Beam Energy, Collision Centrality And Charge Asymmetry Dependence Of The Pion Event Anisotropy In Au+Au Collisions At RHIC

In high energy heavy ion collisions, a new state of hot and dense matter has been created. A series of evidences indicate that such a new state of matter is a strongly coupled Quark-Gluon Plasma (QGP). The Relativistic Heavy Ion Collider (RHIC), built at Brookhaven National Laboratory, is a dedicated machine to study properties of QGP as well as the QCD phase diagram. Theoretical studies suggest that the chiral symmetry is restored at the early stage of heavy ion collisions, and with an extremely strong magnetic field generated in middle central Au+Au collisions, the chiral magnetic effect and chiral separation effect can couple with each other and create density waves of electric and chiral charge, namely, the chiral magnetic wave. The chiral magnetic wave in heavy ion collisions induces an electric quadrupole momentum and leads to a charge asymmetry dependence of azimuthal anisotropy v2of charged particles. It is argued that such effect can be observed via charged pions.In this thesis, the integrated v2of π+and π-with0.15<pr<0.5GeV and|η|1in all centralities of Au+Au collisions at (?)=200,62.4,39,27and19.6GeV are presented. The measured u2is studied as a function of charge asymmetry Ach.It is observed that v2of π+(π-) linearly decrease (increase) with the increase of charge asymmetry. The v2difference between π+and π-, Δv2=v2π--u2π+, is proportional to the charge asymmetry Ach. The slope parameter of Δv2(Ach), which is believed to be related to the electric quadrupole momentum induced by the chiral magnetic wave, is extracted for all the centralities and all energies under study. The slope parameter is shown to have a rise and fall feature from central to peripheral collisions, with a weak energy dependence from (?)=200GeV to27GeV. These observations are qualitatively consistent with theoretical calculations based on chiral magnetic wave.On the experiment-development side, the implementation and upgrade of the STAR High-Level Trigger (HLT) is discussed. The increasing RHIC luminosity and STAR data-taking capability pose a great challenge to data production and storage. To assure the fast physics output for a selective physics topics based on rare events, as well as to monitor RHIC/STAR real-time performing during RHIC low energy runs, a High Level Trigger (HLT) is implemented for STAR. The HLT is a software system built on high performance PC cluster to performer real time tracking and event assembling. It has been used to select events containing high pr, di-electron, light nuclei signals. Based on the HLT selected data sample in year2010, STAR has observed the anti-helium-4nuclei, and studied the elliptic flow of J/ψ.Since2010, the author has participated in the development of the STAR HLT, especially the upgrade towards high performance parallel computing. The author’s con-tributions to HLT include, first, the estimation of HLT tracking efficiency, which sets a baseline for the understanding the HLT trigger performance. Secondly, to cope with the intensive computing requirement of online secondary vertex reconstruction, the au-thor has studied the application of general purpose GPU acceleration and obtained a prototype of GPU based secondary vertex finder which runs60times faster than the con-ventional implementation. In the recent year, the author studied the cellular automaton (CA) based tracker and is adopting it to the STAR HLT.