A 4D IIB Flux Vacuum and Supersymmetry Breaking. I. Fermionic Spectrum

We consider the type-IIB supergravity vacua that include an internal $T^5$, depend on a single coordinate $r$ and respect a four-dimensional Poincar\'e symmetry, with the aim of highlighting low-energy spectra with broken supersymmetry and a bounded string coupling. These vacua are characterized by the flux $\Phi$ of the self-dual five form in the internal torus, the length $\ell$ of the interval described by the coordinate $r$, a dilaton profile that is inevitably constant and a strictly positive dimensionless parameter $h$. As $\ell\rightarrow\infty$ while retaining finite values for $\Phi$ and $\ell \,h^{-\,\frac{5}{4}}$, half of the original ten-dimensional supersymmetry is recovered, while finite values of $\ell$ break it completely. In the large-$\ell$ limit one boundary disappears but the other is still present, and is felt as a BPS orientifold by a probe brane. In this paper we focus on the fermionic zero modes and show that, although supersymmetry is broken for finite values of $\ell$, they are surprisingly those of four-dimensional $N=4$ supergravity coupled to five vector multiplets. The gravitini can acquire masses via radiative corrections, absorbing four of the massless spin-$\frac{1}{2}$ modes.