Showing papers on "Parton published in 1997"
TL;DR: In this article, the medium-induced p⊥-broadening and induced gluon radiation spectrum of a high energy quark traversing a large nucleus is studied.
1,062 citations
TL;DR: In this paper, the authors studied the physics of deeply virtual Compton scattering (DVCS) in the large s and small t kinematic region and derived an Altarelli-Parisi type of evolution equations for these distributions.
Abstract: We study in QCD the physics of deeply virtual Compton scattering (DVCS){emdash}the virtual Compton process in the large s and small t kinematic region. We show that DVCS can probe a new type of {ital off-forward} parton distributions. We derive an Altarelli-Parisi-type of evolution equations for these distributions. We also derive their sum rules in terms of nucleon form factors of the twist-two quark and gluon operators. In particular, we find that the second sum rule is related to fractions of the nucleon spin carried separately by quarks and gluons. We estimate the cross section for DVCS and compare it with the accompanying Bethe-Heitler process at CEBAF and HERMES kinematics. {copyright} {ital 1997} {ital The American Physical Society}
863 citations
TL;DR: In this paper, a generalization of usual parton distributions for the case when long-distance information is accumulated in nonforward matrix elements of quark and gluon light-cone operators is presented.
Abstract: Applications of perturbative QCD to deeply virtual Compton scattering and hard exclusive electroproduction processes require a generalization of usual parton distributions for the case when long-distance information is accumulated in nonforward matrix elements of quark and gluon light-cone operators. They describe two types of nonperturbative functions parameterizing such matrix elements: double distributions F(x,y;t) and nonforward distribution functions F{_}/zeta (X;t), discuss their spectral properties, evolution equations which they satisfy, basic uses and general aspects of factorization for hard exclusive processes.
722 citations
TL;DR: In this article, a QCD factorization theorem for hard exclusive electroproduction of mesons in QCD was proved for the leading power in Q and all logarithms.
Abstract: We formulate and prove a QCD factorization theorem for hard exclusive electroproduction of mesons in QCD. The proof is valid for the leading power in Q and all logarithms. This generalizes previous work on vector meson production in the diffractive region of small x. The amplitude is expressed in terms of off-diagonal generalizations of the usual parton densities. The full theorem applies to all kinds of meson and not just to vector mesons. The parton densities used include not only the ordinary parton density, but also ! ,
575 citations
TL;DR: In this article, the impact of precision measurements of DIS structure functions and inclusive jet production at the Fermilab Tevatron on the global QCD analysis of parton distribution functions is studied in detail.
Abstract: The impact of recent precision measurements of DIS structure functions and inclusive jet production at the Fermilab Tevatron on the global QCD analysis of parton distribution functions is studied in detail. Particular emphasis is placed on exploring the range of variation of the gluon distribution $G(x,Q)$ allowed by these new data. The strong coupling of $G(x,Q)$ with ${\ensuremath{\alpha}}_{s}$ is fully taken into account. A new generation of CTEQ parton distributions, CTEQ4, is presented. It consists of the three standard sets [modified minimal subtraction $(\overline{\mathrm{MS}})$, deep inelastic scattering (DIS), and leading order (LO)], a series that gives a range of parton distributions with corresponding ${\ensuremath{\alpha}}_{s}$'s, and a set with a low starting value of $Q$. Previously obtained gluon distributions that are consistent with the high ${E}_{t}$ jet cross section are also discussed in the context of this new global analysis.
563 citations
TL;DR: In this paper, the Schrodinger equation for compound states of an arbitrary number of reggeized gluons in the multi-colour QCD is reviewed and the gauge-invariant effective action describing the gluon-Reggeon interactions is constructed.
361 citations
TL;DR: In this paper, a Fortran 77 Monte Carlo program is used to simulate complete events in deep inelastic lepton-nucleon scattering, where the parton level interaction is based on the standard model electroweak cross sections, which are fully implemented in leading order for any lepton of arbitrary polarization.
324 citations
TL;DR: In this article, a review of the Higgs boson production at the LHC within the Standard Model and its minimal supersymmetric extension is presented, and the predictions for decay rates and production cross sections are updated by choosing the present value of the top quark mass and recent parton density sets.
Abstract: Higgs boson production at the LHC within the Standard Model and its minimal supersymmetric extension is reviewed. The predictions for decay rates and production cross sections are updated by choosing the present value of the top quark mass and recent parton density sets. Moreover, all relevant higher order corrections, some of which have been obtained only recently, are included in a consistent way.
318 citations
TL;DR: In this paper, a review of the Higgs boson production at the LHC within the Standard Model and its minimal supersymmetric extension is presented, and the predictions for decay rates and production cross sections are updated by choosing the present value of the top quark mass and recent parton density sets.
Abstract: Higgs boson production at the LHC within the Standard Model and its minimal supersymmetric extension is reviewed. The predictions for decay rates and production cross sections are updated by choosing the present value of the top quark mass and recent parton density sets. Moreover, all relevant higher order corrections, some of which have been obtained only recently, are included in a consistent way.
310 citations
TL;DR: In this article, a strong signal for double parton (DP) scattering is observed in a 16pb−1 sample of p¯p→γ/π0+3jets+X data from the CDF experiment at the Fermilab Tevatron.
Abstract: A strong signal for double parton (DP) scattering is observed in a 16pb−1 sample of p¯p→γ/π0+3jets+X data from the CDF experiment at the Fermilab Tevatron. In DP events, two separate hard scatterings take place in a single p¯p collision. We isolate a large sample of data (∼14000events) of which 53% are found to be DP. The process-independent parameter of double parton scattering, σeff, is obtained without reference to theoretical calculations by comparing observed DP events to events with hard scatterings in separate p¯p collisions. The result σeff=(14.5±1.7−2.3+1.7)mb represents a significant improvement over previous measurements, and is used to constrain simple models of parton spatial density. The Feynman x dependence of σeff is investigated and none is apparent. Further, no evidence is found for kinematic correlations between the two scatterings in DP events.
241 citations
TL;DR: In this paper, a pQCD-based model for parton production and equilibration in ultrarelativistic heavy-ion collisions is reviewed, which combines initial and final state radiations together with string phenomenology for nonperturbative soft processes.
TL;DR: In this paper, the authors compute various kinematical distributions for one jet and two jet inclusive photoproduction at HERA and perform a thorough study of the reliability of QCD predictions; in particular, they consider the scale dependence of their results and discuss the cases when the perturbative expansion might break down.
TL;DR: In this article, the analytical perturbative approach, based on perturbation QCD and local parton hadron duality (LPHD), and its application to multiparticle production in jets in the semisoft region are discussed.
Abstract: In this review we discuss the analytical perturbative approach, based on perturbative QCD and local parton hadron duality (LPHD), and its application to multiparticle production in jets in the semisoft region. Analytical formulae are presented for various observables within the accuracy of the modified leading logarithmic approximation (MLLA), i.e. with terms of relative order taken into account systematically, and in some cases with even higher accuracy. These predictions are confronted with existing experimental data. Many details of the perturbative approach to multiple hadroproduction have been consolidated in recent years, and the overall picture is remarkably healthy. The prospects of future studies of the semisoft jet physics are also discussed.
TL;DR: In this paper, the effect of medium-induced parton energy loss on jet fragmentation is studied in high-energy heavy-ion collisions, and an effective jet fragmentation function can be extracted from the inclusive spectrum of charged particles in the opposite direction of a tagged direct photon with a fixed transverse energy.
Abstract: The effect of medium-induced parton energy loss on jet fragmentation is studied in high-energy heavy-ion collisions. It is shown that an effective jet fragmentation function can be extracted from the inclusive ${p}_{T}$ spectrum of charged particles in the opposite direction of a tagged direct photon with a fixed transverse energy. We study the modification of the effective jet fragmentation function due to parton energy loss in $\mathrm{AA}$ as compared to $\mathrm{pp}$ collisions, including ${E}_{T}$ smearing from initial-state radiations for the photon-tagged jets. The effective fragmentation function at ${z=p}_{T}{/E}_{T}^{\ensuremath{\gamma}}\ensuremath{\sim}1$ in $\mathrm{pA}$ collisions is shown to be sensitive to the additional ${E}_{T}$ smearing due to initial multiple parton scatterings whose effect must be subtracted out in $\mathrm{AA}$ collisions in order to extract the effective parton energy loss. Jet quenching in deeply inelastic lepton-nucleus scatterings as a measure of the parton energy loss in cold nuclear matter is also discussed. We also comment on the experimental feasibilities of the proposed study at the Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies and some alternative measurements such as using ${Z}^{0}$ as a tag at the LHC energy.
TL;DR: In this paper, an extensive theoretical analysis of off-forward parton distributions (OFPDs) and the form factors of the quark energy-momentum tensor are estimated at a low energy scale using a bag model.
Abstract: An extensive theoretical analysis of off-forward parton distributions (OFPDs) is presented. The OFPDs and the form factors of the quark energy-momentum tensor are estimated at a low energy scale using a bag model. Relations among the second moments of OFPDs, the form factors, and the fraction of the nucleon spin carried by quarks are discussed. {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: The isosinglet unpolarized and isovector polarized twist-2 quark distributions of the nucleon at low normalization point are calculated in the large-N{sub c} limit.
Abstract: The isosinglet unpolarized and isovector polarized twist-2 quark distributions of the nucleon at low normalization point are calculated in the large- N{sub c} limit The nucleon is described as a soliton of the effective chiral theory We derive the expressions for the distribution functions in the large-N{sub c} limit starting from their definition as numbers of partons carrying a momentum fraction x in the infinite momentum frame We develop a numerical method for computation of the quark and antiquark distributions as sums over the quark single-particle levels in the pion field of the soliton The contributions of the discrete bound-state level as well as the Dirac continuum are taken into account The quark and antiquark distributions obtained explicitly satisfy all general requirements The results are in reasonable agreement with parametrizations of the data at a low normalization point {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: In this article, the effects of the top quark mass, parton distribution functions, QCD scale, and collision energy, on each of the component production mechanisms, and study the kinematic distributions for standard model electroweak production were examined.
Abstract: We present a calculation of the single top quark cross section for proton-antiproton interactions with {radical}(s)=1.8TeV at the Fermilab Tevatron collider. We examine the effects of the top quark mass, parton distribution functions, QCD scale, and collision energy, on each of the component production mechanisms, and study the kinematic distributions for standard model electroweak production. At the upgraded Tevatron with {radical}(s)=2.0TeV and high luminosity, it will be possible to test the nature of the Wtb coupling using single top quark production. We estimate the sensitivity to measure the single top quark cross section, and thus to directly measure V{sub tb} and the top quark partial width. We show what happens to the V{sub tb} measurement when an anomalous (V+A) component is added to the Wtb coupling, and how the top quark polarization affects the kinematic distributions. {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: The current status of measurements of the nucleon structure functions and their understanding is reviewed in this article, with particular attention paid to much improved information on the gluon momentum distribution.
Abstract: The current status of measurements of the nucleon structure functions and their understanding is reviewed. The fixed target experiments E665, CCFR and NMC and the HERA experiments H1 and ZEUS are discussed in some detail. The extraction of parton momentum distribution functions from global fits is described, with particular attention paid to much improved information on the gluon momentum distribution. The status of alpha_s measurements from deep inelastic data is reviewed. Models and non-perturbative approaches for the parton input distributions are outlined. The impact on the phenomenology of QCD of the data at very low values of the Bjorken x variable is discussed in detail. Recent advances in the understanding of the transition from deep inelastic scattering to photoproduction are summarised. Some brief comments are made on the recent HERA measurements of the ep NC and CC cross-sections at very high Q2.
01 Feb 1997
TL;DR: Parton distribution functions give the probability to find partons (quarks and gluons) in a hadron as a function of the fraction η of the proton's momentum carried by the parton as mentioned in this paper.
Abstract: Parton distribution functions give the probability to find partons (quarks and gluons) in a hadron as a function of the fraction η of the proton's momentum carried by the parton. They are conventionally defined in terms of matrix elements of certain operators. They are determined from experimental results on short distance scattering of the partons. Integrals of these functions weighted with ηn are calculable using lattice QCD. Some simple models for their behavior have implications that could also be tested in lattice QCD.
TL;DR: In this article, color fluctuations in the quark-gluon plasma produced at the early stage of nucleus-nucleus collision at RHIC or LHC were studied.
TL;DR: This article used perturbative QCD to estimate the gluon density of the proton at small values of Bjorken x and showed that the available data for diffractive J/ψ photoproduction can discriminate between the Gluon distributions of the most recent sets of partons.
Abstract: We use perturbative QCD, beyond the leading lnQ 2 approximation, to show how measurements of diffractive J/ψ production at HERA can provide a sensitive probe of the gluon density of the proton at small values of Bjorken x. We estimate both the effect of the relativistic motion of the c and \(\bar c\) within the J/ψ and of the rescattering of the \(c\bar c\) quark pair on the proton. We find that the available data for diffractive J/ψ photoproduction can discriminate between the gluon distributions of the most recent sets of partons.
TL;DR: In this paper, the authors studied the production of high-energy hadron collisions of a pair of jets with large rapidity separation in an improved Balitskii-Fadin-Kuraev-Lipatov (BFKL) formalism by recasting the analytic solution of the BFKL equation as an explicit order-by-order sum over emitted gluons.
Abstract: The production in high-energy hadron collisions of a pair of jets with large rapidity separation is studied in an improved Balitskii-Fadin-Kuraev-Lipatov (BFKL) formalism. By recasting the analytic solution of the BFKL equation as an explicit order-by-order sum over emitted gluons, the effects of phase space constraints and the running coupling are studied. Particular attention is paid to the azimuthal angle decorrelation of the jet pair. The inclusion of subleading effects significantly improves the agreement between the theoretical predictions and recent preliminary measurements from the D0 Collaboration. {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: In this article, the transverse momentum spectra of charged particles produced in deep inelastic scattering are measured as a function of the kinematic variables x and Q 2 using the H1 detector at the ep collider HERA.
TL;DR: In this paper, the authors determined the two-loop Altarelli-Parisi splitting functions for fragmentation functions via analytic continuation of the corresponding space-like splitting functions, appearing in the next-to-leading order Q2-evolution equations.
TL;DR: In this article, a formalism based on a laboratory partonic description which connects the parton distributions with the momentum distributions of a quark model has been developed, using Next to Leading Order evolution and has been defined to produce the right support for the Parton distributions.
TL;DR: In this article, the results of a next-to-leading order event generator for purely gluonic jet production were reported, which is the first step in the construction of a full next to-leading-order calculation of three-jet production at hadron colliders.
Abstract: We report the results of a next-to-leading order event generator of purely gluonic jet production. This calculation is the first step in the construction of a full next-to-leading order calculation of three-jet production at hadron colliders. Several jet algorithms commonly used in experiments are implemented and their numerical stability is investigated. {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: The W W and Higgs Physics with PHACT (WPHACT) MC program as discussed by the authors is a MC program and unweighted event generator which computes all Standard Model processes with four fermions in the final state at e+e− colliders.
TL;DR: In this article, the authors describe the application of renormalization group improved perturbative QCD to inelastic lepton-hadron scattering at high centre-of-mass energy but comparatively low photon virtuality.
TL;DR: In this article, a program to simulate the production of heavy quarks through the boson-gluon fusion process in e±p collisions is presented, where the full electroweak structure of the electron-gluon interaction is taken into account as well as the masses of the produced heavy quark.
TL;DR: In this paper, the authors investigated the chemical equilibrium of the parton distributions in collisions of two heavy nuclei, using initial conditions obtained from a self-screened parton cascade calculation and for comparison with the HIJING model.
Abstract: We investigate the chemical equilibration of the parton distributions in collisions of two heavy nuclei. We use initial conditions obtained from a self-screened parton cascade calculation and, for comparison, from the HIJING model. We consider a one-dimensional as well as a three-dimensional expansion of the parton plasma and find that the onset of the transverse expansion impedes the chemical equilibration. At energies of 100 GeV/nucleon, the results for one-dimensional and three-dimensional expansions are quite similar except at large values of the transverse radius. At energies of several TeV/nucleon, the plasma initially approaches chemical equilibrium, but then is driven away from it, when the transverse velocity gradients develop. We find that the total parton multiplicity density remains essentially unaffected by the flow, but the individual concentrations of quarks, antiquarks, and gluons are sensitive to the transverse flow. The consequences of the flow are also discernible in the transverse momenta of the partons and in the lepton pair spectra, where the flow causes a violation of the so-called ${M}_{T}$ scaling.