Spacetime Instanton Corrections in 4D String Vacua - The Seesaw Mechanism for D-Brane Models

TL;DR

This paper analyzes how instanton effects from Euclidean D-branes influence the superpotential in 4D string models, enabling phenomena like neutrino mass generation via the seesaw mechanism.

Contribution

It provides a systematic method to compute instanton corrections in D-brane models, linking string theory effects to phenomenologically relevant couplings.

Findings

Instanton effects can generate Majorana neutrino masses.

Constraints from gauge invariance limit allowed superpotential terms.

The framework applies to Type IIA, IIB, and heterotic string vacua.

Abstract

We systematically investigate instanton corrections from wrapped Euclidean D-branes to the matter field superpotential of various classes of N=1 supersymmetric D-brane models in four dimensions. Both gauge invariance and the counting of fermionic zero modes provide strong constraints on the allowed non-perturbative superpotential couplings. We outline how the complete instanton computation boils down to the computation of open string disc diagrams for boundary changing operators multiplied by a one-loop vacuum diagram. For concreteness we focus on E2-instanton effects in Type IIA vacua with intersecting D6-branes, however the same structure emerges for Type IIB and heterotic vacua. The instantons wrapping rigid cycles can potentially destabilise the vacuum or generate perturbatively absent matter couplings such as proton decay operators, mu-parameter or right-handed neutrino Majorana mass terms. The latter allow the realization of the seesaw mechanism for MSSM-like intersecting D-brane models.