A New Supersymmetry Index for Symmetric Orbifold CFTs

Abstract: We propose a new supersymmetry index for symmetric orbifold CFTs in the setup of the AdS$_3$/CFT$_2$ correspondence. In a novel formulation for symmetric orbifold CFTs based on the Schur-Weyl duality, we show how this index naturally emerges and its protection follows from the detailed nature of exactly marginal operators in these theories. This index is a one-parameter generalization of the standard index and gives more fine-grained information about the structure of microstates than previously available. For the case of the D1-D5 CFT for $T<sup>4$ we demonstrate precise matching of the new index between supergravity and CFT below the black-hole threshold, where the standard index -- the modified elliptic genus -- is trivial. Above the threshold, we uncover a decomposition of black-hole microstates into distinct sectors, invisible to the modified elliptic genus.