Soft SUSY Breaking, Dilaton Domination and Intermediate Scale String Models

TL;DR

This work investigates the low-energy implications of an intermediate-scale string theory, around $M_X \sim 10^{11}$ GeV, focusing on how renormalization group running from the string scale affects soft SUSY-breaking terms in Type I orientifold models. It develops a unified framework for gaugino masses, scalar masses, and A-terms with three source fields $S$, $T$, and $M$, and analyzes dilaton-dominated, $M$-dominated, and moduli-dominated breaking patterns. A key finding is that dilaton-dominated SUSY breaking, previously ruled out by charge-and-colour breaking constraints at the GUT scale, becomes viable at the intermediate scale, especially when extra leptons are added to achieve gauge unification; the intermediate scale also reduces fine-tuning and relaxes certain phenomenological bounds. The results have practical significance by predicting Higgs and sparticle spectra within reach of near-future experiments and by highlighting how brane-world string constructions can accommodate realistic unification and CP/FCNC considerations.

Abstract

We present an analysis of the low-energy implications of an intermediate scale ~10^{11} GeV string theory. We mainly focus on the evolution of the physical parameters under the renormalisation group equations (RGEs) and find several interesting new features that differ from the standard GUT scale or Planck scale scenarios. We give a general discussion of soft supersymmetry breaking terms in type I theories and then investigate the renormalization group running. In the dilaton domination scenario, we present the sparticle spectra, analyzing constraints from charge and colour breaking, fine tuning and radiative electroweak symmetry breaking. We compare with the allowed regions of parameter space when the RGEs start running at the standard GUT or the intermediate scales, and find quite remarkably that the dilaton dominated supersymmetry breaking scenario, which is essentially ruled out from constraints on charge and colour breaking if the fundamental scale is close to the Planck mass, is allowed in a large region of parameter space if the fundamental scale is intermediate.