Lifting 1/4-BPS States in $AdS_3\times S^3 \times T^4$

Abstract: We establish a framework for doing second order conformal perturbation theory for the symmetric orbifold Sym$<sup>N(T<sup>4)$ to all orders in $N$. This allows us to compute how 1/4-BPS states of the D1-D5 system on $AdS_3\times S^3\times T<sup>4$ are lifted as we move away from the orbifold point. As an application we confirm a previous observation that in the large $N$ limit not all 1/4-BPS states that can be lifted do get lifted. This provides evidence that the supersymmetric index actually undercounts the number of 1/4-BPS states at a generic point in the moduli space.