Geometry Driven Particle Correlations in Small Collision Systems

Abstract

Relativistic heavy-ion collisions with a center-of-mass energy per nucleon pair of 200 GeV were performed at Brookhaven National Laboratory between small ions (proton, deuteron, Helium-3) and gold ions with the purpose of exploring the system size limit in which quark-gluon plasma can be produced. These systems were chosen because they have intrinsically circular, elliptic, and triangular initial geometries, respectively, which in the case of quark-gluon plasma formation, would translate to the azimuthal anisotropy of the produced particles. The PHENIX detector measured the azimuthal particle distributions from these three collision systems. The second and third harmonic flow coefficients, v2 and v3 were extracted as a function of transverse momentum. The relative ordering of the magnitude of these coefficients between systems was found to be consistent with a geometric origin. Calculations from hydrodynamical models were found to give the best simultaneous description of the measurements, as determined by a statistical analysis. The relative contributions of geometry and particle density were explored, comparing large and small collision systems. All systems were found to follow the same overall trend. Together these results provide strong evidence for quark-gluon plasma production in small collision systems.