Partonic Medium Research Articles

Charmed hadron signals of partonic medium

We present a short review of our results on the collectivity and the suppression pattern of charmed mesons in heavy-ion collisions based on the microscopic Hadron-String-Dynamics (HSD) transport approach in different scenarios of charm interactions with the surrounding matter—the ‘comover’ dissociation by mesons with further recreation by channels and ‘pre-hadronic’ interaction scenarios. While at SPS energies the hadronic ‘comover’ absorption scenario is found to be compatible with the experimental data, the dynamics of quarks at RHIC are dominated by partonic or ‘pre-hadronic’ interactions in the strongly coupled quark–gluon plasma stage and cannot be modelled by pure ‘hadronic’ interactions. We find that the collective flow of charm in the purely hadronic scenario appears compatible with the data at SPS energies but underestimates the data at top RHIC energies. Thus, the large elliptic flow v2 of D mesons and the low RAA(pT) of J/Ψ seen experimentally at RHIC have to be attributed to the early interactions of non-hadronic degrees of freedom. Simultaneously, we observe that non-hadronic interactions are mandatory in order to describe the narrowing of the J/Ψ rapidity distribution from pp to central Au + Au collisions at the top RHIC energy. We demonstrate additionally that the strong quenching of low-pTJ/Ψ's in central Au + Au collisions indicates that a fraction of final J/Ψ mesons is created by a coalescence mechanism close to the phase boundary.

Probe of the QCD phase diagram with ϕ-mesons in high-energy nuclear collisions

High-energy nuclear collisions provide a unique tool to study a strongly interacting medium. Recent results from the Relativistic Heavy Ion Collider (RHIC) on ϕ-meson production have revealed the formation of a dense partonic medium. The medium constituents are found to exhibit collective behaviour initiated due to partonic interactions in the medium. We present a brief review of the recent results on ϕ production in heavy-ion collisions at the RHIC. One crucial question is where, in the phase diagram, does the transition happen for the matter changing from hadronic to partonic degrees of freedom. We discuss how the ϕ-meson elliptic flow in heavy-ion collisions can be used for the search of the QCD phase boundary.

Resonance production from jet fragmentation

Short lived resonances are sensitive to the medium properties in heavy-ion collisions. Heavy hadrons have larger probability to be produced within the quark gluon plasma phase due to their short formation times. Therefore heavy mass resonances are more likely to be affected by the medium, and the identification of early produced resonances from jet fragmentation might be a viable option to study chirality. The high momentum resonances on the away-side of a triggered di-jet are likely to be the most modified by the partonic or early hadronic medium. We will discuss first results of triggered hadron-resonance correlations in Cu+Cu heavy ion collisions.

Energy and system size dependence of ϕ meson production in [formula omitted] and [formula omitted] collisions

We study the beam-energy and system-size dependence of ϕ meson production (using the hadronic decay mode ϕ→K+K−) by comparing the new results from Cu+Cu collisions and previously reported Au+Au collisions at sNN=62.4 and 200 GeV measured in the STAR experiment at RHIC. Data presented in this Letter are from mid-rapidity (|y|<0.5) for 0.4<pT<5 GeV/c. At a given beam energy, the transverse momentum distributions for ϕ mesons are observed to be similar in yield and shape for Cu+Cu and Au+Au colliding systems with similar average numbers of participating nucleons. The ϕ meson yields in nucleus–nucleus collisions, normalized by the average number of participating nucleons, are found to be enhanced relative to those from p+p collisions. The enhancement for ϕ mesons lies between strange hadrons having net strangeness =1 (K− and Λ¯) and net strangeness =2 (Ξ). The enhancement for ϕ mesons is observed to be higher at sNN=200 GeV compared to 62.4 GeV. These observations for the produced ϕ(ss¯) mesons clearly suggest that, at these collision energies, the source of enhancement of strange hadrons is related to the formation of a dense partonic medium in high energy nucleus–nucleus collisions and cannot be alone due to canonical suppression of their production in smaller systems.

LHC capabilities for quarkonium

The measurement of the charmonium and bottomonium resonances in nucleus-nucleus collisions provides crucial information on high-density QCD matter. First, the suppression of quarkonia production is generally agreed to be one of the most direct probes of quark-gluon plasma formation. The observation of anomalous J/ψ suppression at the CERN-SPS and at RHIC is well established but the clarification of some important remaining questions requires equivalent studies of the ϒ family, will be possible at the LHC. Second, the production of heavy-quarks proceeds mainly via gluon-gluon fusion processes and, as such, is sensitive to saturation of the gluon density at low-x in the nucleus. Measured departures from the expected vacuum quarkonia cross-sections in Pb+Pb collisions at the LHC will thus provide valuable information not only on the thermodynamical state of the produced partonic medium, but also on the initial-state modifications of the nuclear parton distribution functions. The capabilities of the LHC detectors (ALICE, ATLAS and CMS) to study quarkonia production in Pb+Pb collisions at 5.5 TeV per nucleon pair are discussed.

Empirical constraints on parton energy loss in nucleus–nucleus collisions at RHIC

We present empirical features of parton energy loss in nucleus–nucleus collisions at RHIC through studies of the spectra and nuclear modification factors ( R A A ) for charged hadrons, neutral pions ( π 0 ) and non-photonic electrons. The flat distribution of R A A at high transverse momentum ( p T ) for a given collision centrality is consistent with a scenario where parton energy loss Δ p T is proportional to p T . The centrality dependence of the parton energy loss indicates the absence of path length dependence in the magnitude of energy loss. The lack of strong path length dependence suggests a dynamical picture where the dense partonic medium undergoes rapid expansion and the density of the medium falls rapidly in the first a few Fermi interval, which may be much shorter than the full path length. Implications of the empirical constraints on the parton energy loss will also be discussed.

Fragmentation function in non-equilibrium QCD using closed-time path integral formalism

In this paper we implement the Schwinger–Keldysh closed-time path integral formalism in non-equilibrium QCD in accordance to the definition of the Collins–Soper fragmentation function. We consider a high-p T parton in QCD medium at initial time τ 0 with an arbitrary non-equilibrium (non-isotropic) distribution function $f(\vec{p})$ fragmenting to a hadron. We formulate the parton-to-hadron fragmentation function in non-equilibrium QCD in the light-cone quantization formalism. It may be possible to include final-state interactions with the medium via a modification of the Wilson lines in this definition of the non-equilibrium fragmentation function. This may be relevant to the study of hadron production from a quark–gluon plasma at RHIC and LHC.

Low mass dilepton production in relativistic heavy ion collisions

Mechanisms of formation for low mass dilepton excess observed in the relativistic heavy ion collisions are considered. The experimental data are reviewed. In addition to discussing the standard mechanisms of dilepton production specific to the collisions of relativistic nuclei (the pion annihilation in the hadron-gas stage and the quark-antiquark annihilation in the quark-gluon phase), the mechanism of dilepton production in the mixed phase of nuclear matter is proposed, and its contribution to the low mass dilepton spectrum is estimated. In addition, the first-order corrections in the strong-interaction coupling constant to the dilepton production in the parton medium and the nonperturbative approaches are considered.

Momentum kick model description of the near-side ridge and jet quenching

In the momentum kick model, a near-side jet emerges near the surface, kicks medium partons, loses energy, and fragments into the trigger particle and fragmentation products. The kicked medium partons subsequently materialize as the observed ridge particles, which carry direct information on the magnitude of the momentum kick and the initial parton momentum distribution at the moment of jet--medium-parton collisions. The initial parton momentum distribution extracted from the STAR ridge data for central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV has a thermal-like transverse momentum distribution and a rapidity plateau structure with a relatively flat distribution at mid-rapidity and sharp kinematic boundaries at large rapidities. Such a rapidity plateau structure may arise from particle production in flux tubes, as color charges and anticolor charges separate at high energies. The centrality dependence of the ridge yield and the degree of jet quenching can be consistently described by the momentum kick model.

Parton Momentum Distribution at the Moment of Jet-Parton Collisions

The early parton momentum distribution is extracted by using the STAR collaboration data of ridge particles associated with a near-side jet in central A uAu collisions at . The ridge particles are identified as medium partons kicked by the jet near the surface and they carry direct information on the parton momentum distribution at the moment of jet-parton collisions. The extracted parton momentum distribution has a thermal-like transverse momentum distribution but a rapidity plateau structure with a relatively flat rapidity distribution at mid-rapidities with sharp kinematic boundaries at large rapidities that depend on the transverse momentum.

The ‘shoulder’ and the ‘ridge’ in PHENIX

Comparison of two-particle correlations in heavy-ion collisions with measurements in proton collisions shows strong modification of the correlation shape and particle yields. Two new structures are created; both extended in Δη, one centered at Δϕ = 0 (‘ridge’) and the other occurring at Δϕ ≈ π ± 1.1 (‘shoulder’). In these proceedings, we describe the measurements of these structures that show consistency with a scenario of parton–medium interaction and response. We discuss a new analysis with improved resolution in selections on the angle of trigger particles relative to the reaction plane. New measurements of the centrality and pT dependences of the structures raise the possibility that the same production mechanism may give rise to both phenomena.

Exploring early parton momentum distribution with the ridge from the near-side jet

In a central nucleus–nucleus collision at high energies, medium partons kicked by a near-side jet acquire a momentum along the jet direction and subsequently materialize as the observed ridge particles. They carry direct information on the early parton momentum distribution which can be extracted by using the ridge data for central AuAu collisions at GeV. The extracted parton momentum distribution has a thermal-like transverse momentum distribution but a non-Gaussian, relatively flat rapidity distribution at mid-rapidity with sharp kinematic boundaries at large rapidities that depend on the transverse momentum.

Ridge structure in theΔϕ−Δηcorrelation function associated with a near-side jet

In the $\ensuremath{\Delta}\ensuremath{\phi}\text{\ensuremath{-}}\ensuremath{\Delta}\ensuremath{\eta}$ correlation associated with a near-side jet observed by the STAR Collaboration in heavy-ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC), the ridge structure can be explained by the momentum kick model in which the ridge particles are identified as medium partons which suffer a collision with the jet and acquire a momentum kick along the jet direction. If this is indeed the correct mechanism, the ridge structure associated with the near-side jet may be used to probe the parton momentum distribution at the moment of the jet-parton collision, leading to the result that at that instant the parton temperature is slightly higher and the rapidity width substantially greater than corresponding quantities of their evolution product inclusive particles at the end point of the nucleus-nucleus collision.

PARTON EVOLUTION FOR CHEMICAL EQUILIBRATION AT RHIC: DEPENDENCE ON INITIAL CONDITIONS

An important ingredient for studying non-equilibrium evolution of partons at RHIC and LHC is to have physical initial conditions for the partonic system. In this paper we investigate the initial conditions with chemical non-equilibrium at RHIC. Considering the parton production, we studied evolution of the partonic medium with chemical non-equilibrium toward a fully equilibrated partonic system. By using a set of rate equations which describe the chemical equilibration of gluons and quarks, and putting different initial conditions, we obtained the transverse energy production dET/dy and particle multiplicities dN/dy. Comparing the experimental data of dET/dy and dN/dy with numerical results, we get following initial conditions for the formation of partonic system: the initial temperature is around 550 MeV and the initial fugacity λg is from 0.4 to 0.75. It is shown that, until the partonic medium freezes out the partonic system hasn't been totally chemical equilibrated.

Interpretation of recent SPS dilepton data

We summarize our current theoretical understanding of in-medium properties of the electromagnetic current correlator in view of recent dimuon data from the NA60 experiment in In(158 AGeV)–In collisions at the CERN-SPS. We discuss the sensitivity of the results to spacetime evolution models for the hot and dense partonic and hadronic medium created in relativistic heavy-ion collisions and the contributions from different sources to the dilepton-excess spectra.

Photon-tagged correlations in heavy-ion collisions: kinematic requirements and a case study**Based on talks given at Quark Matter 2006, Shanghai, China, 14–20 November 2006, and at the ALICE Physics Working Group 4, CERN, 13 December 2006.

Photon-tagged correlations may be useful to determine how the dense partonic medium produced in heavy-ion collisions affects the fragmentation of high-energy quarks and gluons into a leading hadron. In these proceedings, I discuss the kinematic requirements for the hadron and the prompt photon transverse momentum cuts. A case study at LHC energy, tagging on and photons, is then briefly examined.

Hard and soft probe–medium interactions in a 3D hydro+micro approach at RHIC

We utilize a 3D hybrid hydro+micro model for a comprehensive and consistent description of soft and hard particle production in ultra-relativistic heavy-ion collisions at RHIC. In the soft sector we focus on the dynamics of (multi-) strange baryons, where a clear strangeness dependence of their collision rates and freeze-out is observed. In the hard sector we study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. While the nuclear suppression factor RAA does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows us to study different centralities and in particular the angular variation of RAA with respect to the reaction plane, allowing for a controlled variation of the in-medium path length.

Jet quenching in a three-dimensional hydrodynamic medium

We study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. The soft medium is described by a 3D hydrodynamical model and we treat the averaging over all possible parton paths through the medium without approximation. While the nuclear suppression factor ${R}_{AA}$ does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows to study different centralities and in particular the angular variation of ${R}_{AA}$ with respect to the reaction plane, allowing for a controlled variation of the in-medium path-length. We study the angular dependence of ${R}_{AA}$ for different centralities, discuss the influence of hydrodynamical expansion and flow and comment on the comparison with preliminary data.

Latest results on the hot dense partonic matter at RHIC

At the Relativistic Heavy-Ion Collider (RHIC) collisions of heavy ions at nucleon-nucleon energies of 200GeV appear to have created a new form of matter thought to be a deconfined state of the partons that ordinarily are bound in nucleons. We discuss the evidence that a thermalized partonic medium, usually called a Quark Gluon Plasma (QGP), has been produced. Then, we discuss the effect of this high-density medium on the production of jets and their pair correlations. Next, we look at direct photons as a clean electro-magnetic probe to constrain the initial hard scatterings. Finally, we review the developing picture for the effect of this medium on the production of open heavy quarks and on the screening by the QGP of heavy-quark bound states.

High transverse momentum η meson production inp+p,d+Au, and Au+Au collisions atsNN=200GeV

Inclusive transverse momentum spectra of η mesons in the range pT≈2–12 GeV/c have been measured at midrapidity (|η|<0.35) by the PHENIX experiment at RHIC in p+p,d+Au, and Au+Au collisions at sNN=200 GeV. The η mesons are reconstructed through their η→γ γ channel for the three colliding systems as well as through the η→π0π+π− decay mode in p+p and d+Au collisions. The nuclear modification factor in d+Au collisions, RdAu(pT)≈1.0–1.1, suggests at most only modest pT broadening (“Cronin enhancement”). In central Au+Au reactions, the η yields are significantly suppressed, with RAuAu(pT)≈0.2. The ratio of η to π0 yields is approximately constant as a function of pT for the three colliding systems in agreement with the high-pT world average of Rη/π0≈0.5 in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range of center-of-mass energies (sNN≈3–1800 GeV) as well as, for high scaled momentum xp, in e+e− annihilations at s=91.2 GeV. These results are consistent with a scenario where high-pT η production in nuclear collisions at the Relativistic Heavy Ion Collider is largely unaffected by initial-state effects but where light-quark mesons (π0,η) are equally suppressed due to final-state interactions of the parent partons in the dense medium produced in Au+Au reactions.22 MoreReceived 6 November 2006DOI:https://doi.org/10.1103/PhysRevC.75.024909©2007 American Physical Society