Wall Crossing, Discrete Attractor Flow and Borcherds Algebra

Abstract

The appearance of a generalized (or Borcherds-) Kac-Moody algebra in the spectrum of BPS dyons in N = 4, d = 4 string theory is elucidated. From the low-energy supergravity analysis, we identify its root lattice as the lattice of the T -duality invariants of the dyonic charges, the symmetry group of the root system as the extended S-duality group P GL(2, Z) of the theory, and the walls of Weyl chambers as the walls of marginal stability for the relevant two-centered solutions. This leads to an interpretation for the Weyl group as the group of wall-crossing, or the group of discrete attractor flows. Furthermore we propose an equivalence between a "second-quantized multiplicity" of a charge-and modulidependent highest weight vector and the dyon degeneracy, and show that the wall-crossing formula following from our proposal agrees with the wall-crossing formula obtained from the supergravity analysis. This can be thought of as providing a microscopic derivation of the wall-crossing formula of this theory.