Showing papers on "Parton published in 2005"
TL;DR: In this article, different components of a multiphase transport (AMPT) model that uses the heavy ion jet interaction generator (HIJING) for generating the initial conditions, Zhang's parton cascade (ZPC) for modeling partonic scatterings, the Lund string fragmentation model or a quark coalescence model for hadronization, and a relativistic transport (ART) model for treating hadronic scatterings are improved and combined to give a coherent description of the dynamics of relativist heavy ion collisions.
Abstract: We describe in detail how the different components of a multiphase transport (AMPT) model that uses the heavy ion jet interaction generator (HIJING) for generating the initial conditions, Zhang's parton cascade (ZPC) for modeling partonic scatterings, the Lund string fragmentation model or a quark coalescence model for hadronization, and a relativistic transport (ART) model for treating hadronic scatterings are improved and combined to give a coherent description of the dynamics of relativistic heavy ion collisions. We also explain the way parameters in the model are determined and discuss the sensitivity of predicted results to physical input in the model. Comparisons of these results to experimental data, mainly from heavy ion collisions at the BNL Relativistic Heavy Ion Collider, are then made in order to extract information on the properties of the hot dense matter formed in these collisions.
992 citations
TL;DR: The generalized parton distribution (GPD) as discussed by the authors was introduced as a universal tool to describe hadrons in terms of quark and gluonic degrees of freedom, and has been used for a long time in studies of hadronic structure.
705 citations
TL;DR: In this article, a semi-inclusive deep-inelastic scattering with hadrons in the current fragmentation region detected at low transverse momentum is presented, which is valid to all orders in perturbation theory.
Abstract: We argue a factorization formula for semi-inclusive deep-inelastic scattering with hadrons in the current fragmentation region detected at low transverse momentum. To facilitate the factorization, we introduce the transverse-momentum dependent parton distributions and fragmentation functions with gauge links slightly off the light cone, and with soft-gluon radiations subtracted. We verify the factorization to one-loop order in perturbative quantum chromodynamics and argue that it is valid to all orders in perturbation theory.
480 citations
TL;DR: The results from the STAR Collaboration on directed flow (v(1), elliptic flow, and the fourth harmonic in the anisotropic azimuthal distribution of particles from Au+Au collisions at root s(NN) = 200 GeV are summarized and compared with results from other experiments and theoretical models.
Abstract: 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 root s(NN) = 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 are discussed. For v(4), scaling with v(2)(2) and quark coalescence are discussed.
448 citations
TL;DR: In this paper, a global parton analysis of deep inelastic and related hard-scattering data was performed, including the parton evolution of u and d type quarks.
Abstract: We perform a global parton analysis of deep inelastic and related hard-scattering data, including ${{\mathcal{O}}}(\alpha_{\rm QED})$
corrections to the parton evolution. Although the quality of the fit is essentially unchanged, there are two important physical consequences. First, the different DGLAP evolution of u and d type quarks introduces isospin violation, i.e. $u^p
eq d^n$
, which is found to be unambiguously in the direction to reduce the NuTeV $\sin^2\theta_W$
anomaly. A second consequence is the appearance of photon parton distributions $\gamma(x,Q^2)$
of the proton and the neutron. In principle these can be measured at HERA via the deep inelastic scattering processes $e N \to e\gamma X$
; our predictions are in agreement with the present data.
386 citations
TL;DR: In this paper, a 3 + 1 dimensional Monte Carlo cascade was developed to solve the kinetic on-shell Boltzmann equations for partons including the inelastic gg{r_reversible}ggg pQCD processes.
Abstract: We develop a new 3 + 1 dimensional Monte Carlo cascade solving the kinetic on-shell Boltzmann equations for partons including the inelastic gg{r_reversible}ggg pQCD processes. The back reaction channel is treated - for the first time - fully consistently within this scheme. An extended stochastic method is used to solve the collision integral. The frame dependence and convergency are studied for a fixed tube with thermal initial conditions. The detailed numerical analysis shows that the stochastic method is fully covariant and that convergency is achieved more efficiently than within a standard geometrical formulation of the collision term, especially for high gluon interaction rates. The cascade is then applied to simulate parton evolution and to investigate thermalization of gluons for a central Au+Au collision at RHIC energy. For this study the initial conditions are assumed to be generated by independent minijets with p{sub T}>p{sub 0}=2 GeV. With that choice it is demonstrated that overall kinetic equilibration is driven mainly by the inelastic processes and is achieved on a scale of 1 fm/c. The further evolution of the expanding gluonic matter in the central region then shows almost an ideal hydrodynamical behavior. In addition, full chemical equilibration of the gluons follows onmore » a longer time scale of about 3 fm/c.« less
384 citations
TL;DR: Produced partons have a large local relative orbital angular momentum along the direction opposite to the reaction plane in the early stage of noncentral heavy-ion collisions.
Abstract: Produced partons have large local relative orbital angular momentum along the direction opposite to the reaction plane in the early stage of non-central heavy-ion collisions. Parton scattering is shown to polarize quarks along the same direction due to spin-orbital coupling.Such global quark polarization will lead to many observable consequences,such as left-right asymmetry of hadron spectra, global transverse polarization of thermal photons, dileptons and hadrons. Hadrons from the decay of polarized resonances will have azimuthal asymmetry similar to the elliptic flow. Global hyperon polarization is predicted with indifferent hadronization scenarios and can be easily tested.
379 citations
TL;DR: In this paper, a Monte-Carlo tool was introduced to simulate the rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions, and the model was applied to analysis of the jet quenching pattern at RHIC.
Abstract: The method to simulate the rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions has been developed. The model is a fast Monte-Carlo tool introduced to modify a standard PYTHIA jet event. The full heavy ion event is obtained as a superposition of a soft hydro-type state and hard multi-jets. The model is applied to analysis of the jet quenching pattern at RHIC.
264 citations
TL;DR: In this article, the Parton Quenching Model (PQM) was used for heavy-ion collisions with BDMPS quenching weights and a realistic collision geometry, and the results for leading particle suppression at nucleon-nucleon centre-of-mass energies of 62.4 and 5500 GeV were presented.
Abstract: Parton energy loss effects in heavy-ion collisions are studied with the Monte Carlo program PQM (Parton Quenching Model) constructed using the BDMPS quenching weights and a realistic collision geometry. The merit of the approach is that it contains only one free parameter that is tuned to the high-pt nuclear modification factor measured in central Au-Au collisions at √ sNN = 200 GeV. Once tuned, the model is consistently applied to all the high-pt observables at 200 GeV: the centrality evolution of the nuclear modification factor, the suppression of the away-side jet-like correlations, and the azimuthal anisotropies for these observables. Predictions for the leading-particle suppression at nucleon-nucleon centre-of-mass energies of 62.4 and 5500 GeV are presented. The limits of the eikonal approximation in the BDMPS approach, when applied to finite-energy partons, are discussed.
257 citations
TL;DR: In this article, it was shown that charm quarks behave essentially like light quarks in Pb-Pb collisions at the LHC and that at RHIC, the kinematical regime best suited for such an analysis of D mesons is 7 < pT < 12 GeV, whereas the study of lower transverse momenta is further complicated.
Abstract: � 20 GeV) in Pb–Pb collisions at the LHC, we find that charm quarks behave essentially like light quarks. However, since light-flavored hadron yields are dominated by gluon parents, the heavy-to-light ratio of D mesons is a sensitive probe of the color charge dependence of parton energy loss. In contrast, due to the larger b quark mass, the medium modification of B mesons in the same kinematical regime provides a sensitive test of the mass dependence of parton energy loss. At RHIC energies, the strategies for identifying and disentangling the color charge and mass dependence of parton energy loss are more involved because of the smaller kinematical range accessible. We argue that at RHIC, the kinematical regime best suited for such an analysis of D mesons is 7 < � pT < 12 GeV, whereas the study of lower transverse momenta is further complicated due to the known dominant contribution of additional, particle species dependent, non-perturbative effects.
249 citations
TL;DR: In this article, the suppression of high-p T hadron spectra in nuclear collisions was studied and the nuclear modification factor R A A (p T ), which quantifies the degree of suppression, is almost p T -independent both for RHIC and for the LHC.
TL;DR: In this article, the authors discuss the links between generalized parton distributions (GPDs) and elastic nucleon form factors, in the form of sum rules, represent powerful constraints on parametrizations of GPDs.
Abstract: We discuss the links between generalized parton distributions (GPDs) and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for GPDs at small momentum transfer, is extended to the large momentum transfer region and it is found to describe the basic features of proton and neutron electromagnetic form factor data. This parametrization is used to estimate the quark contribution to the nucleon spin.
TL;DR: In this paper, the authors employ a Fokker-planck equation to approximate the collision integral of the Boltzmann equation and augment earlier studies based on perturbative parton cross sections by resonant heavy-light quark interactions.
Abstract: Charm- and bottom-quark rescattering in a quark-gluon plasma (QGP) is investigated with the objective of assessing the approach toward thermalization. Employing a Fokker-Planck equation to approximate the collision integral of the Boltzmann equation we augment earlier studies based on perturbative parton cross sections by introducing resonant heavy-light quark interactions. The latter are motivated by recent QCD lattice calculations that indicate the presence of ``hadronic'' states in the QGP. We model these states by colorless (pseudo-) scalar and (axial-) vector D and B mesons within a heavy-quark effective theory framework. We find that the presence of these resonances at moderate QGP temperatures substantially accelerates the kinetic equilibration of c quarks as compared to using perturbative interactions. We also comment on consequences for D-meson observables in ultrarelativistic heavy-ion collisions.
TL;DR: In this paper, the authors make quantitative predictions for the rapidity and centrality dependencies of hadron multiplicities in AA, pA and pp collisions at the LHC energies basing on the ideas of parton saturation in the color glass condensate.
TL;DR: In this paper, the authors considered bremsstrahlung energy loss for hard partons traversing a quark-gluon plasma (QCP), and they found that the suppression of the spectrum is nearly flat, with the most suppression at energies $E~30\phantom{\rule{0.3em}{0ex}}T$ (where T is the QGP temperature).
Abstract: We consider bremsstrahlung energy loss for hard partons traversing a quark-gluon plasma (QCP). Accounting correctly for the probabilistic nature of the energy loss, and making a leading-order-accurate treatment of bremsstrahlung, we find that the suppression of the spectrum is nearly flat, with the most suppression at energies $E~30\phantom{\rule{0.3em}{0ex}}T$ (where T is the QGP temperature), without the need for initial state effects such as shadowing and the Cronin effect. This flat pattern should also be observed at the LHC (Large Hadron Collider) out to an energy of $~30\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$.
Journal Article•
TL;DR: In this article, the nuclear parton distribution functions (NPDFs) are determined by global analyses of experimental data on structure-function ratios F{sub 2}{sup A}/F{sub 1/2/3] and Drell-Yan cross-section ratios {sigma}{sub DY/{sigma ∆/DY ∆ A'}.
Abstract: Nuclear parton distribution functions (NPDFs) are determined by global analyses of experimental data on structure-function ratios F{sub 2}{sup A}/F{sub 2}{sup A'} and Drell-Yan cross-section ratios {sigma}{sub DY}{sup A}/{sigma}{sub DY}{sup A'}. The analyses are done in the leading order (LO) and next-to-leading order (NLO) of running coupling constant {alpha}{sub s}. Uncertainties of the NPDFs are estimated in both LO and NLO for finding possible NLO improvement. Valence-quark distributions are well determined, and antiquark distributions are also determined at x 0.2. Gluon modifications cannot be fixed at this stage. Although the advantage of the NLO analysis, in comparison with the LO one, is generally the sensitivity to the gluon distributions, gluon uncertainties are almost the same in the LO and NLO. It is because current scaling-violation data are not accurate enough to determine precise nuclear gluon distributions. Modifications of the PDFs in the deuteron are also discussed by including data on the proton-deuteron ratio F{sub 2}{sup D}/F{sub 2}{sup p} in the analysis. A code is provided for calculating the NPDFs and their uncertainties at given x and Q{sup 2} in the LO and NLO.
CERN1
TL;DR: In this article, a simple empirical parameterization of the x-and t-dependence of generalized parton distributions at zero skewness is presented, using forward parton distribution as input.
Abstract: We present a simple empirical parameterization of the x- and t-dependence of generalized parton distributions at zero skewness, using forward parton distributions as input. A fit to experimental data for the Dirac, Pauli and axial form factors of the nucleon allows us to discuss quantitatively the interplay between longitudinal and transverse partonic degrees of freedom in the nucleon (“nucleon tomography”). In particular we obtain the transverse distribution of valence quarks at given momentum fraction x. We calculate various moments of the distributions, including the form factors that appear in the handbag approximation to wide-angle Compton scattering. This allows us to estimate the minimal momentum transfer required for reliable predictions in that approach to be around $\vert t\vert \simeq 3 {\rm GeV}^2$
. We also evaluate the valence contributions to the energy-momentum form factors entering Ji’s sum rule.
TL;DR: In this article, the collisional energy loss of a charm quark for a static medium was investigated, and it proved to be almost the same order as that of radiative ones estimated to first-order opacity expansion.
Abstract: In considering the collisional energy loss rates of heavy quarks from hard light parton interactions, we computed the total energy loss of a charm quark for a static medium. For the energy range $E~5$--10 GeV of charm quark, it proved to be almost the same order as that of radiative ones estimated to a first-order opacity expansion. The collisional energy loss becomes much more important for lower energy charm quarks, and this feature could be very interesting for the phenomenology of hadrons spectra. Using such collisional energy loss rates, we estimate the momentum loss distribution employing a Fokker-Planck equation and the total energy loss of a charm quark for an expanding quark-gluon plasma under conditions resembling the energies presently available at the BNL Relativistic Heavy Ion Collider. The fractional collisional energy loss is found to be suppressed by a factor of 5 as compared to the static case and does not depend linearly on the system size. We also investigate the heavy to light hadrons $D/\ensuremath{\pi}$ ratio at moderately large (5--10 GeV/c) transverse momenta and comment on its enhancement.
TL;DR: In this paper, the infrared structure of the colour-ordered neutralino decay matrix elements at NLO and NNLO is shown to agree with the structure observed for parton radiation off a quark-gluon antenna.
TL;DR: In this article, it was shown that parton saturation may have started to set in central heavy ion collisions already around the highest CERN SPS energy, and made predictions based on high density QCD for the forthcoming 22$ GeV run at RHIC.
Abstract: The experimental results on hadron production obtained recently at RHIC offer a new prospective on the energy dependence of the nuclear collision dynamics. In particular, it is possible that parton saturation---the phenomenon likely providing initial conditions for the multiparticle production at RHIC energies---may have started to set in central heavy ion collisions already around the highest CERN SPS energy. We examine this scenario, and make predictions based on high density QCD for the forthcoming $\sqrt{s}=22$ GeV run at RHIC.
TL;DR: In this article, the antenna functions for hard gluon-gluon pairs can be systematically derived from the effective Lagrangian describing Higgs boson decay into gluons.
TL;DR: In this article, the authors show that high energy scattering is a statistical process essentially similar to reaction diffusion in a system made of a finite number of particles, and derive universal terms in the rapidity dependence of the saturation scale and the scaling form of the amplitude, which come as the leading terms in a large rapidity and small coupling expansion.
TL;DR: In this article, the authors derived the massless limit from the massive theory and established a massive theory with MS-bar subtraction which approached the mass-less theory with increasing transverse momentum.
Abstract: We calculate the next-to-leading order cross section for the inclusive production of D^{*+-} mesons in p p-bar collisions as a function of the transverse momentum and the rapidity in two approaches using massive or massless charm quarks. For the inclusive cross section, we derive the massless limit from the massive theory. We find that this limit differs from the genuine massless version with MS-bar factorization by finite corrections. By adjusting subtraction terms, we establish a massive theory with MS-bar subtraction which approaches the massless theory with increasing transverse momentum. With these results and including the contributions due to the charm and anti-charm content of the proton and anti-proton, we calculate the inclusive D^{*+-} cross section in p p-bar collisions using realistic evolved non-perturbative fragmentation functions and compare with recent data from the CDF Collaboration at the Fermilab Tevatron at center-of-mass energy root(S) = 1.96 TeV. We find reasonable, though not perfect, agreement with the measured cross sections.
TL;DR: In this article, a partonic subprocess is calculated to the lowest order of perturbative QCD taking into account the transverse momenta of the quark and antiquark as well as Sudakov suppressions.
Abstract: We analyze electroproduction of light vector mesons (
$V = \rho, \phi$
) at small Bjorken-x in an approach that includes the gluonic generalized parton distributions and a partonic subprocess, $\gamma g \to (q\bar{q}) g$
, $q\bar{q}\to V$
. The subprocess is calculated to lowest order of perturbative QCD taking into account the transverse momenta of the quark and antiquark as well as Sudakov suppressions. Our approach allows us to investigate the transition amplitudes for all kinds of polarized virtual photons and polarized vector mesons. Modeling the generalized parton distributions through double distributions and using simple Gaussian wavefunctions for the vector mesons, we compute the longitudinal and transverse cross sections at large photon virtualities as well as the spin-density matrix elements for the vector mesons. Our results are in fair agreement with the findings of recent experiments performed at HERA.
TL;DR: In this paper, the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer was estimated by using a quark-parton representation of virtual Compton scattering.
Abstract: We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer by using a quark-parton representation of virtual Compton scattering. We thus can relate the two-photon exchange amplitude to the generalized parton distributions which also enter in other wide-angle scattering processes. We find that the interference of one- and two-photon exchange contribution is able to substantially resolve the difference between electric form factor measurements from Rosenbluth and polarization transfer experiments. Two-photon exchange has additional consequences which could be experimentally observed, including nonzero polarization effects and a positron-proton/electron-proton scattering asymmetry. The predicted Rosenbluth plot is no longer precisely linear; it acquires a measurable curvature, particularly at large laboratory angle.
TL;DR: In this paper, the authors proposed a new thermalization scenario for heavy ion collisions, which at sufficiently high energies implies the phase transition to the quark-gluon plasma, and they considered a specific example of chiral symmetry restoration induced by a rapid deceleration of the colliding nuclei.
TL;DR: In this article, the collinear subtraction terms needed to establish a massive variable-flavor-number scheme for the one-particle inclusive production of heavy quarks in hadronic collisions are discussed.
Abstract: We present a detailed discussion of the collinear subtraction terms needed to establish a massive variable-flavor-number scheme for the one-particle inclusive production of heavy quarks in hadronic collisions. The subtraction terms are computed by convoluting appropriate partonic cross sections with perturbative parton distribution and fragmentation functions relying on the method of mass factorization. We find (with one minor exception) complete agreement with the subtraction terms obtained in a previous publication by comparing the zero-mass limit of a fixed-order calculation with the genuine massless results in the ${\overline{\mathrm{MS}}}$
scheme. This presentation will be useful for extending the massive variable-flavor-number scheme to other processes.
TL;DR: In this paper, the general form of the transverse momentum dependent correlator entering various hard scattering processes is derived and two new time-reversal odd parton distributions appear at the twist-3 level.
TL;DR: In this paper, the parton distribution function of the pion in the valence region is extracted in a next-to-leading order analysis from Fermilab E-615 pionic Drell-Yan data.
Abstract: The parton distribution function of the pion in the valence region is extracted in a next-to-leading order analysis from Fermilab E-615 pionic Drell-Yan data. The effects of the parametrization of the pion's valence distributions are examined. Modern nucleon parton distributions and nuclear corrections were used and possible effects from higher twist contributions were considered in the analysis. In the next-to-leading order analysis, the high-x dependence of the pion structure function differs from that of the leading order analysis, but not enough to agree with the expectations of pQCD and Dyson-Schwinger calculations.
TL;DR: In this article, a physical mechanism for the $T$-odd Sivers effect in semi-inclusive deep-inelastic scattering was proposed, based on the distortion of chirally odd impact parameter dependent parton distributions.
Abstract: Impact parameter dependent parton distributions are transversely distorted when one considers transversely polarized nucleons and/or quarks. This provides a physical mechanism for the $T$-odd Sivers effect in semi-inclusive deep-inelastic scattering. The transverse distortion can also be connected with Ji's quark angular momentum relation. The distortion of chirally odd impact parameter dependent parton distributions is related to chirally odd generalized parton distributions (GPDs). This result is used to provide a decomposition of the quark angular momentum with respect to (w.r.t.) quarks of definite transversity. Chirally odd GPDs can thus be used to determine the correlation between quark spin and quark angular momentum in unpolarized nucleons. Based on the transverse distortion, we also suggest a qualitative connection between chirally odd GPDs and the Boer-Mulders effect.