Systematic study of nuclear effects in $p$ $+$Al, $p$ $+$Au, $d$ $+$Au, and $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV using $\pi^0$ production

The collaboration
549 authors from 81 institutions, 21 pages, 13 figures, and 3 tables. Data from 2008, 2014, and 2015. Physical Review C. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at http://www.phenix.bnl.gov/papers.html, 2021.

Abstract (data abstract)
PHENIX presents a systematic study of $\pi^0$ production from p+p, p+Al, p+Au, d+Au, and $^{3}$He+Au collisions at $\sqrt{s}$=200GeV. Measurements were done 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 the same 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 $^3$He interact mostly independently with the Au nucleus and that the counter intuitive 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 > $^{3}$He+Au > p+Al. For collisions with Au ions, current calculations based on initial state cold nuclear matter effects obtain the reversed order, suggesting the presence of other contributions to nuclear modifications, in particular at lower $p_T$.