We present measurements of the transverse-momentum dependence of elliptic flow $v_2$ for identified pions and (anti)protons at midrapidity ($\left|\eta \right|<0.35$), in 0%–5% central $p+\mathrm{Au}$ and ${}^3\mathrm{He}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV. When taken together with previously published measurements in $d+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV, the results cover a broad range of small-collision-system multiplicities and intrinsic initial geometries. We observe a clear mass-dependent splitting of $v_2\left(p_T\right)$ in $d+\mathrm{Au}$ and ${}^3\mathrm{He}+\mathrm{Au}$ collisions, just as in large nucleus-nucleus ($A+A$) collisions, and a smaller splitting in $p+\mathrm{Au}$ collisions. Both hydrodynamic and transport model calculations successfully describe the data at low $p_T$ ($<1.5\mathrm{GeV}/c$), but fail to describe various features at higher $p_T$. In all systems, the $v_2$ values follow an approximate quark-number scaling as a function of the hadron transverse kinetic energy per constituent quark $(K{E}_T/n_q$), which was also seen previously in $A+A$ collisions.

• Received 27 October 2017

DOI:https://doi.org/10.1103/PhysRevC.97.064904