Type II string theory on AdS 3 × S 3 × T 4 and symmetric orbifolds

We discuss in detail the 1 + 1 -dimensional superconformal field theory dual to type II string theory on AdS 3 × S 3 × T 4 , emphasizing the string theoretic aspects of this duality. For one unit of Neveu-Schwarz (NS-NS) 5-brane flux ( Q 5 = 1 ), this string theory has been suggested to be dual to a grand-canonical ensemble of T 4 N / S N free symmetric orbifold conformal field theories (CFTs). We show how the string genus expansion emerges to all orders for the free orbifold grand-canonical correlation functions. We also discuss how the strong coupling limit of the NS-NS string theory arises (even at large N ) in the free orbifold description, and argue why this limit does not have a weakly coupled RR description. The dual conformal field theory (CFT) includes (for all values of Q 5 ) an extra T 4 factor that is decoupled from perturbative string theory. We discuss the exactly marginal deformations that relate the different values of Q 5 , including the precise J J ¯ deformations mixing this extra T 4 with the symmetric orbifold.