The PHENIX Collaboration presents a systematic study of inclusive ${\pi }^0$ production from $p+p, p+\mathrm{Al}, p+\mathrm{Au}, d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. 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 $8\mathrm{GeV}/c$, 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-$p_T\text{−}{\pi }^0$ production, the nucleons in the $d$ and ${}^3\mathrm{He}$ 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 $p+\mathrm{Au}>d+\mathrm{Au}>{}^3\mathrm{He}+\mathrm{Au}>p+\mathrm{Al}$. 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 $p_T$.

• Received 11 November 2021
• Accepted 3 May 2022

DOI:https://doi.org/10.1103/PhysRevC.105.064902