Cascades and Collapses, Great Walls and Forbidden Cities: Infinite   Towers of Metastable Vacua in Supersymmetric Field Theories
  In this paper, we present a series of supersymmetric models exhibiting an entirely new vacuum structure: towers of metastable vacua with higher and higher energies. As the number of vacua grows towards infinity, the energy of the highest vacuum remains fixed while the energy of the true ground state tends towards zero. We study the instanton-induced tunneling dynamics associated with such vacuum towers, and find that many distinct decay patterns along the tower are possible: these include not only regions of vacua experiencing direct collapses and/or tumbling cascades, but also other regions of vacua whose stability is protected by ``great walls'' as well as regions of vacua populating ``forbidden cities'' into which tunnelling cannot occur. We also discuss possible applications of this setup for the cosmological-constant problem, for studies of the string landscape, for supersymmetry breaking, and for Z' phenomenology. Finally, we point out that a limiting case of our setup yields theories with yet another new vacuum structure: infinite numbers of degenerate vacua. As a result, the true ground states of such theories are Bloch waves, with energy eigenvalues approximating a continuum and giving rise to a vacuum ``band'' structure. 
Keith R. Dienes, Brooks Thomas