Localized Tachyons and RG Flows

We study condensation of closed string tachyons living on defects, such as orbifold fixed planes and Neveu-Schwarz fivebranes. We argue that the high energy density of localized states decreases in the process of condensation of such tachyons. In some cases this means that ceff decreases along the flow; in others, ceff remains constant and the decreasing quantity is a closed string analog, gcl, of the “boundary entropy” of D-branes. We discuss the non-supersymmetric orbifoldsC/ZZn andC /ZZn. In the first case tachyon condensation decreases n and in some cases connects type II and type 0 vacua. In the second case nonsingular orbifolds are related by tachyon condensation to both singular and non-singular ones. We verify that gcl decreases in flows between non-singular orbifolds. The main tools in the analysis are the structure of the chiral ring of the perturbed theory, the geometry of the resolved orbifold singularities, and the throat description of singular conformal field theories.