Relativistic Heavy Ion Reactions Research Articles

Probing the high-density nuclear symmetry energy with the Ξ−/Ξ0 ratio in heavy-ion collisions at sNN≈3 GeV

Recent beam energy scan experiments at the BNL Relativistic Heavy Ion Collider by the STAR Collaboration [Phys. Lett. B 827, 137003 (2022) and Phys. Rev. Lett. 128, 202303 (2022)] found that hadronic interactions dominate the collective flow and the proton cumulant ratios are driven by baryon number conservation in a region of high baryon density in $\sqrt{{s}_{NN}}=3$ GeV $\mathrm{Au}+\mathrm{Au}$ reactions, indicating that the dense medium formed in such collisions is likely hadronic matter. Within an updated a relativistic transport model with momentum dependent isoscalar and isovector single-nucleon mean-field potentials corresponding to different symmetry energies at suprasaturation densities, the $n/p, {\ensuremath{\pi}}^{\ensuremath{-}}/{\ensuremath{\pi}}^{+}, {K}_{s}^{0}/{K}^{+}, {\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Sigma}}}^{+}$, and ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratios are studied for central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=3$ GeV, where the maximum central density reaches about $(3.6--4.0){\ensuremath{\rho}}_{0}$. The doubly strange ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratio is found to have the strongest sensitivity to the variation of high-density nuclear symmetry energy. Thus, the ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratio in relativistic heavy-ion reactions at $\sqrt{{s}_{NN}}\ensuremath{\sim}3$ GeV may help probe sensitively the poorly known symmetry energy of dense neutron-rich matter critically important for understanding various properties of neutron stars.

Stochastic Variational Method for Viscous Hydrodynamics

In this short review, we focus on some of the subjects, related to J. Cleymans’ pioneering contribution of statistical approaches to the particle production process in heavy-ion collisions. We discuss these perspectives from the effects of stochastic processes in collective variables of hydrodynamic description, which is described by a stochastic variational method. In this connection, we stress also the necessity of the inclusion of surface and quantum effects in the study of relativistic heavy-ion reactions.

Centrality Dependent Lévy-Stable Two-Pion Bose-Einstein Correlations in \( {\sqrt{s_{NN}}} \) = 200 GeV Au+Au Collisions at the PHENIX Experiment

Investigation of femtoscopic correlation functions in relativistic heavy ion reactions is an important tool to access the space-time structure of particle production in the sQGP. The shape of the correlation functions is often assumed to be Gaussian, but a detailed analysis reveals that the statistically correct assumption could be the so-called L\'evy distribution. The detailed analysis of correlation functions in various systems may shed light on the location of the critical endpoint on the QCD phase diagram. It could also reveal if there is partially coherent pion production or could indicate the possible in-medium mass modification of the $\eta'$ meson due to the (partial) restoration of the $U_A(1)$ axial symmetry. In this paper we present the status of the centrality dependent measurements of two-pion L\'evy Bose-Einstein correlation functions in Au+Au collisions at PHENIX.

Theoretical Study of Projectile Fragmentations in Relativistic Heavy-Ion Reactions

We have investigated and interpreted the production cross sections and isotopic distributions of projectile-like residues in the reactions 124 Sn + 124 Sn and 112 Sn + 112 Sn at an incident beam energy of 1 GeV/nucleon measured with the FRS fragment separator at the GSI laboratory. For the interpretation of the data, calculations within the statistical multifragmentation model (SMM) for an ensemble of excited sources were performed with ensemble parameters. The possible modification of symmetry energy parameter, in the multifragmentation region at the low density and hot freeze-out environment, is studied. It is reconfirmed that a significant reduction of the symmetry energy term is found necessary to reproduce experimental results at these conditions. We have also found a decreasing trend of the symmetry energy for large neutron-rich fragments of low excitation energy which is interpreted as a nuclear-structure effect.

Vorticity in peripheral collisions at the Facility for Antiproton and Ion Research and at the JINR Nuclotron-based Ion Collider fAcility

The flow vorticity development is studied in the reaction plane of peripheral relativistic heavy-ion reactions at energies just above the threshold of the transition to a quark-gluon plasma (QGP). Earlier calculations at higher energies with larger initial angular momentum predicted significant vorticity leading to measurable $\ensuremath{\Lambda}$ polarization. Here we discuss the possibility of vorticity and circulation in dense plasma at lower temperatures. In low-viscosity QGP this vorticity still remains significant.

Flow probe of symmetry energy in relativistic heavy-ion reactions

Flow observables in heavy-ion reactions at incident energies up to about 1 GeV per nucleon have been shown to be very useful for investigating the reaction dynamics and for determining the parameters of reaction models based on transport theory. In particular, the elliptic flow in collisions of neutron-rich heavy-ion systems emerges as an observable sensitive to the strength of the symmetry energy at supra-saturation densities. The comparison of ratios or differences of neutron and proton flows or neutron and hydrogen flows with predictions of transport models favors an approximately linear density dependence, consistent with ab-initio nuclear-matter theories. Extensive parameter searches have shown that the model dependence is comparable to the uncertainties of existing experimental data. Comprehensive new flow data of high accuracy, partly also through providing stronger constraints on model parameters, can thus be expected to improve our knowledge of the equation of state of asymmetric nuclear matter.

The Lund–York–Cologne Calorimeter (LYCCA): Concept, design and prototype developments for a FAIR-NUSTAR detector system to discriminate relativistic heavy-ion reaction products

The concept, design and prototype developments for the Lund–York–Cologne CAlorimeter (LYCCA) is presented. LYCCA is a modular device for the NUclear STructure, Astrophysics and Reactions (NUSTAR) science pillar of the Facility for Antiproton and Ion Research (FAIR) at Darmstadt, Germany. LYCCA is designed to discriminate heavy ions produced in nuclear reactions induced by relativistic radioactive ion beams. Measurements of energy loss, total energy, and time-of-flight allow the derivation of proton number, Z, and mass number, A, of the reaction products. LYCCA-inherent tracking of the flight paths of the reaction products enables coincident HIgh-resolution in-beam γ-ray SPECtroscopy (HISPEC) of atomic nuclei far from the line of β-stability.

Flow vorticity in peripheral high-energy heavy-ion collisions

Vorticity development is studied in the reaction plane of peripheral relativistic heavy-ion reactions where the initial state has substantial angular momentum. The earlier predicted rotation effect and Kelvin Helmholtz instability lead to significant initial vorticity and circulation. In low-viscosity quark gluon plasma this vorticity remains still significant at the time of freeze-out of the system, even if damping due to explosive expansion and dissipation decreases the vorticity and circulation. In the reaction plane the vorticity arises from the initial angular momentum, and it is stronger than in the transverse plane, where vorticity is caused by random fluctuations only.

Cold nuclear matter effects on dijet productions in relativistic heavy-ion reactions at LHC

We investigate the cold nuclear matter (CNM) effects on dijet productions in high-energy nuclear collisions at LHC with the next-to-leading order perturbative QCD. The nuclear modifications for dijet angular distributions, dijet invariant mass spectra, dijet transverse momentum spectra and dijet momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS, HKN and DS parametrization sets of parton distributions in nucleus. It is found that dijet angular distributions and dijet momentum imbalance are insensitive to the initial-state CNM effects and thus provide optimal tools to study the final-state hot QGP effects such as jet quenching. On the other hand, the invariant mass spectra and the transverse momentum spectra of dijet are generally enhanced in a wide region of the invariant mass or transverse momentum due to CNM effects with a feature opposite to the expected suppression because of the final-state parton energy loss effect in the QGP. The difference of EPS, EKS, HKN and DS parametrization sets of nuclear parton distribution functions is appreciable for dijet invariant mass spectra and transverse momentum spectra at p+Pb collisions, and becomes more pronounced for those at Pb+Pb reactions.

ENTROPY CREATION IN RELATIVISTIC HEAVY ION COLLISIONS

We review current ideas on entropy production during the different stages of a relativistic nuclear collision. This includes recent results on decoherence entropy and the entropy produced during the hydrodynamic phase by viscous effects. We start by a discussion of decoherence caused by gluon bremsstrahlung in the very first interactions of gluons from the colliding nuclei. We then present a general framework, based on the Husimi distribution function, for the calculation of entropy growth in quantum field theories, which is applicable to the early ("glasma") phase of the collision during which most of the entropy is generated. The entropy calculated from the Husimi distribution exhibits linear growth when the quantum field contains unstable modes and the growth rate is asymptotically equal to the Kolmogorov–Sinaï entropy. We outline how the approach can be used to investigate the problem of entropy production in a relativistic heavy ion reaction from first principles. We show that the same result can be obtained in the framework of a completely different approach called eigenstate thermalization hypothesis. Finally we discuss some recent results on entropy production in the strong coupling limit, as obtained from AdS/CFT duality.

HADRONS IN STRONGLY INTERACTING MATTER

We review the current status of theories and experiments aiming at an understanding and a determination of the properties of light vector and scalar mesons inside strongly interacting hadronic matter. Starting from a discussion of the relevant symmetries of QCD and their connection with the hadronic description through QCD sum rules we then discuss hadronic models used to calculate the in-medium self-energies of hadrons and their spectral functions. The difficulties to link these calculated properties to actual observables are emphasized. Finally, we review in detail all the running experiments searching for in-medium changes of vector and scalar mesons, both with relativistic heavy-ion reactions as well as with elementary reactions on (cold) nuclei. Inconsistencies among experimental results are discussed. While almost all experiments observe a considerable broadening of vector mesons inside the nuclear medium, no evidence for mass changes is observed in the majority of the experiments.

From 0 to 5000 in 2 × 10 −24 seconds: Entropy production in relativistic heavy-ion collisions

We review what is known about the contributions to the final entropy from the different stages of a relativistic nuclear collision, including recent results on the decoherence entropy and the entropy produced during the hydrodynamic phase by viscous effects. We then present a general framework, based on the Husimi distribution function, for the calculation of entropy growth in quantum field theories, which is applicable to the earliest (“glasma”) phase of the collision during which most of the entropy is generated. The entropy calculated from the Husimi distribution exhibits linear growth when the quantum field contains unstable modes and is asymptotically equal to the Kolmogorov-Sinaï (KS) entropy. We outline how the approach can be used to investigate the problem of entropy production in a relativistic heavy-ion reaction from first principles.

Fully integrated transport approach to heavy ion reactions with an intermediate hydrodynamic stage

We present a coupled Boltzmann and hydrodynamics approach to relativistic heavy ion reactions. This hybrid approach is based on the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport approach with an intermediate hydrodynamical evolution for the hot and dense stage of the collision. Event-by-event fluctuations are directly taken into account via the nonequilibrium initial conditions generated by the initial collisions and string fragmentations in the microscopic UrQMD model. After a (3+1)-dimensional ideal hydrodynamic evolution, the hydrodynamical fields are mapped to hadrons via the Cooper-Frye equation and the subsequent hadronic cascade calculation within UrQMD proceeds to incorporate the important final state effects for a realistic freeze-out. This implementation allows us to compare pure microscopic transport calculations with hydrodynamic calculations using exactly the same initial conditions and freeze-out procedure. The effects of the change in the underlying dynamics--ideal fluid dynamics versus nonequilibrium transport theory--is explored. The freeze-out and initial state parameter dependencies are investigated for different observables. The time evolution of the baryon density and particle yields are also discussed. We find that the final pion and proton multiplicities are lower in the hybrid model calculation owing to the isentropic hydrodynamic expansion whereas the yields for strange particles are enhanced owing to the localmore » equilibrium in the hydrodynamic evolution. The results of the different calculations for the mean transverse mass excitation function, rapidity, and transverse mass spectra for different particle species at three different beam energies are discussed in the context of the available data.« less

Charmonium dissociation by mesons in heavy-ion collisions

The charmonium dissociation by mesons in relativistic heavy-ion reactions is analyzed within the Regge approach. It is shown that the inclusion of the initial and final state interactions in the dissociation of J / Ψ to D ¯ ∗ D ∗ close to threshold increases the cross section significantly and can not be neglected in comparison to the total dissociation rate. This is due to resonant D ¯ ∗ − D ∗ interactions in s close to the masses of the Ψ ( 4.04 ) and Ψ ( 4.16 ) mesons. We also investigate thermal effects of the ( c c ¯ ) width for such processes in the medium. All these effects change both the magnitude and the shape of the cross section as a function of s . The results obtained should be applied in the analysis of open and hidden charm production in heavy-ion collisions.

Covariant description of kinetic freeze-out through a finite spacelike layer

The problem of freeze-out (FO) in relativistic heavy-ion reactions is addressed. We develop and analyze an idealized one-dimensional model of FO in a finite layer, based on the covariant FO probability. The resulting post FO phase-space distributions are discussed for different FO probabilities and layer thicknesses.

Medium effects on φ decays to dilepton and kaon-antikaon pairs in relativistic heavy ion reactions

We consider the role of rescattering of secondary kaons on the dilepton branching ratio of the phi meson. In-medium mass modifications and broadening of kaons and phi mesons are taken into account. We find in the framework of a Bjorken scenario for the time evolution of the expanding fireball that the phi yield from dimuons is moderately or at least only slightly enhanced compared to that from kaon-antikaon pairs. The relation to experimental yields measured by the NA49, NA50 and CERES Collaborations at CERN SPS and the PHENIX Collaboration at RHIC is discussed.

A novel Calorimeter Telescope for identification of relativistic heavy-ion reaction channels

A novel Calorimeter Telescope (CATE) is employed in the fast beam Rare Isotope Investigations at GSI (RISING) γ -campaign with relativistic energies at the Fragment Separator (FRS) at GSI. CATE consists of nine modular Si – CsI ( Tl ) detector telescopes for position and Δ E - E res measurements. It registers the scattering angle and identifies the charge ( Z) and the mass ( A) of exotic heavy ions produced after secondary fragmentation or Coulomb excitation.

Multiplicity fluctuations in a hadron gas with exact conservation laws

The study of fluctuations of particle multiplicities in relativistic heavy-ion reactions has drawn much attention in recent years, because they have been proposed as a probe for underlying dynamics and possible formation of quark-gluon plasma. Thus, it is of uttermost importance to describe the baseline of statistical fluctuations in the hadron gas phase in a correct way. We have performed a comprehensive study of multiplicity distributions in the full ideal hadron-resonance gas in different ensembles, namely grand-canonical, canonical and microcanonical, using two different methods: asymptotic expansions and full Monte Carlo simulations. The method based on asymptotic expansion allows a quick numerical calculation of dispersions in the hadron gas with three conserved charges at primary hadron level, while the Monte-Carlo simulation is suitable to study the effect of resonance decays. Even though mean multiplicities converge to the same values, major differences in fluctuations for these ensembles persist in the thermodynamic limit, as pointed out in recent studies. We observe that this difference is ultimately related to the non-additivity of the variances in the ensembles with exact conservation of extensive quantities.

Prethermalization, non-extensive statistics and hydrodynamics

We argue that the success of hydrodynamic description does not necessarily imply realization of the local thermodynamical equilibrium in relativistic heavy ion reactions. A simple and general argument shows that a class of dynamical correlations give rise to the so-called Tsallis non-extensive statistics for which the usual thermodynamical relations are valid. An example of a system having such a dynamics is given, exhibiting a non-Maxwellian energy distribution.

Transverse pressure and strangeness dynamics in relativistic heavy-ion reactions

Transverse hadron spectra from proton–proton, proton–nucleus and nucleus–nucleus collisions from 2 A GeV to 21.3 A TeV are investigated within two independent transport approaches (HSD and UrQMD). For central Au+Au (Pb+Pb) collisions at energies above Elab ∼ 5 A GeV, the measured K± transverse mass spectra have a larger inverse slope parameter than expected from the default calculations. The additional pressure—as suggested by lattice QCD calculations at finite quark chemical potential μq and temperature T—might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au+Au (Pb+Pb) collisions. This is supported by a non-monotonic energy dependence of v2/⟨pT⟩ in the present transport model.