Collective behavior in small systems

Abstract

The presence of correlations between particles significantly separated in pseudorapidity in proton-proton and proton-nucleus collisions has raised questions about whether collective effects are observed in small collision systems as well as in heavy-ion collisions. The quantification of these long-range correlations by vn coefficients is of particular interest. A selection of the latest vn measurements is presented, including results from the recent d+Au beam energy scan at RHIC where a significant non-zero v2 is measured down to low center-of-mass energies (sNN=39GeV). Results from a collision system scan – comprising p+Au, d+Au, and 3He+Au collisions – are also shown to address the role of the initial nuclear geometry in the final state anisotropy. Finally, the challenge of measuring multi-particle cumulants, particularly c2{4}, in p+p collisions is discussed, and new methods for reducing the effects of non-flow are shown to produce a more robust measurement of v2{4} in p+p collisions.