Gravitational Couplings and $Z_2$ Orientifolds

TL;DR

This paper explores how gravitational couplings relate to fractional flux and charges in low-dimensional orientifolds, emphasizing the role of orientifold planes and flux quantization in string theory compactifications.

Contribution

It introduces the necessity of assigning gravitational couplings to orientifold planes and explains fractional charges through flux quantization and modified Bianchi identities.

Findings

Fractional charges of orientifold planes are explained via flux quantization.

Gravitational couplings are extended to include orientifold planes.

Detailed analysis of type IIB orientifold on T^6/Z_2 and its dual F-theory description.

Abstract

The interplay between gravitational couplings on branes and the occurrence of fractional flux in low dimensional orientifolds is examined. It is argued that gravitational couplings need to be assigned not only to D-branes but also to orientifold planes. The fractional charges of the orientifold $d$-planes can be understood in terms of flux quantization of the $d-3$ form potential and modified Bianchi identities. Detailed results are presented for the case of the type IIB orientifold on $T^6/Z_2$, which is dual to F-theory on a complex 4-fold with terminal singularities.