Stable non-BPS D-particles

TL;DR

This work constructs a stable non-BPS D-particle in the orbifold of type IIB theory by $(-1)^{F_L}{\cal I}_4$, localized on the fixed plane. Using boundary-state techniques, it combines untwisted NSNS and twisted RR sectors to form a D0 boundary state that is consistent with open-closed duality and yields a tachyon-free open-string spectrum, with the particle carrying a twisted RR $SO(2)$ charge and forming a long ${\cal N}=(1,1)$ multiplet in $D=6$. The results align with S-dual expectations for the dual of a perturbative stable non-BPS state and offer a potential realization of Sen’s bound-state picture, while highlighting open questions about projection choices and mass normalization. The analysis provides a concrete framework for understanding non-BPS D-brane spectra in orbifold backgrounds and suggests avenues for connecting to related dualities in heterotic theories.

Abstract

It is shown that the orbifold of type IIB string theory by (-1)^{F_L} I_4 admits a stable non-BPS Dirichlet particle that is stuck on the orbifold fixed plane. It is charged under the SO(2) gauge group coming from the twisted sector, and transforms as a long multiplet of the D=6 supersymmetry algebra. This suggests that it is the strong coupling dual of the perturbative stable non-BPS state that appears in the orientifold of type IIB by ΩI_4.