Figure. RAB as a function of pT for six centrality classes at sqrt(snn) = 130 GeV.

The first Au+Au collisions at RHIC were recorded in 2000 at center of mass energy sqrt(s_nn) = 130 GeV. STAR measured the spectra of charged hadrons as a function of transverse momentum and centrality. These data were compared to a composite reference spectrum of p+p collisions for the same beam energy in order to evaluate nuclear effects on hadron production at high pT. The ratio R_AB = (d²N^AB/dpT deta) / (T_AB d²sigma^pp/dpT deta) where d²N^AB/dpT deta is the differential yield per event in the nuclear collision A+B, T_AB=< N_binary> sigma^pp_inel describes the nuclear geometry, and d²sigma^pp/dpT deta is for p+p inelastic collisions. < N_binary> is the mean number of binary NN interactions for the given centrality class of A+B collisions. In the absence of nuclear effects such as shadowing, the Cronin effect, or gluon saturation, hard process rates are expected to scale with < N_binary> and R_AB=1.

However, STAR observed that at the highest pT, hadron suppression of approximately a factor 4-5 is observed for the most central collisions: inclusive hadron production is strongly suppressed at high pT in central Au+Au collisions. For the most peripheral collisions R_AB is consistent with unity, while intermediate centralities interpolate smoothly between the extremes.

These data are consistent with a scenario in which jets produced early in the collision interact strongly with the medium, reducing the yields of "leading" charged hadrons compared to the yields from p+p collisions, where no medium is present.