The fate of the type I non-BPS D7-brane

TL;DR

This paper investigates the decay processes and final states of non-BPS D7-branes in type I string theory, revealing their topological charges, decay mechanisms, and implications for supersymmetry and M-theory.

Contribution

It provides explicit examples of non-BPS D-branes with non-trivial K-theory charges and explores their decay, M-theory lifts, and the relation between K-theory and cohomology.

Findings

Decay of non-BPS D7-branes leads to topologically non-trivial gauge configurations.

Construction of non-supersymmetric orientifolds with tachyon condensation restoring supersymmetry.

Identification of non-BPS states with unique topological and cohomological properties.

Abstract

We describe the fate of the Type I non-BPS D7-brane, which is tachyonic but carries a non-trivial K-theory $\IZ_2$ charge. It decays to topologically non-trivial gauge field configurations on the background D9-branes. In the uncompactified theory the decay proceeds to infinity, while with a transverse torus the decay reaches a final state, a toron gauge configuration with vanishing Chern classes but non-trivial $\IZ_2$ charge. A similar behaviour is obtained for the type I non-BPS D8-brane, and other related systems. We construct explicit examples of type IIB orientifolds with non-BPS D7-branes, which are hence non-supersymmetric, but for which supersymmetry is restored upon condensation of the tachyon. We also report on the interesting structure of non-BPS states of type IIA theory in the presence of an O6-plane, their M-theory lifts, the relation between string theory K-theory and M-theory cohomology, and its interplay with NS-NS charged objects. We discuss several new effects, including: i) transmutation between NS-NS and RR torsion charges, ii) non-BPS states classified by K-theory but not by cohomology in string theory, but whose lift to M-theory is cohomological.