Description Of Heavy-ion Collisions Research Articles

Shape of atomic nuclei in heavy ion collisions

In the hydrodynamic model description of heavy ion collisions, the final-state anisotropic flow $v_n$ are linearly related to the strength of the multi-pole shape of the distribution of nucleons in the transverse plane $\varepsilon_n$, $v_n\propto \varepsilon_n$. The $\varepsilon_n$, for $n=1,2,3,4$, are sensitive to the shape of the colliding ions, characterized by the quadrupole $\beta_2$, octupole $\beta_3$ and hexadecapole $\beta_4$ deformations. This sensitivity is investigated analytically and also in a Monte Carlo Glauber model. One observes a robust linear relation, $\langle\varepsilon_n^2\rangle = a_n'+b_n'\beta_n^2$, for events in a fixed centrality. The $\langle\varepsilon_1^2\rangle$ has a contribution from $\beta_3$ and $\beta_4$, and $\langle\varepsilon_3^2\rangle$ from $\beta_4$. In the ultra-central collisions, there are little cross contributions between $\beta_2$ and $\varepsilon_3$ and between $\beta_3$ and $\varepsilon_2$, but clear cross contributions are present in non-central collisions. Additionally, $\langle\varepsilon_n^2\rangle$ are insensitive to non-axial shape parameters such as the triaxiality. This is good news because the measurements of $v_2$, $v_3$ and $v_4$ can be used to constrain simultaneously the $\beta_2$, $\beta_3$, and $\beta_4$ values. This is best done by comparing two colliding ions with similar mass numbers and therefore nearly identical $a_n'$, to obtain simple equation that relates the $\beta_n$ of the two species. This opens up the possibility to map the shape of the atomic nuclei at a timescale ($<10^{-24}$s) much shorter than probed by low-energy nuclear structure physics ($<10^{-21}$s), which ultimately may provide information complementary to those obtained in the nuclear structure experiments.

Net-particle number fluctuations in a hydrodynamic description of heavy-ion collisions

Utilizing viscous hydrodynamic simulations of heavy-ion collisions, we study the behavior of cumulants of (net-)(anti)proton number distributions at RHIC beam energy scan energies, incorporating non-critical contributions like baryon conservation and excluded volume. The experimental data on net-proton cumulants at √SNN &gt; 20 GeV are consistent with simultaneous effects of global baryon conservation and repulsive interactions in baryon sector, whereas the data at lower collision energies show possible indications for sizable attractive interactions among baryons. We discuss the behavior of factorial cumulants in addition to the ordinary cumulants, and also address the quantitative difference between proton and baryon number cumulants.

Charged-particle angular correlations in XeXe collisions at sNN=5.44 TeV

Azimuthal correlations of charged particles in xenon-xenon collisions at a center-of-mass energy per nucleon pair of $ \sqrt{s_{_\mathrm{NN}}} =$ 5.44 TeV are studied. The data were collected by the CMS experiment at the LHC with a total integrated luminosity of 3.42 $\mu$b$^{-1}$. The collective motion of the system formed in the collision is parameterized by a Fourier expansion of the azimuthal particle density distribution. The azimuthal anisotropy coefficients $v_{2}$, $v_{3}$, and $v_{4}$ are obtained by the scalar-product, two-particle correlation, and multiparticle correlation methods. Within a hydrodynamic picture, these methods have different sensitivities to non-collective and fluctuation effects. The dependence of the Fourier coefficients on the size of the colliding system is explored by comparing the xenon-xenon results with equivalent lead-lead data. Model calculations that include initial-state fluctuation effects are also compared to the experimental results. The observed angular correlations provide new constraints on the hydrodynamic description of heavy ion collisions.

Signatures of chiral symmetry restoration and its survival throughout the hadronic phase interactions

The effect of the chiral symmetry restoration (CSR) on observables from heavy-ion collisions is studied in the energy range [see formula in PDF]=3–20 GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the non-linear σ - ω model, which enter in the computation of the quark scalar density for the CSR mechanism, in order to estimate the uncertainty in our calculations. For the pion-nucleon ∑-term we adopt ∑π ≈ 45 MeV which corresponds to a ’world average’. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at [see formula in PDF]=3–20 GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. We identify particle abundances and rapidity spectra to be suitable probes in order to extract information about CSR, while transverse mass spectra are less sensitive ones. Our results provide a microscopic explanation for the "horn" structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to [see formula in PDF] ≈ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium.

Evidence for chiral symmetry restoration in heavy-ion collisions

We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range sNN=3–20GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for particle production. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at sNN=3–20GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. Our results provide a microscopic explanation for the horn structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to sNN≈7GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the horn structure is investigated as a function of the system size. We additionally present an analysis of strangeness production in the (T,μB)-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.

Chiral symmetry restoration versus deconfinement in heavy-ion collisions at high baryon density

The effect of the chiral symmetry restoration (CSR) on observables from heavy-ion collisions is studied in the energy range within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the non-linear σ − ω model, which enter in the computation of the quark scalar density for the CSR mechanism, in order to estimate the uncertainty in our calculations. For the pion-nucleon Σ-term we adopt Σπ ≈ 45 MeV which corresponds to some ‘world average’. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at , realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. We identify particle abundances and rapidity spectra to be suitable probes in order to extract information about CSR, while transverse mass spectra are less sensitive. Our results provide a microscopic explanation for the “horn” structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to , while the drop at higher energies is associated to the appearance of a deconfined partonic medium.

Dynamical description of heavy-ion collisions at Fermi energies

Descriptions of heavy-ion collisions at Fermi energies require to take into account in-medium dissipation and phase-space fluctuations. The interplay of these correlations with the one-body collective behaviour determines the properties (kinematics and fragment production) and the variety of mechanisms (from fusion to neck formation and multifragmentation) of the exit channel. Starting from fundamental concepts tested on nuclear matter, we build up a microscopic description which addresses finite systems and applies to experimental observables.

Challenges in description of heavy-ion collisions with microscopic time-dependent approaches

Important efforts have been dedicated in the past few years to describe near-barrier heavy-ion collisions with microscopic quantum theories like the time-dependent Hartree–Fock approach and some of its extensions. However, this field is still facing important challenges such as the description of cluster dynamics, the prediction of fragment characteristics in damped collisions, and sub-barrier fusion by quantum tunnelling. These challenges are discussed and possible approaches to solve them are presented.

Characterization of initial fluctuations for the hydrodynamical description of heavy ion collisions

Event-by-event fluctuations in the initial conditions for a hydrodynamical description of heavy ion collisions are characterized. We propose a Bessel-Fourier decomposition with respect to the azimuthal angle, the radius in the transverse plane, and rapidity. This allows for a complete characterization of fluctuations in all hydrodynamical fields including energy density, pressure, fluid velocity, shear stress, and bulk viscous pressure. It has the advantage that fluctuations can be ordered with respect to their wavelength and that they can be propagated mode by mode within the hydrodynamical formalism. Event ensembles can then be characterized in terms of a functional probability distribution. For the event ensemble of a Monte Carlo Glauber model, we provide evidence that the latter is close to Gaussian form, thus allowing for a particularly simple characterization of the event distribution.

Instability induced pressure isotropization in a longitudinally expanding system

In two previous works [arXiv:1009.4363,arXiv:1107.0668], we studied the time evolution of a system of real scalar fields with quartic coupling which shares important features with the Color Glass Condensate description of heavy ion collisions. Our primary objective was to understand how such a system, when initialized with a non-perturbatively large classical field configuration, reaches thermal equilibrium. An essential goal of these works was to highlight the role played by the quantum fluctuations. However, these studies considered only a system confined within a box of fixed volume. In the present paper, we extend this work to a system that expands in the longitudinal direction thereby more closely mimicking a heavy ion collision. We conclude that the microscopic processes that drive the system towards equilibrium are able to keep up with the expansion of the system; the pressure tensor becomes isotropic despite the anisotropic expansion.

Transport properties of the quark-gluon plasma

Transport properties of a thermal medium determine how its conserved charge densities (for instance the electric charge, energy or momentum) evolve as a function of time and eventually relax back to their equilibrium values. Here the transport properties of the quark-gluon plasma are reviewed from a theoretical perspective. The latter play a key role in the description of heavy-ion collisions, and are an important ingredient in constraining particle production processes in the early universe. We place particular emphasis on lattice QCD calculations of conserved current correlators. These Euclidean correlators are related by an integral transform to spectral functions, whose small-frequency form determines the transport properties via Kubo formulae. The universal hydrodynamic predictions for the small-frequency pole structure of spectral functions are summarized. The viability of a quasiparticle description implies the presence of additional characteristic features in the spectral functions. These features are in stark contrast with the functional form that is found in strongly coupled plasmas via the gauge/gravity duality. A central goal is therefore to determine which of these dynamical regimes the quark-gluon plasma is qualitatively closer to as a function of temperature. We review the analysis of lattice correlators in relation to transport properties, and tentatively estimate what computational effort is required to make decisive progress in this field.

Dynamical freeze-out condition in ultrarelativistic heavy ion collisions

We determine the decoupling surfaces for the hydrodynamic description of heavy ion collisions at RHIC and LHC by comparing the local hydrodynamic expansion rate with the microscopic pion-pion scattering rate. The pion ${p}_{T}$ spectra for nuclear collisions at BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) are computed by applying the Cooper-Frye procedure on the dynamical-decoupling surfaces and compared with those obtained from the constant-temperature freeze-out surfaces. Comparison with RHIC data shows that the system indeed decouples when the expansion rate becomes comparable with the pion scattering rate. The dynamical decoupling based on the rates comparison also suggests that the effective decoupling temperature in central heavy ion collisions remains practically unchanged from RHIC to LHC.

Three-body collisions in Boltzmann-Uehling-Uhlenbeck theory

Aiming at a microscopic description of heavy ion collisions in the beam energy region of about $10A$ GeV, we extend the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) transport model by including a relativistic mean field, in-medium baryon-baryon cross sections, and three-body collisions. The model is then compared with experimental data for central Au+Au collisions at $2A\text{\ensuremath{-}}10A$ GeV and central Pb+Pb collisions at $30A$ and $40A$ GeV on the proton rapidity spectra, the midrapidity yields of ${\ensuremath{\pi}}^{+},{K}^{\ifmmode\pm\else\textpm\fi{}},$ and $(\ensuremath{\Lambda}+{\ensuremath{\Sigma}}^{0})$, and the transverse mass spectra of ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${K}^{\ifmmode\pm\else\textpm\fi{}}$. The three-body collisions increase the inverse slope parameters of the hadron ${m}_{\ensuremath{\perp}}$ spectra to good agreement with the data.

Partonic transport description of heavy ion collisions

Transport models that take into account non-equilibrium partonic dynamics of relativistic heavy ion collisions are useful tools for relating the properties of the produced partonic matter to the experimental observables. In this talk, we review results obtained from some of the transport models, particularly the multiphase AMPT transport model, regarding the partonic effects on observables from relativistic heavy ion collisions. Also discussed are recent results from the AMPT model for the dependence of the ratio of parton hexadecupole flow to the square of its elliptic flow on the parton collision number and the separation distribution of pion pairs emitted from a heavy ion collision.

Hydrodynamic description of heavy ion collisions

We give a short review of hydrodynamic models at heavy ion collisions from the point of view of initial conditions, an equation of states (EoS) and a freezeout process. Then we show our latest results of a combined fully three-dimensional macroscopic/microscopic transport approach. In this model for the early, dense, deconfined stage relativistic 3D hydrodynamics of the reaction and a microscopic non-equilibrium model for the later hadronic stage where the equilibrium assumptions are no longer valid are employed. Within this approach we study the dynamics of hot, bulk QCD matter, which is being created in ultra-relativistic heavy ion collisions at RHIC.

Equation of state for hot and dense matter: [formula omitted] model with scaled hadron masses and couplings

The proposed earlier relativistic mean-field model with hadron masses and coupling constants depending on the σ-meson field is generalized to finite temperatures. Within this approach we simulate the in-medium behavior of the hadron masses motivated by the Brown–Rho scaling. The high-lying baryon resonances and boson excitations as well as excitations of the σ, ω and ρ fields interacting via mean fields are incorporated into this scheme. Thermodynamic properties of hot and dense hadronic matter are elaborated with the constructed equation of state. Even at zero baryon density, effective masses of σ – ω – ρ – N excitations abruptly drop down for T ≳ 170 MeV and reach zero at a critical temperature T = T c σ ∼ 210 MeV . Below T c σ (at T ∼ 190 MeV ) the specific heat gets a peak like at crossover. We demonstrate that our EoS can be matched with that computed on the lattice for high temperatures provided the baryon resonance couplings with nucleon are partially suppressed. In this case the quark liquid would masquerade as the hadron one. The model is applied to the description of heavy ion collisions in a broad collision energy range. It might be especially helpful for studying phase diagram in the region near possible phase transitions.

Thoughts on non-perturbative thermalization and jet quenching in heavy ion collisions

We start by presenting physical arguments for the impossibility of perturbative thermalization leading to (non-viscous) Bjorken hydrodynamic description of heavy ion collisions. These arguments are complimentary to our more formal argument presented in [Yu.V. Kovchegov, hep-ph/0503038]. We argue that the success of hydrodynamic models in describing the quark–gluon system produced in heavy ion collisions could only be due to non-perturbative strong coupling effects. We continue by studying non-perturbative effects in heavy ion collisions at high energies. We model non-perturbative phenomena by an instanton ensemble. We show that non-perturbative instanton vacuum fields may significantly contribute to jet quenching in nuclear collisions. At the same time, the instanton ensemble contribution to thermalization is likely to be rather weak, leading to non-perturbative thermalization time comparable to the time of hadronization. This example illustrates that jet quenching is not necessarily a signal of a thermalized medium. Indeed, since the instanton models do not capture all the effects of QCD vacuum (e.g., they do not account for confinement), there may be other non-perturbative effects facilitating thermalization of the system.

Medium corrections in the formation of light charged particles in heavy ion reactions

Within a microscopic statistical description of heavy ion collisions, we investigate the effect of the medium on the formation of light clusters. The dominant medium effects are self-energy corrections and Pauli blocking that produce the Mott effect for composite particles and enhanced reaction rates in the collision integrals. Microscopic description of composites in the medium follows the Dyson equation approach combined with the cluster mean-field expansion. The resulting effective few-body problem is solved within a properly modified Alt-Grassberger-Sandhas formalism. The results are incorporated in a Boltzmann-Uehling-Uhlenbeck simulation for heavy ion collisions. The number and spectra of light charged particles emerging from a heavy ion collision changes in a significant manner in effect of the medium modification of production and absorption processes.

Deuteron formation in nuclear matter

We investigate deuteron formation in nuclear matter at finite temperatures within a systematic quantum statistical approach. We consider formation through three-body collisions relevant already at rather moderate densities because of the strong correlations. The three-body in-medium reaction rates driven by the break-up cross section are calculated using exact three-body equations (Alt–Grassberger–Sandhas type) that have been suitably modified to consistently include the energy shift and the Pauli blocking. Important quantities are the lifetime of deuteron fluctuations and the chemical relaxation time. We find that the respective times differ substantially while using in-medium or isolated cross sections. We expect implications for the description of heavy ion collisions in particular for the formation of light charged particles at low to intermediate energies.

Alpha particle evaporation as a probe for dynamical deformations

Evaporation of alpha particles in heavy ion reactions followed by fusion has proved to be a powerful probe for the properties of emitting nuclei. Detailed experimental data and different model calculations allow us to probe whether the foundation of the statistical model holds for the compound nucleus populated in these reactions. It has been observed that in the case of composite nuclei at moderate energies and angular momenta, evaporation spectra are well explained in terms of the standard statistical model CASCADE code employing optical model transmission coefficients in the description of particle evaporation. However, it has been observed that experimental particle spectra from heavy ion induced fusion reactions are no longer consistent with the predictions of such models. It has been predicted by some authors that in these systems the emission barriers are lower than those expected from optical model transmission coefficients calculated for the respective inverse absorption channels. Some authors claim that these spectra may be well explained in terms of the statistical model incorporating only spin dependent level density and without lowering the emission barriers. The field is not yet free from the controversies. Furthermore, the assumption of the very short formation time in statistical model analysis is one extreme of the general evolution process which in fact is a continuous relaxation process, leading to the composite system from the entrance channel to the equilibrated configuration. Recent dynamical description of heavy ion collisions do not support this assumption in many cases. In symmetric entrance channels and for collisions where centre of mass energy is well above the Coulomb barrier, formation time can be even larger than decay time of the resulting composite system. In such cases realistic approach will be to couple the dynamical evolution of the intrinsic excitation of the composite system to a time dependent statistical model calculation. The above question has been addressed in the light of the alpha particle spectra taken in coincidence with the evaporation residues for the asymmetric 28Si+51V and the symmetric 28Si+27Al systems. The experimental data have been interpreted in the framework of dynamical trajectory model calculations.