A ladder of topologically non-trivial non-BPS states

We consider a simple quiver gauge theory with gauge group U(r1)×U(r2) and a Higgs field in the bi-fundamental representation. The background for this theory is a compact Kähler manifold MM. For a careful but natural choice of Higgs field potential the second order field equations can be replaced with a set of first order BPS equations. We show that the theory admits two energy gaps: The vacuum is topologically trivial but has finite, non-zero energy and is not a BPS state. The second gap lies between the vacuum and the first BPS state. In this gap we find a ladder of states with non-trivial topology, at equidistant energy levels. We give a semi-explicit construction for such topologically non-trivial non-BPS states.