Comments on Open-String Orbifolds with a Non-Vanishing B_{ab}

TL;DR

This paper analyzes how a non-zero NS-NS B-field flux affects open-string orbifolds, influencing gauge group structures, couplings, and supersymmetry breaking mechanisms.

Contribution

It provides a detailed study of the impact of B_{ab} flux on gauge groups, amplitudes, and supersymmetry in open-string orbifold models.

Findings

Gauge group rank reduction proportional to B_{ab}

Continuous connection between U(n) and Sp(n) groups via Moebius amplitude signs

Emergence of non-universal couplings from the Green-Schwarz mechanism

Abstract

We study the effect of a non-vanishing flux for the NS-NS antisymmetric tensor in open-string orbifolds. As in toroidal models, the total dimension of the Chan-Paton gauge group is reduced proportionally to the rank of $B_{ab}$, both on D9 and on D5-branes, while the Moebius amplitude involves some signs that, in the $Z_2$ case, allow one to connect continuously ${\rm U} (n)$ groups to $Sp(n) \otimes Sp(n)$ groups on each set of D-branes. In this case, non-universal couplings between twisted scalars and gauge vectors arise, as demanded by the generalised Green-Schwarz mechanism. We also comment on the role of the NS-NS antisymmetric tensor in a recently proposed type scenario, where supersymmetry is broken on the D-branes, while it is preserved in the bulk.