We present first measurements of the evolution of the differential transverse momentum correlation function, { \it C}, with collision centrality in Au+Au interactions at $\sqrt{ s_{ NN}} = 200$ GeV. { \it C} exhibits a strong dependence on collision centrality that is qualitatively similar to that of number correlations previously reported. We use the observed longitudinal broadening of the near-side peak of { \it C} with increasing centrality to estimate the ratio of the shear viscosity to entropy density, $\eta/s$, of the matter formed in central Au+Au interactions. We obtain an upper limit estimate of $\eta/s$ that suggests that the produced medium has a small viscosity per unit entropy.
Categories: nucl-ex