T.R. Taylor

Moduli stabilization in chiral type IIB orientifold models with fluxes

Abstract We consider type IIB orientifold models on Calabi–Yau spaces with three-form G-flux turned on. These fluxes freeze some of the complex structure moduli and the complex dilaton via an F-term scalar potential. By introducing pairs of D9– D9 branes with Abelian magnetic fluxes it is possible to freeze also some of the Kahler moduli via a D-term potential. Moreover, such magnetic fluxes in general lead to chiral fermions, which make them interesting for string model-building. These issues are demonstrated in a simple toy model based on a Z 2 × Z 2 ′ orbifold.

Strings and D=4

Abstract It is argued that the unique infrared behaviour of non-abelian gauge couplings in four dimensions could be responsible, in closed super- (or heterotic) string theory, for dynamical supersymmetry breaking and for compactification to D=4 with string-scale compactified radii.

Aspects of type I– type II– heterotic triality in four dimensions

We discuss the equivalence between Type I, Type II and Heterotic N = 2 superstring theories in four dimensions. We study the effective field theory of Type I models obtained by orientifold reductions of Type IIB compactifications on K3 × T 2 . We show that the perturbative prepotential is determined by the one-loop corrections to the Planck mass and is associated to an index. As is the case for threshold corrections to gauge couplings, this renormalization is entirely due to N = 2 BPS states that originate from D = 6 massless string modes. We apply our result to the so-called S-T-U model which admits simultaneous Type II and Heterotic descriptions, and show that all three prepotentials agree in the appropriate limits as expected from the superstring triality conjecture.

Dilaton Couplings at Large Distances

Abstract The large-distance behaviour of dilaton couplings is studied within the framework of four-dimensional heterotic superstring theories. String loop corrections violate universality of dilaton and graviton couplings, increasing dilaton couplings at large distances. This implies a serious problem for string theories, unless a non-zero dilaton mass is generated.

Dilaton, gaugino condensation and supersymmetry breaking

Abstract Non-perturbative condensation of gauginos belonging to a gauge group of the form G1 ⊗ G2 ⊗…⊗ Gp, whre Gn are non-abelian subgroups, is examined in superstring theory by utilizing the effective lagrangian techniques. A supersymmetric vacuum with stable expectation values of the dilation and moduli fields is shown to exist provided that the gauge group satisfies one non-trivial constraint.

Amplitude for N-Gluon Superstring Scattering

We consider scattering processes involving N gluonic massless states of open superstrings with a certain Regge slope alpha. At the semiclassical level, the string world-sheet sweeps a disk and N gluons are created or annihilated at the boundary. We present exact expressions for the corresponding amplitudes, valid to all orders in alpha, for the so-called maximally helicity violating configurations, with N = 4, 5 and N = 6. We also obtain the leading O(alpha 2) string corrections to the zero-slope N-gluon Yang-Mills amplitudes.

Axion couplings and effective cut-offs in superstring compactifications

Abstract We use the linear supermultiplet formalism of supergravity to study axion couplings and chiral anomalies in the context of field-theoretical lagrangians describing orbifold compactifications beyond the classical approximation. By matching amplitudes computed in the effective low-energy theory with the results of string loop calculations we determine the appropriate counterterm in this effective theory that assures modular invariance to all loop order. We use supersymmetry consistency constraints to identify the correct ultraviolet cut-offs for the effective low-energy theory. Our results have a simple interpretation in terms of two-loop unification of gauge coupling constants at the string scale.

Complete Six-Gluon Disk Amplitude in Superstring Theory

We evaluate all next-to-maximal helicity violating (NMHV) six-gluon amplitudes in type I open superstring theory in four dimensions, at the disk level, to all orders in α′. Although the computation utilizes supersymmetric Ward identities, the result holds for all compactifications, even for those that break supersymmetry and is completely model-independent. Together with the maximally helicity violating (MHV) amplitudes presented in the previous work, our results provide the complete six-gluon disk amplitude.

Direct Production of Lightest Regge Resonances

Abstract We discuss direct production of Regge excitations in the collisions of massless four-dimensional superstring states, focusing on the first excited level of open strings ending on D-branes extending into higher dimensions. We construct covariant vertex operators and identify “universal” Regge states with the internal parts either trivial or determined by the world-sheet SCFT describing superstrings propagating on an arbitrary Calabi–Yau manifold. We evaluate the amplitudes involving one such massive state and up to three massless ones and express them in the helicity basis. The most important phenomenological applications of our results are in the context of low-mass string (and large extra dimensions) scenarios in which excited string states are expected to be produced at the LHC as soon as the string mass threshold is reached in the center-of-mass energies of the colliding partons. In order to facilitate the use of partonic cross sections, we evaluate them and tabulate for all production processes: gluon fusion, quark absorbing a gluon, quark–antiquark annihilation and quark–quark scattering.

Duality of N = 2 heterotic-type I compactifications in four dimensions

We discuss type I-heterotic duality in four-dimensional models obtained as a Coulomb phase of the six-dimensional U(16) orientifold model compactified on T2 with arbitrary SU(16) Wilson lines. We show that Kahler potentials, gauge threshold corrections and the infinite tower of higher derivative F-terms agree in the limit that corresponds to weak coupling, large T2 heterotic compactifications. On the type I side, all these quantities are completely determined by the spectrum of N = 2 BPS states that originate from D = 6 massless superstring modes.