Showing papers on "Parton published in 1995"
TL;DR: In this article, the perturbative properties of parton distributions generated radiatively from a valence-like input at some low resolution scale are discussed with the aim of explaining the physical aspects underlying the reliability of the predicted distributions in the small-x region.
Abstract: The perturbative properties of parton distributions generated radiatively from a valence-like input at some low resolution scale are discussed with the aim of explaining the physical aspects underlying the reliability of the predicted distributions in the small-x region. Aspects of higher-twist (shadowing) effects as well as small-x resummations are discussed. Utilizing recent improved data atx≳10−2 and a factorization scheme in which the heavy quarksc, b, ..., arenot entailed among the intrinsic (massless) parton distributions, we readjust our valencelike input and provide parametrizations of the slightly modified dynamical LO and NLO\((\overline {MS} ,DIS)\) predictions for parton distributions.
654 citations
TL;DR: Based on QCD-inspired models for multiple jets production, the authors developed a Monte Carlo program to study jet and associated particle production in high energy $pp$, $pA$ and $AA$ collisions.
Abstract: Based on QCD-inspired models for multiple jets production, we developed a Monte Carlo program to study jet and the associated particle production in high energy $pp$, $pA$ and $AA$ collisions. The physics behind the program which includes multiple minijet production, soft excitation, nuclear shadowing of parton distribution functions and jet interaction in dense matter is briefly discussed. A detailed description of the program and instructions on how to use it are given.
535 citations
TL;DR: In this paper, the fundamental theorems of quantum chromodynamics are discussed and a discussion of the global fitting approach to the determination of parton distributions in nucleons is presented.
Abstract: The elements, theoretical basis, and experimental status of perturbative quantum chromodynamics are presented. Relevant field-theoretic methods are introduced at a nonspecialist level, along with a review of the basic ideas and methods of the parton model. This is followed by an account of the fundamental theorems of quantum chromodynamics, which generalize the parton model. Summaries of the theoretical and experimental status of the most important hard-scattering processes are then given, including electron-positron annihilation, deeply inelastic scattering, and hard hadron-hadron scattering, as induced both by electoweak interactions and by quantum chromodynamics directly. In addition, a discussion is presented of the global fitting approach to the determination of parton distributions in nucleons.
381 citations
TL;DR: A new set of distributions, CteQ3, incorporating several new types of data is reported and compared to the two previous sets of CTEQ distributions, and the remaining uncertainties in the parton distributions are assessed.
Abstract: The CTEQ program for the determination of parton distributions through a global QCD analysis of data for various hard scattering processes is fully described. A new set of distributions, CTEQ3, incorporating several new types of data is reported and compared to the two previous sets of CTEQ distributions. A comparison with current data is discussed in some detail. The remaining uncertainties in the parton distributions and methods to further reduce them are assessed. Comparisons with the results of other global analyses are also presented.
350 citations
TL;DR: RAPGAP as mentioned in this paper generates ep collisions where the electron is scattered on a pomeron P coupled to the proton p. A gap in rapidity between the fast moving proton and the remaining hadronic system is observed.
347 citations
TL;DR: In this paper, it was shown that large formation times of bremsstrahlung quanta determine the QCD radiation intensity and derive the gluon energy spectrum, and that the energy loss of fast partons in a QCD medium depends on the incident energy.
320 citations
TL;DR: The production of high transverse momentum gluons in the McLerran-Venugopalan model of nuclear structure functions is considered and the nuclear target size dependence of the distribution of produced glUons is computed.
Abstract: We consider the production of high transverse momentum gluons in the McLerran-Venugopalan model of nuclear structure functions. We explicitly compute the high momentum component in this model. We compute the nuclear target size {ital A} dependence of the distribution of produced gluons.
276 citations
TL;DR: The non-Abelian analogue of the Landau-Pomeranchuk-Migdal effect is investigated in perturbative QCD and the effective formation time of gluon radiation due to the color interference is shown to depend on the color representation of the emitting parton.
Abstract: The non-Abelian analogue of the Landau-Pomeranchuk-Migdal effect is investigated in perturbative QCD. Extending our previous studies, the suppression of induced soft bremsstrahlung due to multiple scatterings of quarks in the spinor representation is considered. The effective formation time of gluon radiation due to the color interference is shown to depend on the color representation of the emitting parton, and an improved formula for the radiative energy loss is derived that interpolates between the factorization and Bethe-Heitler limits.
275 citations
TL;DR: In this article, an approximation scheme to describe high energy photoproduction processes is presented based on the distinction between direct, resolved soft, and resolved hard interaction processes, which can be used to construct effective impact parameter amplitudes.
Abstract: In the framework of the Dual Parton Model an approximation scheme to describe high energy photoproduction processes is presented. Based on the distinction between direct, resolved soft, and resolved hard interaction processes we construct effective impact parameter amplitudes. In order to treat low mass diffraction within the eikonal formalism in a consistent way a phenomenological ansatz is proposed. The free parameters of the model are determined by fits to high energy hadro- and photoproduction cross sections. We calculate the partial photoproduction cross sections and discuss predictions of the model at HERA energies. Using hadro- and photoproduction data together, the uncertainties of the model predictions are strongly reduced.
273 citations
TL;DR: In this paper, the origin of angular ordering in soft parton emission was investigated and it was shown that at small x this coherent structure is masked in the structure function while it can be detected in the associated distributions.
245 citations
TL;DR: New positivity constraints on the spin-dependent structure functions of the nucleon are derived and their domain of allowed values are reduced, in particular, for the chiral-odd parton distribution.
Abstract: We derive new positivity constraints on the spin-dependent structure functions of the nucleon These model-independent results reduce considerably their domain of allowed values, in particular, for the chiral-odd parton distribution [ital h][sub 1]([ital x])
TL;DR: In this article, a combination of gluon and sea quark distributions at small x that is significantly different from those of existing parton sets is presented, and two new global fits to deepinelastic and related data are performed.
TL;DR: In this article, the dual parton model was studied in the fragmentation region up to the cosmic ray energy region, and it was found that accelerator data in this region are indeed relevant for the cosine energy region.
Abstract: The dual parton model for hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions is studied in the fragmentation region up to the cosmic ray energy region. Because of the excellent Feynman scaling behavior of the model outside the regions around [ital x][sub [ital F]]=1 and [ital x][sub [ital F]]=0, it is found that accelerator data in the fragmentation region are indeed relevant for the cosmic ray energy region. However, not enough data are available in the fragmentation region of hadron collisions with light target nuclei. Therefore many features of hadron production in collisions involving nuclei can only be extracted from the study of models.
01 Jan 1995
TL;DR: It is found that accelerator data in the fragmentation region are indeed relevant for the cosmic ray energy region and many features of hadron production in collisions involving nuclei can only be extracted from the study of models.
Abstract: The dual parton model for hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions is studied in the fragmentation region up to the cosmic ray energy region. Because of the excellent Feynman scaling behavior of the model outside the regions around [ital x][sub [ital F]]=1 and [ital x][sub [ital F]]=0, it is found that accelerator data in the fragmentation region are indeed relevant for the cosmic ray energy region. However, not enough data are available in the fragmentation region of hadron collisions with light target nuclei. Therefore many features of hadron production in collisions involving nuclei can only be extracted from the study of models.
TL;DR: The coefficient functions for heavy flavour production in deeply inelastic electron hadron scattering have been calculated previously and tabulated as two-dimensional arrays as is often done for the scale dependent parton densities.
TL;DR: In this paper, the two-loop correction to the gluon trajectory is expressed in terms of these discontinuities, leading to the independence of the trajectory on the properties of the scattered particles, confirming the Gluon Reggeization.
TL;DR: In this article, it was shown that nonzero single spin asymmetries in hadron-hadron high energy and moderately large p T inclusive processes can be obtained, even in massless perturbative QCD, provided the quark intrinsic motion is taken into account.
TL;DR: Hard single diffractive processes are studied within the framework of the triple-Pomeron approximation using a Pomeron structure function motivated by Regge theory and parton distribution functions which do not obey the momentum sum rule.
Abstract: Hard single diffractive processes are studied within the framework of the triple-Pomeron approximation Using a Pomeron structure function motivated by Regge theory we obtain parton distribution functions which do not obey the momentum sum rule Based on Regge factorization, cross sections for hard diffraction are calculated Furthermore, the model is applied to hard diffractive particle production in photoproduction and in pp\ifmmode\bar\else\textasciimacron\fi{} interactions
TL;DR: The cross section at leading order in 1/Q for polarized Drell-Yan scattering at measured lepton-pair transverse momentum QT is studied to find that for a hadron with spin 1/2 the quark content at lead order is described by six distribution functions for each flavor.
Abstract: In this paper we study the cross section at leading order in 1/Q for polarized Drell-Yan scattering at measured lepton-pair transverse momentum QT. We find that for a hadron with spin 1/2 the quark content at leading order is described by six distribution functions for each flavor, which depend on both the lightcone momentum fraction x, and the quark transverse momentum k 2 . These functions are illustrated for a free-quark ensemble. The cross sections for both longitudinal and transverse polarizations are expressed in terms of convolution integrals over the distribution functions.
TL;DR: In this article, it was shown that non zero single spin asymmetries in hadron-hadron high energy and moderately large $p_T$ inclusive processes can be obtained, even in massless perturbative QCD, provided the quark intrinsic motion is taken into account.
Abstract: Within the QCD-improved parton model and assuming the factorization theorem to hold in the helicity basis and for higher twist contributions, we show how non zero single spin asymmetries in hadron-hadron high energy and moderately large $p_T$ inclusive processes can be obtained, even in massless perturbative QCD, provided the quark intrinsic motion is taken into account. A simple model is constructed which reproduces the main features of the data on the single spin asymmetry observed in inclusive pion production in $p\,p$ collisions.
TL;DR: In this paper, a new global fit to deep-inelastic and related data is presented, which finds a gluon distribution which is larger for $x \lapproxeq 0.01$ and smaller for$x \sim 0.1$ and a flatter input sea quark distribution.
Abstract: The latest measurements of $F_2$ at HERA allow for a {\it combination} of gluon and sea quark distributions at small $x$ that is significantly different from those of existing parton sets. We perform a new global fit to deep-inelastic and related data. We find a gluon distribution which is larger for $x \lapproxeq 0.01$, and smaller for $x \sim 0.1$, and a flatter input sea quark distribution than those obtained in our most recent global analysis. The new fit also gives $\alpha_s(M_Z^2) = 0.114$. We study other experimental information available for the gluon including, in particular, the constraints coming from fixed-target and collider prompt $\gamma$ production data.
CERN1
TL;DR: In this paper, the Parton Cascade Model is proposed to describe the early stage of heavy ion collisions at collider energies. But the model is not suitable for the case of particle collisions in complete phase space.
TL;DR: In this paper, a detailed expose on the calculation of O( α s ) corrections to the cross section for the semi-weak production of single top quarks in hadronic interactions is given.
TL;DR: It is argued that parton distributions in coordinate space provide a more natural object for nonperturbative methods compared to the usual momentum distributions in which the physics of different longitudinal distances is being mixed.
Abstract: We argue that parton distributions in coordinate space provide a more natural object for nonperturbative methods compared to the usual momentum distributions in which the physics of different longitudinal distances is being mixed. To illustrate the advantages of the coordinate space formulation, we calculate the coordinate space distributions for valence quarks in the proton using the QCD sum rule approach. A remarkable agreement is found between the calculated and the experimentally measured u-quark distribution up to light-cone distances ${\mathrm{\ensuremath{\Delta}}}^{\mathrm{\ensuremath{-}}}$=${\mathrm{\ensuremath{\Delta}}}^{0}$-${\mathrm{\ensuremath{\Delta}}}^{3}$ of order \ensuremath{\sim}1 fm in the proton rest frame. The calculation for valence d quarks gives much worse results; the reasons for this discrepancy are discussed.
TL;DR: In this paper, the structure of the general solution of the inhomogeneous evolution equations allows the separation of a photon structure function into perturbative (anomalous) and non-perturbative contributions.
Abstract: The structure of the general solution of the inhomogeneous evolution equations allows the separation of a photon structure function into perturbative (“anomalous”) and non-perturbative contributions. The former part is fully calculable, and can be identified with the high-mass contributions to the dispersion integral in the photon mass. Properly normalized “state” distributions can be defined, where the
$$\gamma \to q\bar q$$
splitting probability is factored out. These state distributions are shown to be useful in the description of the hadronic event properties, and necessary for a proper eikonalization of jet cross sections. Convenient parametrizations are provided both for the state and for the full anomalous parton distributions. The non-perturbative parts of the parton distribution functions of the photon are identified with the low-mass contributions to the dispersion integral. Their normalizations, as well as the value of the scaleQ
0 at which the perturbative parts vanish, are fixed by approximating the low-mass contributions by a discrete, finite sum of vector mesons. The shapes of these hadronic distributions are fitted to the available data onF
2
γ
(x, Q
2). Parametrizations are provided forQ
0=0.6 GeV andQ
0=2 GeV, both in the DIS and the
$$\overline {MS}$$
factorization schemes. The full parametrizations are extended towards virtual photons. Finally, the often-used “FKP-plus-TPC/2γ” solution forF
2
γ
(x, Q
2) is commented upon.
TL;DR: Open charm production during the equilibration of a gluon dominated parton plasma is calculated, with both the time-dependent temperature and parton densities given by a set of rate equations.
Abstract: Open charm production during the equilibration of a gluon dominated parton plasma is calculated, with both the time-dependent temperature and parton densities given by a set of rate equations. Including prethermal production, the total enhancement of open charm production over the initial gluon fusion depends sensitively on the initial parton density and the effective temperature. The dependence of the prethermal charm production on the space-momentum correlation in the initial parton phase-space distribution is also discussed.
TL;DR: First evidence for the interplay between the direct- and resolved-photon mechanisms and for the existence of a gluon density inside the photon is found.
Abstract: We present new sets of fragmentation functions for charged pions and kaons, both at leading and next-to-leading order. They are fitted to data on inclusive chargedhadron production in e + e annihilation taken by TPC at PEP ( p s = 29 GeV) and to similar data by ALEPH at LEP, who discriminated between events with charm, bottom, and light-flavour fragmentation in their charged-hadron sample. In contrast to our previous analysis, where we only distinguished between valence-quark, sea-quark, and gluon fragmentation, we are now able to treat all partons independently and to properly incorporate the charm and bottom thresholds. Due to the sizeable energy gap between PEP and LEP, we are sensitive to the scaling violation in the fragmentation process, which allows us to extract a value for the asymptotic scale parameter of QCD, �. Recent data on inclusive charged-hadron production in tagged three-jet events by OPAL and similar data for longitudinal electron polarization by ALEPH allow us to pin down the gluon fragmentation functions. Our new fragmentation functions lead to an excellent description of a multitude of other e + e data on inclusive charged-hadron production, ranging from p s = 5.2 GeV to LEP energy. In addition, they agree nicely with the transverse-momentum spectra of single charged hadrons measured by H1 and ZEUS in photoproduction at the ep collider HERA, which represents a nontrivial check of the factorization theorem of the QCD-improved parton model. In this comparison, we also find first evidence for the interplay between the direct- and resolved-photon mechanisms and for the existence of a gluon density inside the photon.
TL;DR: In this paper, the pseudorapidity and transverse momentum distributions for charm and bottom production at HERA were derived and compared with those of a full Monte Carlo simulation using HERWIG.
TL;DR: In this paper, the global properties of the "rapidity gap" events, observed at HERA, can be understood based on electron-gluon scattering as the underlying partonic process.
TL;DR: In this article, the authors investigated the evolution of parton densities at small values of the momentum fraction, x, by including resummed anomalous dimensions in the renormalization group equations.