Showing papers on "Parton published in 2004"

High-precision QCD at hadron colliders: Electroweak gauge boson rapidity distributions at next-to-next-to leading order

Abstract: We compute the rapidity distributions of W and Z bosons produced at the Tevatron and the LHC through next-to-next-to leading order in QCD. Our results demonstrate remarkable stability with respect to variations of the factorization and renormalization scales for all values of rapidity accessible in current and future experiments. These processes are therefore ''gold-plated'': current theoretical knowledge yields QCD predictions accurate to better than one percent. These results strengthen the proposal to use $W$ and $Z$ production to determine parton-parton luminosities and constrain parton distribution functions at the LHC. For example, LHC data should easily be able to distinguish the central parton distribution fit obtained by MRST from that obtained by Alekhin.

Nuclear parton distributions at next to leading order

Abstract: We perform a next to leading order QCD global analysis of nuclear deep inelastic scattering and Drell-Yan data using the convolution approach to parametrize nuclear parton densities. We find both a significant improvement in the agreement with data compared to previous extractions, and substantial differences in the scale dependence of nuclear effects compared to leading order analyses.

Medium induced gluon radiation off massive quarks fills the dead cone

Abstract: We calculate the transverse momentum dependence of the medium-induced gluon energy distribution radiated off massive quarks in spatially extended QCD matter. In the absence of a medium, the distribution shows a characteristic mass-dependent depletion of the gluon radiation for angles $\ensuremath{\theta}lm/E,$ the so-called dead cone effect. Medium modifications of this spectrum are calculated as a function of quark mass m, initial quark energy E, in-medium path length and density. Generically, medium-induced gluon radiation is found to fill the dead cone, but it is reduced at large gluon energies compared to the radiation off light quarks. We quantify the resulting mass dependence for momentum-averaged quantities (gluon energy distribution and average parton energy loss), compare it to simple approximation schemes and discuss its observable consequences for nucleus-nucleus collisions at the BNL RHIC and CERN LHC. In particular, our analysis does not favor the complete disappearance of energy loss effects from leading open charm spectra at the RHIC.

Universality of soft and collinear factors in hard-scattering factorization

Abstract: Universality in QCD factorization of parton densities, fragmentation functions, and soft factors is endangered by the process dependence of the directions of Wilson lines in their definitions. We find a choice of directions that is consistent with factorization and that gives universality between e(+)e(-) annihilation, semi-inclusive deep-inelastic scattering, and the Drell-Yan process. Universality is only modified by a time-reversal transformation of the soft function and parton densities between Drell-Yan and the other processes, whose only effect is the known reversal of sign for T-odd parton densities such as the Sivers function. The modifications of the definitions needed to remove rapidity divergences with lightlike Wilson lines do not affect the results.

Multiple interactions and the structure of beam remnants

Abstract: Recent experimental data have established some of the basic features of multiple interactions in hadron-hadron collisions. The emphasis is therefore now shifting, to one of exploring more detailed aspects. Starting from a brief review of the current situation, a next-generation model is developed, wherein a detailed account is given of correlated flavour, colour, longitudinal and transverse momentum distributions, encompassing both the partons initiating perturbative interactions and the partons left in the beam remnants. Some of the main features are illustrated for the Tevatron and the LHC.

MHV rules for Higgs plus multi-gluon amplitudes.

Abstract: We present MHV-rules for constructing perturbative amplitudes for a Higgs boson and an arbitrary number of partons. We give explicit expressions for amplitudes involving a Higgs and three negative helicity partons and any number of positive helicity partons - the NMHV amplitudes. We also present a recursive formulation of MHV rules that incorporates the Higgs, quarks and gluons. The recursion relations are valid for all non-MHV amplitudes. The general results agree numerically with all of the available Higgs + n-parton amplitudes and in some cases provide considerably shorter expressions.

Quark imaging in the proton via quantum phase-space distributions

Abstract: We develop the concept of quantum phase-space (Wigner) distributions for quarks and gluons in the proton. To appreciate their physical content, we analyze the contraints from special relativity on the interpretation of elastic form factors, and examine the physics of the Feynman parton distributions in the proton's rest frame. We relate the quark Wigner functions to the transverse-momentum dependent parton distributions and generalized parton distributions, emphasizing the physical role of the skewness parameter. We show that the Wigner functions allow us to visualize quantum quarks and gluons using the language of classical phase space. We present two examples of the quark Wigner distributions and point out some model-independent features.

The tbar-t cross-section at 1.8 and 1.96 TeV: a study of the systematics due to parton densities and scale dependence

Abstract: We update the theoretical predictions for the t t production cross-section at the Tevatron, taking into account the most recent determinations of systematic uncertainties in the extraction of the proton parton densities.

Deep Inelastic Scattering

Abstract: 1. Introduction 2. The quark-parton model 3. Quantum chromodynamics (QCD) and formal methods 4. QCD improved parton model 5. Deep inelastic scattering (DIS) experiments and data 6. Extraction of parton densities 7. Alpha s from scaling violations and jets at high Q squared 8. DIS at high Q squared 9. DIS at low x 10. Hadron induced DIS 11. Polarized DIS 12. Beyond the standard model A. Dirac equations and some other conventions B. Phase space and cross-sections C. DIS cross-sections D. Feynman rules E. Monte Carlo codes F. Data sources G. Parton parameterizations

Higgs radiation off bottom quarks at the Fermilab Tevatron and the CERN LHC

Abstract: Higgs-boson production in association with bottom quarks, $p\overline{p}/pp\ensuremath{\rightarrow}b\overline{b}H+X,$ 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 $q\overline{q},gg\ensuremath{\rightarrow}b\overline{b}H$ 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 dependence and thus stabilize the theoretical predictions, especially when the Higgs-boson is produced in association with high-${p}_{T}$ 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 $b\overline{b}\ensuremath{\rightarrow}H$.

Generalized parton distributions

Abstract: ▪ Abstract This review explains why generalized parton distributions and the related quantum phase-space distributions are useful in exploring the quark and gluon structure of the proton and neutron. It starts with the physics of form factors and parton distributions. Then quantum phase-space distributions and their offspring are discussed. The properties of generalized parton distributions are described. In particular, I elucidate the relation to the spin structure of the nucleon. Finally, various methods to determine the new distributions are explained.

MHV Rules for Higgs Plus Multi-Gluon Amplitudes

Abstract: We use tree-level perturbation theory to show how non-supersymmetric one-loop scattering amplitudes for a Higgs boson plus an arbitrary number of partons can be constructed, in the limit of a heavy top quark, from a generalization of the scalar graph approach of Cachazo, Svrcek and Witten. The Higgs boson couples to gluons through a top quark loop which generates, for large top mass, a dimension-5 operator H tr G^2. This effective interaction leads to amplitudes which cannot be described by the standard MHV rules; for example, amplitudes where all of the gluons have positive helicity. We split the effective interaction into the sum of two terms, one holomorphic (selfdual) and one anti-holomorphic (anti-selfdual). The holomorphic interactions give a new set of MHV vertices -- identical in form to those of pure gauge theory, except for momentum conservation -- that can be combined with pure gauge theory MHV vertices to produce a tower of amplitudes with more than two negative helicities. Similarly, the anti-holomorphic interactions give anti-MHV vertices that can be combined with pure gauge theory anti-MHV vertices to produce a tower of amplitudes with more than two positive helicities. A Higgs boson amplitude is the sum of one MHV-tower amplitude and one anti-MHV-tower amplitude. We present all MHV-tower amplitudes with up to four negative-helicity gluons and any number of positive-helicity gluons (NNMHV). These rules reproduce all of the available analytic formulae for Higgs + n-gluon scattering (n<=5) at tree level, in some cases yielding considerably shorter expressions.

Partonic calculation of the two photon exchange contribution to elastic electron proton scattering at large momentum transfer

Abstract: We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. We relate the two-photon exchange amplitude to the generalized parton distributions which appear in hard exclusive processes. We find that when taking the polarization transfer determinations of the form factors as input, adding in the 2-photon correction does reproduce the Rosenbluth cross sections.

Uncertainties of predictions from parton distributions II: theoretical errors

Abstract: We study the uncertainties in parton distributions, determined in global fits to deep inelastic and related hard scattering data, due to so-called theoretical errors. Amongst these, we include potential errors due to the change of perturbative order (NLO $\to$ NNLO), $\ln(1/x)$ and $\ln(1-x)$ effects, absorptive corrections and higher-twist contributions. We investigate these uncertainties both by including explicit corrections to our standard global analysis and by examining the sensitivity to changes of the x,Q 2,W 2 cuts on the data that are fitted. In this way we expose those kinematic regions where the conventional DGLAP description is inadequate. As a consequence we obtain a set of NLO, and of NNLO, conservative partons where the data are fully consistent with DGLAP evolution, but over a restricted kinematic domain. We also examine the potential effects of such issues as the choice of input parametrisation, heavy target corrections, assumptions about the strange quark sea and isospin violation. Hence we are able to compare the theoretical errors with those uncertainties due to errors on the experimental measurements, which we studied previously. We use W and Higgs boson production at the Tevatron and the LHC as explicit examples of the uncertainties arising from parton distributions. For many observables the theoretical error is dominant, but for the cross section for W production at the Tevatron both the theoretical and experimental uncertainties are small, and hence the NNLO prediction may serve as a valuable luminosity monitor.

Recombination of shower partons at high p T in heavy-ion collisions

Abstract: A formalism for hadron production at high ${p}_{T}$ in heavy-ion collisions has been developed such that all partons hadronize by recombination The fragmentation of a hard parton is accounted for by the recombination of shower partons that it creates Such shower partons can also recombine with the thermal partons to form particles that dominate over all other possible modes of hadronization in the $3l{p}_{T}l8\phantom{\rule{03em}{0ex}}\mathrm{GeV}$ range The results for the high ${p}_{T}$ spectra of pion, kaon, and proton agree well with experiments Energy loss of partons in the dense medium is taken into account on the average by an effective parameter by fitting data, and is found to be universal independent of the type of particles produced, as it should Due to the recombination of thermal and shower partons, the structure of jets produced in nuclear collisions is different from that in $pp$ collisions The consequence on same-side correlations is discussed

Measuring the collective flow with jets.

Abstract: In nucleus-nucleus collisions, high-p(T) partons interact with a dense medium, which possesses strong collective flow components Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow Both components can be disentangled by the measurement of particle production associated with high-p(T) trigger particles, jetlike correlations, and jets In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the eta x phi plane We argue that data on the medium-induced broadening of jetlike particle correlations in Au + Au collisions at the Relativistic Heavy-Ion Collider may provide evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow

CTEQ6 parton distributions with heavy quark mass effects

Abstract: Previously published CTEQ6 parton distributions adopt the conventional zero-mass parton scheme since the corresponding hard cross sections are universally available. For precision observables which are sensitive to charm and bottom quark mass effects, we provide in this paper an improved CTEQ6HQ parton distribution set determined in the more general variable flavor number scheme that incorporates heavy flavor mass effects. We describe in detail the QCD scheme and analysis procedure used, examine the predominant features of the new distributions, and compare them with previous distributions.

Precision measurement of the neutron spin asymmetries and spin-dependent structure functions in the valence quark region

Abstract: We report on measurements of the neutron spin asymmetries $A_{1,2}^n$ and polarized structure functions $g_{1,2}^n$ at three kinematics in the deep inelastic region, with $x=0.33$, 0.47 and 0.60 and $Q^2=2.7$, 3.5 and 4.8 (GeV/c)$^2$, respectively. These measurements were performed using a 5.7 GeV longitudinally-polarized electron beam and a polarized $^3$He target. The results for $A_1^n$ and $g_1^n$ at $x=0.33$ are consistent with previous world data and, at the two higher $x$ points, have improved the precision of the world data by about an order of magnitude. The new $A_1^n$ data show a zero crossing around $x=0.47$ and the value at $x=0.60$ is significantly positive. These results agree with a next-to-leading order QCD analysis of previous world data. The trend of data at high $x$ agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (pQCD) assuming hadron helicity conservation. Results for $A_2^n$ and $g_2^n$ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment $d_2^n$ was evaluated and the new result has improved the precision of this quantity by about a factor of two. When combined with the world proton data, polarized quark distribution functions were extracted from the new $g_1^n/F_1^n$ values based on the quark parton model. While results for $\Delta u/u$ agree well with predictions from various models, results for $\Delta d/d$ disagree with the leading-order pQCD prediction when hadron helicity conservation is imposed.

Generalized parton distributions from lattice QCD.

Abstract: We perform a quenched lattice calculation of the first moment of twist-two generalized parton distribution functions of the proton, and assess the total quark (spin and orbital angular momentum) contribution to the spin of the proton.

Interactions of heavy stable hadronising particles

Abstract: In this article, we study the interactions of stable, hadronising new states, arising in certain extensions of the standard model. A simple model, originally intended for stable gluino hadrons, is developed to describe the nuclear interactions of hadrons containing any new colour triplet or octet stable parton. Hadron mass spectra, nuclear scattering cross sections and interaction processes are discussed. Furthermore, an implementation of the interactions of heavy hadrons in GEANT 3 is presented, signatures are studied, and a few remarks about possible detection with the ATLAS experiment are given.

Unintegrated parton distributions and electroweak boson production at hadron colliders

Abstract: We describe the use of doubly unintegrated parton distributions in hadron-hadron collisions, using the ${(z,k}_{t})$-factorization prescription where the transverse momentum of the incoming parton is generated in the last evolution step. We apply this formalism to calculate the transverse momentum ${(P}_{T})$ distributions of produced W and Z bosons and compare the predictions to Tevatron run 1 data. We find that the observed ${P}_{T}$ distributions can be generated almost entirely by the leading order ${q}_{1}{q}_{2}\ensuremath{\rightarrow}W,Z$ subprocesses, using known and universal doubly unintegrated quark distributions. We also calculate the ${P}_{T}$ distribution of the standard model Higgs boson at the LHC, where the dominant production mechanism is by gluon-gluon fusion.

Parton intrinsic motion in inclusive particle production: unpolarized cross sections, single spin asymmetries, and the Sivers effect

Abstract: The relevance of intrinsic (or primordial) transverse momentum of partons in the inclusive production of particles at high energy and moderately large ${p}_{T}$ has been known for a long time, beginning with Drell-Yan and diphoton processes, and continuing with photon and meson production in hadronic collisions. In view of its renewed interest in the context of polarized processes and single spin asymmetries we perform, in the framework of perturbative QCD with the inclusion of spin and ${\mathbit{k}}_{\ensuremath{\perp}}$ effects, a detailed analysis of several such processes in different kinematical situations. We show that the inclusion of these effects leads, at the level of accuracy reachable in this approach, to an overall satisfactory agreement between theoretical predictions and experimental unpolarized data, thus giving support to the study of spin effects and single spin asymmetries within the same scheme. We present results for transverse single spin asymmetries, generated by the so-called Sivers effect, in inclusive pion and photon production in proton-proton collisions. We compare our results with the available experimental data and with previous results obtained using simplified versions of this approach.

Determination of polarized parton distribution functions and their uncertainties

Abstract: We investigate the polarized parton distribution functions (PDFs) and their uncertainties by using the world data on the spin asymmetry A_1. The uncertainties of the polarized PDFs are estimated by the Hessian method. The up and down valence-quark distributions are determined well. However, the antiquark distributions have large uncertainties at this stage, and it is particularly difficult to fix the gluon distribution. The \chi^2 analysis produces a positively polarized gluon distribution, but even \Delta g(x)=0 could be allowed according to our uncertainty estimation. In comparison with the previous AAC (Asymmetry Analysis Collaboration) parameterization in 2000, accurate SLAC-E155 proton data are added to the analysis. We find that the E155 data improve the determination of the polarized PDFs, especially the polarized antiquark distributions. In addition, the gluon-distribution uncertainties are reduced due to the correlation with the antiquark distributions. We also show the global analysis results with the condition \Delta g(x)=0 at the initial scale, Q^2=1 GeV^2, for clarifying the error correlation effects with the gluon distribution.