was given by Karch and Katz \[72\]. In these models, two stacks of flavor branes, branes and anti-branes, are added to the geometry as a probe, so that the back reaction of the flavor branes is negligible (probe approximation). This approximation is reliable when $N_f \ll N_c $, where $N_c$ and $N_f$ refer to the number of colors and flavors, respectively.

Of course, the brane/anti-brane system is unstable, since the branes and anti-branes will tend to annihilate. This is reflected in the presence of tachyons in the spectrum. But, it should make sense within the context of perturbation theory. The point where the tachyon field vanishes corresponds to a local maximum of the tachyon potential, and thus it is part of a classical solution. The one-loop effective action in an expansion around this solution should be well defined, even though the solution is unstable, and in particular it should have a well-defined phase. It was conjectured that at the minimum of the tachyon potential, the negative contribution to the energy density from the potential exactly cancels the sum of the tensions of the brane and the anti-brane, thereby giving a configuration of zero energy density (and hence restoring space-time supersymmetry). 