Quark Recombination Model Research Articles

Insight from the elliptic flow of identified hadrons measured in relativistic heavy-ion collisions

In this paper, we have discussed the transverse momentum dependence of elliptic flow measured in relativistic heavy-ion collisions. Under the assumption of quark recombination model, [Formula: see text] of light and strange quarks is obtained from the measured [Formula: see text] of identified hadrons. The [Formula: see text] of strange quarks obtained from [Formula: see text]-meson [Formula: see text] is found to be similar to that of from [Formula: see text] [Formula: see text] in Au+Au collisions at [Formula: see text][Formula: see text]GeV. This may indicate that both [Formula: see text] and [Formula: see text] are produced through quark recombination at top RHIC energy. However, we find that the [Formula: see text] of light quarks is consistently higher than that of strange quarks at the low transverse momentum. We have shown that [Formula: see text] of light and strange quarks is consistent with each other when plotted as a function of its transverse kinetic energy. The observed scaling of [Formula: see text] works at both RHIC and LHC energies. It is also shown that the observed scaling holds for other flow harmonics, e.g., [Formula: see text]. The new observed scaling is purely empirical and needs more theoretical input to understand the physics behind it.

Jet Hadronization via Recombination of Parton Showers in Vacuum and in Medium

We have studied the hadronization of jet parton showers based on the quark recombination model. This is achieved by letting gluons at the end of the perturbative shower evolution undergo a non-perturbative splitting into quark and antiquark pairs, then applying a Monte-Carlo version of instantaneous quark recombination, and finally subjecting remnant quarks (those which have not found a recombination partner) to Lund string fragmentation. When applied to parton showers from the PYTHIA Monte Carlo generator, the final hadron spectra from our calculation compare quite well to PYTHIA jets that have been hadronized with the default Lund string fragmentation. Modeling the quark gluon plasma produced in heavy ion collisions by a blast wave model, we have further studied medium effects on the hadronization of jet shower partons by also including their recombination with the thermal partons from the quark gluon plasma. We find that the latter leads to a significant enhancement of intermediate transverse momentum pions and protons at both RHIC and LHC. Our results thus suggest that medium modification of jet fragmentation provides a plausible explanation for the enhanced production of intermediate transverse momentum hadrons observed in experiments.

Distributions and correlations of constituent quarks in jets

In the frame of the quark recombination model, we study the momentum distributions and correlations of constituent quarks in jets by analyzing the final state hadrons generated by PYTHIA for the hard parton fragmentation processes in vacuum. Parameterizations for the distributions are tabulated.

Parton transverse momentum distribution at the moment of hadronization at RHIC

We extract the transverse momentum distribution of effective partons using the spectra of Ω, Ξ, Λ and ϕ hadrons measured by the STAR Collaboration from Au + Au collisions at \( \sqrt {s_{NN} } \) = 200 GeV at RHIC. The extracted momentum distribution of strange quarks is flatter than that of up/down quarks consistent with hydrodynamics expansion in partonic phase prior to hadronization. Consistency in quark ratios derived from various hadron spectra gives clear evidence for hadron production as suggested by quark coalescence or recombination model.

Cooper–Frye formula and non-extensive coalescence at RHIC energy

Transverse spectra are calculated for various types of hadrons stemming from AuAu collisions at s=200 GeV. We utilize a quark recombination model based on generalized Boltzmann–Gibbs thermodynamics for local hadron production at various break-up scenarios.

Dynamical heavy-quark recombination and the nonphotonic single-electron puzzle at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

We show that the single, nonphotonic electron nuclear modification factor ${R}_{\mathit{AA}}^{e}$ is affected by the thermal enhancement of the heavy-baryon-to-heavy-meson ratio in relativistic heavy-ion collisions with respect to proton-proton collisions. We make use of the dynamical quark recombination model to compute such a ratio and show that this produces a sizable suppression factor for ${R}_{\mathit{AA}}^{e}$ at intermediate transverse momenta. We argue that this suppression factor needs to be considered, in addition to the energy loss contribution, in calculations of ${R}_{\mathit{AA}}^{e}$.

Resonance recombination model and quark distribution functions in the quark-gluon plasma

We investigate the consequences of space-momentum correlations in quark phase-space distributions for coalescence processes at the hadronization transition. Thus far it has been proved difficult to reconcile such correlations with the empirically observed constituent quark number scaling (CQNS) at the Relativistic Heavy Ion Collider (RHIC). To address this problem we combine our earlier developed quark recombination model with quark phase-space distributions computed from relativistic Langevin simulations in an expanding quark-gluon plasma (QGP). Hadronization is based on resonance formation within a Boltzmann equation that recovers thermal equilibrium and obeys energy conservation in the quark-coalescence process, while the fireball background is adjusted to hydrodynamic simulations of semicentral $\mathrm{Au}\text{\ensuremath{-}}\mathrm{Au}$ collisions at RHIC. To facilitate the applicability of the Langevin process, we focus on strange and charm quarks. Their interactions in the QGP are modeled using leading-order perturbative QCD augmented by effective Lagrangians with resonances that smoothly merge into hadronic states formed at ${T}_{c}$. The interaction strength is adjusted to reproduce the empirical saturation value for the quark-elliptic flow, ${v}_{2,q}^{\mathrm{sat}}\ensuremath{\simeq}7\text{\ensuremath{-}}8%$. The resulting \ensuremath{\phi} and $J/\ensuremath{\psi}$ elliptic flow recover CQNS over a large range in transverse momentum (${p}_{T}$) within a few percent. As a function of transverse kinetic energy, both the quark spectra from the Langevin simulations and the meson spectra generated via resonance recombination recover CQNS from zero to at least $3 \mathrm{GeV}$.

POPULATION OF MULTI-QUARK STATES IN EXOTIC MULTIPLETS AND THERMALIZATION IN ULTRA-RELATIVISTIC HEAVY ION COLLISIONS

The recent discussion about experimental evidence for pentaquark states has revitalized the interest in exotic hadrons. If such states really exist, it is natural to assume that they will be formed at the late hadronization stage of ultra-relativistic heavy ion collisions, given the success of quark recombination models in the description of hadronization. Here, we apply the qMD model to study the formation of color neutral exotic multi-quark clusters at hadronization. We search for color neutral clusters made up of up to six color charges, respectively. We thus obtain estimates for the numbers and phase space distributions of exotic hadronic states produced by clustering in heavy ion collisions, including the members of the pentaquark multiplets. We obtain particle abundances that are smaller than thermal model predictions. Moreover, the results obtained in recombination from ultra-relativistic heavy ion collisions can be compared to the estimates based on equal population of the corresponding multiplets, and to results from fully thermalized systems. We find that the distribution of exotic hadrons from recombination over large multiplets provides a sensitive signal for thermalization and decorrelation of the initial, non-equilibrium state of the collision.

Uncertainties of parton distributions in a shower from the recombination model

Uncertainties of five shower parton distributions in jets at fixed Q2 = 100 GeV2 are investigated from three sets of fragmentation functions in the framework of a quark recombination model.

The effect of dynamical parton recombination on event-by-event observables

Within a dynamical quark recombination model, we explore various proposed event-by-event observables sensitive to the microscopic structure of the QCD-matter created at RHIC energies. Charge ratio fluctuations, charge transfer fluctuations and baryon-strangeness correlations are computed from a sample of central Au + Au events at the highest RHIC energy available (sNN=200 GeV). We find that for all explored observables, the calculations yield the values predicted for a quark–gluon plasma only at early times of the evolution, whereas the final state approaches the values expected for a hadronic gas. We argue that the recombination-like hadronization process itself is responsible for the disappearance of the predicted deconfinement signals. This might explain why no fluctuation signatures for the transition between quark and hadronic matter was ever observed in the experimental data up to now.

PARTICLE PRODUCTION AT RHIC FROM QUARK RECOMBINATION

Particle production at RHIC is discussed within the framework of quark recombination model. After a short introduction on the anomalies discovered at RHIC, we will first present the basic physics ideas common for all the quark recombination models and different implementations in different research groups. Then the main focus turns to the approach of Oregon group. Feynman and Field's independent fragmentation process is re-interpreted in the quark recombination model. In this approach, hard partons with high virtuality evolves into a shower of semihard partons by radiating gluons which can convert into quark-antiquark pairs. Then different fragmentation functions can be rewritten in terms of only a few shower parton distributions within the recombination model. By fitting the data on the fragmentation functions, the shower parton distributions can be determined and then can be used in other processes together with the assumed exponential distributed soft parton distributions. Applications to the particle production at RHIC for both Au + Au and d + Au collisions in mid-rapidity are discussed. In particular, the suppression for meson production in central Au + Au collisions and Cronin effect in d + Au collisions are naturally explained within the recombination models. Finally, strange particle production is briefly discussed.

Hadron Correlations and Parton Recombination

Parton recombination has been found to be an extremely useful model to understand hadron production at the Relativistic Heavy Ion Collider. It is particularly important to explore its connections with hard processes. This article reviews some of the aspects of the quark recombination model and places particular emphasis on hadron correlations.

Suppression ofp-wave baryons in quark recombination

We show that the observed suppression of the $\ensuremath{\Lambda}(1520)/\ensuremath{\Lambda}$ ratio in central Au+Au collisions at the BNL Relativistic Heavy Ion Collider can be naturally understood in the constituent quark recombination model.

Evolution of Shower Parton Distributions in a Jet from QuarkRecombination Model

The evolution of shower parton distributions in a jet is investigated inthe framework of a quark recombination model. The distributions areparameterized and the Q2 dependence of the parameters is given bypolynomials of ln Q2 for a wide range of Q2.

Possible resolutions of theD-puzzle

We explore possible ways of explaining the net charge event-by-event fluctuations in Au+Au collisions observed in experiments at the Relativistic Heavy Ion Collider within a quark recombination model. We estimate the number of quarks at recombination and their implications for the predicted net charge fluctuations. We also discuss the consequences of diquark and quark-antiquark clustering above the deconfinement temperature.

Hadron correlations from recombination and fragmentation

We review the formalism of quark recombination applied to the hadronization of a quark–gluon plasma. Evidence in favour of the quark recombination model is outlined. Recent work on parton correlations, leading to detectable correlations between hadrons, is discussed. Hot spots from completely quenched jets are a likely source of such correlations which appear to be jet like. It will be discussed how such a picture compares with measurement of associated hadron yields at RHIC.

Hadron production at RHIC: recombination of quarks

We discuss quark recombination applied to the hadronization of a quark gluon plasma. It has been shown that the quark recombination model can explain essential features of hadron production measured in high energy heavy ion collisions.

Nuclear and hadronic reaction mechanisms producing spin asymmetry

We briefly review concept of the quark recombination (QRC) model and a general success of the model. To solve the existing problem, so called anomalous spin observables, in the high energy hyperon spin phenomena, we propose a mechanism; the primarily produced quarks, which are predominantly u and d quarks, act as the leading partons to form the hyperons. Extension of the quark recombination concept with this mechanism is successful in providing a good account of the anomalous spin observables. Another kind of anomaly, the non-zero analysing power and spin depolarization in the Λ hyperon productions, are also discussed and well understood by the presently proposed mechanism. Recently, a further difficulty was observed in an exclusive ΛK+p production and wel will indicate a possible diagram for resolving it.

ANALYZING POWERS IN INCLUSIVE PION PRODUCTION AT HIGH ENERGY AND THE NUCLEON SPIN STRUCTURE

Analyzing powers in inclusive pion production in high energy transversely polarized proton–proton collisions are studied theoretically in the framework of the quark recombination model. Calculations by assuming the SU(6) spin-flavor symmetry for the nucleon structure disagree with the experiments. We solve this difficulty by taking into account the realistic spin distribution functions of the nucleon, which differs from the SU(6) expectation at large x, with a perturbative QCD constraint on the ratio of the unpolarized valence distributions, u/d→5 as x→ 1. We also discuss the kaon spin asymmetry and find AN(K+)=-AN(K0) in the polarized proton–proton collisions at large x F .

New Mechanisms for the Anomalous Spin Observables in High Energy Hyperon Productions

To solve the problem of the anomalous spin observables in high energy hyperon production, we propose a mechanism in which primarily produced quarks, which are predominantly u and d quarks, act as leading partons to form the hyperons. The quark recombination model with the new mechnism is found to provide a good description of the anomalous spin observables. Interesting relationships among various hyperon spin observables are predicted from the new mechanism for further experimental study. Other anomalous observables, the observed non-zero analyzing power and the spin depolarization in hyperon production, can be understood in terms of a spin transfer process also proposed here.