Showing papers on "Parton published in 2000"
TL;DR: In this paper, an up-to-date global QCD analysis of high energy lepton-hadron and hadronhadron interactions is performed to better determine the gluon and quark parton distributions in the nucleon.
Abstract: An up-to-date global QCD analysis of high energy lepton-hadron and hadron-hadron interactions is performed to better determine the gluon and quark parton distributions in the nucleon. Improved experimental data on inclusive jet production, in conjunction with precise deep inelastic scattering data, place good constraints on the gluon over a wide range of x; while new data on asymmetries in Drell-Yan processes contribute to better determine the d/u ratio. Comparisons with results of other recent global analyses are made, and the differences are described. Open issues and the general problem of determining the uncertainties of parton distributions are discussed.
1,299 citations
TL;DR: HERWIG as mentioned in this paper is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, lepton hadron and hadron hadron scattering and soft hadron-hadron collisions in one package.
Abstract: HERWIG is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron scattering and soft hadron-hadron collisions in one package. It uses the parton-shower approach for initial- and final-state QCD radiation, including colour coherence effects and azimuthal correlations both within and between jets. This article updates the description of HERWIG published in 1992, emphasising the new features incorporated since then. These include, in particular, the matching of first-order matrix elements with parton showers, a more correct treatment of heavy quark decays, and a wide range of new processes, including many predicted by the Minimal Supersymmetric Standard Model, with the option of R-parity violation. At the same time we offer a brief review of the physics underlying HERWIG together with details of the input and control parameters and the output data, to provide a self-contained guide for prospective users of the program. This version of the manual (version 3) is updated to HERWIG version 6.5, which is expected to be the last major release of Fortran HERWIG. Future developments will be implemented in a new C++ event generator, HERWIG++.
1,238 citations
CERN1
TL;DR: In this paper, the relation between the Baier-Dokshitzer-Mueller-Peigne-Schiff (BDMPS) and Zakharov formalisms for medium-induced gluon radiation off hard quarks, and the radiation off very few scattering centers was studied.
503 citations
TL;DR: In this paper, it was shown that the off-forward parton distribution for {zeta} = 0 can be interpreted as a simultaneous measurement of the longitudinal momentum and transverse position of partons in the infinite momentum frame.
Abstract: It is shown that the off-forward parton distribution for {zeta}=0, i.e., where the initial and final states differ only in their transverse momenta, can be interpreted as a simultaneous measurement of the longitudinal momentum and transverse position of partons in the infinite momentum frame. (c) 2000 The American Physical Society.
479 citations
TL;DR: In this paper, a new set of fragmentation functions for charged pions, charged kaons, and protons, both at the leading and next-to-leading orders, are fitted to the scaled-momentum distributions of these hadrons measured in e+e− annihilation on the Z-boson resonance at CERN LEP1 and SLAC SLC.
391 citations
TL;DR: Modified quark fragmentation functions in deeply inelastic eA collisions and their QCD evolution equations are derived for the first time in the framework of multiple parton scattering and the results in the next-leading-twist depend on both diagonal and off-diagonal twist-four parton distributions.
Abstract: Modified quark fragmentation functions in deeply inelastic $\mathrm{eA}$ collisions and their QCD evolution equations are derived for the first time in the framework of multiple parton scattering. Induced radiation gives rise to additional terms in the evolution equations and thus softens the modified quark fragmentation functions. The results in the next-leading-twist depend on both diagonal and off-diagonal twist-four parton distributions and the combination of which clearly manifests the LPM interference pattern. The predicted modification depends quadratically on the nuclear size ( ${A}^{2/3}$). Generalization to the case of hot QCD medium is also discussed.
349 citations
TL;DR: In this article, the parton distribution dependence of the total production cross sections and rapidity distributions was studied, paying particular attention to the uncertainties arising from uncertainties in the Parton distributions themselves.
Abstract: W and Z bosons will be produced copiously at the LHC proton-proton collider. We study the parton distribution dependence of the total production cross sections and rapidity distributions, paying particular attention to the uncertainties arising from uncertainties in the parton distributions themselves. Variations in the gluon, the strong coupling, the sea quarks and the overall normalisation are shown to lead to small but non-negligible variations in the cross section predictions. Ultimately, therefore, the measurement of these cross sections will provide a powerful cross check on our knowledge of parton distributions and their evolution.
252 citations
TL;DR: In this paper, a solution is given to the non-linear equation which describes the evolution of the parton cascade in the case of the high parton density, and related physics is discussed as well as some applications to heavy ion-ion collisions.
244 citations
TL;DR: In this paper, the authors considered the limit of a thin quark-gluon plasma, where the number of rescatterings of the jet and gluons is small.
241 citations
CERN1
TL;DR: In this paper, the soft limit of one-loop QCD amplitudes was studied and the process-independent factorization formula that controlled the singular behaviour in this limit was derived from the traditional eikonal factorization.
241 citations
TL;DR: In this paper, a multiphase transport model that includes both initial partonic and final hadronic interactions was developed to study heavy ion collisions at energies available from the Relativistic Heavy Ion Collider (RHIC).
Abstract: To study heavy ion collisions at energies available from the Relativistic Heavy Ion Collider (RHIC), we have developed a multiphase transport model that includes both initial partonic and final hadronic interactions. Specifically, the Zhang's parton cascade (ZPC) model, which uses as input the parton distribution from the heavy ion jet interaction generator (HIJING) model, is extended to include the quark-gluon--to--hadronic-matter transition and also final-state hadronic interactions based on a relativistic transport (ART) model. Predictions of the model for central Au on Au collisions at RHIC are reported.
TL;DR: In this paper, a review on the theoretical aspects and experimental results of polarized deep inelastic scattering and of other hard scattering processes is presented, with a focus on the first moment of the polarized structure function in various factorization schemes, which is related to the axial anomaly, and on its relevance for understanding the origin of the proton spin.
TL;DR: In this paper, the authors studied the evolution of flavour singlet parton densities and structure functions in massless perturbative QCD and derived parametrizations of these quantities, including Bjorken- x dependent estimates of their residual uncertainties.
TL;DR: In this article, the authors present experimental knowledge and theoretical understanding of deep-inelastic scattering on nuclear targets, focusing mainly on nuclear coherence phenomena, such as shadowing, where the key physics issue is the exploration of hadronic and quark-gluon fluctuations of a high-energy virtual photon and their passage through the nuclear medium.
TL;DR: In this paper, a new parametrization of the parton distribution functions is adopted by taking into account the positivity and the counting rule, and the polarized distribution functions of u and d valence quarks, sea quarks and gluon are determined by using world data from the longitudinally polarized deep inelastic scattering experiments.
Abstract: Polarized parton distribution functions are determined by using world data from the longitudinally polarized deep inelastic scattering experiments. A new parametrization of the parton distribution functions is adopted by taking into account the positivity and the counting rule. From the fit to the asymmetry data A_1, the polarized distribution functions of u and d valence quarks, sea quarks, and gluon are obtained. The results indicate that the quark spin content is \Delta\Sigma=0.20 and 0.05 in the leading order (LO) and the next-to-leading-order (NLO) MS-bar scheme, respectively. However, if x dependence of the sea-quark distribution is fixed at small x by "perturbative QCD" and Regge theory, it becomes \Delta \Sigma=0.24 ~ 0.28 in the NLO. The small-x behavior cannot be uniquely determined by the existing data, which indicates the importance of future experiments. From our analysis, we propose one set of LO distributions and two sets of NLO ones as the longitudinally-polarized parton distribution functions.
TL;DR: In this paper, the transverse momentum spectra of the incoming partonic system were used to calculate the shape of the prompt photon spectra in high energy pp and p\bar{p} collisions.
Abstract: We introduce a general expression which enables the parton distribution, unintegrated over the parton transverse momentum, to be obtained from the conventional parton densities. We use the formalism to study the effects of the transverse momentum \(q_t\) of the incoming partonic system on the calculation of the transverse momentum spectra of prompt photons produced in high energy pp and \(p\bar{p}\) collisions. For the purposes of illustration, we use the double logarithm approximation. For large \(q_t\) we calculate the effect directly from the perturbative formalism, whereas for small \(q_t\) we bound the effect using two extreme hypotheses. In both \(q_t\) domains we find that the shapes of the prompt photon spectra are not significantly modified, although the cross sections are enhanced.
TL;DR: In this paper, the authors performed a global study of inclusive charged-hadron production in p anti-p, gamma p, and gamma gamma collisions at next-to-leading order in the parton model of quantum chromodynamics.
Abstract: Using fragmentation functions for charged pions, charged kaons, and (anti)protons recently extracted from experimental data of e^+e^- annihilation at the Z-boson resonance and at centre-of-mass energy root(s) = 29 GeV, we perform a global study of inclusive charged-hadron production in p anti-p, gamma p, and gamma gamma collisions at next-to-leading order in the parton model of quantum chromodynamics. Comparisons of our results with p anti-p data from CERN S p anti-p S and the Fermilab Tevatron, gamma p data from DESY HERA, and gamma gamma data from CERN LEP2 allow us to test the universality of the fragmentation functions predicted by the factorization theorem. Furthermore, we perform comparisons with (e^+e^-)-annihilation data from LEP2 so as to test the scaling violations predicted by the Altarelli-Parisi evolution equations.
TL;DR: In this article, the inclusion of single and double differential crosssections for neutral and charged current processes with four-momentum transfer squared Q^2 between 150 and 30,000 GeV2 and with Bjorken x between 0.0032 and 0.65 are measured in e^+ p collisions.
Abstract: The inclusive single and double differential cross-sections for neutral and charged current processes with four-momentum transfer squared Q^2 between 150 and 30,000 GeV2 and with Bjorken x between 0.0032 and 0.65 are measured in e^+ p collisions. The data were taken with the H1 detector at HERA between 1994 and 1997, and they correspond to an integrated luminosity of 35.6 pb^-1. The Q^2 evolution of the parton densities of the proton is tested, yielding no significant deviation from the prediction of perturbative QCD. The proton structure function F_2(x,Q^2) is determined. An extraction of the u and d quark distributions at high x is presented. At high Q^2 electroweak effects of the heavy bosons Z0 and W are observed and found to be consistent with Standard Model expectation.
TL;DR: In this article, an improvement of the splitting functions at small x was proposed, which overcomes the apparent problems encountered by the BFKL approach and leads to good apparent convergence, and a description of scaling violations valid both at large and small x.
TL;DR: A new QCD analysis of deep inelastic scattering (DIS) data is presented in this article, where all available neutrino and antineutrino cross sections are reanalyzed and included in the fit, along with charged-lepton DIS and Drell-Yan data.
Abstract: A new QCD analysis of deep inelastic scattering (DIS) data is presented. All available neutrino and antineutrino cross sections are reanalyzed and included in the fit, along with charged-lepton DIS and Drell–Yan data. A massive factorization scheme is used to describe the charm component of the structure functions. Next-to-leading-order parton distribution functions are provided. In particular, the strange-sea density is determined with a higher accuracy with respect to other global fits.
TL;DR: In this paper, the scale dependence of the twist-3 quark-gluon parton distributions was studied using the observation that in the multi-color limit the corresponding QCD evolution equations possess an additional integral of motion and turn out to be effectively equivalent to the Schrodinger equation.
TL;DR: In this paper, the authors derived an evolution equation describing the high energy behavior of the cross section for the single diffractive dissociation in deep inelastic scattering on a hadron or a nucleus.
TL;DR: In this paper, the authors point out that like-sign W pair production provides a relatively clean way of searching for, and calibrating, double parton scattering at the LHC.
TL;DR: In this paper, the authors calculate excitation functions for various dynamical quantities as well as experimental observables from SIS to RHIC energies within the HSD transport approach which is based on string, quark, diquark ( q, q, qq, q q ) and hadronic degrees of freedom without including any explicit phase transition to a quark-gluon plasma (QGP).
TL;DR: In this paper, the parton momentum density distributions of the proton are determined from a next-to-leading order QCD analysis of structure functions measured at HERA and by fixed target experiments.
Abstract: The parton momentum density distributions of the proton are determined from a next-to-leading order QCD analysis of structure functions measured at HERA and by fixed target experiments. Also included are data on the difference of the up and down anti-quark distributions. The uncertainties in the parton densities, structure functions and related cross sections are estimated from the experimental errors and those on the input parameters of the fit. Several QCD predictions based on the parton densities obtained from this analysis are calculated and compared to data.
TL;DR: In this paper, a new solution of 3+1D covariant kinetic theory is presented for nuclear collisions in the energy domain. But it is shown that the transport evolution differs significantly from hydrodynamics.
Abstract: New solutions of 3+1D covariant kinetic theory are presented for nuclear collisions in the energy domain ${E}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\sim}200A$ GeV. They are obtained using MPC, a new Monte Carlo parton transport technique that employs very high parton subdivision that is necessary to preserve covariance. The transport results are compared with ideal hydrodynamics solutions. We show that the transport evolution differs significantly from hydrodynamics. In addition, we compare the transport freeze-out distributions to those obtained from ideal hydrodynamics with the Cooper-Frye isotherm freeze-out prescription. The transport freeze-out four-volume is shown to be sensitive to the reaction rates and deviates from both timelike and spacelike freeze-out 3D hypersurfaces commonly assumed. In particular, we find that there does not exist a universal freeze-out temperature. Finally, the transverse momentum distributions are found to deviate by up to an order of magnitude from (Cooper-Frye frozen) hydrodynamics for a wide range of possible initial conditions and reaction rates at RHIC energies.
TL;DR: In this article, the amplitude of deeply virtual Compton scattering in the parton model was computed and it was shown that the amplitude up to the accuracy O(1/Q) depends on new skewed parton distributions.
Abstract: We compute amplitude of deeply virtual Compton scattering in the parton model. We found that the amplitude up to the accuracy O(1/Q) depends on new skewed parton distributions (SPD's). These additional contributions make the DVCS amplitude explicitly transverse.
TL;DR: In this paper, the authors calculate excitation functions for various dynamical quantities as well as experimental observables from SIS to RHIC energies within the HSD transport approach, which is based on string, quark, diquark ($q, \bar{q}, qq, q q, \bar {q}\bar{qs}$) and hadronic degrees of freedom without including any explicit phase transition to a quark-gluon plasma (QGP).
Abstract: We calculate excitation functions for various dynamical quantities as well as experimental observables from SIS to RHIC energies within the HSD transport approach which is based on string, quark, diquark ($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom without including any explicit phase transition to a quark-gluon plasma (QGP). It is argued that the failure of this more 'conventional' approach in comparison to experimental data should indicate the presence of a different phase which might be either attributed to space-time regions of vanishing scalar quark condensate ($ $ = 0) or to the presence of a QGP phase with strongly interacting partons. We study the $K/\pi$ ratio, the low mass dilepton enhancement in the invariant mass regime from 0.2 -- 1.2 GeV as well as charmonium suppression for central Au + Au collisions as a function of the bombarding energy and present predictions for these observables as well as hadron rapiditiy distributions at RHIC energies. Whereas all observables studied within HSD show smooth increasing/decreasing excitation functions, the experimental $K^+/\pi^+$ ratio indicates a maximum at 11 A$\cdot$GeV (or above) which is interpreted as a signature for a chirally restored phase in the course of the reaction.
TL;DR: In this paper, the parton distribution dependence of the total production cross sections and rapidity distributions was studied, paying particular attention to the uncertainties arising from uncertainties in the Parton distributions themselves.
Abstract: W and Z bosons will be produced copiously at the LHC proton-proton collider. We study the parton distribution dependence of the total production cross sections and rapidity distributions, paying particular attention to the uncertainties arising from uncertainties in the parton distributions themselves. Variations in the gluon, the strong coupling, the sea quarks and the overall normalisation are shown to lead to small but non-negligible variations in the cross section predictions. Ultimately, therefore, the measurement of these cross sections will provide a powerful cross check on our knowledge of parton distributions and their evolution.