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Relativistic Heavy Ion Collision
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Relativistic Heavy Ion Collision

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: closed (30 December 2023) | Viewed by 11227

Special Issue Editor

Prof. Dr. Carlos Pajares

E-Mail Website
Guest Editor
Departamento de Fisica de Particulas, Universidale de Santiago de Compostela and Instituto Galego de Fisica de Altas Enerxias (IGFAE), 15782 Santiago, Spain
Interests: quark gluon plasma; quantum cromodynamics; multiparticle producton; percolation

Special Issue Information

Dear Colleagues,

The RHIC and LHC experimental data on heavy ion collisions have obtained evidence of the existence of a fluid of strong interacting quarks and gluons. The collective behavior of this formed matter have been intensively studied in the last years. Most of the properties seen in AA collisions, as elliptic flow, rapidity long range correlations, ridge structure and strangeness enhancement have been as well observed in small colliding objects as pp, pPb, dAu, HeAu rising questions related to how the collective behavior is formed. The required early thermalization is due to final state interactions or there are another possible mechanisms? What is the order of the different harmonics for different size colliding objects and therefore what kind of interaction produces the anisotropy? How are arranged the color fields stretched between the partons of the colliding particles when the number of collisions becomes very large? Are there repulsion between these color conductors? Why is there a suppression of the near side baryon-baryon correlations? At what temperature is expected that the strong quark gluon matter starts to be free? On the other hand, some questions related to the phase transition diagram are necessary to study further, as the experimental dependence of the energy and the entropy density on the temperature, the determination of the tri-critical point, the phase diagram with a magnetic field or the dependence of the transport coefficients, like the shear and bulk viscosity and the electrical conductivity on the temperature. The study of these and other related topics are the goal of this special issue.

Prof. Dr. Carlos Pajares
Guest Editor

Manuscript Submission Information

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Published Papers (10 papers)

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Research

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15 pages, 407 KiB  
Article
Particle Production in pA Collisions at Mid-Rapidity in the Color Glass Condensate
by Pedro Agostini, Tolga Altinoluk and Néstor Armesto
Universe 2024, 10(2), 58; https://0-doi-org.brum.beds.ac.uk/10.3390/universe10020058 - 26 Jan 2024
Viewed by 1017
Abstract
Particle correlations in small collisions systems, like proton–nucleus, lie at the core of the discussion about whether quark–gluon plasma is produced in small systems. Both initial and final state explanations have been essayed to describe such correlations. In this work, we focus on [...] Read more.
Particle correlations in small collisions systems, like proton–nucleus, lie at the core of the discussion about whether quark–gluon plasma is produced in small systems. Both initial and final state explanations have been essayed to describe such correlations. In this work, we focus on the initial state explanations provided by the quantum effects in the initial wave function of the incoming hadrons, in the framework of the Color Glass Condensate effective theory. We describe the formalism indicating the different inputs required for phenomenological applications. We compare the results from two different models, finding that the results for azimuthal harmonics agree qualitatively, but show quantitative differences, particularly at transverse momenta above the saturation scale. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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17 pages, 493 KiB  
Article
Multiplicity Distributions and Modified Combinants in the Multipomeron Model of pp Interaction at High Energies
by Vladimir Vechernin, Evgeny Andronov, Vladimir Kovalenko and Andrei Puchkov
Universe 2024, 10(2), 56; https://0-doi-org.brum.beds.ac.uk/10.3390/universe10020056 - 26 Jan 2024
Viewed by 1095
Abstract
The multiplicity distributions of charged particles and their combinants for pp collisions at LHC energies are studied within the Multipomeron Exchange Model (MEM) that takes into account the phenomenon of string fusion. It is shown that the use of Gaussian-type distributions for multiplicity [...] Read more.
The multiplicity distributions of charged particles and their combinants for pp collisions at LHC energies are studied within the Multipomeron Exchange Model (MEM) that takes into account the phenomenon of string fusion. It is shown that the use of Gaussian-type distributions for multiplicity distributions at a fixed number of pomerons allows, within the MEM framework, the reproduction of the resulting multiplicity distributions and the oscillatory behavior of combinants, found in the ALICE and CMS pp collision data at LHC energies. It is important that in the proposed approach, the parameters of these Gaussian-type distributions are not considered free, but are calculated from the two-particle correlation function of a single string. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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17 pages, 545 KiB  
Article
Equation of State of Quark–Gluon Matter in the Clustering-of-Color-Sources Approach
by Aditya Nath Mishra, Guy Paić, Carlos Vales Pajares, Rolf P. Scharenberg and B. K. Srivastava
Universe 2024, 10(2), 55; https://0-doi-org.brum.beds.ac.uk/10.3390/universe10020055 - 23 Jan 2024
Viewed by 1083
Abstract
In the first few microseconds after the Big Bang, the hot dense matter was in the form of quark–gluon plasma consisting of free quarks and gluons. By colliding heavy nuclei at RHIC and LHC at a velocity close to the speed of light, [...] Read more.
In the first few microseconds after the Big Bang, the hot dense matter was in the form of quark–gluon plasma consisting of free quarks and gluons. By colliding heavy nuclei at RHIC and LHC at a velocity close to the speed of light, we were able to recreate primordial matter and observe that matter after expansion and cooling. In the present work, we have analyzed the transverse-momentum spectra of charged particles in high-multiplicity pp collisions at LHC energies s= 5.02 and 13 TeV, published by the ALICE Collaboration, using the Color-String Percolation Model. For heavy ions, Pb–Pb at sNN= 2.76 and 5.02 TeV along with Xe–Xe at sNN= 5.44 TeV have been analyzed. The initial temperature was extracted both in low- and high-multiplicity events in pp collisions. For A−A collisions, the temperature was obtained as a function of centrality. A universal scaling in the temperature from pp and A−A collisions was obtained when multiplicity was scaled by the transverse interaction area. For the higher-multiplicity events in pp collisions at s= 5.02 and 13 TeV, the initial temperature was above the universal hadronization temperature and was consistent with the creation of deconfined matter. From the measured energy density ε and the temperature, the dimensionless quantity ε/T4 was obtained, to obtain the degree of freedom of the deconfined matter. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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10 pages, 554 KiB  
Article
Study of Transverse-Spherocity Biased pp Collisions at the LHC Energies Using the PYTHIA 8 Event Generator
by Antonio Ortiz, Lizardo Valencia Palomo and Victor Manuel Minjares Neriz
Universe 2024, 10(1), 30; https://0-doi-org.brum.beds.ac.uk/10.3390/universe10010030 - 11 Jan 2024
Cited by 2 | Viewed by 1009
Abstract
The ALICE collaboration recently reported the mean transverse momentum as a function of charged-particle multiplicity for different pp-collision classes defined based on the “jettiness” of the event. The event “jettiness” is quantified using transverse spherocity that is measured at midpseudorapidity ( [...] Read more.
The ALICE collaboration recently reported the mean transverse momentum as a function of charged-particle multiplicity for different pp-collision classes defined based on the “jettiness” of the event. The event “jettiness” is quantified using transverse spherocity that is measured at midpseudorapidity (|η|<0.8) considering charged particles with transverse momentum within 0.15<pT<10 GeV/c. Comparisons to PYTHIA 8 (tune Monash) predictions show a notable disagreement between the event generator and data for jetty events that increases as a function of charged-particle multiplicity. This paper reports on the origin of such a disagreement. Since at intermediate and high pT (2<pT<10 GeV/c), the spectral shape is expected to be modified by color reconnection or jets, their effects on the average pT are studied. The results indicate that the origin of the discrepancy is the overpredicted multijet yield by PYTHIA 8, which increases with the charged-particle multiplicity. This finding is important to understand the way transverse spherocity and multiplicity bias the pp collisions and how well models like PYTHIA 8 reproduce those biases. The studies are pertinent since transverse spherocity is currently used as an event classifier by experiments at the LHC. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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22 pages, 377 KiB  
Article
An Effective Field Theory Study of Medium Heavy Quark Evolution
by Miguel Ángel Escobedo
Universe 2024, 10(1), 23; https://0-doi-org.brum.beds.ac.uk/10.3390/universe10010023 - 5 Jan 2024
Viewed by 1127
Abstract
The evolution of hard probes in a medium is a complex multiscale problem that significantly benefits from the use of Effective Field Theories (EFTs). Within the EFT framework, we aim to define a series of EFTs in a way that addresses each energy [...] Read more.
The evolution of hard probes in a medium is a complex multiscale problem that significantly benefits from the use of Effective Field Theories (EFTs). Within the EFT framework, we aim to define a series of EFTs in a way that addresses each energy scale individually in separate steps. However, studying hard probes in a medium presents challenges. This is because an EFT is typically constructed by formulating the most general Lagrangian compatible with the problem’s symmetries. Nevertheless, medium effects may not always be encoded adequately in an effective action. In this paper, we construct an EFT that is valid for studying the evolution of a heavy quark in a QCD plasma containing few other heavy quarks, where degrees of freedom with an energy of the order of the temperature scale are integrated out. Through this example, we explicitly demonstrate how to handle the doubling of degrees that arise in non-equilibrium field theory. As a result, we derive a Fokker–Planck equation using only symmetry and power counting arguments. The methods introduced in this paper will pave the way for future developments in the study of quarkonium suppression. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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13 pages, 1507 KiB  
Article
Structure of the Medium Formed in Heavy Ion Collisions
by J. R. Alvarado García, D. Rosales Herrera, A. Fernández Téllez, Bogar Díaz and J. E. Ramírez
Universe 2023, 9(6), 291; https://0-doi-org.brum.beds.ac.uk/10.3390/universe9060291 - 15 Jun 2023
Cited by 1 | Viewed by 1369
Abstract
We investigate the structure of the medium formed in heavy ion collisions using three different models: the Color String Percolation Model (CSPM), the Core–Shell-Color String Percolation Model (CSCSPM), and the Color Glass Condensate (CGC) framework. We analyze the radial distribution function of the [...] Read more.
We investigate the structure of the medium formed in heavy ion collisions using three different models: the Color String Percolation Model (CSPM), the Core–Shell-Color String Percolation Model (CSCSPM), and the Color Glass Condensate (CGC) framework. We analyze the radial distribution function of the transverse representation of color flux tubes in each model to determine the medium’s structure. Our results indicate that the CSPM behaves as an ideal gas, while the CSCSPM exhibits a structural phase transition from a gas-like to a liquid-like structure. Additionally, our analysis of the CGC framework suggests that it produces systems that behave like non-ideal gases for AuAu central collisions at RHIC energies and liquid-like structures for PbPb central collisions at LHC energies. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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17 pages, 463 KiB  
Article
Production of Cumulative Pions and Percolation of Strings
by Mikhail Aleksandrovich Braun
Universe 2023, 9(4), 195; https://0-doi-org.brum.beds.ac.uk/10.3390/universe9040195 - 19 Apr 2023
Cited by 1 | Viewed by 613
Abstract
Production of pions in high-energy collisions with nuclei in the kinematics prohibited for free nucleons (“cumulative pions”) is studied in the fusing color string model. The model describes the so-called direct mechanism for cumulative production. The other (spectator) mechanism dominates in production of [...] Read more.
Production of pions in high-energy collisions with nuclei in the kinematics prohibited for free nucleons (“cumulative pions”) is studied in the fusing color string model. The model describes the so-called direct mechanism for cumulative production. The other (spectator) mechanism dominates in production of cumulative protons, and is suppressed for pions. In the model, cumulative pions are generated by string fusion, which raises the maximal energy of produced partons above the level of the free nucleon kinematics. Momentum and multiplicity sum rules are used to determine the spectra in the deep fragmentation region. Predicted spectra of cumulative pions exponentially fall with the scaling variable x in the interval 1<x<3 with a slope between 5.1 and 5.6, which agrees well with the raw data obtained in the recent experiment at RHIC involving Cu–Au collisioins. However, the agreement is worse for the so-called unfolded data, presumably taking into account corrections due to the experimental setup and having rather a power-like form. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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11 pages, 494 KiB  
Article
Diagram of High-Energy Nuclear Collisions
by Evgeny Andronov, Magdalena Kuich and Marek Gazdzicki
Universe 2023, 9(2), 106; https://0-doi-org.brum.beds.ac.uk/10.3390/universe9020106 - 18 Feb 2023
Cited by 2 | Viewed by 1087
Abstract
Many new particles, mostly hadrons, are produced in high-energy collisions between atomic nuclei. The most popular models describing the hadron-production process are based on the creation, evolution and decay of resonances, strings or quark–gluon plasma. The validity of these models is under vivid [...] Read more.
Many new particles, mostly hadrons, are produced in high-energy collisions between atomic nuclei. The most popular models describing the hadron-production process are based on the creation, evolution and decay of resonances, strings or quark–gluon plasma. The validity of these models is under vivid discussion, and it seems that a common framework for this discussion is missing. Here, for the first time, we explicitly introduce the diagram of high-energy nuclear collisions, where domains of the dominance of different hadron-production processes in the space of laboratory-controlled parameters, the collision energy and nuclear-mass number of colliding nuclei are indicated. We argue that the recent experimental results suggest the location of boundaries between the domains, allowing for the first time to sketch an example diagram. Finally, we discuss the immediate implications for experimental measurements and model development following the proposed sketch of the diagram. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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10 pages, 535 KiB  
Review
Multiplicity Dependence of Quarkonium Production
by Zaida Conesa del Valle
Universe 2024, 10(2), 59; https://0-doi-org.brum.beds.ac.uk/10.3390/universe10020059 - 29 Jan 2024
Viewed by 842
Abstract
Recent measurements on heavy-flavour production as a function of charged-particle multiplicity at the LHC are discussed. Focus is given to quarkonium results in small (pp or pPb) collision systems. The measurements of relative yields, i.e., the ratio of the particle yields in given [...] Read more.
Recent measurements on heavy-flavour production as a function of charged-particle multiplicity at the LHC are discussed. Focus is given to quarkonium results in small (pp or pPb) collision systems. The measurements of relative yields, i.e., the ratio of the particle yields in given multiplicity intervals to the multiplicity integrated yield are presented and compared to model calculations from Monte Carlo event generators as well as to models considering effects at play in the initial and/or final state of the collision. The absolute inclusive J/ψ yield as a function of the absolute charged-particle multiplicity is evaluated; a smooth behaviour of the absolute yield is observed across collision systems, from pp to pPb and PbPb collisions. Analogous measurements of the excited-to-ground state quarkonium ratios as a function of charged-particle multiplicity are also reviewed. Finally, the study of exotic particle production as a function of charged-particle multiplicity is introduced as a complementary tool to investigate the nature of the χc1(3872) hadron. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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17 pages, 1463 KiB  
Review
String Interactions as a Source of Collective Behaviour
by Christian Bierlich
Universe 2024, 10(1), 46; https://0-doi-org.brum.beds.ac.uk/10.3390/universe10010046 - 17 Jan 2024
Cited by 1 | Viewed by 920
Abstract
The discovery of collective effects in small collision systems has spurred a renewed interest in hadronization models, and is also a source for collective effects all the way to large collision systems, where they are usually ascribed to the creation of a Quark–Gluon [...] Read more.
The discovery of collective effects in small collision systems has spurred a renewed interest in hadronization models, and is also a source for collective effects all the way to large collision systems, where they are usually ascribed to the creation of a Quark–Gluon Plasma. In this topical mini-review, the microscopic model for string interactions, based on the Lund string hadronization model, developed with exactly this aim in mind is reviewed, and some prospects for the future are presented. Full article
(This article belongs to the Special Issue Relativistic Heavy Ion Collision)
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