Anomalous centrality variation of minijet angular correlations in Au–Au collisions at 62 and 200 GeV from STAR

Abstract

We have measured 2D autocorrelations for all charged hadrons in STAR with pt> 0.15 GeV/c and |η|< 1 from Au+Au collisions at 62 and 200 GeV. The correlation structure is dominated by a peak centered at zero relative opening angles on η and ϕ which we hypothesize is caused by minimum-bias jets (minijets). We observe a large excess of minijet correlations in more-central Au–Au collisions relative to binary-collision scaling (more correlated pairs than expected from surface emission or even volume emission). We also observe a sudden increase of the minijet peak amplitude and η width relative to binary-collision scaling of scattered partons which occurs at an energy-dependent centrality point. There is a possible scaling of the transition point with transverse particle density. The large minijet correlations bring into question the degree of thermalization in RHIC collisions.