Pions In Heavy-ion Collisions Research Articles

Collective flows of clusters and pions in heavy-ion collisions at GeV energies

Collective flows of clusters and pions in heavy-ion collisions at GeV energies

Shear viscosity of nucleons and pions in heavy-ion collisions at energies of NICA

The shear viscosity is calculated microscopically via the Green-Kubo relation for the series of snapshots in the central region in an ongoing relativistic collision simulated via the UrQMD framework for various bombarding energies in the anticipated NICA experiments. In previous works the shear viscosity was calculated as function of temperature, while the chemical potential of baryon charge was kept constant. In present work we extract, in various time windows, the average energy density, the net baryon density and the small though nonzero net strangeness density. By fitting these parameters to statistical model, one can get temperature and both chemical potentials of baryon charge and strangeness. Simultaneously, these parameters are used as input to simulations in a box, again within the UrQMD transport model. The autocorrelations in time of the energy stress tensor are extracted, and subsequently via the Green-Kubo identities the shear viscosity coefficient of that equilibrium hadronic system is obtained. Then we calculate partial viscosity both for nucleons and pions for five collision energies from Elab = 5 to 40 AGeV. It appears that substantial part of the contribution to total shear viscosity of the system comes out from pion-nucleon and other correlators.

Production of Subthreshold Pions in Heavy-Ion Collisions Using a Hydrodynamic Approach with a Nonequilibrium Equation of State

A hydrodynamic approach with a nonequilibrium equation of state is used to describe collisions between heavy ions with medium energies. The double differential cross sections for the production of subthreshold $${{\pi }^{0}}$$ mesons and $${{\pi }^{ + }}$$ mesons during collisions between various nuclei are calculated to develop this approach with the inclusion of nuclear viscosity effects. They are in good agreement with the available experimental data.

Resonance decay dynamics and their effects on pT spectra of pions in heavy-ion collisions

We examine the influence of resonance decay dynamics on momentum spectra of pions in heavy-ion collisions. Taking the decay $\omega \rightarrow 3 \pi$ as an example, we demonstrate how details of the decay matrix element can modify the physical observables. Such a dynamical effect is commonly neglected in statistical models. To remedy the situation, we formulate a theoretical framework for incorporating hadron dynamics into the analysis, which can be straightforwardly extended to describe general N-body decays.

Directed Flow in Heavy-Ion Collisions and Its Implications for Astrophysics

Analysis of directed flow ( v 1 ) of protons, antiprotons and pions in heavy-ion collisions is performed in the range of collision energies s N N = 2.7–39 GeV. Simulations have been done within a three-fluid model employing a purely hadronic equation of state (EoS) and two versions of the EoS with deconfinement transitions: a first-order phase transition and a smooth crossover transition. The crossover EoS is unambiguously preferable for the description of experimental data at lower collision energies s N N ≲ 20 Gev. However, at higher collision energies s N N ≳ 20 Gev. the purely hadronic EoS again becomes advantageous. This indicates that the deconfinement EoS in the quark-gluon sector should be stiffer at high baryon densities than those used in the calculation. The latter finding is in agreement with that discussed in astrophysics in connection with existence of hybrid stars with masses up to about two solar masses.

Directed flow is a sensitive probe of deconfinement transition

Analysis of available data on directed flow (\(v_{1}\)) of protons, antiprotons and pions in heavy-ion collisions is performed in the range of incident energies relevant to the Nuclotron-based Ion Collider Facility (NICA). Simulations have been done within a three-fluid model employing a purely hadronic equation of state (EoS) and two versions of the EoS involving deconfinement transitions: a first-order phase transition and a smooth crossover transition. High sensitivity of the directed flow, especially the proton one, to onset of the deconfinement transition is found. The crossover EoS is favored by the majority of the considered experimental data. Future data from NICA could clarify certain inconsistency of data at \( E_{lab} = 2-8\)A·GeV.

Elliptic flow difference of charged pions in heavy-ion collisions

Recently, the STAR Collaboration at RHIC has presented experimental evidence for the correlation between the elliptic flow difference of charged pions and charge asymmetry as a possible signal of the chiral magnetic wave. We demonstrate that the STAR results can be understood within the standard viscous hydrodynamics.

What can we learn from the directed flow in heavy-ion collisions at BES RHIC energies?

Analysis of directed flow ($v_1$) of protons, antiprotons and pions in heavy-ion collisions is performed in the range of collision energies $\sqrt{s_{NN}}$ = 2.7--39 GeV. Simulations have been done within a three-fluid model employing a purely hadronic equation of state (EoS) and two versions of the EoS with deconfinement transitions: a first-order phase transition and a smooth crossover transition. The crossover EoS is unambiguously preferable for the description of the most part of experimental data in this energy range. The directed flow indicates that the crossover deconfinement transition takes place in semicentral Au+Au collisions in a wide range of collision energies 4 \lsim\sqrt{s_{NN}}\lsim$ 30 GeV. The obtained results suggest that the deconfinement EoS's in the quark-gluon sector should be stiffer at high baryon densities than those used in the calculation. The latter finding is in agreement with that discussed in astrophysics.

Bose-Einstein condensation of pions in heavy-ion collisions at energies available at the CERN Large Hadron Collider

We analyse in detail the possibility of Bose-Einstein condensation of pions produced in heavy-ion collisions at the beam energy $\sqrt{s_{\rm NN}}$ = 2.76 TeV. Our approach is based on the chemical non-equilibrium thermal model of hadron production which has been generalised to include separately the contribution from the local zero-momentum state. In order to study both the hadronic multiplicities and the transverse-momentum spectra, we use the Cracow freeze-out model which parameterises the flow and space-time geometry of the system at freeze-out in a very economic way. Our analysis indicates that about 5% of all pions may form the Bose-Einstein condensate.

Directed flow indicates a cross-over deconfinement transition in relativistic nuclear collisions

Analysis of directed flow ($v_1$) of protons, antiprotons and pions in heavy-ion collisions is performed in the range of incident energies $\sqrt{s_{NN}}$ = 2.7--27 GeV. Simulations have been done within a three-fluid model employing a purely hadronic equation of state (EoS) and two versions of the EoS involving deconfinement transitions: a first-order phase transition and a smooth crossover transition. High sensitivity of the directed flow, especially the proton one, to the EoS is found. The crossover EoS is favored by the most part of considered experimental data. A strong wiggle in the excitation function of the proton $v_1$ slope at the midrapidity obtained with the first-order-phase-transition EoS and a smooth proton $v_1$ with positive midrapidity slope, within the hadronic EoS unambiguously disagree with the data. The pion and antiproton $v_1$ also definitely testify in favor of the crossover EoS. The results obtained with deconfinement EoS's apparently indicate that these EoS's in the quark-gluon sector should be stiffer at high baryon densities than those used in the calculation.

Energy dependence of the high [formula omitted] pion production and [formula omitted] azimuthal correlation in heavy ion collisions in the experiments with fixed target

The suppression of particle emission at high p T in central heavy ion collisions can be interpreted as a sign of parton energy loss in strongly interacting matter. Measurements of the azimuthal correlation C 2 ( δ ϕ ) can provide the information of the properties modification of mini-jets originating from hard scattering partons. The high p T pions in heavy ion collisions can be sensitive to the partonic phase of the nuclear matter at the energy domain of Nuclotron (JINR) and SIS100 (FAIR). The simulation study of the experimental measurement the π + ( π − ) at high p T spectra, π + / π − yield ratios and the C 2 ( δ ϕ ) azimuthal correlation for the central A u + A u collisions at 3.5 ⋅ A , 5 ⋅ A , 10 ⋅ A and 25 ⋅ A GeV are presented.

PROBING THE SYMMETRY ENERGY AT SUPRA-SATURATION DENSITIES FROM PION EMISSION IN HEAVY-ION COLLISIONS

Within the framework of the improved isospin dependent quantum molecular dynamics (ImIQMD) model, the emission of pion in heavy-ion collisions in the region 1 A GeV as a probe of nuclear symmetry energy at supra-saturation densities is investigated systematically, in which the pion is considered to be mainly produced by the decay of resonances Δ(1232) and N *(1440). The π-/π+ yields are calculated for selected Skyrme parameters SkP, SLy6, Ska and SIII, and also for the cases of different stiffness of symmetry energy with the parameter SLy6. Preliminary results compared with the measured data by the FOPI collaboration favor a hard symmetry energy of the potential term proportional to (ρ/ρ0)γs with γs = 2.

Shadowing effects in direct photon and hadron production in heavy-ion collisions

Nuclear shadowing effects are checked in the production of direct photons and neutral pions in heavy-ion collisions at RHIC/LHC energy within a next-to-leading order perturbative QCD parton model. The transverse momentum dependence of the nuclear modification factors is presented in p + A and A + A collisions to distinguish different parameterizations for nuclear shadowing effects. Our numerical results show that in central A + A collisions the nuclear modification factor for direct photons is more sensitive to shadowing effects than the factor for pion hadrons, and it is a powerful observable to discriminate different parameterizations for parton distributions.

Probing high-density behavior of symmetry energy from pion emission in heavy-ion collisions

Within the framework of the improved isospin dependent quantum molecular dynamics (ImIQMD) model, the emission of pion in heavy-ion collisions in the region 1 A GeV as a probe of nuclear symmetry energy at supra-saturation densities is investigated systematically, in which the pion is considered to be mainly produced by the decay of resonances Δ(1232) and N∗(1440). The total pion multiplicities and the π−/π+ yields are calculated for selected Skyrme parameters SkP, SLy6, Ska and SIII, and also for the cases of different stiffness of symmetry energy with the parameter SLy6. Preliminary results compared with the measured data by the FOPI Collaboration favor a hard symmetry energy of the potential term proportional to (ρ/ρ0)γs with γs=2.

Forward production of protons and pions in heavy-ion collisions

The problem of forward production of hadrons in heavy-ion collisions at RHIC is revisited with a modification of the theoretical treatment on the one hand and with the use of new data on the other. The basic formalism for hadronization remains the same as before, namely, recombination, but the details of momentum degradation and quark regeneration are improved. Recent data on the $p/\ensuremath{\pi}$ and $\overline{p}/p$ ratios are used to constrain the value of the degradation parameter. The transverse momentum (${p}_{T}$) spectrum of the average charged particles is well reproduced. A prediction on the ${p}_{T}$ dependence of the $\overline{p}/p$ ratio at $\ensuremath{\eta}=3.2$ is made.

Parton and hadron correlations in jets

Correlation between shower partons is first studied in high-${p}_{T}$ jets. Then within the framework of parton recombination the correlation between pions in heavy-ion collisions is investigated. Since thermal partons play very different roles in central and peripheral collisions, it is found that the correlation functions of the produced hadrons behave very differently at different centralities, especially at intermediate ${p}_{T}$. The correlation function that can best exhibit the distinctive features is suggested. There is not a great deal of overlap between what we can calculate and what has been measured. Nevertheless, some aspects of our results compare favorably with experimental data.

Anomalous suppression of $\pi^0$ production at large transverse momentum in Au + Au and d + Au collisions at $\sqrt{{\bf s}_{\bf NN}} = 200$ GeV

We propose a model of suppression of large $p_T$-pions in heavy ion collisions based on the interaction of the large $p_T$ pion with the dense medium created in the collision. The model is practically the same as the one previously introduced to describe $J/\psi$ suppression. Both the $p_T$ and the centrality dependence of the data are reproduced. In deuteron-gold collisions, the effect of the final state interaction with the dense medium turns out to be negligibly small. Here the main features of the data are also reproduced both at mid and at forward rapidities.

System size dependence of strangeness production at 158 A GeV

An enhanced production of strange particles per pion in heavy ion collisions compared to elementary p + p interactions was observed in Pb + Pb reactions at the CERN SPS. New results obtained on small colliding systems, C + C and Si + Si, also show an enhancement. It increases steeply with system size and reaches in the larger system almost the level measured in Pb + Pb reactions. The shapes of the transverse mass spectra of strange mesons (K+, K−, ϕ) and baryons are consistent with the presence of collective transverse flow, which is already visible in small systems (C + C) and increases with the system size. The ratio together with the K+/K− ratio yields information about the baryochemical potential μB. The potential, determined at midrapidity, increases with the system size since the stronger stopping shifts baryons from near beam and projectile rapidity towards midrapidity. For the 4π yields μB is almost independent of the size of the colliding system.

Universal pion freeze-out in heavy-ion collisions.

Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions lambda(f) reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider.

Kinematics and dynamics of independent pion emission

Multiparticle boson states, proposed recently for ``independently'' emitted pions in heavy-ion collisions, are reconsidered in standard second quantized formalism and shown to emerge from a simplistic chaotic current dynamics. Compact equations relate the density operator, the generating functional of multiparticle counts, and the correlator of the external current to each other. ``Bose-Einstein condensation'' is related to the external pulse. A quantum master equation is advocated for future Monte Carlo simulations.