Return of the Torsion D-Branes

TL;DR

This paper investigates torsion D-branes on Calabi-Yau manifolds, analyzing their monodromies, stability, and bound states, with implications for string compactifications and D-brane charge classifications.

Contribution

It provides a detailed study of torsion D-branes, including monodromy calculations, stability analysis, and the proposal for D-brane charge groups with discrete fluxes, extending previous understanding.

Findings

Monodromies of torsion D-brane charges are determined around singularities.

Evidence for new threshold bound states in nonabelian fundamental groups.

Comparison with Gepner model computations supports the theoretical framework.

Abstract

We study D-branes on Calabi-Yau manifolds, carrying charges which are torsion elements of the K-theory. Interesting physics ensues when we follow these branes into nongeometrical phases of the compactification. On the level of K-theory, we determine the monodromies of the group of charges as we circle singular loci in the closed string moduli space. Going beyond K-theory, we discuss the stability of torsion D-branes as a function of the K\"ahler moduli. When the fundamental group of the Calabi-Yau is nonabelian, we find evidence for new threshold bound states of BPS branes. In a two-parameter example, we compare our results with computations in the Gepner model. Our study of the torsion D-branes in the compactification of [FHSV] sheds light on the physics of that model. In particular, we develop a proposal for the group of allowed D-brane charges in the presence of discrete RR fluxes.