String States on AdS_3 x S^3 from the Supergroup

Abstract: In the hybrid formulation of string theory on AdS_3 x S^3, the compactification-independent physical spectrum at the first massive level is determined. We find that these states transform in representations of the Lie superalgebra psl(2|2) and present a cohomological characterisation of them within the context of the PSL(2|2) WZW model. We conjecture that a similar cohomological description of the physical states also exists at higher mass levels and give partial evidence for this by comparing our results to the RNS string spectrum.