Partonic Phase Research Articles

Breaking of the number-of-constituent-quark scaling for identified-particle elliptic flow as a signal of phase change in low-energy data taken at the BNL Relativistic Heavy Ion Collider (RHIC)

We argue that measurements of identified-particle elliptic flow in a wide energy range could shed light on the possible phase change in high-energy heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC). When the hadronization process is dominated by quark coalescence, the number-of-constituent-quark (NCQ) scaling for the identified-particle elliptic flow can serve as a probe for studying the strong interacting partonic matter. In the upcoming RHIC low-energy runs, the NCQ scaling behavior may be broken because of the change of the effective degrees of freedom of the hot dense matter, which corresponds to the transition from the dominant partonic phase to the dominant hadronic phase. A multiphase transport model is used to present the dependence of NCQ scaling behavior on the different hadronization mechanisms.

Shear-Viscosity to Entropy-Density Ratio of a Relativistic Hadron Gas

Ultrarelativistic heavy-ion collisions at the Brookhaven National Laboratory Relativistic Heavy Ion Collider (RHIC) are thought to have produced a state of matter called the quark-gluon plasma, characterized by a very small shear-viscosity to entropy-density ratio eta/s, near the lower bound predicted for that quantity by anti-de Sitter space/conformal field theory methods. As the produced matter expands and cools, it evolves through a phase described by a hadron gas with rapidly increasing eta/s. We calculate eta/s as a function of temperature in this phase both in and out of chemical equilibrium and find that its value poses a challenge for viscous relativistic hydrodynamics, which requires small values of eta/s in order to successfully describe the collective flow observables at the RHIC. We therefore conclude that the origin of the low viscosity matter at the RHIC must be in the partonic phase of the reaction.

Nonlinear diffusion and particle production in relativistic systems

The short parton production phase in high-energy heavy-ion collisions is treated analytically as a nonlinear diffusion process. The initial buildup of the rapidity density distributions of produced charged hadrons within τ p ≃ 0.25 fm/c occurs in three sources during the colored partonic phase. In a two-step approach, the subsequent diffusion in pseudorapidity space during the interaction time of τ i n t ≃ 7 – 10 fm/c (mean duration of the collision) is essentially linear as expressed in the Relativistic Diffusion Model (RDM) which yields excellent agreement with the data at RHIC energies, and allows for predictions at LHC energies. Results for d+Au are discussed in detail.

Formation and decay of hadronic resonances in the QGP

Hadronic resonances can play a pivotal role in providing experimental evidence for partial chiral symmetry restoration in the deconfined quark–gluon phase produced at RHIC and the LHC. Their lifetimes, which are comparable to the lifetime of the partonic plasma phase, make them an invaluable tool to study medium modifications to the resonant state due to the chiral transition. In this Letter we show that the heavier, but still abundant, light and strange quark resonances K∗, ϕ, Δ and Λ∗ have large probability to be produced well within the plasma phase due to their short formation times. We demonstrate that, under particular kinematic conditions, these resonances can be formed and will decay inside the partonic state, but still carry sufficient momentum to not interact strongly with the hadronic medium after the QCD phase transition. Thus, K∗, ϕ, Δ and Λ∗ should exhibit the characteristic property modifications which can be attributed to chiral symmetry restoration, such as mass shifts, width broadening or branching ratio modifications.

Parton distributions at hadronization from bulk dense matter produced inAu+Aucollisions atsNN=200GeV

We present an analysis of $\ensuremath{\Omega},\ensuremath{\Xi},\ensuremath{\Lambda}$, and $\ensuremath{\phi}$ spectra from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV in terms of distributions of effective constituent quarks at hadronization. Consistency in quark ratios derived from various hadron spectra provides clear evidence for hadron formation dynamics as suggested by quark coalescence or recombination models. We argue that the constituent quark distribution reflects properties of the effective partonic degree of freedom at hadronization. Experimental data indicate that strange quarks have a transverse-momentum distribution flatter than that of up/down quarks consistent with hydrodynamic expansion in partonic phase prior to hadronization. Our extracted parton transverse-momentum distributions at the hadronization provide a unique constraint on the partonic evolution history. After the multiphase transport model is tuned to reproduce the strange and up/down quark distributions, the model can describe the measured spectra of hyperons and $\ensuremath{\phi}$ mesons very well where hadrons are formed through dynamical coalescence.

Particle production and nonlinear diffusion in relativistic systems

AbstractThe short parton production phase in high‐energy heavy‐ion collisions is treated analytically as a nonlinear diffusion process. The initial buildup of the rapidity density distributions of produced charged hadrons within τp≃ 0.25 fm/c occurs in three sources during the colored partonic phase. In a two‐step approach, the subsequent diffusion in pseudorapidity space during the interaction time of τint≃ 7‐10 fm/c (mean duration of the collision) is essentially linear as expressed in the Relativistic Diffusion Model (RDM) which yields excellent agreement with the data at RHIC energies, and allows for predictions at LHC energies. Results for d+Au are discussed in detail.

Particle production and nonlinear diffusion in relativistic systems

The short parton production phase in high-energy heavy-ion collisions is treated analytically as a nonlinear diffusion process. The initial buildup of the rapidity density distributions of produced charged hadrons within tau_p = 0.25 fm/c occurs in three sources during the colored partonic phase. In a two-step approach, the subsequent diffusion in pseudorapidity space during the interaction time of tau_int = 7-10 fm/c (mean duration of the collision) is essentially linear as expressed in the Relativistic Diffusion Model (RDM) which yields excellent agreement with the data at RHIC energies, and allows for predictions at LHC energies. Results for d+Au are discussed in detail.

Partonic effect on anisotropic flows of Ω baryon for Au+Au at 62.4 and 200 GeV/c

The elliptic flow v2 and the fourth order anisotropic flow v4 of (Ω+Ω) have been studied in the framework a parton–hadronic transport model, namely a multi-phase transport (AMPT) model, for 197Au+197Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and 62.4 GeV. The transverse momentum (pT) and the transverse kinetic energy (mT-m0) dependence of v2 and v4 are presented. The calculation in the AMPT model seems consistent with the STAR data. The results show that the v2 of (Ω+Ω) in 200 GeV obeys the constituent quark number scaling that has been observed for other mesons and baryons. Comparison of (Ω+Ω) elliptic flow v2 in the default version of AMPT, the melting version of AMPT and the RQMD model calculation, shows that the parton cascade process is important to reproduce the sizeable v2, and the string melting AMPT model preferably reproduces (Ω+Ω) elliptic flow v2 in 197Au+197Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The v2 of (Ω+Ω) in the 62.4 and 200 GeV collisions seem similar, and the pT dependence of (Ω+Ω) baryons’ v4 in 62.4 GeV and 200 GeV looks also similar in the string melting AMPT model, which indicates that a similar partonic matter phase has been reached in both energies.

Baryon-strangeness correlations in parton/hadron transport model for Au+Au collisions at 200 GeV

Baryon-strangeness correlation (CBS) has been investigated with a multi-phase transport model (AMPT) in 197Au+197Au collisions at 200 GeV. The centrality dependence of CBS is presented within the model, from partonic phase to hadronic matter. We find that the system still reserves partial predicted signatures of CBS after parton coalescence. But after hadronic rescattering, the predicted signatures will be obliterated completely. So it seems that both coalescence hadronization process and hadronic rescattering are responsible for the disappearance of the CBS signatures.

Examining the Crossover from the Hadronic to Partonic Phase in QCD

A mechanism, consistent with color confinement, for the transition between perturbative and physical vacua during the gradual crossover from the hadronic to partonic phase is proposed. The essence of this mechanism is the appearance and growing up of a kind of grape-shape perturbative vacuum inside the physical one. A percolation model based on simple dynamics for parton delocalization is constructed to exhibit this mechanism. The crossover from hadronic matter to sQGP (strongly coupled quark-gluon plasma) as well as the transition from sQGP to weakly coupled quark-gluon plasma with increasing temperature is successfully described by using this model.

Time-odd distribution functions, breaking of long range correlations, and sudden entropy changes, in Drell-Yan high-energy processes

Time-odd parton distribution functions in a Drell-Yan process are here studied by examining the evolution of the internal statistical properties of the interacting hadrons. Time-odd functions are shown to be a signature of the irreversible process in which a hadronic state characterized by long range correlation properties (hadronic phase) decays to produce a cloud of independent partons (partonic phase) because of initial/final state interactions. The relevant considered variable is the rate of increase of the entropy of the hadronic system. This quantity is shown to be roughly equal to the decay rate of the hadronic state. Conditions for getting a leading twist time-odd effect are established on this basis. Last, the relevant case of a large entropy increase associated with transverse-dominated initial/final state interactions is analyzed.

Net charge transfer fluctuation in quark–gluon matter and hadron matter

We investigated the net charge transfer fluctuation at mid-rapidity region in Au + Au collisions at root s(NN) = 200 GeV. A partonic and hadronic cascade model, PACIAE, is applied to follow the particle transport in both the partonic and hadronic phases. We have determined the factor, kappa, which characterizes the net charge transfer fluctuation. Considering the pure hadronic and pure partonic scenarios, we obtain a factor of 3-5 difference in kappa. However, by switching on the hadronization of partonic matter and introducing the secondary hadron-hadron interactions, the factor kappa will increase and finally approach the value of the pure hadronic scenario within an accuracy of 20%. (c) 2006 Elsevier B.V. All rights reserved.

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.

Statistical hadronization as a snapshot of a dynamical fireball evolution

Statistical hadronization models are extremely successful in describing measured ratios of hadrons produced in heavy-ion collisions for a wide range of beam energies from SIS to RHIC. Using the idea of statistical hadronization at the phase boundary within the framework of a recently proposed dynamical model for the thermodynamics of fireballs, we establish a relation between the equation of state in the partonic phase probed at the phase transition temperature ${T}_{C}$ and the measured hadron ratios. In this way, the ratios can be predicted parameter free. We demonstrate that this framework gives a consistent description of conditions both at SPS and RHIC. As the data on dilepton emission from heavy-ion collisions give evidence for strong in-medium modifications of hadronic properties, we schematically investigate the sensitivity of our results to such modifications and qualitatively sketch a consistent scenario involving such in-medium effects.

Hadron production in heavy ion collisions: Fragmentation and recombination from a dense parton phase

We discuss hadron production in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). We argue that hadrons at transverse momenta ${P}_{T}l5\phantom{\rule{0.3em}{0ex}}\text{GeV}$ are formed by recombination of partons from the dense parton phase created in central collisions at RHIC. We provide a theoretical description of the recombination process for ${P}_{T}g2\phantom{\rule{0.3em}{0ex}}\text{GeV}$. Below ${P}_{T}=2\phantom{\rule{0.3em}{0ex}}\text{GeV}$ our results smoothly match a purely statistical description. At high transverse momentum hadron production is well described in the language of perturbative QCD by the fragmentation of partons. We give numerical results for a variety of hadron spectra, ratios, and nuclear suppression factors. We also discuss the anisotropic flow ${v}_{2}$ and give results based on a flow in the parton phase. Our results are consistent with the existence of a parton phase at RHIC hadronizing at a temperature of $175\phantom{\rule{0.3em}{0ex}}\text{MeV}$ and a radial flow velocity of $0.55c$.

J/ψproduction in relativistic heavy ion collisions from a multiphase transport model

Using A Multi-Phase Transport (AMPT) model, we study J/psi production from interactions between charm and anti-charm quarks in initial parton phase and between D and Dbar mesons in final hadron phase of relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). Including also the inverse reactions of J/psi absorption by gluons and light mesons, we find that the net number of J/psi from the parton and hadron phases is smaller than that expected from the superposition of initial nucleon-nucleon collisions, contrary to the J/psi enhancement predicted by the kinetic formation model. The production of J/psi is further suppressed if one includes the color screening effect in the parton phase. We have also studied the dependence of J/psi production on the charm quark mass and the effective charm meson mass.

Model for dilepton rates from a fireball

We calculate the dilepton emission rate from a fireball created in an ultra-relativistic heavy-ion collision. For the partonic phase, we complement the perturbative results by a phenomenological approach based on lattice QCD results, whereas in the hadronic phase temperature and finite baryon density effects on the spectral function are considered. The resulting rates are compared to data from CERES/NA45.

The next Yukawa phase of QCD at RHIC

QCD predicts the existence of a new partonic Yukawa phase above a critical temperature $T_c\sim 150$ MeV. I review some of the key observables which will be used in heavy ion reactions to search for this new phase at RHIC. Systematics of collective observables ($N_{ch},E_\perp)$, flow patterns, meson interferometry, jet quenching, and $J/\psi$ suppression are discussed. This talk is updated with the first data from RHIC.

Chemical eguilibration and the hadron–QGP phase transition

We discuss recent experimental results on hadron multiplicities in ultra-relativistic nuclear collisions. The data for central collisions are in quantitative agreement with predictions of a thermal model assuming full chemical equilibration. It is argued that this provides strong, albeit indirect, evidence for the formation of a partonic phase in the collision prior to hadron production.

Photon physics at ultrarelativistic-energy heavy-ion collisions

Photons are discussed as potential probes of the dynamics of nucleus-nucleus reactions at ultrarelativistic energies. The signals conveyed by photons produced in the early partonic phase of the collision are explored. The discussion is illustrated by presently existing calculations which include photon production during the entire duration of the collision.