Showing papers on "Parton published in 2001"
TL;DR: HERWIG as mentioned in this paper is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, leptonhadron and hadron-hadron scattering and soft hadronhadron 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.
2,068 citations
TL;DR: In this paper, a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic final states in e+e− annihilation is proposed, which provides a leading-order description of hard multi-jet configurations together with jet fragmentation, while avoiding the most serious problems of double counting.
Abstract: We propose a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic final states in e+e− annihilation. The matrix element and parton shower domains are separated at some value yini of the jet resolution, defined according to the kT-clustering algorithm. The matrix elements are modified by Sudakov form factors and the parton showers are subjected to a veto procedure to cancel dependence on yini to next-to-leading logarithmic accuracy. The method provides a leading-order description of hard multi-jet configurations together with jet fragmentation, while avoiding the most serious problems of double counting. We present first results of an approximate implementation using the event generator APACIC++.
972 citations
TL;DR: In this article, a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic final states in $e+e^-$ annihilation is proposed, which provides a leading-order description of hard multi-jet configurations together with jet fragmentation.
Abstract: We propose a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic final states in $e^+e^-$ annihilation. The matrix element and parton shower domains are separated at some value $y_{ini}$ of the jet resolution, defined according to the $k_T$-clustering algorithm. The matrix elements are modified by Sudakov form factors and the parton showers are subjected to a veto procedure to cancel dependence on $y_{ini}$ to next-to-leading logarithmic accuracy. The method provides a leading-order description of hard multi-jet configurations together with jet fragmentation, while avoiding the most serious problems of double counting. We present first results of an approximate implementation using the event generator APACIC++.
500 citations
TL;DR: The QCD corrections to these processes in the standard model at next-to-leading order reduce the renormalization and factorization scale dependence considerably and stabilize the theoretical predictions for the cross sections.
Abstract: Higgs-boson production in association with bottom quarks, p¯ p/pp ! bbH+X, is one of the most important discovery channels for supersymmetric Higgs particles at the Tevatron and the LHC. We have calculated the next-to-leading order QCD corrections to the parton processes q¯ q,gg ! bbH and present results for total cross sections and for distributions in the transverse momenta of the bottom quarks. The QCD corrections reduce the renormalization and factorization scale depen- dence and thus stabilize the theoretical predictions, especially when the Higgs boson is produced in association with high-pT bottom quarks. The next-to-leading order predictions for the total cross section are in reasonable numerical agreement with calculations based on bottom-quark fusion bb ! H.
420 citations
TL;DR: In this article, the effect of multiple scattering of a highly off-shell quark and the induced parton energy loss is expressed in terms of the modification to the quark fragmentation functions.
393 citations
TL;DR: In this article, the energy, centrality, rapidity, and atomic number dependences of hadron multiplicities in nuclear collisions are derived, and the centrality and rapidity dependences with energy appear different from other approaches, and can be used as an important test of the ideas based on parton saturation and classical Chromo-Dynamics.
370 citations
TL;DR: In this paper, the authors developed a general method to quantify the uncertainties of parton distribution functions and their physical predictions, with emphasis on incorporating all relevant experimental constraints, and applied it to calculate uncertainties of gluon and quark distribution functions, W boson rapidity distributions, and the correlation between W and Z production cross sections.
Abstract: We develop a general method to quantify the uncertainties of parton distribution functions and their physical predictions, with emphasis on incorporating all relevant experimental constraints. The method uses the Hessian formalism to study an effective chi-squared function that quantifies the fit between theory and experiment. Key ingredients are a recently developed iterative procedure to calculate the Hessian matrix in the difficult global analysis environment, and the use of parameters defined as components along appropriately normalized eigenvectors. The result is a set of 2D eigenvector basis parton distributions (where $d\ensuremath{\approx}16$ is the number of parton parameters) from which the uncertainty on any physical quantity due to the uncertainty in parton distributions can be calculated. We illustrate the method by applying it to calculate uncertainties of gluon and quark distribution functions, W boson rapidity distributions, and the correlation between W and Z production cross sections.
334 citations
TL;DR: In this article, a QCD-inspired parton model for the initial stage of nuclear collisions at very high energies (RHIC, LHC and beyond) is presented, which is a self-consistent treatment, using the same formalism for calculating cross sections and particle production.
321 citations
TL;DR: In this paper, the yield of large transverse momentum hadrons is modified due to induced gluon radiation off a hard parton traversing a QCD medium.
Abstract: We determine how the yield of large transverse momentum hadrons is modified due toinduced gluon radiation off a hard parton traversing a QCD medium. The quenching factoris formally a collinear- and infrared-safe quantity and can be treated perturbatively. In spiteof that, in the p ⊥ region of practical interest, its value turns out to be extremely sensitive tolarge distances and can be used to unravel the properties of dense quark-gluon final statesproduced in heavy ion collisions. We also find that the standard modelling of quenching byshifting p ⊥ in the hard parton cross section by the mean energy loss is inadequate. 1 Introduction The so-called jet quenching [1–6] is considered an important signal of the production of a newstate of dense matter (quark-gluon plasma) in ultrarelativistic heavy ion collisions. This isunderstood as the suppression of the yield of large transverse momentum jets or particles withrespect to proton-proton collisions.In this paper we concentrate on the quenching effect in inclusive particle spectra, due to theenergy loss by medium induced gluon radiation [7–24].
299 citations
TL;DR: In this paper, the effect of multiple scattering of a highly off-shell quark and the induced parton energy loss is expressed in terms of the modification to the quark fragmentation functions.
Abstract: Multiple parton scattering and induced parton energy loss is studied in deeply inelastic scattering (DIS) off nuclei. The effect of multiple scattering of a highly off-shell quark and the induced parton energy loss is expressed in terms of the modification to the quark fragmentation functions. We derive such modified quark fragmentation functions and their QCD evolution equations in DIS using generalized factorization of higher twist parton distributions. We consider double-hard, hard-soft parton scattering as well as their interferences in the same framework. The final result, which depends on both the diagonal and off-diagonal twist-four parton distributions in nuclei, demonstrates clearly the Landau-Pomeranchuk-Migdal interference features and predicts a unique nuclear modification of the quark fragmentation functions.
297 citations
Book•
25 Apr 2001
TL;DR: In this paper, the Parton model is used for scaling violations of the scaling violation rule of the Neutrino Deep-Inelastic Scattering (DIS) model.
Abstract: Introduction ELECTROMAGNETIC STRUCTURE OF THE NUCLEON Elastic Electron Scattering Electromagnetic Form Factors of the Nucleon Nucleon Resonances Compton Scattering WEAK PROBES OF NUCLEON STRUCTURE Matrix Elements of the Weak Currents The Axial Matrix Elements in Charged Current Neutrino Reactions Beta-Decay Myon Capture Near Threshold Pion Electroproduction Neutral Current Interactions DEEP-INELASTIC LEPTON SCATTERING ON THE NUCLEON The Parton Model Scaling Violations Sum Rules Neutrino Deep-Inelastic Scattering Experimental Results for Unpolarised Targets Spin Dependent Structure Functions Qualitative Understanding of Parton Distributions ELEMENTS OF QCD Basic Lagrangian Feynman Rules Renormalization The Renormalization Group Deep Inelastic Scattering: Parton Model Deep Inelastic Scattering within QCD ASPECTS OF NON-PERTUBATIVE QCD Symmetries, Currents and Anomalies Structure of the QCD Vacuum: Brief Survey QCD Sum Rules Lattice QCD and the Nucleon CHIRAL SYMMETRY AND NUCLEON STRUCTURE Chiral Effective Field Theory Baryon Masses and Sigma Terms Chiral Low Energy Theorems Summary: The Pion Cloud of the Nucleon MODELS OF THE NUCLEON Survey Non-relativistic Quark Models Soliton Models Bag Models Chiral Quark Models Chiral Solitons Final Remarks Appendix A: Notations and Conventions Appendix B: Cross Sections for Lepton and Photon Nucleon Scattering.
TL;DR: In this article, the yield of large transverse momentum hadrons is modified due to induced gluon radiation off a hard parton traversing a QCD medium, and the standard modelling of quenching by shifting $p_\perp$ in the hard partons cross section by the mean energy loss is inadequate.
Abstract: We determine how the yield of large transverse momentum hadrons is modified due to induced gluon radiation off a hard parton traversing a QCD medium. The quenching factor is formally a collinear- and infrared-safe quantity and can be treated perturbatively. In spite of that, in the $p_\perp$ region of practical interest, its value turns out to be extremely sensitive to large distances and can be used to unravel the properties of dense quark-gluon final states produced in heavy ion collisions. We also find that the standard modelling of quenching by shifting $p_\perp$ in the hard parton cross section by the mean energy loss is inadequate.
TL;DR: In this paper, the authors give a complete representation of virtual Compton scattering γ ∗ p→γp at large initial photon virtuality Q 2 and small momentum transfer squared t in terms of the light-cone wavefunctions of the target proton.
TL;DR: In this paper, the structure of infrared and ultraviolet singularities in on-shell QCD and supersymmetric QCD amplitudes at one-loop order is discussed and the dependence on the regularization scheme is also considered.
TL;DR: In this article, the authors studied the production of hadronic collisions with W gauge bosons, heavy quark pairs, and n extra jets, with n up to 4 jets.
Abstract: We study in this paper the production, in hadronic collisions, of final states with W gauge bosons, heavy quark pairs and n extra jets (with n up to 4). The complete partonic tree-level QCD matrix elements are evaluated using the ALPHA algorithm, and the events generated at the parton level are then evolved through the QCD shower and eventually hadronised using the coherent shower evolution provided by the HERWIG Monte Carlo. We discuss the details of our Monte Carlo implementation, and present results of phenomenological interest for the Tevatron Collider and for the LHC. We also comment on the impact of our calculation on the backgrounds to W (H -> b bbar) production, when only one b jet is tagged.
TL;DR: In this paper, the complete and exact overlap representation of skewed parton distributions for unpolarised and polarised quarks and gluons is derived within the framework of light-cone quantisation.
TL;DR: In this paper, the parton distributions of nuclei from deuteron to heavy ones with the mass number A~208 were investigated in the leading order of alpha s. They were provided either analytical expressions or computer subroutines for practical usage.
Abstract: Parametrization of nuclear parton distributions is investigated in the leading order of alpha_s. The parton distributions are provided at Q^2=1 GeV^2 with a number of parameters, which are determined by a chi^2 analysis of the data on nuclear structure functions. Quadratic or cubic functional form is assumed for the initial distributions. Although valence quark distributions in the medium x region are relatively well determined, the small x distributions depend slightly on the assumed functional form. It is difficult to determine the antiquark distributions at medium x and gluon distributions. From the analysis, we propose parton distributions at Q^2=1 GeV^2 for nuclei from deuteron to heavy ones with the mass number A~208. They are provided either analytical expressions or computer subroutines for practical usage. Our studies should be important for understanding the physics mechanism of the nuclear modification and also for applications to heavy-ion reactions. This kind of nuclear parametrization should also affect existing parametrization studies in the nucleon because "nuclear" data are partially used for obtaining the optimum distributions in the "nucleon".
TL;DR: In this article, it was shown that for both quarks and gluons there are eight generalized parton distributions in the proton: four which conserve parton helicity and four which do not.
Abstract: We show that for both quarks and gluons there are eight generalized parton distributions in the proton: four which conserve parton helicity and four which do not. We explain why time reversal invariance does not reduce this number from eight to six, as previously assumed in the literature.
TL;DR: In this paper, a measurement of elastic deeply virtual Compton scattering was made using e^+ p collision data corresponding to a luminosity of 46.5 pb^{-1], taken with the H1 detector at HERA, and the cross section was measured as a function of the photon virtuality, Q^2, the invariant mass of the \gamma* p system, W, and for the first time, differentially in the squared momentum transfer at the proton vertex, t, in the kinematic range 2 < q^2 < 80 Ge
TL;DR: This Letter presents for the reactions pp, pp-->tt+X-->l+l'-+X the first calculation of the dilepton angular distribution at next-to-leading order in the QCD coupling, keeping the full dependence on the spins of the intermediate tt state.
Abstract: The collider experiments at the Tevatron and the LHC will allow for detailed investigations of the properties of the top quark This requires precise predictions of the hadronic production of tt pairs and of their subsequent decays In this Letter we present for the reactions pp, pp-->tt+X-->l+l'-+X the first calculation of the dilepton angular distribution at next-to-leading order in the QCD coupling, keeping the full dependence on the spins of the intermediate tt state The angular distribution reflects the degree of correlation of the t and t spins which we determine for different choices of t and t spin bases In the case of the Tevatron, the QCD corrections are sizable, and the distribution is quite sensitive to the parton content of the proton
TL;DR: In this article, the authors performed a global study of charged-hadron production in p p, γp, and γγ collisions at next to leading order in the parton model of quantum chromodynamics.
TL;DR: The heavy top-mass approximation appears to work well for intermediate Higgs-boson masses, provided that the transverse momenta of the final-state partons are smaller than the top-quark mass.
Abstract: Real-emission corrections to gg-->H, which lead to H+2 jet events, are calculated at order alpha(4)(s). Contributions include top-quark triangles, boxes, and pentagon diagrams and are evaluated analytically for arbitrary top mass m(t). This new source of H+2 jet events is compared to the weak-boson fusion cross section for a range of Higgs boson masses. The heavy top-mass approximation appears to work well for intermediate Higgs-boson masses, provided that the transverse momenta of the final-state partons are smaller than the top-quark mass.
TL;DR: A simple extrapolation formula for the moments of the nonsinglet quark distribution u-d, as a function of quark mass, is proposed, which embodies the general constraints imposed by the chiral symmetry of QCD.
Abstract: We present the resolution of a long-standing discrepancy between the moments of parton distributions calculated from lattice QCD and their experimental values. We propose a simple extrapolation formula for the moments of the nonsinglet quark distribution u-d, as a function of quark mass, which embodies the general constraints imposed by the chiral symmetry of QCD. The inclusion of the leading nonanalytic behavior leads to an excellent description of both the lattice data and the experimental values of the moments.
TL;DR: In this article, a parton model for heavy-ion collisions at the RHIC energies is studied and the effect of parton energy loss on the azimuthal anisotropy of hadron spectra at large transverse momentum in non-central high-energy heavy ion collisions is studied.
Abstract: Parton energy loss inside a dense medium leads to the suppression of large pT hadrons and can also cause azimuthal anisotropy of hadron spectra at large transverse momentum in non-central high-energy heavy-ion collisions. Such azimuthal anisotropy is studied qualitatively in a parton model for heavy-ion collisions at the RHIC energies. The coefficientv2(pT) of the elliptic anisotropy at large pT is found to be very sensitive to parton energy loss. It decreases slowly with pT contrary to its low pT behavior where v2 increases very rapidly with pT. The turning point signals the onset of contributions of hard processes and the magnitude of parton energy loss. The centrality dependence of v2(pT) is shown to be sensitive to both size and density dependence of the parton energy loss and the later can also be studied via variation of the colliding energy. The anisotropy coefficient v2/" normalized by the spatial ellipticity " is found to decrease significantly toward semi-peripheral collisions, differing from the hydrodynamic results for low pT hadrons. Constrained by the existing WA98 experimental data at the SPS energy on parton energy loss, both hadron spectra suppression and azimuthal anisotropy at high pT are predicted to vanish for b >7-8 fm in Au + Au collisions at p s =130-200 GeV when the hadron rapidity density per unit area of the initial overlapped region is less than what is achieved in the central Pb + Pb collisions at the SPS energy.
TL;DR: In this paper, the current status of our knowledge of the flavor structure of the nucleon sea is reviewed and the implications of various theoretical models as well as possible future measurements are also discussed.
TL;DR: The basic properties of generalized quark and gluon distributions in the deuteron are discussed and it is pointed out how they probe the interplay of nucleon and parton degrees of freedom in theDeuteron wave function.
Abstract: We introduce generalized quark and gluon distributions in the deuteron, which can be measured in exclusive processes like deeply virtual Compton scattering and meson electroproduction We discuss the basic properties of these distributions and point out how they probe the interplay of nucleon and parton degrees of freedom in the deuteron wave function
TL;DR: In this paper, a QCD study of fragmentation processes for light and heavy quarks in the semi-inclusive region of large x is presented, where large logarithmic terms due to soft-gluon radiation are evaluated and resummed to all perturbative orders in the QCD coupling αS.
TL;DR: Though decreasing asymptotically as T/E with the parton energy, the relative reduction is found to be important for intermediate energies.
Abstract: Stimulated gluon emission and thermal absorption, in addition to induced radiation, are considered for an energetic parton propagating inside a quark-gluon plasma. In the presence of thermal gluons, stimulated emission reduces, while absorption increases, the parton's energy. The net effect is a reduction of the parton energy loss. Though decreasing asymptotically as $T/E$ with the parton energy, the relative reduction is found to be important for intermediate energies. The modified energy dependence of the energy loss will affect the shape of suppression of moderately high ${p}_{T}$ hadrons due to jet quenching in high-energy heavy-ion collisions.
TL;DR: In this paper, a QCD study of fragmentation processes for light and heavy quarks in the semi-inclusive region of large x is presented, and large logarithmic terms due to soft-gluon radiation are evaluated and resummed to all perturbative orders in the QCD coupling alpha_s.
Abstract: We present a QCD study of fragmentation processes for light and heavy quarks in the semi-inclusive region of large x. Large logarithmic terms, due to soft-gluon radiation, are evaluated and resummed to all perturbative orders in the QCD coupling alpha_s. Complete analytical results to next-to-leading logarithmic accuracy are given for one-particle and two-particle inclusive distributions in e+e- annihilation and DIS. Factorization of parton radiation at low transverse momenta is exploited to identify the universal (process-independent) perturbative fragmentation function that controls heavy-quark processes, and to perform next-to-leading logarithmic resummation of its soft-gluon contributions. To gauge the quantitative impact of resummation, we perform numerical studies of light- and heavy-quark fragmentation in e+e- collisions.
TL;DR: In this article, the crossing properties of the matrix elements of non-local operators, parameterized by generalized parton distribution, are considered and ambiguities analogous to the ones for the vector potential in the two-dimensional magneto-statics are outlined.