sectors, and find that the negative value is indeed realized in this specific model.Comment: 20 page

Moduli Forces in String Models with Large Internal Dimensions

We estimate the strength and range of forces mediated by string moduli in type I string models with two (or more) large internal dimensions. We find that forces mediated by twisted moduli which live on the brane world--volume can mediate forces which are orders of magnitude stronger than gravity with a range up to a milimeter. If they exist, these forces can be easily observed in present experiments. On the other hand, forces mediated by the dilaton and untwisted moduli are about a hundred times stronger than gravity and may be observed depending on their range.Comment: 18 pages in phyzzx.tex, minor changes and additions, two references added and abstract rephrase

Open string topological amplitudes and gaugino masses

We discuss the moduli-dependent couplings of the higher derivative F-terms (\Tr W^2)^{h-1}, where $W$ is the gauge N=1 chiral superfield. They are determined by the genus zero topological partition function $F^{(0,h)}$, on a world-sheet with $h$ boundaries. By string duality, these terms are also related to heterotic topological amplitudes studied in the past, with the topological twist applied only in the left-moving supersymmetric sector of the internal $N=(2,0)$ superconformal field theory. The holomorphic anomaly of these couplings relates them to terms of the form $\Pi^n({\rm Tr}W^2)^{h-2}$, where $\Pi$'s represent chiral projections of non-holomorphic functions of chiral superfields. An important property of these couplings is that they violate R-symmetry for $h\ge 3$. As a result, once supersymmetry is broken by D-term expectation values, (\Tr W^2)^2 generates gaugino masses that can be hierarchically smaller than the scalar masses, behaving as $m_{1/2}\sim m_0^4$ in string units. Similarly, $\Pi{\rm Tr}W^2$ generates Dirac masses for non-chiral brane fermions, of the same order of magnitude. This mechanism can be used for instance to obtain fermion masses at the TeV scale for scalar masses as high as $m_0\sim{\cal O}(10^{13})$ GeV. We present explicit examples in toroidal string compactifications with intersecting D-branes.Comment: 57 pages, 6 figures; Abstract and references correcte

D-branes and the Standard Model

We perform a systematic study of the Standard Model embedding in a D-brane configuration of type I string theory at the TeV scale. We end up with an attractive model and we study several phenomenological questions, such as gauge coupling unification, proton stability, fermion masses and neutrino oscillations. At the string scale, the gauge group is U(3)_color x U(2)_weak x U(1)_1 x U(1)_bulk. The corresponding gauge bosons are localized on three collections of branes; two of them describe the strong and weak interactions, while the last abelian factor lives on a brane which is extended in two large extra dimensions with a size of afew microns. The hypercharge is a linear combination of the first three U(1)s. All remaining U(1)s get masses at the TeV scale due to anomalies, leaving the baryon and lepton numbers as (perturbatively) unbroken global symmetries at low energies. The conservation of baryon number assures proton stability, while lepton number symmetry guarantees light neutrino masses that involve a right-handed neutrino in the bulk. The model predicts the value of the weak angle which is compatible with the experiment when the string scale is in the TeV region. It also contains two Higgs doublets that provide tree-level masses to all fermions of the heaviest generation, with calculable Yukawa couplings; one obtains a naturally heavy top and the correct ratio m_b/m_tau. We also study neutrino masses and mixings in relation to recent solar and atmospheric neutrino data.Comment: 42 pages, Latex2e, 6 figures, final version to be published in Nucl. Phys.

The Neutrino Mass Spectrum in the Supersymmetric Flipped SU(5)xU(1) GUT Model

In the context of the supersymmetric flipped $SU(5)\otimes U(1)$ GUT model, we have studied in detail the neutrino mass spectrum, obtaining an approximate formula for the corresponding effective three-generation mass matrix. It is found that at least two neutrinos are expected to have extremely small masses of no particular interest. The third neutrino, on the other hand, may eventually get a mass of order $10^{-3}$ eV, of relevance for the solution of the solar neutrino problem with the MSW mechanism, as a consequence of vacuum-expectation-values at the GUT mass scale for some of the scalar partners of the right-handed neutrinos.Comment: 11 pages UAB-FT-32

Topological Amplitudes in Heterotic Superstring Theory

We show that certain heterotic string amplitudes are given in terms of correlators of the twisted topological (2,0) SCFT, corresponding to the internal sector of the N=1 spacetime supersymmetric background. The genus g topological partition function $F^g$ corresponds to a term in the effective action of the form $W^{2g}$, where W is the gauge or gravitational superfield. We study also recursion relations related to holomorphic anomalies, showing that, contrary to the type II case, they involve correlators of anti-chiral superfields. The corresponding terms in the effective action are of the form $W^{2g}\Pi^n$, where $\Pi$ is a chiral superfield obtained by chiral projection of a general superfield. We observe that the structure of the recursion relations is that of N=1 spacetime supersymmetry Ward identity. We give also a solution of the tree level recursion relations and discuss orbifold examples.Comment: LaTeX, 60 pages, 2 PS figure

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