Instanton Counting and Dielectric Branes

We consider the Hanany-Witten type brane configuration in a background of RR 4-form field strength and examine the behavior of Euclidean D0-branes propagating between two NS5-branes. We evaluate the partition function of the D0-branes and show that it coincides with the Nekrasov partition function of instantons for four-dimensional N=2 supersymmetric Yang-Mills theory. In this analysis, the Myers effect plays a crucial role. We apply the same method to the brane configuration realizing four-dimensional N=2 theory with hypermultiplets in the fundamental representation and reproduce the corresponding Nekrasov partition function.