Fields In Relativistic Heavy-ion Collisions Research Articles

Energy-momentum tensor of the dilute (3+1)D glasma

We present a succinct formulation of the energy-momentum tensor of the glasma characterizing the initial color fields in relativistic heavy-ion collisions in the color glass condensate effective theory. We derive concise expressions for the (3+1)D dynamical evolution of symmetric nuclear collisions in the weak field approximation employing a generalized McLerran-Venugopalan model with nontrivial longitudinal correlations. Utilizing Monte Carlo integration, we calculate in unprecedented detail non-trivial rapidity profiles of early time observables at RHIC and LHC energies, including transverse energy densities and eccentricities. For our setup with broken boost invariance, we carefully discuss the placement of the origin of the Milne frame and interpret the components of the energy-momentum tensor. We find longitudinal flow that deviates from standard Bjorken flow in the (3+1)D case and provide a geometric interpretation of this effect. Furthermore, we observe a universal shape in the flanks of the rapidity profiles regardless of collision energy and predict that limiting fragmentation should also hold at LHC energies. Published by the American Physical Society 2024

Towards Experimental Confirmation of Quarkonia Melting in Quark–Gluon Plasma: A Review of Recent Measurements of Quarkonia Production in Relativistic Heavy-Ion Collisions

The dissociation, or “melting”, of heavy quarkonia states due to color charge screening is a predicted signature of quark–gluon plasma (QGP) formation, with a quarkonium state predicted to dissociate when the temperature of the medium is higher than the binding energy of the quarkonium state. A conclusive experimental observation of quarkonium melting coupled with a detailed theoretical understanding of the melting mechanism would enable the use of quarkonia states as temperature probes of the QGP, a long-sought goal in the field of relativistic heavy-ion collisions. However, the interpretation of quarkonia suppression measurements in heavy-ion collisions is complicated by numerous other cold nuclear matter effects that also result in the dissociation of bound quarkonia states. A comprehensive understanding of these cold nuclear matter effects is therefore needed in order to correctly interpret quarkonia production measurements in heavy-ion collisions and to observe the melting of quarkonium states experimentally. In this review, recent measurements of quarkonia production in pA and AA collisions and their state-of-the-art theoretical interpretations will be discussed, as well as the future measurements needed to further the knowledge of cold nuclear matter effects and realize a measurement of quarkonia melting in heavy-ion collisions.

Pair invariant mass to isolate background in the search for the chiral magnetic effect in Au + Au collisions at sNN=200 GeV

Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME)—an electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable (Δγ) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy (v2). We report here differential measurements of the correlator as a function of the pair invariant mass (minv) in 20–50% centrality Au+Au collisions at sNN=200 GeV by the STAR experiment at the BNL Relativistic Heavy Ion Collider. Strong resonance background contributions to Δγ are observed. At large minv where this background is significantly reduced, the Δγ value is found to be significantly smaller. An event-shape-engineering technique is deployed to determine the v2 background shape as a function of minv. We extract a v2-independent and minv-averaged signal Δγsig=(0.03±0.06±0.08)×10−4, or (2±4±5)% of the inclusive Δγ(minv>0.4 GeV/c2)=(1.58±0.02±0.02)×10−4, within pion pT=0.2–0.8 GeV/c and averaged over pseudorapidity ranges of −1<η<−0.05 and 0.05<η<1. This represents an upper limit of 0.23×10−4, or 15% of the inclusive result, at 95% confidence level for the minv-integrated CME contribution.Received 11 June 2020Revised 12 July 2022Accepted 12 August 2022DOI:https://doi.org/10.1103/PhysRevC.106.034908©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle correlations & fluctuationsRelativistic heavy-ion collisionsNuclear Physics

How to measure the magnetic field in relativistic heavy-ion collisions

The intense magnetic field created in heavy-ion collisions has often been calculated with classical electrodynamics. This description is thought to be accurate if the occupation number of every field mode is high enough. However, this hypothesis has never been tested. Recently, we have studied a process involving the magnetic field, which we believe to be the simplest: pion production in ultraperipheral collisions. We have started using the classical approximation to study the forward pion production by the magnetic excitation of nucleons through the process N → Δ → N + π. Then, in a subsequent work, we have replaced this classical field by a flux of photons and have calculated the cross section for photoproduction of pions, which is the quantum version of magnetic excitation. We compared the cross sections obtained with both formalisms, which turned out to be compatible with each other. In this note, we review those works, enlarge the discussion and suggest some improvements that can be done to make our results more accurate. We emphasize that these pions could be detected by the ZDC’s installed at ATLAS and ALICE. Since the theoretical uncertainties are under control, this detection could be used to perform an indirect measurement of the magnetic field intensity.

A GPU-based general numerical framework for plasma simulations in terms of microscopic kinetic equations with full collision terms

We have proposed a general numerical framework for plasma simulations on graphics processing unit clusters based on microscopic kinetic equations with full collision terms. Our numerical algorithm consistently deals with both long-range (classical forces in the Vlasov term) and short-range (quantum processes in the collision term) interactions. Providing the relevant particle masses, charges and types (classical, fermionic or bosonic), as well as the external forces and the matrix elements (in the collisional integral), the algorithm consistently solves the coupled multi-particle kinetic equations. Currently, the framework is being tested and applied in the field of relativistic heavy-ion collisions; extensions to other plasma systems are straightforward. Our framework is a potential and competitive numerical platform for consistent plasma simulations.

Magnetic field in relativistic heavy-ion collisions: Testing the classical approximation

It is believed that in noncentral relativistic heavy-ion collisions a very strong magnetic field is formed. There are several studies of the effects of this field, where $\stackrel{P\vec}{B}$ is calculated with the expressions of classical electrodynamics. A quantum field may be approximated by a classical one when the number of field quanta in each field mode is sufficiently high. This may happen if the field sources are intense enough. In heavy-ion physics the validity of the classical treatment was not investigated. In this work we propose a test of the quality of the classical approximation. We calculate an observable quantity using the classical magnetic field and also using photons as input. If the results of both approaches coincide, this will be an indication that the classical approximation is valid. More precisely, we focus on the process in which a nucleon is converted into a delta resonance, which then decays into another nucleon and a pion, i.e., $N\ensuremath{\rightarrow}\mathrm{\ensuremath{\Delta}}\ensuremath{\rightarrow}{N}^{\ensuremath{'}}\ensuremath{\pi}$. In ultraperipheral relativistic heavy-ion collisions this conversion can be induced by the classical magnetic field of one of the ions acting on the other ion. Alternatively, we can replace the classical magnetic field by a flux of equivalent photons, which are absorbed by the target nucleons. We calculate the cross sections in these two independent ways and find that they differ from each other by $\ensuremath{\simeq}10$ % in the considered collision energy range. This suggests that the two formalisms are equivalent and that the classical approximation for the magnetic field is reasonable.

Dynamics of QCD matter — current status

In this article, there are 18 sections discussing various current topics in the field of relativistic heavy-ion collisions and related phenomena, which will serve as a snapshot of the current state of the art. Section 1 reviews experimental results of some recent light-flavored particle production data from ALICE collaboration. Other sections are mostly theoretical in nature. Very strong but transient magnetic field created in relativistic heavy-ion collisions could have important observational consequences. This has generated a lot of theoretical activity in the last decade. Sections 2, 7, 9, 10 and 11 deal with the effects of the magnetic field on the properties of the QCD matter. More specifically, Sec. 2 discusses mass of [Formula: see text] in the linear sigma model coupled to quarks at zero temperature. In Sec. 7, one-loop calculation of the anisotropic pressure are discussed in the presence of strong magnetic field. In Sec. 9, chiral transition and chiral susceptibility in the NJL model is discussed for a chirally imbalanced plasma in the presence of magnetic field using a Wigner function approach. Sections 10 discusses electrical conductivity and Hall conductivity of hot and dense hadron gas within Boltzmann approach and Sec. 11 deals with electrical resistivity of quark matter in presence of magnetic field. There are several unanswered questions about the QCD phase diagram. Sections 3, 11 and 18 discuss various aspects of the QCD phase diagram and phase transitions. Recent years have witnessed interesting developments in foundational aspects of hydrodynamics and their application to heavy-ion collisions. Sections 12 and 15–17 of this article probe some aspects of this exciting field. In Sec. 12, analytical solutions of viscous Landau hydrodynamics in 1+1D are discussed. Section 15 deals with derivation of hydrodynamics from effective covariant kinetic theory. Sections 16 and 17 discuss hydrodynamics with spin and analytical hydrodynamic attractors, respectively. Transport coefficients together with their temperature- and density-dependence are essential inputs in hydrodynamical calculations. Sections 5, 8 and 14 deal with calculation/estimation of various transport coefficients (shear and bulk viscosity, thermal conductivity, relaxation times, etc.) of quark matter and hadronic matter. Sections 4, 6 and 13 deal with interesting new developments in the field. Section 4 discusses color dipole gluon distribution function at small transverse momentum in the form of a series of Bells polynomials. Section 6 discusses the properties of Higgs boson in the quark–gluon plasma using Higgs–quark interaction and calculate the Higgs decays into quark and anti-quark, which shows a dominant on-shell contribution in the bottom-quark channel. Section 13 discusses modification of coalescence model to incorporate viscous corrections and application of this model to study hadron production from a dissipative quark–gluon plasma.

Wigner function and kinetic theory for massive spin-1/2 particles

We calculate the Wigner function for massive spin-1/2 particles in an inhomogeneous electromagnetic field to leading order in the Planck constant ħ. Going beyond leading order in ħ we then derive a generalized Boltzmann equation in which the force exerted by an inhomogeneous electromagnetic field on the particle dipole moment arises naturally. Furthermore, a kinetic equation for this dipole moment is derived. Carefully taking the massless limit we find agreement with previous results. The case of global equilibrium with rotation is also studied. Our framework can be used to study polarization effects induced by vorticity and magnetic field in relativistic heavy-ion collisions.

Chemical freeze-out conditions from net-kaon fluctuations at RHIC

We compare the mean-over-variance ratio of the net-kaon distribution calculated within a state-of-the-art hadron resonance gas model to the latest experimental data from the Beam Energy Scan at RHIC by the STAR collaboration. Our analysis indicates that it is not possible to reproduce the experimental results using the freeze-out parameters from the existing combined fit of net-proton and net-electric charge mean-over-variance. The strange mesons need about 10-15 MeV higher temperatures than the light hadrons at the highest collision energies. In view of the ongoing lambda fluctuation measurements, we predict the net-lambda variance-over-mean at the light and strange chemical freeze-out parameters. We observe that the lambda fluctuations are sensitive to the difference in the freeze-out temperatures established in this analysis. Our results have implications for other phenomenological models in the field of relativistic heavy-ion collisions.

Centrality dependence of photon yield and elliptic flow from gluon fusion and splitting induced by magnetic fields in relativistic heavy-ion collisions

We compute the photon yield and elliptic flow coefficient in relativistic heavy-ion collisions from gluon fusion and splitting processes induced by a magnetic field for different centralities. The calculation accounts for the intense magnetic field and the high gluon occupation number at early times. The photon production induced by these process represents an excess contribution over calculations without magnetic field effects. We compare this excess to the difference between PHENIX data and recent hydrodynamic calculations for the photon transverse momentum distribution and elliptic flow coefficient $v_2$. The time evolution of the field strength and reaction volume is computed using UrQMD. We show that with reasonable values for the saturation scale, the calculation helps to better describe the experimental results obtained at RHIC energies for the lowest part of the transverse photon momentum at different centralities.

Particle Polarization and Structure of Vortical Field in Relativistic Heavy-Ion Collisions

We review studies of vortical motion and the resulting global polarization of Λ and hyperons in heavy-ion collisions within 3FD model. The 3FD simulations indicate that energy dependence of the observed global polarization of hyperons in the midrapidity region is a consequence of the decrease of the vorticity in the central region with the collision energy rise because of pushing out the vorticity field into the fragmentation regions. At high collision energies this pushing-out results in a peculiar vortical structure consisting of two vortex rings: one ring in the target fragmentation region and another one in the projectile fragmentation region with matter rotation being opposite in these two rings.

Kinetic theory for massive spin−1/2 particles from the Wigner-function formalism

We calculate the Wigner function for massive spin-1/2 particles in an inhomogeneous electromagnetic field to leading order in the Planck constant $\hbar$. Going beyond leading order in $\hbar$ we then derive a generalized Boltzmann equation in which the force exerted by an inhomogeneous electromagnetic field on the particle dipole moment arises naturally. Carefully taking the massless limit we find agreement with previous results. The case of global equilibrium with rotation is also studied. Finally, we outline the derivation of fluid-dynamical equations from the components of the Wigner function. The conservation of total angular momentum is promoted as an additional fluid-dynamical equation of motion. Our framework can be used to study polarization effects induced by vorticity and magnetic field in relativistic heavy-ion collisions.

Charge-dependent pair correlations relative to a third particle in p + Au and d + Au collisions at RHIC

Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics. This can lead to electric charge separation along the strong magnetic field in relativistic heavy-ion collisions – the chiral magnetic effect (CME). We report measurements by the STAR collaboration of a CME-sensitive observable in p+Au and d+Au collisions at 200 GeV, where the CME is not expected, using charge-dependent pair correlations relative to a third particle. We observe strong charge-dependent correlations similar to those measured in heavy-ion collisions. This bears important implications for the interpretation of the heavy-ion data.

Probing the transverse size of initial inhomogeneities with flow observables

Disentangling the effect of initial conditions and medium properties is an open question in the field of relativistic heavy-ion collisions. We argue that, while one can study the impact of initial inhomogeneities by varying their size, it is important to maintain the global properties fixed. We present a method to do this. We show that many observables are insensitive to the the hot spot sizes, including integrated vn, scaled distributions of vn, symmetric cumulants, event-plane correlations, and differential vn(pT). We find however that the factorization breaking ratio rn and sub-leading component in a Principal Component Analysis are more sensitive to the initial granularity and can be used to probe short-scale features of the initial density.

Investigating different Λ and [formula omitted] polarizations in relativistic heavy-ion collisions

Based on the chiral kinetic equations of motion, spin polarizations of various quarks, due to the magnetic field induced by spectator protons as well as the quark-antiquark vector interaction, are studied within a partonic transport approach. Although the magnetic field in QGP enhances the splitting of the spin polarizations of partons compared to the results under the magnetic field in vacuum, the spin polarizations of s and s¯ quarks are also sensitive to the quark-antiquark vector interaction, challenging that the different Λ and Λ¯ spin polarization is a good measure of the magnetic field in relativistic heavy-ion collisions. It is also found that there is no way to obtain the large splitting of the spin polarization between Λ and Λ¯ at sNN=7.7 GeV with partonic dynamics.

Erratum: Prompt photon yield and elliptic flow from gluon fusion induced by magnetic fields in relativistic heavy-ion collisions [Phys. Rev. D 96 , 014023 (2017)

Erratum: Prompt photon yield and elliptic flow from gluon fusion induced by magnetic fields in relativistic heavy-ion collisions [Phys. Rev. D <b>96</b> , 014023 (2017)

Prompt photon yield and elliptic flow from gluon fusion induced by magnetic fields in relativistic heavy-ion collisions

We compute photon production at early times in semi-central relativistic heavy-ion collisions from non-equilibrium gluon fusion induced by a magnetic field. The calculation accounts for the main features of the collision at these early times, namely, the intense magnetic field and the high gluon occupation number. The gluon fusion channel is made possible by the magnetic field and would otherwise be forbidden due to charge conjugation invariance. Thus, the photon yield from this process is an excess over calculations without magnetic field effects. We compare this excess to the difference between PHENIX data and recent hydrodynamic calculations for the photon transverse momentum distribution and elliptic flow coefficient $v_2$. We show that with reasonable values for the saturation scale and magnetic field strength, the calculation helps to better describe the experimental results obtained at RHIC energies for the lowest part of the transverse photon momentum.

Evidence for creation of strong electromagnetic fields in relativistic heavy-ion collisions

It is proposed to identify a strong electric field created during relativistic collisions of asymmetric nuclei via observation of pseudorapidity and transverse momentum distributions of hadrons with the same mass but opposite charges. The detailed calculation results for the directed flow within the Parton-Hadron String Dynamics model are given for Cu-Au interactions at the NICA collision energies of $\sqrt{s_{NN}}=9$ and $5$ GeV. The separation effect is observable at 9 GeV as clearly as at 200 GeV

Chiral electric field in relativistic heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider**Supported by National Natural Science Foundation of China (11375069, 11435054, 11075061, 11221504) and Key Laboratory Foundation of Quark and Lepton Physics (Hua-Zhong Normal University)(QLPL2014P01)

It has been proposed that electric fields may lead to chiral separation in quark-gluon plasma (QGP). This is called the chiral electric separation effect. The strong electromagnetic field and the QCD vacuum can both be completely produced in off-central nuclear-nuclear collision. We use the Woods-Saxon nucleon distribution to calculate the electric field distributions of off-central collisions. The chiral electric field spatial distribution at Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC) energy regions are systematically studied in this paper. The dependence of the electric field produced by the thermal quark in the central position with different impact parameters on the proper time with different collision energies in the RHIC and LHC energy regions are studied in this paper.

A Systematic Study of Magnetic Field in Relativistic Heavy-Ion Collisions in the RHIC and LHC Energy Regions

The features of magnetic field in relativistic heavy-ion collisions are systematically studied by using a modified magnetic field model in this paper. The features of magnetic field distributions in the central point are studied in the RHIC and LHC energy regions. We also predict the feature of magnetic fields at LHCsNN=900, 2760, and 7000 GeV based on the detailed study at RHICsNN=62.4, 130, and 200 GeV. The dependencies of the features of magnetic fields on the collision energies, centralities, and collision time are systematically investigated, respectively.