It is shown that for small relative angle and kinetic energy two type I $U(1)$ strings can form bound states upon collision instead of the more familiar intercommuting configuration. The velocity below which this may happen is estimated as function of the ratio of the coupling constants in the theory, crossing angle and initial kinetic energy.
We determine the spectrum of D-string bound states in various classes of generalized type I vacuum configurations with sixteen and eight supercharges. The precise matching of the BPS spectra confirms the duality between unconventional type IIB orientifolds with quantized NS-NS antisymmetric tensor and heterotic CHL models in D=8. A similar analysis puts the duality between type II (4,0) models and type I strings {it without open strings} on a firmer ground. The analysis can be extended to type II (2,0) asymmetric orbifolds and their type I duals that correspond to unconventional K3 compactifications. Finally we discuss BPS-saturated threshold corrections to the correponding low-energy effective lagrangians. In particular we show how the exact moduli dependence of some F^4 terms in the eight-dimensional type II (4,0) orbifold is reproduced by the infinite sum of D-instanton contributions in the dual type I theory.
We derive analytic expressions for the interaction energy between two general $U(1)$ cosmic strings as the function of their relative orientation and the ratio of the coupling constants in the model. The results are relevant to the statistic description of strings away from critical coupling and shed some light on the mechanisms involved in string formation and the evolution of string networks.
We consider the propagation of Type I open superstrings on orbifolds with four non-compact dimensions and $N=1$ supersymmetry. In this paper, we concentrate on a non-trivial Z_2xZ_2 example. We show that consistency conditions, arising from tadpole cancellation and algebraic sources, require the existence of three sets of Dirichlet 5-branes. We discuss fully the enhancements of the spectrum when these 5-branes intersect. An amusing attribute of these models is the importance of the tree-level (in Type I language) superpotential to the consistent relationship between Higgsing and the motions of 5-branes.
We describe how the strings, which are classical solutions of the continuum three-dimensional U(1)+Higgs theory, can be studied on the lattice. The effect of an external magnetic field is also discussed and the first results on the string free energy are presented. It is shown that the string free energy can be used as an order parameter when the scalar self-coupling is large and the transition is continuous.
So far the most sophisticated experiments have shown no trace of new physics at the TeV scale. Consequently, new models with unexplored parameter regions are necessary to explain current results, re-examine the existing data, and propose new experiments. In this Letter, we present a modified version of the $mu u$SSM supersymmetric model where a non-Universal extra U(1) gauge symmetry is added in order to restore an effective R-parity that ensures proton stability. We show that anomalies equations cancel without having to add emph{any} exotic matter, restricting the charges of the fields under the extra symmetry to a discrete set of values. We find that it is the viability of the model through anomalies cancellation what defines the conditions in which fermions interact with dark matter candidates via the exchange of $Z$ bosons. The strict condition of universality violation means that LHC constraints for a $Z$ mass do not apply directly to our model, allowing for a yet undiscovered relatively light $Z$, as we discuss both in the phenomenological context and in its implications for possible flavour changing neutral currents. Moreover, we explore the possibility of isospin violating dark matter interactions; we observe that this interaction depends, surprisingly, on the Higgs charges under the new symmetry, both limiting the number of possible models and allowing to analyse indirect dark matter searches in the light of well defined, particular scenarios.