Observation of Collective Behavior in High-Multiplicity Proton-Gold Collisions with PHENIX at RHIC
Quark Gluon Plasma (QGP) is an extremely hot and dense state of nuclear matter in which the quarks and gluons are not confined into hadrons. QGP existed in the first microsecond of the Universe and presently, it is produced in high energy nucleus-nucleus (A-A) collisions at particle accelerators. The most remarkable property of the QGP is its near-perfect fluid behavior. In this thesis we explore the limits and origin of this collective behavior by studying smaller collision systems, which originally were not expected to create QGP. Surprisingly, we find that the collective behavior persists in the smaller systems. We used data from the PHENIX experiment at the Relativistic Heavy Ion Collider and conducted a series of experiments with controlled initial geometry: proton-gold, deuteron-gold, and helium-gold collisions. We measured the second and third Fourier harmonics of the azimuthal distributions of inclusive charged hadrons. The results demonstrate that the initial geometry plays a vital role in the development of final-state momentum anisotropies. The second harmonics was also measured for particles with different masses (pions and protons) and the results indicate that the particles move in a common velocity field, and the flow develops at a partonic level. The results are compared to various theoretical models. Hydrodynamics with small specific viscosity provides the best simultaneous description of the measurements in the three collision systems.