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Universe
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Relativistic Heavy-Ion Collisions: Theory and Observation
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Relativistic Heavy-Ion Collisions: Theory and Observation

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "High Energy Nuclear and Particle Physics".

Deadline for manuscript submissions: 15 July 2026 | Viewed by 3686

Editors

Prof. Dr. Jun Xu

E-Mail Website
Guest Editor
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
Interests: heavy-ion collisions; theoretical nuclear physics
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Chunjian Zhang

E-Mail Website1 Website2
Guest Editor
Institute of Modern Physics, Fudan University, 220 Handan Road, Shanghai 200433, China
Interests: relativistic heavy-ion collisions; nuclear structure

Special Issue Information

Dear Colleagues,

Relativistic heavy-ion collisions, or sometimes called little bangs, are an essential tool in exploring the properties of strong-interacting quantum chromodynamics (QCD) matter and evolution of the early Universe. Significant new physics phenomena have emerged over the past twenty years, including the study of the QCD phase diagram and the critical end point, chiral dynamics and spin polarization, jet physics, collective flows in bulk quark-gluon plasma (QGP) and small systems, correlation and particle production, etc. While experimental observations rely on the high-quality performance of beams and detectors, theoretical studies on the dynamics of relativistic heavy-ion collisions are largely based on transport and hydrodynamics approaches. More recently, it has been found that relativistic heavy-ion collisions could be an alternative way of studying the structure of colliding nuclei. Machine learning, which has significant impacts on various fields, serves as a useful method in both data collecting and theoretical investigation. This Special Issue highlights some of the above topics and may hopefully stimulate some new ideas in high-energy nuclear physics as well as in interdisciplinary research fields.

Prof. Dr. Jun Xu
Prof. Dr. Chunjian Zhang
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-anonymized peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • quark-gluon plasma
  • transport model
  • hydrodynamics model
  • phase diagram
  • spin polarization
  • jet
  • collective flow

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Published Papers (5 papers)

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Research

20 pages, 1873 KB  
Article
Geometric Bias and Centrality Dependence of Jet Quenching in High-Energy Nuclear Collisions
by Changle Sun, Yichao Dang and Shanshan Cao
Universe 2026, 12(5), 150; https://doi.org/10.3390/universe12050150 - 21 May 2026
Viewed by 221
Abstract
Jet quenching provides a valuable measure of the opacity of the quark–gluon plasma (QGP) produced in high-energy heavy-ion collisions. However, substantial suppression of charged hadron spectra is observed in highly peripheral collisions, despite the expectation of negligible jet–QGP interactions in this regime. To [...] Read more.
Jet quenching provides a valuable measure of the opacity of the quark–gluon plasma (QGP) produced in high-energy heavy-ion collisions. However, substantial suppression of charged hadron spectra is observed in highly peripheral collisions, despite the expectation of negligible jet–QGP interactions in this regime. To address this, we develop a HIJING-based initial condition model that accounts for the impact parameter dependence of both inelastic nucleon–nucleon (NN) collisions and the number of hard partonic scatterings per inelastic NN collision. This dependence introduces a geometric bias effect on the jet yield within a given centrality class of nucleus–nucleus (AA) collisions, suppressing the high-pT hadron spectrum in peripheral collisions due to dilute nucleon overlap at large AA impact parameters. By combining this improved initial condition model with a linear Boltzmann transport model for jet–QGP interactions, we obtain a satisfactory description of the centrality dependence of charged hadron suppression in Pb+Pb collisions at sNN=5.02 TeV. Full article
(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)
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12 pages, 552 KB  
Article
Acceptance Effects on the Extracted Spin Alignment of K*0 Mesons in Relativistic Heavy-Ion Collisions
by Shaowei Lan, Qiuhua Liu and Pengfei Ji
Universe 2026, 12(5), 145; https://doi.org/10.3390/universe12050145 - 16 May 2026
Viewed by 271
Abstract
The spin alignment of vector mesons, characterized by the spin-density-matrix element ρ00, is an important observable for studying spin dynamics in relativistic heavy-ion collisions. Experimental measurements have reported deviations of ρ00 from the isotropic expectation of 1/3, [...] Read more.
The spin alignment of vector mesons, characterized by the spin-density-matrix element ρ00, is an important observable for studying spin dynamics in relativistic heavy-ion collisions. Experimental measurements have reported deviations of ρ00 from the isotropic expectation of 1/3, motivating careful evaluation of possible acceptance effects. In this work, we investigate the influence of finite experimental coverage on the extracted ρ00 of K∗0 mesons using a toy model constrained by realistic kinematic distributions from the AMPT model. The reconstructed ρ00 is examined as a function of pseudorapidity (η) and transverse momentum (pT) within typical experimental acceptance ranges. We find that limited pseudorapidity coverage can lead to reconstructed ρ00 values above 1/3, even when the input distribution is isotropic. This behavior originates from the selective removal of decay daughters outside the η window, which modifies the cosθ∗ distribution. A dependence on transverse momentum is also observed, particularly at low pT where daughter particles are more sensitive to longitudinal acceptance constraints. Comparisons with STAR measurements are presented for reference, without attempting to reinterpret the experimental results. Overall, this study provides a systematic examination of acceptance-induced effects and may serve as a useful reference for future measurements of vector-meson spin alignment. Full article
(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)
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14 pages, 372 KB  
Article
Probing Short-Range Nucleon–Nucleon Correlations by Detecting Spectator Neutrons in Collider Experiments
by Aleksandr Svetlichnyi, Savva Savenkov, Polina Iusupova and Igor Pshenichnov
Universe 2026, 12(4), 118; https://doi.org/10.3390/universe12040118 - 17 Apr 2026
Viewed by 495
Abstract
We investigate whether short-range nucleon–nucleon correlations (NN-SRC) and cluster configurations in nuclei can be explored by studying spectator neutrons produced in high-energy nucleus–nucleus collisions. In particular, we propose to measure the multiplicity distributions of forward spectator neutrons in symmetric 12C–12C [...] Read more.
We investigate whether short-range nucleon–nucleon correlations (NN-SRC) and cluster configurations in nuclei can be explored by studying spectator neutrons produced in high-energy nucleus–nucleus collisions. In particular, we propose to measure the multiplicity distributions of forward spectator neutrons in symmetric 12C–12C and 40Ca–40Ca collisions at sNN=11 GeV with the Spin Physics Detector (SPD) at the NICA facility. To assess this method, we simulate the production of spectator nucleons in these reactions using the Abrasion–Ablation Monte Carlo for Colliders model with MST clustering (AAMCC-MST). Short-range nucleon–nucleon correlations inside 12C and 40Ca are implemented via a Monte Carlo rejection sampling procedure. Our results indicate that spectator production exhibits only a weak dependence on the specific features of NN-SRC. We also observe that including α-cluster configurations in 12C leads to a reduction of the average multiplicity of spectator neutrons as a function of collision centrality. Full article
(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)
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13 pages, 4275 KB  
Article
Fluctuations of Temperature in the Polyakov Loop-Extended Nambu–Jona-Lasinio Model
by He Liu, Peng Wu, Hong-Ming Liu and Peng-Cheng Chu
Universe 2026, 12(2), 37; https://doi.org/10.3390/universe12020037 - 28 Jan 2026
Viewed by 450
Abstract
In this study, we investigate temperature fluctuations in hot QCD matter using a three-flavor Polyakov loop-extended Nambu–Jona-Lasinio (PNJL) model. The high-order cumulant ratios Rn2 (n>2) exhibit non-monotonic variations across the chiral phase transition, characterized by slight fluctuations [...] Read more.
In this study, we investigate temperature fluctuations in hot QCD matter using a three-flavor Polyakov loop-extended Nambu–Jona-Lasinio (PNJL) model. The high-order cumulant ratios Rn2 (n>2) exhibit non-monotonic variations across the chiral phase transition, characterized by slight fluctuations in the chiral crossover region and significant oscillations around the critical point. In contrast, distinct peak and dip structures are observed in the cumulant ratios at low-baryon chemical potential. These structures gradually weaken and eventually vanish at high chemical potential as they compete with the sharpening of the chiral phase transition, particularly near the critical point and the first-order phase transition. Our results indicate that these non-monotonic peak and dip structures in high-order cumulant ratios are associated with the deconfinement phase transition. This study quantitatively analyzes temperature fluctuation behavior across different phase transition regions, and the findings are expected to be observed and validated in heavy-ion collision experiments through measurements of event-by-event mean transverse momentum fluctuations. Full article
(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)
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10 pages, 21975 KB  
Article
A Comparison Study of Collisions at Relativistic Energies Involving Light Nuclei
by Hai-Cheng Wang, Song-Jie Li, Jun Xu and Zhong-Zhou Ren
Universe 2025, 11(9), 296; https://doi.org/10.3390/universe11090296 - 1 Sep 2025
Viewed by 1482
Abstract
We present extensive comparisons of 16O+16O collisions at a center-of-mass energy per nucleon pair sNN=200 GeV and 208Pb+16O collisions at sNN=68.5 GeV as well as 20Ne+20Ne [...] Read more.
We present extensive comparisons of 16O+16O collisions at a center-of-mass energy per nucleon pair sNN=200 GeV and 208Pb+16O collisions at sNN=68.5 GeV as well as 20Ne+20Ne collisions at sNN=200 GeV and 208Pb+20Ne collisions at sNN=68.5 GeV based on a multiphase transport (AMPT) model. We recommend measuring the ratio of the elliptic flow to the triangular flow, which shows appreciable sensitivity to the structure of light nuclei, as also found in other studies. This is especially so if the observable is measured near the target rapidity in 208Pb+16O or 208Pb+20Ne collisions, as originally found in the present study. Our study serves as a useful reference for understanding the effect of structure on observables in collisions involving light nuclei under analysis or on the schedule. Full article
(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)
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