Study Quark Gluon Plasma by particle correlations in heavy ion collisions

Li Yi, Purdue University

Abstract

A strongly interacting Quark Gluon Plasma (QGP) is created in relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). Two-particle (dihadron) angular correlations have been used to study the properties of the QGP. The two major contributions to the dihadron correlations are jet correlations and correlations due to anisotropic collective flow. While jet correlations probe jet-medium interactions, anisotropic flow provides information about the thermodynamic properties of the QGP. The third harmonic flow ( v₃) is sensitive to the shear viscosity to entropy density ratio &eegr;/s of the QGP medium and the initial energy density fluctuations. This thesis provides the first v₃ measurement in Au+Au collisions at √s_nn=200 GeV from the STAR experiment. The Δ&eegr;-gap, multiplicity and p_T dependence of the v₃ are reported along with comparisons with hydrodynamic calculations.^ In heavy ion collisions, the two-particle cumulant flow measurement is contaminated by nonflow correlations, such as jet. An accurate flow measurement is crucial for the determination of the &eegr;/s of the QGP. This thesis provides a data-driven isolation of Δ&eegr;-dependent and Δ&eegr;-independent components in the two-particle cumulant measurement. The Δ&eegr;-dependent term is associated with nonflow, while the Δ&eegr;-independent term is associated with flow and flow fluctuations. It is found that in 20-30% centrality Au+Au collisions, the relative flow fluctuation is 34%, and the nonflow relative to flow square is 5% with Δ&eegr;-gap > 0.7 for $0.15 < p^T < 2 GeV/c at :&eegr;: < 1.^ The recent observations of long-range Δ&eegr; correlations (the ridge) in p + p and p+pb collisions at the Large Hadron Collider (LHC) raised the question of collective flow in these small systems, which had been considered control experiments for heavy-ion collision studies. This thesis provides a careful analysis of short- and long-range two-particle correlations in d+Au collisions at 200 GeV from the STAR experiment. The event-activity selection affects the jetlike correlated yield in d+Au collisions. Therefore, a simple difference between high- and low-activity collisions cannot be readily interpreted as nonjet, anisotropic flow correlations. This thesis reports the near-side ridge yield as a function of multiplicity and Δ&eegr;, and its ratio to the away-side jet dominated correlated yield, as well as the ratio to the underlying event multiplicity. This thesis also analyzes the dihadron azimuthal correlations in terms of Fourier coefficients V_n. The V₂ is found to be independent of event multiplicity and similar between Au- and d-going forward/backward rapidities. These dihadron correlation measurements should provide insights into the theoretical understanding of the near-side ridge in d+Au collisions and possible collective flow and QGP formation in these small collision systems.^

Degree

Ph.D.

Advisors

Fuqiang Wang, Purdue University.

Subject Area

Physics, General|Chemistry, Nuclear

COinS