Global string singularities

By analysing the fully coupled equations of motion for a U(1) global string with gravity, we show that global string spacetimes are singular. This singularity is not removable (i.e. due to a bad choice or coordinates) but is a physical curvature singularity.     

Nonrenormalizable superpotential in four-dimensional string models

A construction of an effective nonrenormalizable superpotential has been discussed within the approach of a four-dimensional heterotic superstring with free fermions on the world sheet. On the basis of N=2 superconformal theory, useful selection rules that take into account all global symmetries have been obtained within the formalism of covariant vertices and the ghost-picture-changing operation. The results have been applied to the model featuring the effective observable gauge group U(5)×U(3)_H, which includes the non-Abelian horizontal group and which describes (3+1) generation. The full superpotential of the model up to sixth order inclusive has been obtained, and its coupling constants have been calculated. The quark-lepton mass spectrum of the model has been discussed with allowance for contributions from nonrenormalizable terms.     

Ward identities in four-dimensional string theory

A four-dimensional heterotic string theory is envisaged in the background of its massless gauge Ward identities are derived. The spontaneous generating functional is constructed in the hamiltonian phase space and gauge Ward identities are derived. The spontaneous breaking of gauge symmetry is discussed in this approach.     

Stable singularities in string theory

We study a topological obstruction of a very stringy nature concerned with deforming the target space of anN=2 non-linear -model. This target space has a singularity which may be smoothed away according to the conventional rules of geometry, but when one studies the associated conformal field theory one sees that such a deformation is not possible without a discontinuous change in some of the correlation functions. This obstruction appears to come from torsion in the homology of the target space (which is seen by deforming the theory by an irrelevant operator). We discuss the link between this phenomenon and orbifolds with discrete torsion as studied by Vafa and Witten.Supported in part by NSF grant DMS-9400873.     

Logarithmic divergence in four-dimensional string theory

Logarithmic divergences in the one-loop scattering amplitude of the heterotic string compactified on the Z_n-orbifold are investigated. The coefficient of these divergences is shown to be a function of the indices of the representations to which the massless states of the model belong.     

Naked singularities in a self-similar Tolman-Bondi spacetime

Following Lake and Zannias we show that naked strong curvature singularities develop in Tolman-Bondi inhomogeneous spherically symmetric spacetimes for all the three cases of a bound, unbound and marginally bound gravitational collapse. It is observed that the assumption of self-similarity rather than the spherical symmetry is crucial in determining the nature of the singularity in any gravitationally collapsing configuration.     

Holography and string dynamics in time-dependent backgrounds

We study the dynamics of D-branes in a smooth time-dependent background. The theory on the branes is a time-dependent noncommutative field theory. We find the metric and fluxes that determine the dual holographic closed string theory. This provides a concrete example of holography in a cosmological setting.     

A modular subgroup invariance in a four-dimensional string

A four-dimensional closed supersymmetric string theory is constructed from the left-moving Neveu-Schwarz bosonic string and the right-moving superstring. The gauge group [SU(2)]⁶ arises from an algebraic compactification involving a new affine Kac-Moody construction in terms of Neveu-Schwarz operators. The theory is Lorentz-invariant, tachyon-free and has four-point one-loop amplitudes invariant under a subgroup of the modular group.     

Gauge couplings in four-dimensional type I string orbifolds

We compute threshold effects to gauge couplings in four-dimensional _N orientifold models of type I strings with GN = 2 and GN = 1 supersymmetry, and study their dependence on the geometric moduli. We also compute the tree-level (disk) couplings of the open sector gauge fields to the twisted closed string moduli of the orbifold in various models and study their effects and that of the one-loop threshold corrections on gauge coupling unification. We interpret the results from the (supergravity) effective theory point of view and comment on the conjectured heterotic-type I duality.