Quadrupole anisotropy in dihadron azimuthal correlations in central d+Au collisions at sqrt(s_NN)=200 GeV
The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in d+Au collisions at sqrt(s_NN)=200 GeV. These measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central p+Pb collisions at sqrt(s_NN)=5.02 TeV, which have indicated strong anisotropic long-range correlations in angular distributions of hadron pairs. The origin of these anisotropies is currently unknown. Various competing explanations include parton saturation and hydrodynamic flow. We observe qualitatively similar anisotropies at RHIC to those seen at the LHC, and when both are divided by an estimate of the initial-state eccentricity, the anisotropies follow a common multiplicity scaling. This scaling is also found to extend to heavy ion data at RHIC and the LHC, where the anisotropies are widely thought to be due to hydrodynamic flow. The results presented here, at much lower collision energy and with a deuteron projectile (instead of a proton), provide important new information for understanding the origin of these new long-range correlations.