On the Heterotic Effective Action at One-Loop, Gauge Couplings and the Gravitational Sector

We present in detail the procedure for calculating the heterotic one-loop effective action. We focus on gravitational and gauge couplings. We show that the two-derivative couplings of the gravitational sector are not renormalized at one loop when the ground state is supersymmetric. Arguments are presented that this non-renormalization theorem persists to all orders in perturbation theory. We also derive the full one-loop correction to the gauge coupling. For a class of $N=2$ ground states, namely those that are obtained by toroidal compactification to four dimensions of generic six-dimensional $N=1$ models, we give an explicit formula for the gauge-group independent thresholds, and show that these are equal within the whole family.Comment: LateX, 17pp. A minor correction mad

Supersymmetry breaking in M-theory

We describe the breaking of supersymmetry in M-theory by coordinate dependent (Scherk-Schwarz) compactification of the eleventh dimension. Supersymmetry is spontaneously broken in the gravitational and moduli sector and communicated to the observable sector, living at the end-point of the semicircle, by radiative gravitational interactions. This mechanism shares the generic features of non-perturbative supersymmetry breaking by gaugino condensation, in the presence of a constant antisymmetric field strength, in the weakly coupled regime of the heterotic string, which suggests that both mechanisms could be related by duality. In particular an analysis of supersymmetric transformations in the infinite-radius limit reveals the presence of a discontinuity in the spinorial parameter, which coincides with the result found in the presence of gaugino condensation, while the condensate is identified with the quantized parameter entering the boundary conditionsComment: 10 pages, latex + espcrc2.sty, no figures. Based on talks given at the 5th International Conference on Supersymmetries in Physics, SUSY 97, May 27-31, 1997, University of Pennsylvania, Philadelphia, PA; and, International Europhysics Conference on High Energy Physics, HEP 97, 19-26 August, Jerusalem, Israe

Production of Kaluza-Klein States at Future Colliders

Perturbative breaking of supersymmetry in four-dimensional string theories predict in general the existence of new large dimensions at the TeV scale. Such large dimensions lie in a domain of energies accessible to particle accelerators. Their main signature is the production of Kaluza-Klein excitations which can be detected at future colliders. We study this possibility for hadron colliders (TEVATRON, LHC) and $e^+ e^-$ colliders (LEP-200, NLC-500).Comment: 13 pages, LATEX, 4 postscript figures appended at the end, CPTH-A293.0294 and IEM-FT-84/9

R^2 Corrections and Non-perturbative Dualities of N=4 String ground states

We compute and analyse a variety of four-derivative gravitational terms in the effective action of six- and four-dimensional type II string ground states with N=4 supersymmetry. In six dimensions, we compute the relevant perturbative corrections for the type II string compactified on K3. In four dimensions we do analogous computations for several models with (4,0) and (2,2) supersymmetry. Such ground states are related by heterotic-type II duality or type II-type II U-duality. Perturbative computations in one member of a dual pair give a non-perturbative result in the other member. In particular, the exact CP-even R^2 coupling on the (2,2) side reproduces the tree-level term plus NS 5-brane instanton contributions on the (4,0) side. On the other hand, the exact CP-odd coupling yields the one-loop axionic interaction a.R\wedge R together with a similar instanton sum. In a subset of models, the expected breaking of the SL(2,Z)_S S-duality symmetry to a \Gamma(2)_S subgroup is observed on the non-perturbative thresholds. Moreover, we present a duality chain that provides evidence for the existence of heterotic N=4 models in which N=8 supersymmetry appears at strong coupling.Comment: Latex2e, 51 pages, 1 figur

Non-Perturbative Gravitational Corrections in a Class of N=2 String Duals

We investigate the non-perturbative equivalence of some heterotic/type II dual pairs with N=2 supersymmetry. The perturbative heterotic scalar manifolds are respectively SU(1, 1)/U(1) x SO(2, 2+NV)/ SO(2) x SO(2+NV) and SO(4, 4+NH)/ SO(4) x SO(4+NH) for moduli in the vector multiplets and hypermultiplets. The models under consideration correspond, on the type II side, to self-mirror Calabi-Yau threefolds with Hodge numbers h(1,1)= NV +3= h(2,1)= NH +3, which are K3 fibrations. We consider three classes of dual pairs, with NV=NH=8, 4 and 2. The models with h(1,1)=7 and 5 provide new constructions, while the h(1,1)=11, already studied in the literature, is reconsidered here. Perturbative R2-like corrections are computed on the heterotic side by using a universal operator whose amplitude has no singularities in the (T,U) space, and can therefore be compared with the type II side result. We point out several properties connecting K3 fibrations and spontaneous breaking of the N=4 supersymmetry to N=2. As a consequence of the reduced S- and T- duality symmetries, the instanton numbers in these three classes are restricted to integers, which are multiples of 2, 2 and 4, for NV=8, 4 and 2, respectively

Soft Masses in Theories with Supersymmetry Breaking by TeV-Compactification

We study the sparticle spectroscopy and electroweak breaking of theories where supersymmetry is broken by compactification (Scherk-Schwarz mechanism) at a TeV. The evolution of the soft terms above the compactification scale and the resulting sparticle spectrum are very different from those of the usual MSSM and gauge mediated theories. This is traced to the softness of the Scherk-Schwarz mechanism which leads to scalar sparticle masses that are only logarithmically sensitive to the cutoff starting at two loops. As a result, squarks and sleptons are naturally an order of magnitude lighter than gauginos. In addition, the mechanism is very predictive and the sparticle spectrum depends on just two new parameters. A significant advantage of this mechanism relative to gauge mediation is that a Higgsino mass $\mu\sim M_susy$ is automatically generated when supersymmetry is broken. Our analysis applies equally well to theories where the cutoff is near a TeV or $M_{Pl}$ or some intermediate scale. We also use these observations to show how we may obtain compactification radii which are hierarchically larger than the fundamental cutoff scale.Comment: 26 pages, 1 figure, Late

Dynamical supersymmetry breaking with a large internal dimension

Supersymmetry breaking in string perturbation theory predicts the existence of a new dimension at the TeV scale. The simplest realization of the minimal supersymmetric Standard Model in the context of this mechanism has two important consequences: (i) A natural solution to the $\mu$-problem; (ii) The absence of quadratic divergences in the cosmological constant, which leads to a dynamical determination of the supersymmetry breaking and electroweak scale. We present an explicit example in which the whole particle spectrum is given as a function of the top quark mass. A generic prediction of this mechanism is the existence of Kaluza-Klein excitations for gauge bosons and higgses. In particular the first excitation of the photon could be accessible to future accelerators and give a clear signal of the proposed mechanism.Comment: 27 pages, latex, 6 figures available by FAX upon reques

An Effective Supergravity for the Thermal Phases of N=4 Strings

A universal effective supergravity Lagrangian describing the thermal phases of heterotic strings on T^4 x S^1, IIA and IIB strings on K^3 x S^1 is constructed. The resulting non-perturbative phase structure is discussed.Comment: 9 pages, 6th Hellenic School and Workshop, Corfu, Greece, Sept. 9