Parton Recombination Model Research Articles

A unified study of the production of all identified hadrons over wide ranges of transverse momenta at LHC

The production of all identified hadrons in the heavy-ion collisions at the CERN Large Hadron Collider (LHC) is studied with emphasis on the pT distributions up to 20 GeV/c in central Pb+Pb collisions at . The parton recombination model is used to determine the hadronic pT spectra from the quark distributions. From the heavy hyperon spectra it is known from earlier studies that the u, d, s thermal distributions in pT are exponential with large inverse slopes that cannot be identified with any temperature in conventional fluid models. They are used as inputs in our model together with shower partons determined from our treatment of momentum degradation that uses high-pT pion data as input. Those thermal and shower partons are used to calculate the pT distributions of all observed hadrons (π, K, p, Λ, Ξ, Ω and ϕ) over wide ranges of pT, so the system is highly constrained. We show how well the LHC data can be reproduced with only a few parameters to adjust. Centrality dependence has not been studied. What is learned is that minijets are important, not only in giving rise to abundant shower partons, but also in the conversion of semihard partons in the medium to soft partons that enhance the thermal partons. Since the conversion process can occur throughout the expansion phase of the high-density medium, the results of this work may be taken to imply that advocacy for the assumption of rapid equilibration needs to be reconciled with the phenomenological evidence that we have found for prolonged creation of soft partons.

Hadronization of dense partonic matter

The parton recombination model has turned out to be a valuable tool to describe hadronization in high-energy heavy-ion collisions. I review the model and revisit recent progress in our understanding of hadron correlations. I also discuss higher Fock states in the hadrons, possible violations of the elliptic flow scaling and recombination effects in more dilute systems.

Review of Parton Recombination Models

Parton recombination models have been very successful in explaining data taken at RHIC on hadron spectra and emission patterns in Au+Au collisions at transverse momenta above 2 GeV/c, which have exhibited features which could not be understood in the framework of basic perturbative QCD. In this article I will review the current status on recombination models and outline which future challenges need to be addressed by this class of models.

Nuclear effects on hadron production ind+Aucollisions atsNN=200GeV revealed by comparison withp+pdata

PHENIX has measured the centrality dependence of midrapidity pion, kaon, and proton transverse momentum distributions in d+Au and p+p collisions at root s(NN) = 200 GeV. The p+p data provide a reference for nuclear effects in d+Au and previously measured Au+Au collisions. Hadron production is enhanced in d+Au, relative to independent nucleon-nucleon scattering, as was observed in lower energy collisions. The nuclear modification factor for (anti)protons is larger than that for pions. The difference increases with centrality but is not sufficient to account for the abundance of baryon production observed in central Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The centrality dependence in d+Au shows that the nuclear modification factor increases gradually with the number of collisions encountered by each participant nucleon. We also present comparisons with lower energy data as well as with parton recombination and other theoretical models of nuclear effects on particle production.

Correlations in the Parton Recombination Model

We describe how parton recombination can address the recent measurement of dynamical jet-like two particle correlations. In addition we discuss the possible effect realistic light-cone wave-functions including higher Fock-states may have on the well-known elliptic flow valence-quark number scaling law.

Production of K0s, ϕ, Λ and Ξ from 200 GeV d+Au collisions at RHIC

We present the preliminary measurement of K0s, ϕ, Λ and Ξ transverse momentum (pt) distribution from GeV d+Au collisions at RHIC. The measured spectra can be described by using a double exponential fit. The centrality dependence of the particle yield with respect to the number of binary collision scaling (RCP) will be presented for a pt up to 6 GeV/c. The particle dependence of the RCP among K0s, ϕ, Λ and Ξ is consistent with expectations from parton recombination models for hadron formation. The Cronin effect and its particle dependence appear not due to initial parton scatterings alone as previously assumed in models.

Proton production ind+Aucollisions and the Cronin effect

Proton production in the intermediate p_T region in d+Au collisions is studied in the parton recombination model. The recombination of soft and shower partons is shown to be important in central collisions, but negligible in peripheral collisions. It is found that the large nuclear modification factor for proton production can be well reproduced by a calculation of the 3-quark recombination process.

Nuclear shadowing in the structure function F3( x)

Nuclear modification of the structure function F 3 is investigated. Although it could be estimated in the medium and large x regions from the nuclear structure function F 2 A , it is essentially unknown at small x. The nuclear structure function F 3 A at small x is investigated in two different theoretical models: a parton-recombination model with Q 2 rescaling and an aligned-jet model. We find these models predict completely different behavior at small x: antishadowing in the first parton model and shadowing in the aligned-jet model. Therefore, studies of the ratio F 3 A F 3 D at small x could be useful in discriminating among different models, whith produce similar shadowing behavior in the structure function F 2. We also estimate currently acceptable nuclear modification of F 3 at small x by using F 2 A F 2 D experimental data and baryon-number conservation.

SU(2)-flavor-symmetry breaking in nuclear antiquark distributions

SU (2)-flavor-symmetry breaking in antiquark distributions of the nucleon was suggested by the New Muon Collaboration (NMC) in deep inelastic muon scattering. As an independent test, Drell-Yan data for the tungsten target have been used for examining the asymmetry. We investigate whether there exists significant modification of the ū - d̄ distribution in nuclei in a parton-recombination model. It should be noted that a finite ū - d̄ distribution is theoretically possible in nuclei even if the sea is SU(2) f symmetric in the nucleon. In neutron-excess nuclei such as tungsten, there exist more d-valence quarks than u-valence quarks, so that more d- quarks are lost than u- quarks , due to parton-recombination s in the small x region. Our results suggest that the nuclear modification in the tungsten is a 2–10 % effect on the ū - d̄ distribution suggested by the NMC data. Nuclear effects on the flavor asymmetric distribution could be an interesting topic for future theoretical and experimental investigations.

Nuclear shadowing in a parton recombination model: Q2 variation.

${\mathit{Q}}^{2}$ variation of the nuclear-structure-function ratio ${\mathit{F}}_{2}^{\mathit{A}}$(x,${\mathit{Q}}^{2}$)/${\mathit{F}}_{2}^{\mathit{D}}$(x,${\mathit{Q}}^{2}$) is investigated in a parton model with ${\mathit{Q}}^{2}$-rescaling and parton-recombination effects. Calculated results are compared with the New Muon Collaboration (NMC) and the Fermilab-E665 experimental data. We find that our theoretical results show small ${\mathit{Q}}^{2}$ variations and that they are consistent with the data within present experimental accuracy.

A dependence of hadron-nucleus massive lepton pair production.

Nuclear effects in the small x region of hadron-nucleus Drell-Yan processes at 800 GeV are analyzed. We employ the parton recombination model to describe the suppression in the ${\mathit{R}}_{\mathrm{DY}}^{\mathit{p}\mathit{A}}$(${\mathit{x}}_{2}$) ratio and we compare it with ${\mathit{R}}_{\mathrm{EMC}}$(x), in the same kinematical region. Our results show that shadowing effects in Drell-Yan and deep inelastic scattering processes admit a description based on this model in both cases but with different characteristic parameters. Good agreement with experimental results is obtained.

On the possible measurement of gluon asymmetry in a spinning nucleus

The quadrupole gluon distribution, which exists only for hadrons with spin >or=1, is investigated for a typical light nucleus using the convolution model and a QCD inspired parton recombination model. The effects of an exotic quadrupole gluon component upon asymmetries in prompt photon production and J/ psi lepton production are estimated.

Spin structure functions of nuclei in the QCD parton recombination model.

The multipole $L=2$ structure function, measurable in deeply inelastic scattering of unpolarized leptons off a polarized $J\ensuremath{\ge}1$ nuclear target, is a good indicator of exotic quark-gluon components in the nucleus. We present here estimates of this structure function for two different classes of nuclei---light nuclei describable in an independent-particle model approach and heavy nuclei described by slowly rotating collective variables. An estimate of the exotic effects is made within the context of a model wherein gluons and quarks from separate nucleons fuse together to alter the parton densities relative to that in isolated nucleons.

Nuclear effects in J/ψ hadroproduction

Abstract A phenomenological picture of the parton recombination model for shadowing at small values of x is applied to hadron-nucleus J/ψ production. Agreement with experimental data is obtained provided that a further suppression of J/ψ production inside the nucleus is taken into account.

Spin-parameter measurements in inclusiveΣ0production

We have measured the polarization P, the analyzing power A, and the polarization transfer D of ${\ensuremath{\Sigma}}^{0}$'s produced inclusively by a polarized proton beam at 18.5 GeV/c. Our data cover a region of moderate ${p}_{T}$ (average 1 GeV/c) and Feynman x up to 0.75. We find agreement with a previous measurement of the ${\ensuremath{\Sigma}}^{0}$ polarization P. We observe nonzero values for A and D, but they are significantly smaller than predictions based on a simple parton-recombination model. We have extended this model to include finite transversity spin flips, which improves agreement with the data considerably.

A-dependence in the shadowing region of deep inelastic scattering

The parton recombination model of nuclear shadowing in deep inelastic lepton scattering suggests that shadowing should begin to occur at larger values of Bjorken x as A increases. This expectation as well as that of weak dependence of Q2 of the shadowing phenomenon are consistent with recent data.

Hyperon polarization asymmetry: Polarized beams and Omega - production.

The parton recombination model relates polarizations in inclusive baryon fragmentation to those of the underlying constituent subprocesses. Measurements of spin-spin correlations in experiments using polarized beams will provide detailed information on the quark spin dynamics in the color-confining recombination process. We present a large number of predictions for the octet baryon\ensuremath{\rightarrow}octet baryon transitions. We discuss the production of polarized ${\ensuremath{\Omega}}^{\mathrm{\ensuremath{-}}}$'s, required to measure the magnetic moment of the ${\ensuremath{\Omega}}^{\mathrm{\ensuremath{-}}}$. We also review the data from unpolarized beam experiments, and find very satisfactory agreement between experiment and theory.

Models for polarization asymmetry in inclusive hadron production

The parton-recombination model plus SU(6) symmetry is used to relate polarizations in inclusive baryon production to those of the underlying constituent subprocesses. A description of the origin of the polarization asymmetry in terms of Thomas precession of the quarks' spins in the recombination process accounts for all of the qualitative features of the baryon and antibaryon polarization data.

Quark dynamics of polarization in inclusive hadron production

The parton-recombination model plus SU(6) symmetry is used to relate polarizations in inclusive baryon production to those of the underlying constituent subprocesses. A large number of predictions are given, none of which disagrees with experiment. It is suggested that the polarization arises via Thomas precession of the quarks' spins in the recombination process. This description accounts for all of the qualitative features of the baryon and antibaryon polarization data.

Cumulative production of particles in high energy collisions

We show that the parton recombination model of hadron production combined with a quark-parton model of the nucleus describe cumulative meson production in high energy collisions with nuclei remarkably well.