<a href="https://arxiv.org/abs/hep-th/0510251">An Index for 4 dimensional super conformal theories</a>

Citation:
Kinney, Justin, Juan Martin Maldacena, Shiraz Minwalla, and Suvrat Raju. "An Index for 4 dimensional super conformal theories." Commun.Math.Phys. 275 (2005): 209-254.