One of the "definitive" references for the classical lid driven cavity flow is the paper by Ghia and Ghia. That is, the results of Ghia and Ghia are treated as a benchmark for the cavity flow problem. What are the "definitive" references for flow over a backward facing step?

Not quite sure this is the ulimative (or definitive) reference you are seeking, but it seems that the following experimental study (turbulent, incompressible) is cited by many researchers.

Driver, D.M. and Seegmiller, H.L., "Features of a Reattaching Turbulent Shear Layer in Divergent Channel Flow," AIAA Journal, Vol. 23, No. 2, Feb. 1985, pp. 163-171.