Open Access Publications from the University of California

## Probing Quark-Gluon Plasma and Chiral Effects in Heavy-Ion Collisions: Measurements of Strangeness ($\Omega$ and $\phi$) Production and Identified Particle Correlation in Au+Au Collisions at STAR/RHIC

• Author(s): Wen, Liwen
• et al.
Abstract

Ultra-relativistic heavy-ion collision produces an extremely hot and dense medium of de-confined quarks and gluons, which is called Quark-Gluon Plasma (QGP). The STAR detector at Relativistic Heavy Ion Collider (RHIC) provides powerful experimental capabilities to probe the properties of this new form of matter, as well as novel quantum effects induced by the restoration of fundamental symmetry in qauntum-chromdynamics (QCD). Towards these goals, two research projects have been carried out at STAR/RHIC and will be presented in this thesis: 1) Measurement of mid-rapidity ($|y|<0.5$) multi-strangenss particle ($\Omega$ and $\phi$) production in Au+Au collisions at $\sqrt{s_{NN}}=14.5$ GeV; 2) A systematic search for chiral effects using identified particle correlation.

Production mechanism for strange hadrons could be dramatically different in the presence of QGP compared to regular $pp$ collisions. Thus strangeness signal is used extensively in Beam Energy Scan I (BES-I) program at RHIC to map out the phase diagram of QCD matter. As a part of BES-I, gold nuclei are collided at $\sqrt{s_{NN}} = 14.5$ GeV and the productions of mid-rapidity $\Omega(sss)$ and $\phi(\bar{s}s)$ are measured for the collisions. The ratio of anti-omega ($\bar{\Omega}$) over omega ($\Omega^-$) is calculated and used to extract thermodynamics parameters ($\mu_B/T$ and $\mu_S/T$) of collision system via statistical model. Additionally, the transverse momentum ($p_T$) dependence of nuclear modification factor ($R_{cp}$) is measured for $\phi$ meson and the result shows similar feature to energies lower than 19.6 GeV. As a test of coalescence formation mechanism for strange hadrons, $\textrm{N}(\Omega^-+\bar{\Omega}^+)/2(\textrm{N}(\phi))$ as a function of $p_T$ is studied and the data from central collision is found to deviate from model calculation and higher energy ($\sqrt{s_{NN}}>19.6$ GeV) results in $p_T$ range from $2.0-3.0$ GeV/c, which may imply a transition of created medium whose underlying dominant degrees of freedom change from quarks/gluons to hadrons as collision energy goes below 19.6 GeV.

%whose underlying dominant degrees of freedom change from quarks/gluons to hadrons as...

With excellent particle identification capability of STAR, a systematic search for the Chiral Magnetic Effect (CME) via measurements of $\gamma_{112}$ correlation and $\kappa_K$ parameter for identified particle pairs ($\pi\pi$, $pK$, $\pi K$, $pp$, $p\pi$) in Au+Au collisions has been conducted. The $\kappa_K$ results are compared to expectations from the \textit{A Multi-Phase Transport Model} (AMPT) simulations. Except $\pi\pi$ and $pp$ correlations, the CME signals from other particle pairs are consistent with background model. $\kappa_K$ from $\pi\pi$ shows higher values than background expectation, while the result for $pp$ is even lower than the background, which requires further investigation.

%A study of $\gamma_{112}$ and $\delta$ correlations for $\Lambda p$ in Au+Au 27 GeV shows that in mid-central and mid-peripheral collisions, baryon numbers are separated across reaction plane, which is consistent with the Chiral Vortical Effect (CVE) expectation.

To search for Chiral Vortical Effect (CVE), a measurement of $\gamma_{112}$ and $\delta$ correlations for $\Lambda p$ pairs in Au+Au collisions at $\sqrt{s_{NN}}=27$ GeV was carried out and the results show that the CVE induced baryon number separation may exist in mid-central and mid-peripheral collisions with little contamination from flowing resonance decay background.

Future development of searches for the chirality effect in heavy ion collisions will also be discussed.