We construct a family of equations of state within a quasiparticle model by relating pressure, energy density, baryon density and susceptibilities adjusted to first-principles lattice QCD calculations. The relation between pressure and energy density from lattice QCD is surprisingly insensitive to details of the simulations. Effects from different lattice actions, quark masses and lattice spacings used in the simulations show up mostly in the quark-hadron phase transition region which we bridge over by a set of interpolations to a hadron resonance gas equation of state. Within our optimized quasiparticle model we then examine the equation of state along isentropic expansion trajectories at small net baryon densities, as relevant for experiments and hydrodynamic simulations at RHIC and LHC energies. We illustrate its impact on azimuthal flow anisotropies and transverse momentum spectra of various hadron species.
We analyze the possibilities for studying properties of dense QCD-matter, created in ultrarelativistic nuclear collisions, by hard QCD-production processes, so-called "hard" probes -- heavy quarkonia, hard jets, high mass dimuons. Special attention is paid to the potential of coming heavy ion experiments on Large Hadron Collider to observe the rescattering and energy losses of hard partons in quark-gluon plasma.
The method for simulation of medium-induced rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions is developed. The model is realized as fast Monte-Carlo tool implemented to modify standard PYTHIA jet event.
Hadrons emitted by the pre-surface layer of quark-gluon plasma (QGP) before phase transition into hadronic gas are considered as possible source of direct information about QGP. It is shown that these hadrons dominate at soft p_t if QGP is created in ultrarelativistic heavy ion collisions.
We present a model for radiative energy loss of heavy quarks in quark gluon plasma which incorporates coherence effects. We then study its consequences on the radiation spectra as well as on the nuclear modification factor of open heavy mesons produced in ultrarelativistic heavy ion collisions.
Basing on model concepts we research the monojet-to-dijet ratio as a function of the jet energy detection threshold in ultrarelativistic collisions of nuclei. We provide an comparative analysis of the contribution to monojet yield from gluon radiation before initial hard parton-parton scattering and from non-symmetric dijet energy losses in quark-gluon plasma, which expected to be created in ultrarelativistic heavy ion collisions.
The method to simulate the rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions has been developed. The model is a fast Monte-Carlo tool introduced to modify a standard PYTHIA jet event. The full heavy ion event is obtained as a superposition of a soft hydro-type state and hard multi-jets. The model is applied to analysis of the jet quenching pattern at RHIC.
The possibility to observe the medium-modified fragmentation of hard b-quarks tagged by a leading muon in ultrarelativistic heavy ion collisions is analyzed. We have found that reasonable statistics, ~20000 events per 1 month of LHC run with lead beams, can be expected for the realistic geometrical acceptance and kinematic cuts. The numerical estimates on the effect of the medium-induced softening b-jet fragmentation function are given.
We consider the production of the open charm and J/psi mesons in heavy ion collisions at BNL RHIC. We discuss several recently developed pictures for J/psi production and argue that a measurement at RHIC energies is crucial for disentangling these different descriptions.
We discuss a possible new manifestation of the formation of an initial nonequilibrium flux tube stage in high energy heavy ion collisions. It is shown that a strong asymmetry in particle distributions around axes of hard-transverse jets takes place due to a large difference between longitudinal and transverse forces acting on the hardly scattered quarks or gluons crossing the flux tubes.
Some recent experimental results obtained in collisions of heavy nuclei ($\sqrt{s}=200$ GeV) at BNL Relativistic Heavy-Ion Collider (RHIC) are discussed. The probes of dense matter created in heavy-ion collision by quarkonia, $D$ and $B$ mesons containing heavy charm and beauty quarks are considered. The centrality, rapidity and transverse momentum dependences of the nuclear modification factor and elliptic flow coefficient are presented and their possible theoretical interpretation is provided.
The mean transverse momentum in high energy nuclear collisions is studied. The formation of a Color Glass Condensate results in an increase of the square of the mean transverse momentum with the charged multiplicity per unit rapidity and unit transverse area. This prediction is in agreement with the presently available data, in particular with the heavy-ion data, lending support to the notion that the transverse momentum spectra in ultrarelativistic collisions of heavy nuclei might be controlled by intrinsic transverse momentum broadening and not by hadronic rescattering, i.e. transverse flow.
We discuss the modification of a jet fragmentation function due to medium-induced partonic energy loss in context of leading particle observables in ultrarelativistic nucleus-nucleus interactions. We also analyze the relation between in-medium softening jet fragmentation function and suppression of the jet rates due to energy loss outside the jet cone. The predicted anti-correlation between two effects allows to probe a fraction of partonic energy loss carried out of the jet cone and truly lost to the jet.
We study color fluctuations in the quark-gluon plasma produced at the early stage of nucleus-nucleus collision at RHIC or LHC. The fluctuating color current, which flows along the beam, can be very {\it large} due to the strong anisotropy of the parton momentum distribution. A specific fluctuation, which splits the parton system into the current filaments parallel to the beam direction, is argued to grow exponentially. The physical mechanism responsible for the phenomenon, which is known as a filamentation instability, is discussed.
The azimuthal anisotropy of jet spectra due to energy loss of jet partons in azimuthally non-symmetric volume of dense quark-gluon matter is considered for semi-central nuclear interactions at collider energies. We develop the techniques for event-by-event analysing the jet azimuthal anisotropy using particle and energy elliptic flow, and suggest a method for calculation of coefficient of jet azimuthal anisotropy without reconstruction of nuclear reaction plane.
In ultrarelativistic heavy-ion collisions at $\sqrt{s} >$ 20 (120) A$\cdot$GeV a copious production of charm (bottom) production sets in which, via correlated semileptonic $D \bar D$ ($B \bar B$) decays, gives rise to a dilepton yield at invariant mass $M \approx$ 2 -- 3 GeV in excess of the Drell-Yan yield and the thermal dilepton signal from deconfined matter as well. We show that appropriate single-electron transverse momentum cuts (suitable for ALICE at LHC) cause a threshold like behavior of the dilepton spectra from heavy-quark meson decays and the Drell-Yan process and can allow to obse…
The production and annihilation of antiprotons in the hadronic phase of heavy-ion collisions is evaluated within a thermal equilibrium approach. It is shown that the inverse reaction of $p\bar p$ annihilation (i.e., multi-pion annihilation $N_ππ\to p\bar p$ with $N_π\simeq$~5-7) in connection with oversaturation of pion phase space (i.e., finite pion chemical potentials) plays an important role in understanding the observed antiproton yields at SpS energies within the standard picture of subsequent chemical and thermal freezeout. Implications for RHIC energies are also addressed.
Results from a model study on the formation of exotic quark clusters at the hadronization stage of a heavy ion collision are presented. The dynamical quark molecular dynamics (qMD) model which is used is sketched, and results for exotica made of up to six (anti-)quarks are shown. The second part focuses on pentaquarks. The rapidity distribution are shown, and the distribution of strangeness is found to yield an indicator of thermalization and homogenisation of the deconfined quark system. Relative Theta^+ yields are found to be lower than thermal model estimates.
We study, for the first time, differential distributions for two $ρ^0$ meson production in exclusive ultraperipheral, ultrarelativistic heavy ion collisions via a double-scattering mechanism. The calculations are done in the impact parameter space. The cross section for $γA \to ρ^0 A$ is parametrized based on an existing model. Smearing of $ρ^0$ masses is taken into account. The results of calculations for single and double-$ρ^0$ production are compared to experimental data at the RHIC and LHC energies. The mechanism considered gives a significant contribution to the $AA \to AA π^+π^-π^+π^-$ r…
Based on the factorization formalism of nonrelativistic quantum chromodynamics (NRQCD), we calculate the production cross section for the charmonium and the bottomonium produced by the hard photoproduction processes and fragmentation processes in relativistic heavy ion collisions. It is shown that the existing experimental data on heavy quarkonium production at the Large Hadron Collider (LHC) can be described in the framework of the NRQCD formalism, and the phenomenological values of matrix elements for color-singlet and color-octet components give the main contribution. The numerical results …
We introduce an event-by-event perturbative-QCD + saturation + hydro ("EKRT") framework for ultrarelativistic heavy-ion collisions, where we compute the produced fluctuating QCD-matter energy densities from next-to-leading order perturbative QCD using a saturation conjecture to control soft particle production, and describe the space-time evolution of the QCD matter with dissipative fluid dynamics, event by event. We perform a simultaneous comparison of the centrality dependence of hadronic multiplicities, transverse momentum spectra, and flow coefficients of the azimuth-angle asymmetries, aga…
We develop a 1+1 dimensional hydrodynamical model for central heavy-ion collisions at ultrarelativistic energies. Deviations from Bjorken's scaling are taken into account by implementing finite-size profiles for the initial energy density. The calculated rapidity distributions of pions, kaons and antiprotons in central Au+Au collisions at the c.m. energy 200 AGeV are compared with experimental data of the BRAHMS Collaboration. The sensitivity of the results to the choice of the equation of state, the parameters of initial state and the freeze-out conditions is investigated. Experimental constr…
We study the induced photon bremsstrahlung from a fast quark produced in AA-collisions due to multiple scattering in quark-gluon plasma. For RHIC and LHC conditions the induced photon spectrum is sharply peaked at photon energy close to the initial quark energy. In this region the contribution of the induced radiation to the photon fragmentation function exceeds the ordinary vacuum radiation. Contrary to previous analyses our results show that at RHIC and LHC energies the final-state interaction effects in quark-gluon plasma do not suppress the direct photon production, and even may enhance it…
We ``quantify'' the role of elastic as well as inelastic gg<->ggg pQCD processes in kinetic equilibration within a pQCD inspired parton cascade. The contributions of different processes to kinetic equilibration are manifested by the transport collision rates. We find that in a central Au+Au collision at RHIC energy pQCD Bremstrahlung processes are much more efficient for momentum isotropization compared to elastic scatterings. For the parameters chosen the ratio of their transport collision rates amounts to 5:1.
The transverse momentum distributions for inclusive $η_{c,b}$ meson described by gluon-gluon interactions from photoproduction processes in relativistic heavy ion collisions are calculated. We considered the color singlet (CS) and color octet (CO) components with the framework of non-relativistic Quantum Chromodynamics (NRQCD) into the production of heavy quarkonium. The phenomenological values of the matrix elements for the color-singlet and color-octet components give the main contribution to the production of heavy quarkonium from the gluon-gluon interaction caused by the emission of additi…
We investigate the consequences of a nonzero bulk viscosity coefficient on the transverse momentum spectra, azimuthal momentum anisotropy, and multiplicity of charged hadrons produced in heavy ion collisions at LHC energies. The agreement between a realistic 3D hybrid simulation and the experimentally measured data considerably improves with the addition of a bulk viscosity coefficient for strongly interacting matter. This paves the way for an eventual quantitative determination of several QCD transport coefficients from the experimental heavy ion and hadron-nucleus collision programs.
We present a new cascade-type microscopic simulation of nucleus-nucleus collisions at RHIC energies. The basic elements are partons (quarks and gluons) moving in 8N-dimensional phase space according to Poincare-covariant dynamics. The parton-parton scattering cross sections used in the model are computed within perturbative QCD in the tree-level approximation. The Q^2 dependence of the structure functions is included by an implementation of the DGLAP mechanism suitable for a cascade, so that the number of partons is not static, but varies in space and time as the collision of two nuclei evolve…
The role of minijet production as initial conditions for QGP production at $τ\sim 0.1 fm/c$ in nuclear collisions at the LHC and RHIC energies is discussed.
We calculate the centrality dependence of inclusive cross section of large-pT charmed-meson ($D^{0}$, $D^{*}$, $D^{*+}$, and $D_{s}^{+}$) from heavy quark fragmentation by the hard photoproduction processes for the nucleus-nucleus collisions in ultrarelativistic heavy ion collisions. The numerical results indicate that the modification of the hard photoproduction processes cannot be negligible for the charmed-meson production in Au-Au collisions at Relativistic Heavy Ion Collider (RHIC) and Pb-Pb collisions at Large Hadron Collider (LHC).
Coulomb corrections to perturbation theory for producing electron-positron pairs in ultrarelativistic heavy ion collisions are considered in a part-analytical, part-numerical approach. Production probabilities are reduced from perturbation theory with increasing charge of the colliding heavy ions, as has been previously argued in the literature. It is shown here that the reduction from perturbation theory comes from the appropriate physical spatial cutoff of the electromagnetic potentials arising from the colliding ultrarelativistic heavy ions.
An exact solution of the two center time-dependent Dirac equation for pair production induced by ultrarelativistic heavy ion collisions is presented. Cross sections to specific final states approach those of perturbation theory. Multiplicity rates are reduced from perturbation theory.
An exact solution of the time-dependent Dirac equation for ionization and pair production induced by ultrarelativistic heavy ion collisions is presented. Exact transition probabilities, equivalent to those that would be obtained in an untruncated basis coupled channels calculation, are presented. Exact bound-electron positron pair production probabilities are calculated to be mostly smaller than those calculated with the same potential in perturbation theory at impact parameters small enough for differences to occur.
In recent works [Segev,Wells; Baltz, McLerran; Eichmann, Reinhardt, Schramm, Greiner] it was claimed that the Coulomb correction to the $e^+e^-$ production cross section in relativistic heavy-ion collisions is absent. We point out that this statement is in obvious contradiction to some well known results. We obtain large Coulomb corrections in the pair production cross section for the RHIC and LHC heavy-ion colliders.
We review the production of lepton pairs during high energy nuclear collisions. We highlight the information being carried out of the hot and dense strongly interacting medium. We describe the phenomenon of scalar-vector mixing that can take place in a dense medium and suggest possible measurable signatures of this effect in high energy heavy ion collisions.
If the bulk viscosity of QCD matter is large, the effective pressure of the hot and dense matter created in ultrarelativistic heavy ion collisions can become negative, leading to instabilities in the evolution of the plasma. In the context of heavy ion collisions, this effect is sometimes referred to as cavitation. In this contribution we discuss the onset of cavitation in event-by-event hydrodynamic simulations of ultrarelativistic heavy ion collisions at LHC energies. We estimate how large the bulk viscosity of the QGP has to be in the QCD (pseudo) phase transition region in order for the ef…
We present a new cascade-type microscopic simulation of Nucleus-Nucleus Collisions at RHIC energies. The basic elements are partons (quarks and gluons) moving in 8N-dimensional phase space according to Poincaré-covariant dynamics. The parton-parton scattering cross sections used in the model are computed within perturbative QCD in the tree-level approximation. The $Q^2$ dependence of the structure functions is included by an implementation of the DGLAP mechanism suitable for a cascade, so that the number of partons is not static, but varies in space and time as the collision of two nuclei evol…
The effect of shadowing on the early state of ultrarelativistic heavy ion collisions is investigated along with transverse energy and hard process production, specifically Drell-Yan, $J/ψ$, and $Υ$ production. We choose several parton distributions and parameterizations of nuclear shadowing, as well as the spatial dependence of shadowing, to study the influence of shadowing on relevant observables. Results are presented for Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV and Pb+Pb collisions at $\sqrt{s_{NN}} = 5.5$ TeV.
Recent results on minijet production in nuclear collisions at the RHIC and LHC energies are reviewed. Initial conditions of the QGP at $τ=0.1$ fm$/c$, especially parton chemistry, thermalization and net baryon number-to-entropy ratio are discussed. Also, contribution of minijets from a hard BFKL-pomeron ladder will be estimated.
We show how anisotropy in momentum of the gluon distribution in ultrarelativistic heavy ion collisions gives rise to polarization of direct photons produced via gluon-quark Compton scattering, as well as by quark-antiquark annihilation into a gluon-photon pair. We estimate the polarization asymmetry from the Compton process within a toy model where the polarized photons are produced from thermal gluons scattered by heavy-quark scattering centers moving with Bjorken boost-invariant flow, and find that it could be as large as 10%. We conclude that polarization measurements of directly produced p…
Using the quasiclassical Green's function in the Coulomb field, we analyze the probabilities of single and multiple $e^+e^-$-pair production at fixed impact parameter $b$ between colliding ultrarelativistic heavy nuclei. We perform calculations in the Born approximation with respect to the parameter $Z_Bα$, and exactly in $Z_Aα$, $Z_{A}$ and $Z_{B}$ are the charge numbers of the corresponding nuclei. We also obtain the approximate formulas for the probabilities valid for $Z_Aα, Z_Bα\lesssim 1$.
Global transverse-momentum conservation induces correlations between any number of particles, which contribute in particular to the two- and three-particle correlations measured in heavy-ion collisions. These correlations are examined in detail, and their importance for studies of jets and their interaction with the medium is discussed.
One of the striking features of particle production at high beam energies is the near equal abundance of matter and antimatter in the central rapidity region. In this paper we study how this symmetry is reached as the beam energy is increased. In particular, we quantify explicitly the energy dependence of the approach to matter/antimatter symmetry in proton-proton and in heavy-ion collisions. Expectations are presented also for the production of more complex forms of antimatter like antihypernuclei.
Using next-to-leading order perturbative QCD and a conjecture of saturation to suppress the production of low-energy partons, we calculate the initial energy densities and formation times for the dissipative fluid dynamical evolution of the quark-gluon plasma produced in ultrarelativistic heavy-ion collisions. We identify the framework uncertainties and demonstrate the predictive power of the approach by a good global agreement with the measured centrality dependence of charged particle multiplicities, transverse momentum spectra and elliptic flow simultaneously for the Pb+Pb collisions at the…
We manifest the origin of the wrong conclusion made by several groups of authors on the absence of Coulomb corrections to the cross section of the e^+e^- pair production in ultrarelativistic heavy-ion collisions. The source of the mistake is connected with an incorrect passage to the limit in the expression for the cross section. When this error is eliminated, the Coulomb corrections do not vanish and agree with the results obtained within the Weizsäcker-Williams approximation.
In this paper we consider the production of proton-antiproton pairs in two-photon interactions in electron-positron and heavy-ion collisions. We try to understand the dependence of the total cross section on the photon-photon c.m. energy as well as corresponding angular distributions measured by the Belle Collaboration for the $γγ\to p \bar{p}$ process. To understand the Belle data we include the proton-exchange, the $f_2(1270)$ and $f_2(1950)$ $s$-channel exchanges, as well as the hand-bag mechanism. The helicity amplitudes for the $γγ\to f_{2} \to p \bar{p}$ process are written explicitly ba…
The potential of B-physics in ultrarelativistic heavy ion collisions is discussed. The different mechanisms of heavy quark production at high energies are considered. We analyze the sensitivity of high-mass mu+mu- pairs from BBbar semileptonic decays and secondary J/Psi's from single B decays to the medium-induced bottom quark energy loss at LHC energies.
In this contribution, we present some predictions for the production of D and B mesons in ultrarelativistic heavy ion collisions at RHIC and LHC energies and confront them with experimental results obtained so far by the STAR, PHENIX, ALICE and CMS collaborations. We next discuss some preliminary results obtained with an improved description of the medium based on EPOS initial conditions, and its possible implications on the nuclear modification factor and on the elliptic flow of heavy quarks.
The influence of the nuclear Coulomb field on transverse spectra of $π^+$ and $π^-$ measured in $Pb+Pb$ reactions at 158 A GeV has been investigated. Pion trajectories are calculated in the field of an expanding fireball. The observed enhancement of the $π^-/π^+$ ratio at small momenta depends on the temperature and transverse expansion velocity of the source, the rapidity distribution of the net positive charge, and mainly the time of the freeze-out.
The $γγ\toππ$ reactions are discussed. To describe those processes, we include dipion continuum, resonances, high-energy pion-pion rescatterings, $ρ$ meson exchange and pQCD Brodsky-Lepage mechanisms. The cross section for the production of pion pairs in photon-photon collisions in peripheral heavy ion collisions is calculated with the help of Equivalent Photon Approximaption (EPA) in the impact parameter space. We show predictions at $\sqrt{s_{NN}}=3.5$ TeV which could be measured e.g. by the ALICE collaboration at the LHC.
In this proceedings contribution, we will review the basics of quasiparticle anisotropic hydrodynamics. Then, we will present some recent phenomenological results of 3+1d quasiparticle anisotropic hydrodynamics and Pb-Pb $2.76$ TeV collisions from the ALICE collaboration. We show that 3+1d quasiparticle anisotropic hydrodynamics model is able to describe the experimental results for Pb-Pb collisions at $2.76$ TeV quite well for many observables such as the spectra, the elliptic flow, and HBT radii in many centrality classes.
Anisotropy in heavy-ion collisions leads to polarization of both direct and virtual photons, the latter detected via internal conversion to dileptons pairs. Thus measurement of photon polarization probes the anisotropy at all stages of collisions. In order to characterize the polarization of virtual photons, we derive here the general structure of the photon polarization operator, rho^(mu nu), in an anisotropic medium, in terms of the four spectral functions, two transverse and one longitudinal, as usual, plus a new spectral function that reflects anisotropy in asymmetric collisions; and deriv…
We discuss exclusive production of lepton-antilepton, quark-antiquark, $ππ$ and $ρ^0 ρ^0$ pairs in ultraperipheral, ultrarelativistic heavy-ion collisions. The cross sections for exclusive production of pairs of particles is calculated in Equivalent Photon Approximation (EPA). Realistic (Fourier transform of charge density) charge form factors of nuclei are used and the corresponding results are compared with the cross sections calculated with monopole form factor used in the literature. Absorption effects are discussed and quantified. The cross sections obtained with realistic form factors ar…
In ultrarelativistic heavy ion collisions, charm quark fragmentation is modified due to the melting of strings inside the high density partonic matter. D mesons produced in hadronization can further produce J/psi particles. Using a multiphase transport model, we investigate the effect on the rapidity distribution of the produced J/psi particles with two different charm quark fragmentation functions. It is shown that the J/psi rapidity distribution is sensitive to the nuclear modification of charm quark fragmentation and thus is a good indicator of the onset of string melting and the production…
We discuss the transverse momentum distribution of thermal and prompt photons in ultrarelativistic heavy ion collisions. The prompt photons are mostly produced by fragmentation of quark jets (bremsstrahlung). If these quark jets suffer a large energy loss in heavy-ion reactions, prompt photons are significantly suppressed. Thermal electromagnetic radiation from the quark-gluon plasma and the hadron gas might then dominate the intermediate k_T-range. In central Au+Au collisions at \sqrt{s}=200A GeV, and in the range 2 GeV < k_T < 4 GeV, the inverse slope of the thermal radiation is 360-460 MeV,…
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 RHIC and 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 dec…
