Tachyon condensation in magnetic compactifications

Intersecting D-brane models and their T-dual magnetic compactifications provide an attractive framework for particle physics, allowing for chiral fermions and supersymmetry breaking. Generically, magnetic compactifications have tachyons that are usually removed by Wilson lines. However, quantum corrections prevent local minima for Wilson lines. We therefore study tachyon condensation in the simplest case, the magnetic compactification of type I string theory on a torus to eight dimensions. We find that tachyon condensation restores supersymmetry, which is broken by the magnetic flux, and we compute the Kaluza-Klein mass spectrum. The gauge group SO(32) is broken to USp(16). We give arguments that the vacuum reached by tachyon condensation corresponds to the unique 8d superstring theory already known in the literature, with discrete B_ab background or, in the T-dual version, the type IIB orientifold with three O7₋-planes, one O7₊-plane and eight D7-branes coincident with the O7₊-plane. The ground state after tachyon condensation is supersymmetric and has no chiral fermions.