The most physical and understandable definition of Nekrasov's partition function to me uses five-dimensional gauge theories. Namely, any 4d N=2 susy gauge theory has a 5d version with the same matter content, so that compactifying it on a small S1 brings it back to the original 4d theory.
Then we put the theory on the so-called Omega background: it is R4×[0,β], but (→x,0) and (→x′,β) are identified by a rotation
→x′=(cosβϵ1sinβϵ100−sinβϵ1cosβϵ10000cosβϵ2sinβϵ200−sinβϵ2cosβϵ2)→x.
Then we take the limit β→0, keeping ϵ1,2 fixed. (Strictly speaking we also need to add a background SU(2)R symmetry gauge field, so that some of the susy is preserved.)
Most of what Nekrasov did using his cohomological framework can be seen directly in this higher-dimensional setup. See e.g. Sec. 3.2 of my review article in preparation, available here.
This post imported from StackExchange Physics at 2014-08-23 04:59 (UCT), posted by SE-user Yuji