Comments on string propagation near defects
Davit Sahakyan · CERN Document Server (European Organization for Nuclear Research) · 2004
This thesis investigates various aspects of string propagation near defects. In particular we study the effective theory describing NS5 branes in certain decoupling limit gs → 0, which is called Little String Theory (LST). The LST has a holographical description in terms of closed strings propagating in the near horizon geometry of NS5 branes. Using this description, we show that the high energy thermodynamics of LST is unstable; we exhibit a mode localized near the horizon of the black hole, which has mass that vanishes at high energy. We argue that the high temperature phase of the theory involves the condensation of this mode. Another aspect of the LST, which is addressed in the thesis, is the topological version of this theory. We argue that the topological LST is described in terms of N = 2 string in the background of NS5 branes. We show that this topological string can be used to efficiently compute the half-BPS F 4 terms in the low-energy effective action of the LST. Using the strong-weak coupling string duality relating type IIA strings on K3 and heterotic string on T 4 , the same term may be computed in the heterotic string theory near a point of enhanced gauge symmetry. We study the F 4 terms in the heterotic string and in the LST, and show that they have the same structure, and they agree in the cases for which we compute both of them. We also clarify some additional issues, such as the role of normalizable modes in the holographic linear dilaton backgrounds, the precise identification of vertex operators in these backgrounds with states and operators in the supersymmetric Yang- Mills theory that arises in the low energy limit of LST, and the normalization of two-point functions. We also discuss some aspects of string propagation in AdS 3 spaces, which arises as part of near horizon geometry of collection of k NS5 branes and p fundamental strings. In particular we study the maximally symmetric D-branes in AdS 3. We also comment on the role of the spectral flow symmetry of the underlying SL(2, R )/U(1) coset model in constructing D-branes that correspond to degenerate representations of SL(2, R ).