Parton Coalescence Research Articles

Elliptic flow in heavy ion collisions at varying energies: Partonic versus hadronic dynamics

We examine whether the breakdown in elliptic flow quark number scaling observed at the Relativistic Heavy Ion Collider (RHIC) energy scan is related to the turning off of deconfinement by testing the hypothesis that hydrodynamics and parton coalescence always apply, but are obscured, at lower energies, by variations in the widths of quark and antiquark rapidity distribution. We find that this effect is enough to spoil quark number scaling in elliptic flow. A lack of scaling in data, therefore, does not signal the absence of partonic degrees of freedom and hadronization by coalescence. In a coalescing partonic fluid, however, elliptic flow of antibaryons should be greater than that of baryons, since antibaryons contain a greater admixture of partons from the highly flowing midrapidity region. Intriguingly, purely hadronic dynamics has a similar dependence of baryon-antibaryons elliptic flow as purely partonic dynamics, again because antibaryons tend to come from regions where the deviation of the system from hydrodynamic behavior is at its smallest. The opposite trend observed in experiment is therefore an indication that we might be misunderstanding the origin of elliptic flow. We finish by discussing possible explanations of this and suggest experimental measurements capable of clarifying the situation.

Effect of final state interactions on particle production ind+Aucollisions at energies available at the BNL Relativistic Heavy Ion Collider

We show that particle species dependence of enhanced hadron production at intermediate transverse momentum (${p}_{T}$) for $d+\text{Au}$ collisions at RHIC can be understood in terms of the hadronization from string fragmentation and the subsequent hadronic rescatterings in the final state. A multiphase transport model (AMPT) with two different hadronization mechanisms, string fragmentation or parton coalescence, is used in our study. When the hadrons are formed from string fragmentation, the subsequent hadronic rescatterings will result in particle mass dependence of the nuclear modification factor ${R}_{\mathrm{CP}}$, which is consistent with the present experimental data. On the other hand, in the framework of parton coalescence, the mass dependence disappears and the strangeness plays an important role in hadron production.

Multi-particle interactions within the UrQMD approach

A mechanism for locally density-dependent dynamic parton rearrangement and fusion has been implemented into the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) approach. The same mechanism has been previously built in the Quark Gluon String Model (QGSM). This rearrangement and fusion approach based on parton coalescence ideas enables the description of multi-particle interactions, namely 3 ! 3 and 3 ! 2, between (pre)hadronic states in addition to standard binary interactions. The UrQMD model (v2.3) extended by these additional processes allows to investigate implications of multi-particle interactions on the reaction dynamics of ultrarelativistic heavy ion collisions. The mechanism, its implementation and first results of this investigation are presented and discussed.

The proton-to-pion ratio and the near-side jet correlation in pp and AA collisions

Hadron production consists of two main channels in the intermediate-pT region at RHIC and LHC energies: parton coalescence/recombination and jet fragmentation. The relative contributions of these two channels change with increasing parton density connected to increasing atomic numbers of colliding nuclei and increasing collective behavior. Near-side hadron–hadron correlations carry information about these mechanisms, especially the proton-to-pion ratio. We have studied the dependence of this specific hadron ratio and discuss the possible experimental observables to determine the contributions of parton coalescence and jet fragmentation channels.

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.

Quark number scaling ofv2in transverse kinetic energy and its implications for coalescence models

We find that a simple extension of the coalescence model is sufficient to incorporate the perfect quark number scaling behavior of the elliptic flow in transverse kinetic energy, recently discovered by the PHENIX Collaboration. The flavor dependence of the elliptic flow can be consistently described in the low and intermediate ${p}_{T}$ if the transverse kinetic energy is conserved in the $2\ensuremath{\rightarrow}1$ or $3\ensuremath{\rightarrow}1$ parton coalescence process at the hadronization. Thus suggesting the quark coalescence as a possible hadronization mechanism at low ${p}_{T}$ as well.

Parton cascade and coalescence

This is a review of the parton cascade approach and its implications on parton coalescence at RHIC.

Heavy-quark probes of the quark-gluon plasma and interpretation of recent data taken at the BNL Relativistic Heavy Ion Collider

Thermalization and collective flow of charm (c) and bottom (b) quarks in ultrarelativistic heavy-ion collisions are evaluated based on elastic parton rescattering in an expanding quark-gluon plasma (QGP). We show that resonant interactions in a strongly interacting QGP (sQGP), as well as parton coalescence, can play an essential role in the interpretation of recent data from the BNL Relativistic Heavy-Ion Collider (RHIC), and thus illuminate the nature of the sQGP and its hadronization. Our main assumption, motivated by recent findings in lattice quantum chromodynamics, is the existence of D- and B-meson states in the sQGP, providing resonant cross sections for heavy quarks. Pertinent drag and diffusion coefficients are implemented into a relativistic Langevin simulation to compute transverse-momentum spectra and azimuthal asymmetries (${v}_{2}$) of b- and c-quarks in Au-Au collisions at RHIC. After hadronization into D- and B-mesons using quark coalescence and fragmentation, associated electron-decay spectra and ${v}_{2}$ are compared to recent RHIC data. Our results suggest a reevaluation of radiative and elastic quark energy-loss mechanisms in the sQGP.

Azimuthal anisotropy in Au+Au collisions atsNN=200GeV

The results from the STAR Collaboration on directed flow (v_1), elliptic flow (v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a Blast Wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v_2, scaling with the number of constituent quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and quark coalescence is discussed.

Parton Coalescence and Spacetime

The influence of spacetime dynamics in hadronization via parton coalescence at RHIC is investigated using covariant parton transport theory: Key observables, the quark number scaling of elliptic flow and the enhancement of the p/π ratio, show strong dynamical effects and differ from earlier results based on the simple coalescence formulas.

Production of forward rapidity photons in high energy heavy ion collisions

We consider production of prompt photons in high energy gold–gold and deuteron–gold collisions in the forward rapidity region of RHIC ( y ∼ 3.8 ). In this kinematics, the projectile partons typically have large x b j while the target partons are mostly at very small x b j , so that the primary partonic collisions involve valence quarks from the projectile and gluons from the target. We take the target nucleus to be a color glass condensate while the projectile deuteron or nucleus is treated as a dilute system of partons. We show that the photon production cross section can be written as a convolution of a quark–nucleus scattering cross section, involving a quark–antiquark dipole, with the leading order quark–photon fragmentation function. We consider different models of the quark–antiquark dipole and show that measurement of photons in the forward rapidity region at RHIC can distinguish between different parameterizations of the dipole cross section as well as help clarify the role of parton coalescence models in hadron production at RHIC.

Pentaquark baryon production at the relativistic heavy ion collider

Production of pentaquark Θ + baryons in central relativistic heavy ion collisions is studied in a kinetic model. Assuming that a quark–gluon plasma is produced in the collisions, we first determine the number of Θ + produced from the quark–gluon plasma using a parton coalescence model, and then take into consideration its production and absorption in subsequent hadronic matter via the reactions K N ↔ Θ , K N ↔ π Θ and π N ↔ K ¯ Θ . We find that although the final Θ + number is affected by hadronic interactions, it remains sensitive to the initial number of Θ + produced from the quark–gluon plasma, particularly in the case of a small Θ + width as imposed by the K + N and K + d scattering data. Because of small baryon chemical potential in the hot dense matter produced in these collisions, the number of produced anti- Θ is only slightly smaller than that of Θ + .

Particle correlations at RHIC from parton coalescence dynamics—first results

A new dynamical approach that combines covariant parton transport theory with hadronization channels via parton coalescence and fragmentation is applied to Au+Au at RHIC. Basic consequences of the simple coalescence formulae, such as elliptic flow scaling and enhanced p/π ratio, turn out to be rather sensitive to the spacetime aspects of coalescence dynamics.

Particle-type dependence of azimuthal anisotropy and nuclear modification of particle production in Au+Au collisions at square root of sNN=200 GeV.

We present STAR measurements of the azimuthal anisotropy parameter v(2) and the binary-collision scaled centrality ratio R(CP) for kaons and lambdas (Lambda+Lambda) at midrapidity in Au+Au collisions at square root of s(NN)=200 GeV. In combination, the v(2) and R(CP) particle-type dependencies contradict expectations from partonic energy loss followed by standard fragmentation in vacuum. We establish p(T) approximately 5 GeV/c as the value where the centrality dependent baryon enhancement ends. The K(0)(S) and Lambda+Lambda v(2) values are consistent with expectations of constituent-quark-number scaling from models of hadron formation by parton coalescence or recombination.

Meson and baryon elliptic flow at high pT from parton coalescence

The large and saturating differential elliptic flow v2(p⊥) observed in Au+Au reactions at RHIC so far could only be explained by assuming an order of magnitude denser initial parton system than estimated from perturbative QCD. Hadronization via parton coalescence can resolve this ‘opacity puzzle’ because it enhances hadron elliptic flow at large p⊥ relative to that of partons at the same transverse momentum. An experimentally testable consequence of the coalescence scenario is that v2(p⊥) saturates at about 50% higher values for baryons than for mesons. In addition, if strange quarks have weaker flow than light quarks, hadron v2 at high p⊥ decreases with relative strangeness content.