Showing papers on "Parton published in 1996"
TL;DR: In this paper, the authors show that the radiative energy loss of a high energy quark traversing a large nucleus grows with the length of the nuclear matter, as does the characteristic transverse momentum squared of the parton.
Abstract: The medium-induced $p_{\perp}$-broadening and induced gluon radiation spectrum of a high energy quark or gluon traversing a large nucleus is studied. Multiple scattering of the high energy parton in the nucleus is treated in the Glauber approximation. We show that -dE/dz, the radiative energy loss of the parton per unit length, grows as L, the length of the nuclear matter, as does the characteristic transverse momentum squared of the parton $p_{\perp W}^2$. We find dE/dz = (1/8)\alpha_s N_c $p_{\perp W}^2$ holds independent of the details of the parton-nucleon scatterings so long as L is large. Numerical estimates suggest that $p_{\perp}$-broadening and energy loss may be significantly enhanced in hot matter as compared to cold matter, thus making the study of such quantities a possible signal for quark-gluon plasma formation.
726 citations
TL;DR: In this paper, the authors studied the medium induced energy loss spectrum of a high energy quark or gluon traversing a hot QCD medium of finite volume and modeled the interaction by a simple picture of static scattering centres.
Abstract: The medium induced energy loss spectrum of a high energy quark or gluon traversing a hot QCD medium of finite volume is studied. We model the interaction by a simple picture of static scattering centres. The total induced energy loss is found to grow as $L^2$, where $L$ is the extent of the medium. The solution of the energy loss problem is reduced to the solution of a Schrodinger-like equation whose ``potential'' is given by the single-scattering cross section of the high energy parton in the medium. These resuls should be directly applicable to a quark-gluon plasma.
646 citations
TL;DR: In this paper, the authors construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables, concluding that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gluon density in the photon rather difficult.
Abstract: The high energy photoproduction of jets is being observed at theep collider, HERA. It may be that the HERA centre-of-mass energy is sufficiently large that the production of more than one pair of jets perep collision becomes possible, owing to the large number density of the probed gluons. We construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables. The conclusion is that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gluon density in the photon rather difficult. Total rates for the production of many (i.e.≥3) jets could provide direct evidence for the presence of multiple interactions, although parton showering and hadronization significantly affect low transverse energy jets.
535 citations
TL;DR: In this article, a well-balanced textbook of the field of ultra-relativistic heavy-ion collisions (RHICs) is presented, which is primarily suited to theoretically oriented graduate students and researchers (both theorists and experimental physicists).
Abstract: The author has succeeded in writing a well-balanced textbook of the new field of ultra-relativistic heavy-ion collisions (RHICs). The book is primarily suited to theoretically oriented graduate students and researchers (both theorists and experimental physicists). After introducing the necessary kinematical variables and describing the phenomenology of nucleon--nucleon collisions, the author proceeds to elucidate various concepts of high-energy particle physics which are relevant to the study of RHICs. Among these are the hard-scattering model, the Schwinger particle production mechanism, the classical string model and the dual parton model. He then focuses on the concept of the quark--gluon plasma (QGP) and gives an overview of the methods and results of lattice gauge theory. The final chapters of the book are devoted to the creation of a QGP via heavy-ion collisions and its possible signatures. In great detail the author discusses dilepton production, suppression, photon production, Hanbury--Brown--Twiss interferometry and strangeness enhancement. The book is very well written and I can recommend it to all graduate students and researchers interested in the field of RHICs. However, the book has one shortcoming: it focuses on heavy-ion physics in the energy region E > 200 GeV/nucleon and higher. The physics of the hadron gas, important for the later stages of RHICs at CERN and dominant for beam energies in the BEVALAC/SIS/AGS range, is neglected. To acquire an overview of the entire field of RHICs, the reader would profit from a supplementary reading of the more introductory textbook Introduction to Relativistic Heavy-Ion Collisions by L Csernai.
355 citations
TL;DR: The photon-photon collisions are investigated in the framework of the two-component dual parton model and the model is shown to agree well with hadron production data from hadron-hadron and photon- hadron collisions.
Abstract: Photon-photon collisions are investigated in the framework of the two-component dual parton model. The model contains contributions from direct, resolved soft, and resolved hard interactions. All free parameters of the model are determined in fits to hadron-hadron and photon-hadron cross section data. The model is shown to agree well with hadron production data from hadron-hadron and photon-hadron collisions. The multiparticle production in hadron-hadron, photon-hadron, and photon-photon collisions as predicted by the model is compared. Strong differences are only found as a function of the transverse momentum variable. The hadron production in photon-photon collisions at present and future electron-positron colliders is studied using photon spectra according to bremsstrahlung, beamstrahlung, and backscattered laser radiation.
283 citations
TL;DR: This work determines polarized parton distributions at both leading and next-to-leading order in perturbation theory for quarks and gluons using all available data on the polarized structure function.
Abstract: The distribution of the spin of the nucleon among its constituents can be parametrized in the form of polarized parton distribution functions for quarks and gluons. Using all available data on the polarized structure function {ital g}{sub 1}({ital x},{ital Q}{sup 2}), we determine these distributions at both leading and next-to-leading order in perturbation theory. We suggest three different, equally possible scenarios for the polarized gluon distribution, whose {ital x} dependence is found to be only loosely constrained by current experimental data. We examine various possibilities of measuring polarized parton distributions at future experiments. {copyright} {ital 1996 The American Physical Society.}
173 citations
TL;DR: In this paper, the authors derived expressions for the heavy-flavour components of the deep-inelastic structure functions FL and F2 in the variableflavour number scheme (VFNS) and showed that the VFNS gives a better prediction for the charm structure function at large Q 2 values than FOPT.
Abstract: Starting from fixed-order perturbation theory (FOPT) we derive expressions for the heavy-flavour components of the deep-inelastic structure functions FL and F2 in the variable-flavour number scheme (VFNS). These expressions are valid in all orders of perturbation theory. This derivation establishes a relation between the parton densities parametrized at N and N light flavours. The consequences for the existing parametrizations of the parton densities are discussed. Further we show that in charm electroproduction the exact and asymptotic expressions for the heavy-quark coefficient functions yield identical results for F2 when Q^2>20 (GeV/c)^2. We also study the differences between the FOPT and the VFNS descriptions for F2. It turns out that the charm structure function in the VFNS is larger than the one obtained in FOPT over the whole Q^2-range. Furthermore inspection of the perturbation series reveals that the higher order corrections in the VFNS are smaller than those present in FOPT for Q^2>10 (GeV/c)^2. Therefore the VFNS gives a better prediction for the charm structure function at large Q^2-values than FOPT.
162 citations
TL;DR: In this paper, the authors construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables, concluding that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gluon density in the photon rather difficult.
Abstract: The high energy photoproduction of jets is being observed at the ep collider, HERA. It may be that the HERA centre-of-mass energy is sufficiently large that the production of more than one pair of jets per ep collision becomes possible, owing to the large number density of the probed gluons. We construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables. The conclusion is that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gluon density in the photon rather difficult. Total rates for the production of many (i.e. > 2) jets could provide direct evidence for the presence of multiple interactions, although parton showering and hadronization significantly affect low transverse energy jets.
161 citations
TL;DR: In this article, 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.
Abstract: A pQCD-based model for parton production and equilibration in ultrarelativistic heavy-ion collisions is reviewed. The model combines pQCD processes including initial and final state radiations together with string phenomenology for nonperturbative soft processes. Nuclear effects on the initial parton production, such as multiple parton scattering and nuclear shadowing of parton distribution functions are considered. Comparisons with existing data are made and further tests of the model to constrain model parameters are proposed. With the obtained space-time history of the parton production, evolution of the minijet gas toward a fully equilibrated parton plasma is studied. Direct probes of the early parton dynamics, such as pre-equilibrium photon and dilepton production, open charm production, $J/\psi$ suppressionand jet quenching are also reviewed.
161 citations
TL;DR: In this paper, the authors derived general formulae for the polarized and unpolarized distributions (singlet and non-singlet) in the chiral quark-soliton model.
155 citations
TL;DR: A perturbative QCD formula for inclusive energy spectra of heavy quarks from heavy quark initiated jets, which takes into account collinear and/or soft logarithms in all orders, and two-loop effects, is applied to distributions of heavy flavored hadrons in the framework of the local parton-hadron duality concept.
Abstract: A perturbative QCD formula for inclusive energy spectra of heavy quarks from heavy quark initiated jets, which takes into account collinear and/or soft logarithms in all orders, the exact first order result, and two-loop effects, is applied to distributions of heavy flavored hadrons in the framework of the local parton-hadron duality concept. {copyright} {ital 1995 The American Physical Society.}
TL;DR: The authors performed a global analysis which incorporates HERA measurements of the proton structure function at small $x, and which extends down to 1.5\; GeV^2, and compared the predictions of the single jet inclusive cross section with recent measurements from Fermilab Tevatron.
Abstract: New data, especially HERA measurements of the proton structure function at small $x$, allow the opportunity to improve our knowledge of the gluon and quark distribution functions. We perform a global analysis which incorporates these new precise data, and which extends down to $Q^2 = 1.5\; GeV^2$. We discuss the sensitivity to the value of $\alpha_s$ and the improvement in the determination of the gluon. We compare the predictions of the single jet inclusive cross section with recent measurements from the Fermilab Tevatron.
TL;DR: In this paper, the nucleon is viewed as a soliton of the effective chiral lagrangian, which allows a consistent nonperturbative calculation of the leading-twist parton distributions at a low normalization point.
Abstract: At large N_c the nucleon can be viewed as a soliton of the effective chiral lagrangian. This picture of nucleons allows a consistent nonperturbative calculation of the leading-twist parton distributions at a low normalization point. We derive general formulae for the polarized and unpolarized distributions (singlet and non-singlet) in the chiral quark-soliton model. The consistency of our approach is demonstrated by checking the baryon number, isospin and total momentum sum rules, as well as the Bjorken sum rule. We present numerical estimates of the quark and antiquark distributions and find reasonable agreement with parametrizations of the data at a low normalization point. In particular, we obtain a sizeable fraction of antiquarks, in agreement with the phenomenological analysis.
TL;DR: In this paper, the HERA measurements of the proton structure function at small x allow the opportunity to improve our knowledge of the gluon and quark distribution functions, and they perform a global analysis which incorporates these new precise data, and which extends down to Q 2 = 1.5 GeV2.
TL;DR: In this paper, a generic ansatz for the extension of parton distributions of the real photon to those of the virtual photon is proposed, which allows closed-form parametrizations.
TL;DR: HECTOR as discussed by the authors is a Fortran program for semi-analytical calculations of radiative QED, QCD, and electroweak corrections to the double-differential cross sections of NC and CC deep inelastic charged lepton proton (or lepton deuteron) scattering.
CERN1
TL;DR: In this paper, the authors reexamine the top quark production cross section at the Tevatron and LHC, in the light of recent progress on the resummation of logarithmic soft gluon corrections.
TL;DR: Short-distance QCD is employed to calculate the J/\ensuremath{\psi} survival probability in an equilibrating parton gas whose evolution is governed by a set of master rate equations, and the transverse momentum dependence of the suppression is shown to be sensitive to the initial conditions and the evolution history of the parton plasma.
Abstract: Short-distance QCD is employed to calculate the J/\ensuremath{\psi} survival probability in an equilibrating parton gas whose evolution is governed by a set of master rate equations. Partons in the early stage of high-energy nuclear collisions may initially not be in equilibrium, but their average transverse momentum is sufficiently high to break up a QQ\ifmmode\bar\else\textasciimacron\fi{} bound state. Such a breakup during the evolution of the parton gas is shown to cause a substantial J/\ensuremath{\psi} suppression at both RHIC and LHC energies, using realistic estimates of the initial parton densities. The transverse momentum dependence of the suppression is also shown to be sensitive to the initial conditions and the evolution history of the parton plasma. \textcopyright{} 1996 The American Physical Society.
TL;DR: It is shown that the perturbative result does not depend on the manner nonperturbative or infrared effects are handled in principal-value resummation, and the renormalization or factorization scale dependence of the re- summed cross section is derived.
Abstract: Our calculation of the total cross section for inclusive production of {ital t{bar t}} pairs in hadron collisions is presented. The principal ingredient of the calculation is resummation of the universal leading-logarithm effects of gluon radiation to all orders in the quantum chromodynamics coupling strength, restricted to the region of phase space that is demonstrably perturbative. We derive the perturbative regime of the resummed series, starting from the principal-value resummation approach, and we isolate the perturbative domain in both moment space and, upon inversion of the corresponding Mellin transform, in momentum space. We show that our perturbative result does not depend on the manner nonperturbative or infrared effects are handled in principal-value resummation. We treat both the quark-antiquark and gluon-gluon production channels consistently in the MS factorization scheme. We compare our method and results with other resummation methods that rely on the choice of infrared cutoffs. We derive the renormalization or factorization scale dependence of our re- summed cross section, and we discuss factorization scheme dependence and remaining theoretical uncertainties, including estimates of possible nonperturbative contributions. We include the full content of the exact next-to-leading order calculation in obtaining our final results. We present predictions of the physical cross sectionmore » as a function of top quark mass in proton-antiproton reactions at center-of-mass energies of 1.8 and 2.0 TeV. We also provide the differential cross section as a function of the parton-parton subenergy. {copyright} {ital 1996 The American Physical Society.}« less
TL;DR: The Brodsky-Lepage-Mackenzie scale-setting procedure suggests that higher order perturbative corrections to the first moment of the hadronic invariant mass spectrum are small for bottom decay, and even tractable for charm decay.
Abstract: We calculate the leading perturbative and power corrections to the hadronic invariant mass and energy spectra in semileptonic heavy hadron decays. We apply our results to the $B$ system. Moments of the invariant mass spectrum, which vanish in the parton model, probe gluon bremsstrahlung and nonperturbative effects. Combining our results with recent data on $B$ meson branching ratios, we obtain a lower bound $\overline{\ensuremath{\Lambda}}g410$ MeV and an upper bound ${m}_{b}^{\mathrm{pole}}l4.89$ GeV. The Brodsky-Lepage-Mackenzie scale-setting procedure suggests that higher order perturbative corrections to the first moment of the hadronic invariant mass spectrum are small for bottom decay, and even tractable for charm decay.
TL;DR: In this article, the production of target associated particles and of nuclear fragments in high energy hadronnucleus interactions is studied based on a Monte Carlo realization of the Dual Parton Model, and a formation zone intranuclear cascade of low energy secondaries inside the target nucleus is discussed.
Abstract: Based on a Monte Carlo realization of the Dual Parton Model we study the production of target associated particles and of nuclear fragments in high energy hadronnucleus interactions. A formation zone intranuclear cascade of low energy secondaries inside the target nucleus is discussed. We calculate excitation energies of residual nuclei left after the intranuclear cascade process and treat their further disintegration by introducing models for the evaporation of protons, neutrons, and light fragments, high energy fission, and by applying a Fermi Break-up model to light nuclear fragments. The results are compared to data on target associated particle production. We furthermore calculate cross sections for the production of nuclear fragments.
TL;DR: Diffractive parton distribution functions give the probability to find a parton in a hadron if the hadron is diffractively scattered and their relation to diffractive deeply inelastic scattering and to photoproduction of jets at DESY HERA is discussed.
Abstract: Diffractive parton distribution functions give the probability to find a parton in a hadron if the hadron is diffractively scattered. We provide an operator definition of these functions and discuss their relation to diffractive deeply inelastic scattering and to photoproduction of jets at DESY HERA. We perform a calculation in the style of {open_quote}{open_quote}constituent counting rules{close_quote}{close_quote} for the behavior of these functions when the detected parton carries almost all of the longitudinal momentum transferred from the scattered hadron. {copyright} {ital 1996 The American Physical Society.}
TL;DR: The W W and Higgs Physics with PHACT (WPHACT) MC program as mentioned in this paper 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.
Abstract: WPHACT (W W and Higgs Physics with PHACT) is a MC program and unweighted event generator which computes all Standard Model processes with four fermion in the final state at $e^+ e^-$ colliders. It is based on an helicity amplitude method which allows precise and fast evaluations of the matrix elements both for massless and massive fermions. Fermion masses for $b$ quarks are exactly taken into account. QED initial state and Coulomb corrections are evaluated, while QCD final state corrections are included in an approximate formulation. Cuts can be easily introduced and distributions for any variable at parton level can be implemented. The contributions to the processes of neutral Standard Model or Susy Higgs can be included. Anomalous couplings effects for the triple coupling can be computed. An interface to hadronization is provided and Jetset can be directly called from the program.
TL;DR: It is shown that inelastic processes, and {ital not} elastic processes as is commonly assumed, are dominant in the equilibration of the plasmamore and that gluon branching leads the other processes in entropy generation, and that, within perturbative QCD, processes with higher power in {alpha}{sub {ital s}} need not be less important for the purpose of Equilibration than those with lower power.
Abstract: We investigate the thermalization and the chemical equilibration of a parton plasma created from Au+Au collision at LHC and RHIC energies starting from the early moment when the particle momentum distributions in the central region become for the first time isotropic due to longitudinal cooling. Using the relaxation time approximation for the collision terms in the Boltzmann equations for gluons and for quarks and the real collision terms constructed from the simplest QCD interactions, we show that the collision times have the right behavior for equilibration. The magnitude of the quark (antiquark) collision time remains bigger than the gluon collision time throughout the lifetime of the plasma so that gluons are equilibrating faster than quarks both chemically and kinetically. That is we have a two-stage equilibration scenario as has been pointed out already by Shuryak sometimes ago. Full kinetic equilibration is however slow and chemical equilibration cannot be completed before the onset of the deconfinement phase transition assumed to be at {ital T}{sub {ital c}}=200 MeV. By comparing the collision entropy density rates of the different processes, we show explicitly that inelastic processes, and {ital not} elastic processes as is commonly assumed, are dominant in the equilibration of the plasmamore » and that gluon branching leads the other processes in entropy generation. We also show that, within perturbative QCD, processes with higher power in {alpha}{sub {ital s}} need not be less important for the purpose of equilibration than those with lower power. The state of equilibration of the system has also a role to play. We compare our results with those of the parton cascade model. {copyright} {ital 1996 The American Physical Society.}« less
TL;DR: In this paper, two-jet cross sections in deep inelastic scattering at HERA are calculated in next-to-leading order and QCD corrections are implemented in a new ep → n jets event generator, MEPJET, which allows to analyze arbitrary jet definition schemes and general cuts in terms of parton 4-momenta.
TL;DR: In this paper, it is shown that the divergent cross sections in the calculation of parton production can be regulated self-consistently without an ad hoc cut-off.
TL;DR: In this paper, a semi-analytical approach to the model independent leptonic QED corrections to deep inelastic neutral current lepton-nucleon scattering is presented.
Abstract: We give an exhaustive presentation of the semi-analytical approach to the model independent leptonic QED corrections to deep inelastic neutral current lepton-nucleon scattering. These corrections include photonic bremsstrahlung from and vertex corrections to the lepton current of the order φ(α) with soft photon exponentiation. A common treatment of these radiative corrections in several variables — leptonic, hadronic, mixed, Jaquet-Blondel variables — has been developed and double differential cross sections are calculated. In all sets of variables we use some structure functions, which depend on the hadronic variables and which do not have to be defined in the quark parton model. The remaining numerical integrations are twofold (for leptonic variables) or onefold (for all other variables). For the case of hadronic variables, all phase space integrals have been performed analytically. Numerical results are presented for a large kinematical range, covering fixed target as well as collider experiments at HERA or LEP⊗LHC, with a special emphasis on HERA physics.
TL;DR: The energy distribution function, which can be computed without needing final state parton fragmentation functions, is considered, and the results are used to sum the perturbation series to obtain a result valid for both large and small values of transverse momentum.
Abstract: Measurement of the distribution of hadronic energy in the final state in deeply inelastic electron scattering at DESY HERA can provide a good test of our understanding of perturbative QCD. For this purpose, we consider the energy distribution function, which can be computed without needing final state parton fragmentation functions. We compute this distribution function for finite transverse momentum ${q}_{T}$ at order ${\ensuremath{\alpha}}_{s}$, and use the results to sum the perturbation series to obtain a result valid for both large and small values of transverse momentum.
TL;DR: In this paper, the transverse spin quark distribution in the chiral quark-soliton model is derived and compared to the results obtained in other models, and numerical estimates are given.
TL;DR: In this article, the authors determined the two-loop Altarelli-Parisi splitting functions for fragmentation functions via analytic continuation of the corresponding space-like splitting functions, for the case of unpolarized fragmentation functions and for the first time for the functions describing the fragmentation of a longitudinally polarized parton into a spin-1/2 hadron such as Lambda baryon.
Abstract: We determine the two-loop 'time-like' Altarelli-Parisi splitting functions, appearing in the next-to-leading order Q^2-evolution equations for fragmentation functions, via analytic continuation of the corresponding 'space-like' splitting functions for the evolution of parton distributions. We do this for the case of unpolarized fragmentation functions and - for the first time - also for the functions describing the fragmentation of a longitudinally polarized parton into a longitudinally polarized spin-1/2 hadron such as a Lambda baryon. Our calculation is based on the method proposed and employed by Curci, Furmanski and Petronzio in the unpolarized case in which we confirm their results.