Ultrarelativistic Nucleus-nucleus Collisions Research Articles

Examinations of the early degrees of freedom in ultrarelativistic nucleus-nucleus collisions

The Parton-Hadron-String-Dynamics (PHSD) transport model is used to study the impact on the choice of initial degrees of freedom on the final hadronic and electromagnetic observables in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We find that a non-perturbative system of massive gluons (scenario I) and a system dominated by quarks and antiquarks (scenario II) lead to different hadronic observables when imposing the same initial energy-momentum tensor $T_{\mu \nu}(x)$ just after the passage of the impinging nuclei. In case of the gluonic initial condition the formation of $s,{\bar s}$ pairs in the QGP proceeds rather slow such that the anti-strange quarks and accordingly the $K^+$ mesons do not achieve chemical equilibrium even in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Accordingly, the $K^+$ rapidity distribution is suppressed in the gluonic scenario and in conflict with the data from the BRAHMS Collaboration. The proton and antiproton rapidity distributions also disfavor the scenario I. Furthermore, a clear suppression of direct photon and dilepton production is found for the pure gluonic initial conditions which is not so clearly seen in the present photon and dilepton spectra from Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV due to a large contribution from other channels. It is argued that dilepton spectra in the invariant mass range 1.2 GeV $< M <$ 3 GeV will provide a definitive answer once the background from correlated $D$-meson decays is subtracted experimentally.

Parton-hadron matter at finite temperature and chemical potential

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities or the electric conductivity of the hot QCD medium. The PHSD model reproduces a large variety of observables from SPS to LHC energies, e.g. the quark-number scaling of elliptic flow, transverse mass and rapidity spectra of charged hadrons, dilepton spectra, open and hidden charm production, collective flow coefficients etc., which are associated with the observation of a strongly interacting QGP (sQGP). The ’highlights’ of the latest results from LHC energies are presented with a focus on the correlation between the average transverse momentum < pT > and the number of charged particles Nch in p-p, p-Pb and Pb-Pb collisions at midrapidity as observed by the ALICE Collaboration.

The QGP dynamics in relativistic collisions

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities or the electric conductivity of the hot QCD medium. The PHSD model reproduces a large variety of observables from SPS to LHC energies, e.g. the quark-number scaling of elliptic flow, transverse mass and rapidity spectra of charged hadrons, dilepton spectra, open and hidden charm production, collective flow coefficients etc., which are associated with the observation of a strongly interacting QGP (sQGP). The ‘highlights’ of the latest results from LHC energies are presented with a focus on observable effects from the non-equilibrium initial dynamics.

Resummation of Jet Shapes and Extracting Properties of the Quark-Gluon Plasma

Understanding the properties of the quark-gluon plasma (QGP) that is produced in ultra-relativistic nucleus-nucleus collisions has been one of the top priorities of the heavy ion program at the LHC. Energetic jets are produced and subsequently quenched in the collisions. Such jet quenching phenomena provide promising tools to probe the medium properties by studying the modification of jets due to the medium interactions. Significant modifications of jet shapes have been measured. In this talk we focus on the calculation of jet shapes in both proton-proton and lead-lead collisions using soft-collinear effective theory (SCET), with Glauber gluon interactions in the medium. Large logarithms in jet shapes are resummed at next-to-leading logarithmic (NLL) accuracy by the renormalization-group evolution between hierarchical jet scales. The medium interactions contribute as power corrections, and we calculate the modification of jet shapes at leading order in opacity with the static QGP model. Preliminary results are presented with good agreement with the recent CMS jet shape measurements.

Jet shape resummation using soft-collinear effective theory

The jet shape is a classic jet substructure observable that probes the average transverse energy profile inside a reconstructed jet. The studies of jet shapes in proton-proton collisions have served as precision tests of perturbative Quantum Chromodynamics (QCD). They have also recently become the baseline for studying the in-medium modification of parton showers in ultra-relativistic nucleus-nucleus collisions. The jet shape is a function of two angular parameters $R$ and $r$, which can be at hierarchical scales. Its calculation suffers from large logarithms of the ratio between the two scales, and these phase space logarithms can conveniently be resummed in the framework of soft-collinear effective theory (SCET). We find that, up to power corrections, the integral jet shape can be expressed in a factorized form which involves only the ratio between two jet energy functions. Resummation is performed at next-to-leading logarithmic order using renormalization-group evolution techniques. Comparisons to jet shape measurements at the LHC are presented to verify the dominant role of the collinear parton shower and to identify the kinematic region in which power-suppressed soft modes and non-perturbative effects may play a role.

Pre-equilibrium radial flow from central shock-wave collisions in AdS 5

Using gauge/gravity duality, central ultrarelativistic nucleus-nucleus collisions are modelled as collisions of shock waves in five-dimensional asymptotic AdS space. For early times after the collision, it is possible to analytically match the metric from the past to the future light-cone. This allows extraction of the pre-equilibrium energy-momentum tensor of the strongly coupled, large N gauge theory. For central collisions, this allows qualitative statements concerning the build-up of radial flow at mid-rapidity in AA and pA collisions. We find that the early-time radial flow buildup is identical to that expected from ideal hydrodynamics with an entropy density proportional to the square root of the product of the matter densities in the individual "nuclei".

A perturbative framework for jet quenching

Abstract We present a conceptually new framework for describing jet evolution in the dense medium produced in ultra-relativistic nucleus-nucleus collisions using perturbative QCD and its implementation into the Monte Carlo event generator Jewel. The rescattering of hard partons in the medium is modelled by infrared continued pQCD matrix elements supplemented with parton showers. The latter approximate higher order real-emission matrix elements and thus generate medium-induced gluon emissions. The interplay between different emissions is governed by their formation times. The destructive interference between subsequent scattering processes, the non-Abelian version of the Landau-Pomeranchuk-Migdal effect, is also taken into account. In this way the complete radiation pattern is consistently treated in a uniform way. Results obtained within this minimal and theoretically well constrained framework are compared with a variety of experimental data susceptible to jet-quenching effects at both RHIC and the LHC. Overall, a good agreement between data and simulation is found. This new framework also allows to identify and quantify the dominant uncertainties in the simulation, and we show some relevant examples for this.

Colloquium: Phase diagram of strongly interacting matter

A fundamental question of physics is what ultimately happens to matter as it is heated or compressed. In the realm of very high temperature and density the fundamental degrees of freedom of the strong interaction, quarks and gluons, come into play and a transition from matter consisting of confined baryons and mesons to a state with ``liberated'' quarks and gluons is expected. The study of the possible phases of strongly interacting matter is at the focus of many research activities worldwide. In this paper the physical aspects of the phase diagram, its relation to the evolution of the early Universe, as well as the inner core of neutron stars are discussed. Also recent progress in the experimental study of hadronic or quark-gluon matter under extreme conditions with ultrarelativistic nucleus-nucleus collisions is summarized.

Asymmetric collision of two shock waves in AdS5

We consider high energy collisions of two shock waves in AdS5 as a model of ultrarelativistic nucleus-nucleus collisions in the boundary theory. We first calculate the graviton field produced in the collisions in the NLO and NNLO approximations, corresponding to three- and four-graviton exchanges with the shock waves. We then consider the asymmetric limit where the energy density in one shock wave is much higher than in the other one. In the boundary theory this setup corresponds to proton-nucleus collisions, with the nucleus being the denser of the two shock waves and the proton being the less dense one. Employing the eikonal approximation we find the exact high energy analytic solution for the metric in AdS5 for the asymmetric collision of two delta-function shock waves. The solution resums all-order graviton exchanges with the ``nucleus'' shock wave and a single-graviton exchange with the ``proton'' shock wave. Using the holographic renormalization prescription we read off the energy-momentum tensor of the matter produced in proton-nucleus collisions. We show in explicit detail that in the boundary theory the proton is completely stopped by strong-coupling interactions with the nucleus, in agreement with our earlier results [1]. We also apply the eikonal technique to the asymmetric collision of two unphysical delta-prime shock waves, which we introduced in [1] as a means of modeling nuclear collisions with weak coupling initial dynamics. We obtain a surprising result that, for delta-prime shock waves, the multiple bulk graviton exchange series giving the leading energy-dependent contribution to the energy-momentum tensor terminates at the order of two graviton exchanges with the nucleus.

Kaon and pion spectra in relativistic and ultrarelativistic nucleus-nucleus collisions

The formation of kaon and pion spectra is considered within the relativistic hydrodynamic model of interaction of heavy nuclei. The terms describing the formation of quarks and gluons are included in the state equation of the hydrodynamic model. This approach allowed consideration of nucleus-nucleus collisions in the range from 4 to 160 GeV/nucleon and reproduce the increase in temperature of the kaons formed, which is not reproduced within the ultrarelativistic quantum molecular-dynamics model and other microscopic approaches.

Hydrodynamic source with continuous emission in Au+Au collisions ats=200GeV

We analyze single particle momentum spectra and interferometry radii in central Au+Au collisions at RHIC within a hydro-inspired parametrization accounting for continuous hadron emission through the whole lifetime of hydrodynamically expanding fireball. We found that a satisfactory description of the data is achieved for a physically reasonable set of parameters when the emission from non space-like sectors of the enclosed freeze-out hypersurface is fairly long: $ 9$ fm/c. This protracted surface emission is compensated in outward interferometry radii by positive $r_{out} - t$ correlations that are the result of an intensive transverse expansion. The main features of the experimental data are reproduced: in particular, the obtained ratio of the outward to sideward interferometry radii is less than unity and decreases with increasing transverse momenta of pion pairs. The extracted value of the temperature of emission from the surface of hydrodynamic tube approximately coincides with one found at chemical freeze-out in RHIC Au+Au collisions. A significant contribution of the surface emission to the spectra and to the correlation functions at relatively large transverse momenta should be taken into account in advanced hydrodynamic models of ultrarelativistic nucleus-nucleus collisions.

Selected Contributions to the Poster Session, Future Experiments and Facilities at the Quark Matter 2005, 18th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, August 4–9, 2005, Budapest, Hungary

Selected Contributions to the Poster Session, Future Experiments and Facilities at the Quark Matter 2005, 18th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, August 4–9, 2005, Budapest, Hungary

Direct-photon production in heavy-ion collisions from SPS to RHIC energies

Direct photons are an important tool for the detection of the quark-gluon plasma in ultra-relativistic nucleus-nucleus collisions. Direct-photon measurements were made in Pb + Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 17.2$ GeV and in Au + Au collisions at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV. These results are reviewed and compared with model calculations.

Two particle rapidity, transverse momentum, and azimuthal correlations in relativistic nuclear collisions and transverse radial expansion

At the very first stage of an ultra-relativistic nucleus-nucleus collision new particles are produced in individual nucleon-nucleon collisions. In the transverse plane, all particles from a single $NN$ collision are initially located at the same position. The subsequent transverse radial expansion of the system creates strong position-momentum correlations and leads to characteristic rapidity, transverse momentum, and azimuthal correlations among the produced particles.

Strangeness Production in Ultrarelativistic Nucleus–NucleusCollisions

Based on the relaxation equations describing the chemicalequilibration of gluons, quarks and s quarks at finite baryondensity derived from the Jüttner distribution of partons,with the help of a rapid phase transition scenario from quarkphase to hadron phase, we calculate strangeness productionin the quark phase and hadron phase. It is found that the K−/π−ratio is enhanced to be larger than that in pp collisions by about afactor 3.

Influence of the partonic Pauli blocking on the hadronic final state in relativistic nucleus-nucleus collisions

A simple transport model for ultra-relativistic nucleus-nucleus collisions is proposed to investigate the effect of Pauli blocking, in the transition from the hadron phase to parton phase, on the hadronic final state. A boost invariant study of the Pauli blocking is implemented in the Monte Carlo simulation for the first time. It turns out that this partonic Pauli effect on the final hadron multiplicity in a nucleus-nucleus collision is negligible at SPS energy, around two percent at RHIC energy, but reaching ten percent at LHC energy. The higher reaction energy the stronger is the partonic Pauli effect.

ICPAQGP2001: Conference summary

I review recent progress in ultrarelativistic nucleus-nucleus collisions, and the connection of this field to modern QCD theory of deconfinement and/or chiral symmetry restoration. The talks at this Conference have shown a convergence of data and theory as far as the CERN SPS investigations at √s = 17 GeV are concerned; the parton-hadron phase boundary seems now located atT = 170 ± 10 MeV. New data from RHIC and direct photon production results from CERN have been shown that point out the field’s future direction: analysis of partonic matter atT > 200 MeV. Astrophysics analysis was shown to be linked crucially to further theoretical progress with non-perturbative QCD.

Production and rescattering of heavy quarks in ultrarelativistic nucleus-nucleus collisions

Contributions of various mechanisms to the heavy-quark-production cross section in ultrarelativistic nucleus-nucleus collisions, including the direct pair production \(b\bar b\) and \(c\bar c\) as a result of hard primary collisions and pair production in showers induced by hard-produced partons in the initial and final states, are investigated. The sensitivity of the muon pair spectra with large invariant masses (from semileptonic \(B\bar B\) decays) and spectra of the secondary J/ψ (from individual B decays) to the multiple scattering and the energy losses of b quarks in dense quark-gluon matter is studied. The formation of such matter is expected in heavy-ion collisions at the LHC.

Dynamical scaling law for jet tomography.

Medium modifications of parton fragmentation provide a novel tomographic tool for the study of the hot and dense matter created in ultrarelativistic nucleus-nucleus collisions. Their quantitative analysis, however, is complicated by the strong dynamical expansion of the collision region. Here we establish for the multiple scattering induced gluon radiation spectrum a scaling law which relates medium effects in a collision of arbitrary dynamical expansion to that in an equivalent static scenario. Based on this scaling, we calculate for typical kinematical values of the RHIC and LHC heavy ion programming medium-modified fragmentation functions for collisions with realistic dynamical expansion.

Theoretical conference summary

This talk reviews progress reported at Quark-Matter 2001 on various theoretical aspects of the quark-gluon plasma and discusses how the new results obtained at RHIC change the standard picture of ultrarelativistic nucleus-nucleus collisions.