A fermionic state sum model
Recently Jorma Louko, Steven Kerr and I produced a state sum model with fermionic variables. It is only in 1d space-time, so something of a toy model, but it illustrates some important principles. Firstly, the model is triangulation-independent, which means it has the “diffeomorphism symmetry” of general relativity. Secondly, the fermion integration induces an action for the bosonic gauge field (as expected) but without a cosmological constant (very unexpected). I’m still somewhat puzzled over exactly how that works, but it would be very nice if there are models like that in higher dimensions.