Stage Of Relativistic Heavy-ion Collisions Research Articles

Erratum: Out-of-equilibrium photon production in the late stages of relativistic heavy-ion collisions [Phys. Rev. C 105 , 044910 (2022)

Erratum: Out-of-equilibrium photon production in the late stages of relativistic heavy-ion collisions [Phys. Rev. C <b>105</b> , 044910 (2022)

Polarized photons from the early stages of relativistic heavy-ion collisions

Polarized photons from the early stages of relativistic heavy-ion collisions

Addendum to “Stability of classical chromodynamic fields”

A system of chromodynamic fields, which can be treated as classical, is generated at the earliest stage of relativistic heavy-ion collisions. Numerical simulations show that the system is unstable but the nature of the instability is not well understood. We study the problem systematically. In the first paper, we have performed a linear stability analysis of space-time uniform chromoelectric and chromomagnetic fields. There they have been considered the Abelian configurations of single-color potentials linearly depending on coordinates and non-Abelian ones where the fields are generated by the multicolor noncommuting uniform potentials. Here we extend and supplement the analysis. We discuss the parallel chromoelectric and chromomagnetic fields that occur simultaneously. We also consider a general non-Abelian configuration of the uniform fields. Finally, we discuss the gauge dependence of our results.

Out-of-equilibrium photon production in the late stages of relativistic heavy-ion collisions

In this work, we assess the importance of nonequilibrium dynamics in the production of photons from the late stages of relativistic heavy-ion collisions. The ${p}_{\mathrm{T}}$-differential spectra and ${v}_{2}$ of photons from the late hadronic stage are computed within a nonequilibrium hadron transport approach, and compared to the results of a local equilibrium evolution using ideal relativistic hydrodynamics. It is found that nonequilibrium dynamics enhance the late-stage photon production at low ${p}_{\mathrm{T}}$ and decreases it at higher ${p}_{\mathrm{T}}$ compared to the estimate from hydrodynamics. This same comparison points to a significant increase in the momentum anisotropies of these photons due to nonequilibrium dynamics. Once combined with photons produced above the particlization temperature in the hydrodynamics evolution, the differences between the two approaches appear modest for the ${p}_{\mathrm{T}}$ differential spectra, but are clearly noticeable at low ${p}_{\mathrm{T}}$ for the elliptic flow: nonequilibrium dynamics enhance the photon ${v}_{2}$ below ${p}_{\mathrm{T}} \ensuremath{\approx}1.4$ GeV.

Evolution of Λ polarization in the hadronic phase of heavy-ion collisions

Using the ampt$+$music$+$urqmd hybrid model, we study the global and local spin polarizations of $\mathrm{\ensuremath{\Lambda}}$ hyperons as functions of the freeze-out temperature of the spin degree of freedom in the hadronic phase of Au$+$Au collisions at $\sqrt{{s}_{NN}}=19.6$ GeV. Including contributions from both the thermal vorticity and thermal shear of the hadronic matter, we find that with the spin freeze-out temperature dropping from the hadronization temperature of 160 MeV to 110 MeV at the kinetic freeze-out, both the global and local spin polarizations of $\mathrm{\ensuremath{\Lambda}}$ hyperons due to the thermal vorticity decrease by a factor of 2, while those due to the thermal shear decrease quickly and become negligibly small at 140 MeV. Our results thus suggest that it is important to take into account the evolution of the spin degree of freedom in the hadronic stage of relativistic heavy-ion collisions when theoretically predicted global and local $\mathrm{\ensuremath{\Lambda}}$ spin polarizations are compared with the experimental measurements.

Multimessenger heavy-ion collision physics

This work studies the production of direct photons in relativistic nuclear collisions, along with the production of hadrons. Radiation from the very first instants to the final moments of the evolution is included. The hybrid model used here describes all stages of relativistic heavy-ion collisions. Chronologically, those are an initial state reflecting the collision of nuclei described within the Color Glass Condensate effective theory; a pre-equilibrium phase based on non-equilibrium linear response; relativistic viscous hydrodynamics, and a hadronic afterburner. The effect of the pre-equilibrium phase on both photonic and hadronic observables is highlighted for the first time. The potential of photon observables -- spectrum, differential elliptic and triangular flow -- to reveal the chemical equilibration time is studied. Finally, we consider "small collision systems", including proton+nucleus collisions and collisions of light nuclei, as probed by hadronic and electromagnetic observables. We demonstrate how photon production can signal the formation of quark-gluon plasma in such small systems.

The energy-momentum tensor at the earliest stage of relativistic heavy-ion collisions

Nuclear collisions at high energies produce a gluon field that can be described using the Colour Glass Condensate (CGC) effective theory at proper times \(\tau \lesssim 1\) fm. The theory can be used to calculate the gluon energy-momentum tensor, which provides information about the early time evolution of the chromo-electric and chromo-magnetic fields, energy density, longitudinal and transverse pressures, and other quantities. We obtain an analytic expression for the energy-momentum tensor using an expansion in the proper time, and working to sixth order. The calculation is technically difficult, in part because the number of terms involved grows rapidly with the order of the \(\tau \) expansion, but also because of several subtle issues related to the definition of event-averaged correlators, the method chosen to regulate these correlators, and the dependence of results on the parameters introduced by the regularization and nuclear density profile functions. All of these issues are crucially related to the important question of the extent to which we expect a CGC approach to be able to accurately describe the early stages of a heavy-ion collision. We present some results for the evolution of the energy density and the longitudinal and transverse pressures. We show that our calculation gives physically meaningful results up to values of the proper time which are close to the regime at which hydrodynamic simulations are initialized. In a companion paper [1] we give a detailed analysis of several other experimentally relevant quantities that can be calculated from the energy-momentum tensor.

Emission of electromagnetic radiation from the early stages of relativistic heavy-ion collisions

We estimate the production of electromagnetic radiation (real and virtual photons) from the early, pre-equilibrium, stage of relativistic heavy-ion collisions. The parton dynamics are obtained as a solution of the Boltzmann equation in the Fokker-Planck diffusion limit. The photon and dilepton rates are integrated and the obtained yields are compared with those from standard sources and with available experimental data. Non-equilibrium photon spectra are predicted for Pb+Pb at $\sqrt{s_{\rm NN}} = 5.02$ TeV.

Probing the structure of the initial state of heavy-ion collisions with pT -dependent flow fluctuations

The connection between initial-state geometry and anisotropic flow can be quantified through a well-established mapping between $p_T$-integrated flow harmonics and cumulants of the initial transverse energy distribution. In this paper we successfully extend this mapping to also include $p_T$-differential flow. In doing so, we find that subleading principal components of anisotropic flow can reveal previously unobserved details of the hydrodynamic response, in both the linear and the nonlinear regimes. Most importantly, we show that they provide novel information on the small-scale structures present in the initial stage of relativistic heavy-ion collisions.

A Non-Equilibrium Approach to Photon Emission from the Late Stages of Relativistic Heavy-Ion Collisions

Cross sections for photon production in hadronic scattering processes have been calculated according to an effective chiral field theory. For π + ρ → π + γ and π + π → ρ + γ processes, these cross sections have been implemented into a novel hadronic transport approach (SMASH), which is suitable for collisions at low and intermediate energies. The implementation is verified by systematically comparing the thermal photon rate to theoretical expectations. The photon rates we obtain are compared to previous works, where scattering processes mediated by ω mesons are found to contribute significantly to the total photon production. Finally, the impact of considering the finite width of the ρ meson is investigated, and a significant enhancement of photon production in the low-energy region is observed. This work is the first step towards a consistent treatment of photon emission in hybrid hydrodynamics+transport approaches. The quantification of the importance of the hadronic stage for the resolution of the direct photon flow puzzle is a next step and can be applied to identify equilibrium and non-equilibrium effects in the hadronic afterburner.

Emergent hydrodynamics in microscopic modeling of early stage of relativistic heavy-ion collisions

Time evolution of hot and dense nuclear matter produced in central gold-gold collisions at energies between E_{lab} = 10 and 160 AGeV is studied within two transport string models, UrQMD and QGSM. In contrast to the previous studies, here we investigate the macroscopic characteristics of the system before the state of chemical and thermal equilibrium is attained. For all energies in question two interesting observations are made for times starting already from t ge 1 fm/c. (1) The matter in the cell expands almost isentropically with nearly constant entropy per baryon. (2) Pressure in the cell appears to be very close to the pressure calculated for equilibrated hadron gas with the same values of energy density, baryon density and strangeness density. The pressure linearly depends on the energy density, P = a(sqrt{s}) varepsilon . Therefore, both observations endorses the formal application of relativistic hydrodynamics from the very early stages of heavy-ion collisions, despite of the fact that the matter in the fireball is out of equilibrium.

Heavy quarks embedded in glasma

Heavy quarks, which are produced at the earliest stage of relativistic heavy-ion collisions, probe the entire history of the quark-gluon plasma that is created in the collision. Initially the plasma is populated with chromodynamic fields which can be treated as classical. We study the transport of heavy quarks across such a system, which is called glasma, using a Fokker-Planck equation where the quarks interact with long wavelength chromodynamic fields. We compute field correlators which are used to calculate the collision terms of the transport equation. Finally, the energy loss and momentum broadening of heavy quarks in the glasma are studied. Both of these quantities are sizable and strongly directionally dependent.

Aspects of Relativistic Heavy-Ion Collisions

The rapid thermalization of quarks and gluons in the initial stages of relativistic heavy-ion collisions is treated using analytic solutions of a nonlinear diffusion equation with schematic initial conditions, and for gluons with boundary conditions at the singularity. On a similarly short time scale of t ≤ 1 fm/c, the stopping of baryons is accounted for through a QCD-inspired approach based on the parton distribution functions of valence quarks, and gluons. Charged-hadron production is considered phenomenologically using a linear relativistic diffusion model with two fragmentation sources, and a central gluonic source that rises with ln 3 ( s N N ) . The limiting-fragmentation conjecture that agrees with data at energies reached at the Relativistic Heavy-Ion Collider (RHIC) is found to be consistent with Large Hadron Collider (LHC) data for Pb-Pb at s N N = 2.76 and 5.02 TeV. Quarkonia are used as hard probes for the properties of the quark-gluon plasma (QGP) through a comparison of theoretical predictions with recent CMS, ALICE and LHCb data for Pb-Pb and p-Pb collisions.

Impact of Glasma on heavy quark observables in nucleus-nucleus collisions at LHC

In the pre-thermal equilibrium stage of relativistic heavy-ion collisions, a strong quasi-classical transverse gluon field emerges at about τ0≃0.1fm/c and evolves together with their longitudinal counterparts according to the classical Yang-Mills (CYM) equations. Recently it has been shown that these fields induce a diffusion of charm quarks in momentum space resulting in a tilt of their spectrum without a significant drag. We find that in nucleus-nucleus collisions at LHC such a novel dynamics of charm quarks leads to an initial enhancement of the nuclear modification factor (RAA) at pT larger than 2GeV/c contrary to the standard lore. Moreover, the same dynamics leads to a larger final elliptic flow (v2) inducing a relation between RAA and v2 that is quite close to the experimental measurements. Our study also shows that such an initial pre-thermal stage is unlikely to be described in terms of a standard drag and diffusion dynamics, because even if one tune such coefficients to reproduce the same RAA(pT) this would imply a significantly smaller v2.

Photons from the early stages of relativistic heavy-ion collisions

We present results about photons production in relativistic heavy ion collisions. The main novelty of our study is the calculation of the contribution of the early stage photons to the photon spectrum. The initial stage is modeled by an ensemble of classical gluon fields which decay to a quark-gluon plasma via the Schwinger mechanism, and the evolution of the system is studied by coupling classical field equations to relativistic kinetic theory; photons production is then computed by including the pertinent collision processes into the collision integral. We find that the contribution of the early stage photons to the direct photon spectrum is substantial for $p_T \approx 2$ GeV and higher, the exact value depending on the collision energy; therefore we identify this part of the photon spectrum as the sign of the early stage. Moreover, the amount of photons produced during the early stage is not negligible with respect to those produced by a thermalized quark-gluon plasma: we support the idea that there is no dark age in relativistic heavy ion collisions.

Charge dependent particle correlations motivated by chiral magnetic effect and chiral vortical effect

In the early stage of relativistic heavy-ion collisions, a strong magnetic field can be generated. The interaction between the strong magnetic field and the hot medium would provide a way to study the nontrivial QCD topological structures, such as the chiral magnetic effect (CME) and chiral vortical effect (CVE). The three particle azimuthal correlator has been proposed to measure the separation of electric/baryonic charges with respect to the reaction plane due to the chiral effects. We report the STAR measurements of three particle azimuthal correlations of charged particles and identified particles to search for possible signatures of the CME and CVE in relativistic heavy-ion collisions of various energies and systems.

Modelling early stages of relativistic heavy-ion collisions

In this study we model early time dynamics of relativistic heavy ion collisions by an initial color-electric field which then decays to a plasma by the Schwinger mechanism. The dynamics of the many particles system produced by the decay is described by relativistic kinetic theory, taking into account the backreaction on the color field by solving self-consistently the kinetic and the field equations. Our main results concern isotropization and thermalization for a 1+1D expanding geometry. In case of small η / s ( η / s ≲ 0.3) we find τ isotropization ≈ 0.8 fm/c and τ thermalization ≈ 1 fm/c in agreement with the common lore of hydrodynamics.

Analytic Bjorken flow in one-dimensional relativistic magnetohydrodynamics

In the initial stage of relativistic heavy-ion collisions, strong magnetic fields appear due to the large velocity of the colliding charges. The evolution of these fields appears as a novel and intriguing feature in the fluid-dynamical description of heavy-ion collisions. In this work, we study analytically the one-dimensional, longitudinally boost-invariant motion of an ideal fluid in the presence of a transverse magnetic field. Interestingly, we find that, in the limit of ideal magnetohydrodynamics, i.e., for infinite conductivity, and irrespective of the strength of the initial magnetization, the decay of the fluid energy density e with proper time τ is the same as for the time-honoured “Bjorken flow” without magnetic field. Furthermore, when the magnetic field is assumed to decay ∼τ−a, where a is an arbitrary number, two classes of analytic solutions can be found depending on whether a is larger or smaller than one. In summary, the analytic solutions presented here highlight that the Bjorken flow is far more general than formerly thought. These solutions can serve both to gain insight on the dynamics of heavy-ion collisions in the presence of strong magnetic fields and as testbeds for numerical codes.

Applications of dissipative and anisotropic hydrodynamics in description of early stages of relativistic heavy-ion collisions

Kinetic and hydrodynamic models describing early stages of relativistic heavy-ion collisions are discussed. We emphasise the role of the shear-bulk coupling for the correct determination of the time dependence of the bulk viscous pressure.

Applications of dissipative and anisotropic hydrodynamics in description of early stages of relativistic heavy-ion collisions

Kinetic and hydrodynamic models describing early stages of relativistic heavy-ion collisions are discussed. We emphasise the role of the shear-bulk coupling for the correct determination of the time dependence of the bulk viscous pressure.