Systematic study of nuclear effects in p plus Al, p plus Au, d + Au, and He-3 + Au collisions at root S-NN=200 GeV using pi(0) production

The PHENIX Collaboration presents a systematic study of inclusive pi(0) production from p + p, p + Al, p + Au, d + Au, and He-3 + Au collisions at root S-NN = 200 GeV. Measurements were performed with different centrality selections as well as the total inelastic, 0-100%, selection for all collision systems. For 0-100% collisions, the nuclear-modification factors, R-xA, are consistent with unity for P-T above 8 GeV/c, but exhibit an enhancement in peripheral collisions and a suppression in central collisions. The enhancement and suppression characteristics are similar for all systems for the same centrality class. It is shown that for high-p(T)-pi(0) production, the nucleons in the d and He-3 interact mostly independently with the Au nucleus and that the counterintuitive centrality dependence is likely due to a physical correlation between multiplicity and the presence of a hard scattering process. These observations disfavor models where parton energy loss has a significant contribution to nuclear modifications in small systems. Nuclear modifications at lower p(T) resemble the Cronin effect-an increase followed by a peak in central or inelastic collisions and a plateau in peripheral collisions. The peak height has a characteristic ordering by system size as p + Au > d + Au > He-3 + Au > p + Al. For collisions with Au ions, current calculations based on initial-state cold nuclear matter effects result in the opposite order, suggesting the presence of other contributions to nuclear modifications, in particular at lower p(T).