Some issues in soft SUSY-breaking terms from dilaton/moduli sectors

TL;DR

The paper investigates the structure of soft SUSY-breaking terms in four-dimensional string models under dilaton/moduli-dominated SUSY breaking, extending prior work from an overall modulus to a multimoduli setting. It derives general expressions for soft masses, A-terms, and gaugino masses, including cases with diagonal and off-diagonal Kähler metrics, and identifies sum rules that constrain the spectrum across large classes of models. The authors show that multimoduli effects can yield tachyonic scalars or alter the gaugino–scalar mass hierarchy, with implications for MSSM-like models and string-GUTs. They also analyze μ and B terms, highlighting mechanisms from both the Kähler potential and the superpotential, and demonstrate a robust prediction of tanβ ≈ 1 at the string scale in orbifold constructions, while discussing FCNC concerns from off-diagonal metrics and the sensitivity to non-perturbative corrections.

Abstract

We study the structure of the soft SUSY-breaking terms obtained from some classes of 4-D strings under the assumption of dilaton/moduli dominance in the process of SUSY-breaking. We generalize previous analysis in several ways and in particular consider the new features appearing when several moduli fields contribute to SUSY breaking (instead of an overall modulus $T$). Some qualitative features indeed change in the multimoduli case.A general discussion for symmetric Abelian orbifolds as well as explicit examples are given. Certain general sum-rules involving soft terms of different particles are shown to apply to large classes of models. Unlike in the overall modulus $T$ case, gauginos may be lighter than scalars even at the tree-level. However, if one insists in getting that pattern of soft terms, these sum rules force some of the scalars to get negative mass$^2$. These tachyonic masses could be a problem for standard model 4-D strings but an advantage in the case of string-GUTs.We also discuss the possible effects of off-diagonal metrics for the matter fields which may give rise to flavour-changing neutral currents. Different sources for the bilinear $B$ soft term are studied. It is found that the Giudice-Masiero mechanism for generating a ``$μ$-term'', as naturally implemented in orbifolds, leads to the prediction $|tgβ|=1$ at the string scale, independently of the Goldstino direction.