Mock Modular Index of M2-M5 Brane System

We present BPS indices of the supergroup WZW models that live on intersecting M2-M5 brane systems. They can encode data of the stretched M2-branes between M5-branes and count the BPS states. They are generally expressed in terms of mock theta functions via the Kac-Wakimoto character formula of the affine Lie superalgebra. We give an explicit expression of the index for the $PSL(2|2)_{k=1}$ WZW model in terms of the second order multi-variable Appell-Lerch sum. It indicates that wall-crossing occurs in the BPS state counting due to the $C$-field on the M5-branes.