Measurements of mass-dependent azimuthal anisotropy in central p plus Au, d + Au, and He-3 + Au collisions at root s(NN)=200 GeV

Abstract

We present measurements of the transverse-momentum dependence of elliptic flow upsilon(2) for identified pions and (anti)protons at midrapidity (vertical bar eta vertical bar < 0.35), in 0%-5% central p + Au and He-3 + Au collisions at ,root s(NN) = 200 GeV. When taken together with previously published measurements in d + Au collisions at root 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 upsilon(2) (p(T)) in d + Au and He-3 + Au collisions, just as in large nucleus-nucleus (A + A) collisions, and a smaller splitting in p + Au collisions. Both hydrodynamic and transport model calculations successfully describe the data at low p(T) (<1.5 GeV/c), but fail to describe various features at higher p(T). In all systems, the upsilon(2) values follow an approximate quark-number scaling as a function of the hadron transverse kinetic energy per constituent quark (KET/n(q)), which was also seen previously in A + A collisions.