Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


Yang_thesis_final.pdf (27.14 MB)

ETD Abstract Container

Abstract Header

Azimuthal anisotropy in gold-gold collisions at 4.5 GeV center-of-mass energy per nucleon pair using fixed-target mode at the Relativistic Heavy-Ion Collider

Wu, Yang
http://orcid.org/0000-0003-0692-8590
http://rave.ohiolink.edu/etdc/view?acc_num=kent1562355001935965

Abstract Details

2019, PHD, Kent State University, College of Arts and Sciences / Department of Physics.
Heavy ion collisions allow us to study the formation and characteristics of Quark-Gluon Plasma (QGP), a state of matter now known to exist at very high temperature and/or very high energy density. QGP also existed around a millionth of a second after the Big Bang, and it may persist today at the center of compact stars. A brief time (a few fm/c) after the QGP phase is achieved in a heavy ion collision, the hadronization process starts to take place. This leads to the production of particles such as protons, pions, etc., which can be observed by particle detectors. What symmetry or asymmetry went into the motion of particles during the collision process? There are other interesting, yet unanswered questions, such as what is the order of the phase transition between the QGP and hadronic phases? Such questions intrigue many high-energy nuclear physics researchers. The Relativistic Heavy Ion Collider (RHIC) located at Brookhaven National Laboratory began operation in the year 2000 in a mission to produce controlled heavy ion collisions and QGP. The STAR (Solenoidal Tracker at RHIC) collaboration operates one of four experiments at RHIC, and STAR is at present the only remaining experiment taking data. The Beam Energy Scan (BES) program at RHIC explores collisions over the widest possible range of beam energies, and allows us to study the properties of the Quantum Chromodynamics (QCD) phase diagram in the regions where a first-order phase transition and a critical point may exist. Phase-I of this program (BES-I) collected data during 2011-2014 and interesting results have been observed. For example, there is a minimum in an azimuthal anisotropy parameter called directed flow (v1) for protons and other baryons at collision energies of √sNN = 10-20 GeV, and it qualitatively resembles the predicted signature of a softening of the equation of state associated with the first-order phase transition. To better identify this softest point, we need to make measurements of directed flow at lower energies than the minimum energy reached in BES-I. In this dissertation, I focus on measurements of the directed flow component v1. The momentum-space azimuthal anisotropy of the final-state particles from collisions can be described by Fourier harmonic components. The first harmonic coefficient is called directed flow, v1, and it characterizes the strength of the collective sideward motion in the plane perpendicular to the colliding beam direction. Theory indicates that directed flow is imparted during the very early stage of collisions and is not much altered during the subsequent stages. Therefore it can provide insight into the early stages of a collision, when the QGP phase exists for a short time. This research continues one of the main goals of the BES program, which is to search for a possible first-order phase transition from hadronic matter to QGP and back again. Also, my research helps to pave the way for the second phase of the BES program (BES-II), which will take data in 2019-2021 and is dedicated to obtaining much more statistics and a wider range of collision energies, in order to study the QCD phase diagram in finer detail. Although BES-I has taken useful data down to a collision energy of √sNN = 7.7 GeV, attempts to go even lower than 7.7 GeV were unsuccessful, due to the collision luminosity being too low. RHIC normally collides two accelerated beams from opposite directions, but the new fixed-target mode involves using just one gold beam and steering it to collide with a gold foil target at a suitable position inside the beam pipe. The data in this dissertation correspond to a collision energy of √sNN = 4.5 GeV taken during a short test. About 1.3 million Au+Au collision events at 4.5 GeV were recorded within half an hour. The number of recorded events was limited by the maximum throughput of the data acquisition system, and luminosity was no longer a problem. I present here v1 and its slope with respect to rapidity (dv1/dy) near midrapidity, for identified particle types, namely protons, π+ and π-, and compared with whatever published results are available at about the same beam energy. Previous comparable work mainly came from experiments at the Brookhaven AGS accelerator two decades ago, before RHIC came into operation. Extrapolation to test consistency with STAR BES-I measurements is also of interest. Proton v1 results (as well as other fixed-target measurements) agree well with AGS results under very similar experimental and analysis conditions, which confirms that the STAR detector performs well in the new fixed-target mode. The STAR detector can acquire much more statistics and has cleaner particle identification ability than the AGS experiments in the 1990s. We also have obtained the first measurements of π+ and π- v1 from this energy range, which the old AGS experiments did not report. The observed directed flow slope of charged pions follows the negative trend of directed flow slope for mesons as observed in STAR BES-I. The slope for π+ is lower than for π-, which supports the idea that quarks transported for the initial-state nuclei have a bigger effect on π-. Many fixed-target runs have been scheduled as part of the STAR BES-II program with several beam energy points, and at least 100 times the present statistics at each energy. Furthermore, new detector subsystem upgrades will further expand the coverage of STAR in transverse momentum and rapidity, and will also improve particle identification. Using the measurements reported in this dissertation, followed in a few years by the improved measurements of BES-II, it should be possible to better understand and map-out the main features of the QCD phase diagram.
Declan Keane (Advisor)
Spyridon Margetis (Committee Member)
Michael Strickland (Committee Member)
Diane Stroup (Committee Member)
Arvind Bansal (Committee Chair)
138 p.
Nuclear Physics; Physics
heavy ion collision; nuclear physics; RHIC; STAR; BES; fixed target; anisotropic flow; directed flow; hadron; baryon; meson; quark gluon plasma; QGP; nuclear matter; phase diagram; critical point; first order phase transition; rapidity;

Recommended Citations

Citations

  • Wu, Y. (2019). Azimuthal anisotropy in gold-gold collisions at 4.5 GeV center-of-mass energy per nucleon pair using fixed-target mode at the Relativistic Heavy-Ion Collider [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1562355001935965

    APA Style (7th edition)

  • Wu, Yang. Azimuthal anisotropy in gold-gold collisions at 4.5 GeV center-of-mass energy per nucleon pair using fixed-target mode at the Relativistic Heavy-Ion Collider. 2019. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1562355001935965.

    MLA Style (8th edition)

  • Wu, Yang. "Azimuthal anisotropy in gold-gold collisions at 4.5 GeV center-of-mass energy per nucleon pair using fixed-target mode at the Relativistic Heavy-Ion Collider." Doctoral dissertation, Kent State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=kent1562355001935965

    Chicago Manual of Style (17th edition)

kent1562355001935965
187
© 2019, all rights reserved.
This open access ETD is published by Kent State University and OhioLINK.