Showing papers on "Parton published in 1998"
TL;DR: In this paper, it is shown that within theoretical uncertainties their previous (1994) dynamical/radiative parton distributions are compatible with most recent data and still applicable within the restricted accuracy margins of the presently available next-to-leading order calculations.
Abstract: Dynamical parton densities, generated radiatively from valence-like inputs at some low resolution scale, are confronted with recent small-x data on deep inelastic and other hard scattering processes. It is shown that within theoretical uncertainties our previous (1994) dynamical/radiative parton distributions are compatible with most recent data and still applicable within the restricted accuracy margins of the presently available next-to-leading order calculations. Due to recent high precision measurements we also present an updated, more accurate, version of our (valence-like) dynamical input distributions. Furthermore, our perturbatively stable parameter-free dynamical predictions are extended to the extremely small-x region,
$10^{-8} \lesssim x \lesssim 10^{-5}$
, relevant to questions concerning ultra-high-energy cosmic ray and neutrino astronomy.
817 citations
Abstract: In this paper we derive the complete Wilson renormalization group equation which governs the evolution of the gluon distribution and other gluonic observables at low x and arbitrary density.
582 citations
TL;DR: The first explicit formulae for the complete set of one-loop helicity amplitudes necessary for computing next-to-leading order corrections for e + e − annihilation into four jets, for W, Z or Drell-Yan production in association with two jets at hadron colliders, and for three-jet production in deeply inelastic scattering experiments were presented in this article.
455 citations
TL;DR: In this paper, a new analysis of parton distributions of the proton is presented, which incorporates a wide range of new data, an improved treatment of heavy flavours and a re-examination of prompt photon production.
Abstract: We present a new analysis of parton distributions of the proton. This incorporates a wide range of new data, an improved treatment of heavy flavours and a re-examination of prompt photon production. The new set (MRST) shows systematic differences from previous sets of partons which can be identified with particular features of the new data and with improvements in the analysis. We also investigate the sensitivities of the results to (i) the uncertainty in the determination of the gluon at large
$x$
, (ii) the value of
$\alpha_S (M_Z^2)$
and (iii) the minimum
$Q^2$
cut on the data that are included in the global fit.
450 citations
27 Apr 1998
TL;DR: In this article, the size of the uncertainties in estimating the hadronization corrections is compared to the uncertainties of perturbative predictions, and it is shown that for the inclusive k_\perp ordered jet clustering algorithm the corrections are smallest.
Abstract: The size of non-perturbative corrections to high E_T jet production in deep-inelastic scattering is reviewed. Based on predictions from fragmentation models, hadronization corrections for different jet definitions are compared and the model dependence as well as the dependence on model parameters is investigated. To test whether these hadronization corrections can be applied to next-to-leading order (NLO) calculations, jet properties and topologies in different parton cascade models are compared to those in NLO. The size of the uncertainties in estimating the hadronization corrections is compared to the uncertainties of perturbative predictions. It is shown that for the inclusive k_\perp ordered jet clustering algorithm the hadronization corrections are smallest and their uncertainties are of the same size as the uncertainties of perturbative NLO predictions.
383 citations
TL;DR: Recently, there have been some interesting developments involving off-forward parton distributions of the nucleon, deeply virtual Compton scattering, and hard diffractive vector-meson production.
Abstract: Recently, there have been some interesting developments involving off-forward parton distributions of the nucleon, deeply virtual Compton scattering, and hard diffractive vector-meson production. These developments are triggered by the realization that the off-forward distributions contain information about the internal spin structure of the nucleon and that diffractive electroproduction of vector mesons depends on these unconventional distributions. This paper gives a brief overview of the recent developments.
357 citations
TL;DR: In this article, a Monte Carlo program for solving the Boltzmann equation for partons via the cascade method is presented, where only gluon-gluon elastic scattering is included.
301 citations
TL;DR: In this article, single transverse-spin asymmetries for hadronic pion production at large transverse momenta using QCD factorization were analyzed and the leading contributions were naturally produced by twist-3 parton correlation functions that couple quark fields and gluon field strengths.
Abstract: We analyze single transverse-spin asymmetries for hadronic pion production at large transverse momenta using QCD factorization. In the large ${x}_{F}$ region, leading contributions to the asymmetries are naturally produced by twist-3 parton correlation functions that couple quark fields and gluon field strengths. With a simple model for these matrix elements, leading-order asymmetries calculated from QCD are consistent with data on pion production from Fermilab, and can be used to predict single-spin asymmetries at BNL RHIC. We argue that our perturbative calculation for the asymmetries is relevant to pion transverse momenta as low as a few GeV.
283 citations
TL;DR: Using the NMC and E665 nuclear structure function ratios F2A/F2D and F2C from deep inelastic lepton-nucleus collisions, and the E772 Drell-Yan dilepton cross sections from proton-nuclear collisions, the authors determined nuclear parton distributions at an initial scale Q02.
270 citations
TL;DR: In this paper, it is shown that within theoretical uncertainties their previous (1994) dynamical/radiative parton distributions are compatible with most recent data and still applicable within the restricted accuracy margins of the presently available next-to-leading order calculations.
Abstract: Dynamical parton densities, generated radiatively from valence-like inputs at some low resolution scale, are confronted with recent small-x data on deep inelastic and other hard scattering processes. It is shown that within theoretical uncertainties our previous (1994) dynamical/radiative parton distributions are compatible with most recent data and still applicable within the restricted accuracy margins of the presently available next-to-leading order calculations. Due to recent high precision measurements we also present an updated, more accurate, version of our (valence-like) dynamical input distributions. Furthermore, our perturbatively stable parameter-free dynamical predictions are extended to the extremely small-x region, 10^-8 ~< x ~< 10^-5, relevant to questions concerning ultra-high-energy cosmic ray and neutrino astronomy.
243 citations
Book•
[...]
TL;DR: In this article, a brief reprise of basic concepts in relativity, quantum mechanics of fields and particle physics is discussed, including the dynamics of the massless relativistic string; confinement; causality and relatival covariance; Lund fragmentation processes; QED and QCD Bremsstrahlung; multiplicities and particle-parton distributions.
Abstract: The Lund model, inspired by quantum chromodynamics, has provided a very promising approach to the dynamics of quark and gluon interactions. Starting with a brief reprise of basic concepts in relativity, quantum mechanics of fields and particle physics, this 1998 book discusses: the dynamics of the massless relativistic string; confinement; causality and relativistic covariance; Lund fragmentation processes; QED and QCD Bremsstrahlung; multiplicities and particle-parton distributions. Throughout the book, theory is confronted with current experimental data, and implications for future experiments are also considered. The book also explores the relationships between the Lund model and other models based on field theory (the Schwinger model, S-matrix models, light-cone algebra physics and variations of the parton model), and models based on statistical mechanics (the Feynman-Wilson gas, scaling, iterative cascade models).
TL;DR: In this paper, the problem of going from one description to the other can be solved in a satisfactory manner to all orders, where the key ingredient is the constraint of matching the evolution of the physical structure function F_2 order by order in alpha-s(Q^2) in addition to the matching of the value of Fǫ itself.
Abstract: At low Q^2, charm production in deep-inelastic scattering is adequately described by assuming generation in electroweak boson-light parton scattering (dominantly boson-gluon fusion) which naturally incorporates the correct threshold behaviour. At high Q^2 this description is inadequate, since it does not sum logs in Q^2/m_c^2, and is replaced by the treatment of the charm quark as a light parton. We show how the problem of going from one description to the other can be solved in a satisfactory manner to all orders. The key ingredient is the constraint of matching the evolution of the physical structure function F_2 order by order in alpha_s(Q^2) in addition to the matching of the value of F_2 itself. This leads to new expressions for the coefficient functions associated with the charm parton which are unique in incorporating both the correct threshold and asymptotic behaviours at each order in perturbation theory. The use of these improved coefficients lead to an improvement in global fits and an excellent description of the observed F_2,charm.
TL;DR: In this article, the authors presented a double unresolved Eikonal and Altarelli-Parisi splitting function for tree-level multiparton scattering amplitudes which are appropriate when two partons are unresolved.
TL;DR: In this article, the authors extended the Baier-Dokshitzer-Mueller-Peign formalism applied to static media to the case of a quark which hits successive layers of matter of decreasing temperature, and showed that the resulting radiative energy loss can be as large as 6 times the corresponding one in a static plasma at the reference temperature T=T(L), which is reached after the quark propagates a distance L.
Abstract: We study analytically the medium-induced energy loss of a high energy parton passing through a finite size QCD plasma, which is expanding longitudinally according to Bjorken{close_quote}s model. We extend the Baier-Dokshitzer-Mueller-Peign{acute e}-Schiff formalism already applied to static media to the case of a quark which hits successive layers of matter of decreasing temperature, and we show that the resulting radiative energy loss can be as large as 6 times the corresponding one in a static plasma at the reference temperature T=T(L), which is reached after the quark propagates a distance L. {copyright} {ital 1998} {ital The American Physical Society}
TL;DR: In this paper, the authors illustrate a technique that can be used to efficiently propagate the uncertainties to new observables, assess the compatibility of new data with an initial fit, and, in case the compatibility is good, include the new data in the fit.
Abstract: Standard parton distribution function sets do not have rigorously quantified uncertainties. In recent years it has become apparent that these uncertainties play an important role in the interpretation of hadron collider data. In this paper, using the framework of statistical inference, we illustrate a technique that can be used to efficiently propagate the uncertainties to new observables, assess the compatibility of new data with an initial fit, and, in case the compatibility is good, include the new data in the fit. {copyright} {ital 1998} {ital The American Physical Society}
TL;DR: In this paper, a parton model with QCD-inspired hard scattering and evolution in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically.
Abstract: High-p{sub T} particle spectra in p+p (p(bar sign)+p), p+A, and A+B collisions are calculated in a parton model with QCD-inspired hard scattering and evolution in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically. The intrinsic k{sub T} and its broadening in p+A and A+B collisions due to initial multiple parton scattering are found to be very important at low energies ({radical}(s) 1 GeV/c) in A+B collisions scale very well with the number of binary nucleon-nucleon collisions (modulo effects of multiple initial scattering). This suggests that hard parton scattering is the dominant particle production mechanism underlying the hadron spectra at p{sub T}{approx}2-10 GeV/c. However, there is no evidence of jet quenching or parton energy loss. Assuming this model of parton scattering, nuclear broadening and parton energy loss, one can exclude an effective parton energy loss dE{sub q}/dx>0.01 GeV/fm and a mean free path {lambda}{sub q}<7 fm from the experimental data of A+B collisions at the SPS energies.more » Predictions for high p{sub T} particle spectra in p+A and A+A collisions with and without jet quenching at the RHIC energy are also given. Uncertainties due to initial multiple scattering and nuclear shadowing of parton distributions are also discussed. (c) 2000 The American Physical Society.« less
TL;DR: In this paper, the influence of color exchange on the resummed cross section is contained in the eigenvalues and eigenvectors of an anomalous dimension matrix, which describes the factorization of coherent soft gluons from the hard scattering.
TL;DR: In this article, the one-loop corrections to a general off-forward deeply virtual Compton process at leading twist for both parton helicity-dependent and independent cases were calculated.
Abstract: We calculate the one-loop corrections to a general off-forward deeply virtual Compton process at leading twist for both parton helicity-dependent and independent cases. We show that the infrared divergences can be factorized entirely into off-forward parton distributions, even when one of the two photons is on shell. We argue that this property persists to all orders in perturbation theory. We obtain the next-to-leading order Wilson coefficients for the general leading-twist expansion of the product of two electromagnetic currents in the {ovr MS} scheme. {copyright} {ital 1998} {ital The American Physical Society}
TL;DR: In this paper, the authors consider how much these predictions should be affected by higher order perturbative corrections, by resummation of enhanced corrections to all orders, and by (power-suppressed) non-perturbative correction.
TL;DR: In this paper, effective jet fragmentation functions are constructed in which leading hadrons will be suppressed in high-energy hadronic or nuclear collisions, assuming an effective energy loss dE/dx for the high-sub T partons.
Abstract: Since large-p{sub T} particles in high-energy hadronic or nuclear collisions come from jet fragmentation, jet quenching due to parton energy loss in dense matter will cause the suppression of large-p{sub T} hadron spectra in high-energy heavy-ion collisions. Assuming an effective energy loss dE/dx for the high-E{sub T} partons, effective jet fragmentation functions are constructed in which leading hadrons will be suppressed. Using such effective fragmentation functions, high-p{sub T} hadron spectra and particle suppression factors relative to pp collisions are estimated in central high-energy nuclear collisions with a given range of the assumed dE/dx. It is found that the suppression factors are very sensitive to the value of the effective energy loss. Systematic nuclear and flavor dependence of the hadron spectra are also studied. {copyright} {ital 1998} {ital The American Physical Society}
TL;DR: In this article, the authors show that the soft contribution in both cases can be described in terms of the same universal nonforward parton densities (ND's) F(x;t), which are the simplest hybrids of the usual parton density and hadronic form factors.
Abstract: The authors argue that at moderately large momentum transfer {minus}t {approx_lt} 10 GeV{sup 2}, hadronic form factors and wide-angle Compton scattering amplitudes are dominated by mechanism corresponding to overlap of soft wave functions. They show that the soft contribution in both cases can be described in terms of the same universal nonforward parton densities (ND's) F(x;t), which are the simplest hybrids of the usual parton densities and hadronic form factors. They propose a simple model for ND's possessing required reduction properties. Their model easily reproduces the observed magnitude and the dipole t-dependence of the proton form factor F{sub 1}{sup p}(t) in the region 1 GeV{sup 2} < {minus}t < 10 GeV{sup 2}. Their results for the wide-angle Compton scattering cross section follow the angular dependence of existing data and are rather close to the data in magnitude.
TL;DR: In this article, the authors review the recent developments of virtual Compton scattering and focus on the kinematical regimes which look the most promising, and discuss the experimental perspectives and the role of the Bethe-Heitler background.
01 Jan 1998
TL;DR: In this article, the size of the uncertainties in estimating the hadronization corrections is compared to the uncertainties of perturbative predictions, and it is shown that for the inclusive k_\perp ordered jet clustering algorithm the corrections are smallest.
Abstract: The size of non-perturbative corrections to high E_T jet production in deep-inelastic scattering is reviewed. Based on predictions from fragmentation models, hadronization corrections for different jet definitions are compared and the model dependence as well as the dependence on model parameters is investigated. To test whether these hadronization corrections can be applied to next-to-leading order (NLO) calculations, jet properties and topologies in different parton cascade models are compared to those in NLO. The size of the uncertainties in estimating the hadronization corrections is compared to the uncertainties of perturbative predictions. It is shown that for the inclusive k_\perp ordered jet clustering algorithm the hadronization corrections are smallest and their uncertainties are of the same size as the uncertainties of perturbative NLO predictions.
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 αs measurements from deep inelastic data is reviewed. Models and nonperturbative 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 summarized. Some brief comments are made on the recent HERA measurements of the ep NC and CC cross-sections at very high Q2.
TL;DR: In this paper, the first moments of the proton, deuteron, and neutron structure functions were analyzed and the Bjorken sum rule was found to be in agreement with the theoretical prediction at the level of 10%.
Abstract: We present a next-to-leading order QCD analysis of the presently available data on the spin structure function [Formula Presented] including the final data from the Spin Muon Collaboration. We present results for the first moments of the proton, deuteron, and neutron structure functions, and determine singlet and nonsinglet parton distributions in two factorization schemes. We also test the Bjorken sum rule and find agreement with the theoretical prediction at the level of 10%. © 1998 The American Physical Society.
TL;DR: In this paper, the scale dependence of the ratios of parton distributions in a proton of a nucleus and in the free proton was studied within the framework of the lowest order leading-twist DGLAP evolution.
Abstract: The scale dependence of the ratios of parton distributions in a proton of a nucleus $A$ and in the free proton, $R_i^A(x,Q^2)=f_{i/A}(x,Q^2)/f_i(x,Q^2)$, is studied within the framework of the lowest order leading-twist DGLAP evolution. By evolving the initial nuclear distributions obtained with the GRV-LO and CTEQ4L sets at a scale $Q_0^2$, we show that the ratios $R_i^A(x,Q^2)$ are only moderately sensitive to the choice of a specific modern set of free parton distributions. We propose that to a good first approximation, this parton distribution set-dependence of the nuclear ratios $R_i^A(x,Q^2)$ can be neglected in practical applications. With this result, we offer a numerical parametrization of $R_i^A(x,Q^2)$ for all parton flavours $i$ in any $A>2$, and at any $10^{-6}\le x \le 1$ and any $Q^2\ge 2.25$ GeV$^2$ for computing cross sections of hard processes in nuclear collisions.
TL;DR: In this paper, a next-to-leading order Monte Carlo program for the calculation of heavy quark cross sections in deeply inelastic scattering is described, and several distributions are presented and their variation with respect to charm quark mass, parton distribution set, and renormalization or factorization scale.
Abstract: A next-to-leading order Monte Carlo program for the calculation of heavy quark cross sections in deeply inelastic scattering is described. Concentrating on charm quark and ${D}^{*\ifmmode\pm\else\textpm\fi{}}(2010)$ production at DESY HERA, several distributions are presented and their variation with respect to charm quark mass, parton distribution set, and renormalization or factorization scale is studied.
TL;DR: In this article, a new analysis of parton distributions of the proton is presented, which incorporates a wide range of new data, an improved treatment of heavy flavours and a re-examination of prompt photon production.
Abstract: We present a new analysis of parton distributions of the proton. This incorporates a wide range of new data, an improved treatment of heavy flavours and a re-examination of prompt photon production. The new set (MRST) shows systematic differences from previous sets of partons which can be identified with particular features of the new data and with improvements in the analysis. We also investigate the sensitivities of the results to (i) the uncertainty in the determination of the gluon at large x, (ii) the value of alpha_S(M_Z^2) and (iii) the minimum Q^2 cut on the data that are included in the global fit.
TL;DR: In this article, a recent global analysis of nucleon parton distributions carried out at next-to-leading order (NLO) in the MS scheme was extended to provide distributions in the so-called DIS scheme.
CERN1
TL;DR: In this paper, the authors present explicit expressions for the resummation of large-xT Sudakov effects in the transverse energy distribution of prompt-photons produced in hadronic collisions, to next-to-leading logarithmic (NLL) accuracy.
Abstract: We present the explicit expressions for the resummation of large-xT Sudakov effects in the transverse-energy distribution of prompt-photons produced in hadronic collisions, to next-to-leading logarithmic (NLL) accuracy. Fragmentation processes do not contribute to the Sudakov resummation at NLL level. In Mellin space, the resummed radiative factor is the product of independent radiators for each external coloured parton appearing in the Born process, times a simple factor describing the interferences between initial- and final-state soft-gluon radiation. The formulae are given in terms of Mellin moments, and can be used for phenomenological applications using standard techniques for the inverse-Mellin transforms. The calculations presented in this work, when added to the existing works on DY and DIS production, complete the theoretical ground-work necessary to carry out global fits of parton densities with a uniform NLL accuracy in the large-x region.