Stringy Instanton Effects in Models with Rigid Magnetised D-branes

TL;DR

The work analyzes stringy instanton effects in globally consistent $T^6/\mathbb{Z}_2\times\mathbb{Z}_2$ orientifolds with discrete torsion and magnetised D-branes, focusing on rigid cycles to control zero-mode content. It derives the four-dimensional effective action with anomaly cancellation and Green-Schwarz couplings, and identifies how Euclidean branes, notably ${\rm E}1_3$ instantons, generate non-perturbative superpotential terms. The authors construct three explicit globally consistent models showing instanton-induced mass terms and linear couplings, including a one-loop conformal model whose conformality is broken at a hierarchically small scale by instantons. These results illustrate a mechanism for hierarchical mass generation and potential non-perturbative conformal-symmetry breaking within string compactifications, with implications for moduli stabilization and hierarchy problems.

Abstract

We compute instantonic effects in globally consistent T^6/Z2xZ2 orientifold models with discrete torsion and magnetised D-branes. We consider fractional branes and instantons wrapping the same rigid cycles. We clarify and analyse in detail the low-energy effective action on D-branes in these models. We provide explicit examples where instantons induce linear terms in the charged fields, or non-perturbative mass terms are generated. We also find examples where the gauge theory on fractional branes has conformal symmetry at one-loop, broken by instantonic mass terms at a hierarchically small energy scale.