The Weinberg Operator and a Lower String Scale in Orientifold Compactifications

TL;DR

This paper explores how orientifold string compactifications relate the string scale to phenomenological scales like neutrino masses, analyzing instanton effects and quiver models to understand constraints on the string scale.

Contribution

It provides a systematic analysis of quiver models with MSSM spectrum incorporating instanton-induced neutrino masses and discusses constraints on the string scale from phenomenological considerations.

Findings

String scale constraints from neutrino mass generation mechanisms.

Viable quiver models with instanton effects matching observed neutrino masses.

Analysis of four-stack and five-stack quivers for MSSM spectrum.

Abstract

We investigate the interplay between the string scale and phenomenological scales in orientifold compactifications. Specifically, we discuss in generality the tension that often arises in accounting for neutrino masses, Yukawa couplings, and a mu-term of the correct order and show that it often constrains the string scale M_s. The discussion focuses on two scenarios where, (1) the observed order of the neutrino masses are accounted for by a D-instanton induced "stringy" Weinberg operator, or (2) effectively via the type I seesaw mechanism with an instanton induced Majorana mass term. In both scenarios, the string scale might be further constrained if the suppression factor of a single D-instanton must account for two of the phenomenological scales. For the sake of concreteness, we present phenomenologically viable quivers which exhibit these effects and perform a systematic analysis of four-stack and five-stack quivers which give rise to the exact MSSM spectrum and account for the order of the neutrino masses via the stringy Weinberg operator.