Initial nuclear effects in the improved HIJING code and the production of charged pions, kaons and (anti)protons at STAR BES

doi:10.1016/j.nuclphysa.2020.122033

Nuclear Physics A

Volume 1003, November 2020, 122033

https://doi.org/10.1016/j.nuclphysa.2020.122033 Get rights and content

Abstract

The improved Heavy Ion Jet Interaction Generator (HIJING) code is introduced to explore possible initial nuclear effects on the bulk observables of charged pions, kaons and (anti)protons observed in Au+Au collisions at $\sqrt{s_{NN}} = 7.7$ , 11.5, 19.6, 27 and 39 GeV. The standard HIJING code is extended by (i) different baryon production mechanisms, and (ii) Regge collective cascade recipe. We find that the centrality dependence of the identified particle multiplicity $d N / d y$ at mid-rapidity is quite sensitive to Regge cascading. The overall contribution of baryon production mechanism is shown to enhance the centrality dependence of the mixed particle ratios, $K^{\pm} / π^{\pm}$ and $p^{\pm} / π^{\pm}$ at $< N_{p a r t} > < 200$ (which corresponds to $20 % - 100 %$ centrality), especially at higher collision energy. It turns out that the bulk data are shown to be more consistent with HIJING (plus a suitable hadron rescattering code) employing both an advanced popcorn baryon production mechanism and Regge initial scatterings.

Introduction

The phase transition from partonic degrees of freedom (quarks and gluons) in relativistic heavy ion collisions to hadronic degrees of freedom is a central focus of recent experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and at the Large Hadron Collider (LHC) at CERN. The scientific goal of these experiments is to search for the critical point and phase boundary of a new state of matter, a Quark Gluon plasma (QGP). To this end, RHIC has undertaken the first phase of the Beam Energy Scan (BES) program [1]. The idea is to find how the fraction of matter in the QGP phase changes when lowering the collision energy and what energy the QGP ceases to exist.

Recent measurements by STAR collaboration [1] within the RHIC program provide high precision data of identified particles $π^{\pm}$ , $K^{\pm}$ and (anti)proton at mid-rapidity ( $| y | < 0.1$ ) in Au+Au collisions at the BES of $\sqrt{s_{NN}} = 7.7$ , 11.5, 19.6, 27 and 39 GeV. The bulk properties of produced particles such as transverse momentum ( $p_{T}$ ) spectra, multiplicity density ( $d N / d y$ ), particle ratios may provide information about the particle production mechanisms from the lowest possible energy to the highest possible BES, thereby reveal the evolution in the behavior of the system formed in heavy ion collisions as the collision energy increases.

To explore these evolutionary processes, many microscopic hadronic transport models are used to simulate heavy-ion collisions such as Relativistic Quantum Molecular Dynamics (RQMD) [2], Ultra-relativistic Quantum Molecular Dynamics (UrQMD) [3], a Relativistic Cascade (ARC) [4], a Relativistic Transport (ART) [5] and a MultiPhase Transport (AMPT) [6] models.

Recently, the AMPT model has been used to study the STAR data of $p_{T}$ spectra, $d N / d y$ and particle ratios of charged pions, kaons and (anti)proton, produced in Au+Au collisions at $\sqrt{s_{NN}} = 7.7$ , 27 and 200 GeV [7]. For this study, three different sets of parameters were applied for both the default and string melting version of the AMPT model [8]. It was shown in Ref. [7] that both versions of AMPT model with different initial conditions were not able to describe the bulk properties of the measured data.

In order to overcome these limitations we introduce the improved HIJING approach. In contrast to the standard HIJING code [9], the improved version includes: (i) sampling of Fermi motion of nuclear ground state, (ii) a Regge cascade for initial state interactions [10], and (iii) two different baryon production mechanisms; namely, the simple (SP) and advanced (AP) popcorn mechanisms [11], [12]. This improved version will be denoted as HIJING/SP(AP)/Rcas. A meaningful comparison of STAR data [1] requires the introduction of final state interactions. Similar to AMPT code [6], we adopt the ART model [5] of hadron rescattering.

Using the improved HIJING code [10], [13], we explore the main effects that contribute to the description of the bulk observables of the recent STAR data [1], of $π^{\pm}$ , $k^{\pm}$ and $p^{\pm}$ in Au+Au collisions at $\sqrt{s_{NN}} = 7.7$ , 11.5, 19.6, 27 and 39 GeV. In particular, two main effects are discussed here: (i) the popcorn baryon production mechanisms, and (ii) the initial state interactions.

The manuscript is organized as follows: Section 2 defines the basic ingredients of the HIJING/(SP)AP/Rcas+ART model. In Sec. 3 we confront the model to measurements from STAR [1] experiments. Finally, in Sec. 4 we present our conclusions.

Section snippets

Model description: the HIJING/(SP)AP/Rcas+ART approach

Here we outline the basic ideas of the HIJING/AP/Rcas+ART model. The details of HIJING/AP/Rcas and ART are described in Refs. [5], [10], [13], [14], [15]. Here we concentrate on those points which are important for understanding the results discussed in sec. 3.

The HIJING/(SP)AP/Rcas+ART is a microscopic transport approach of relativistic nuclear collisions which contain (i) Eikonal formalism to determine the number of interacting nucleons and hard scatterings [9], (ii) Regge collective

Results and discussion

In this section we present the results of the HIJING/SP(AP)+ART model, coupled with different initial nuclear effects, with the measured basic bulk observables such as the transverse momentum ( $p_{T}$ ) spectra, centrality dependence of $d N / d y$ , and particle ratios of pions, kaons and (anti)protons at $\sqrt{s_{NN}} = 7.7$ , 11.5, 19.6, 27 and 39 GeV, from STAR BES experiment [1]. The analysis range is restricted to the measured mid-rapidity range of ( $| y | < 0.1$ ) [1].

To study initial nuclear effects on the final hadron

Summary and conclusions

We have studied the identified particle yields of $π^{\pm}$ , $k^{\pm}$ and (anti)proton in Au+Au collisions at $\sqrt{s_{NN}} = 7.7$ , 11.5, 19.6, 27 and 39 GeV by a hybrid model, using HIJING/SP(AP)/Rcas for the initial nuclear stage and a relativistic transport (ART) model of final hadronic re-scatterings. To assess initial nuclear effects of baryon production in Au+Au collisions, the HIJING model is supplemented with the simple (SP) and advanced popcorn (AP) mechanisms of Lund string fragmentation models. The Regge

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author would like to thank Prof. Khaled Abdel-Waged for guidance and support.

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Initial nuclear effects in the improved HIJING code and the production of charged pions, kaons and (anti)protons at STAR BES

Abstract

Introduction

Section snippets

Model description: the HIJING/(SP)AP/Rcas+ART approach

Results and discussion

Summary and conclusions

Declaration of Competing Interest

Acknowledgements

Prog. Part. Nucl. Phys.

Nucl. Phys. A

Nucl. Phys. B

Phys. Rev.

Phys. Rev. C

Phys. Rev. C

Annu. Rev. Nucl. Part. Sci.

Phys. Rev. C

Constraining particle production mechanism in Au+Au collisions at RHIC energies using a multi phase transport model

Phys. Rev. C

Phys. Rev. D

J. Phys. G, Nucl. Part. Phys.

Phys. Scr.