Showing papers on "Parton published in 2006"

Matching matrix elements and shower evolution for top-quark production in hadronic collisions

Abstract: We study the matching of multijet matrix elements and shower evolution in the case of top production in hadronic collisions at the Tevatron and at the LHC. We present the results of the matching algorithm implemented in the ALPGEN Monte Carlo generator, and compare them with results obtained at the parton level, and with the predictions of the MC@NLO approach. We highlight the consistency of the matching algorithm when applied to these final states, and the excellent agreement obtained with MC@NLO for most inclusive quantities. We nevertheless identify also a remarkable difference in the rapidity spectrum of the leading jet accompanying the top quark pair, and comment on the likely origin of this discrepancy.

Parton ladder splitting and the rapidity dependence of transverse momentum spectra in deuteron-gold collisions at the BNL Relativistic Heavy Ion Collider

Abstract: We present a phenomenological approach (EPOS), based on the parton model, but going much beyond by incorporating elastic and inelastic parton ladder splitting. Based on this model, we try to understand proton-proton and deuteron-gold ($d\mathrm{Au}$) collisions, in particular the rapidity dependence of transverse momentum results in $d\mathrm{Au}$ from all four experiments conducted on the BNL Relativistic Heavy Ion Collider.

A model of jet quenching in ultrarelativistic heavy ion collisionsand high- p T hadron spectra at RHIC

Abstract: The method to simulate the rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions has been developed. The model is a fast Monte Carlo tool introduced to modify a standard PYTHIA jet event. The full heavy ion event is obtained as a superposition of a soft hydro-type state and hard multi-jets. The model is applied to the analysis of the jet quenching pattern at RHIC.

Heavy-quark probes of the quark-gluon plasma and interpretation of recent data taken at the BNL Relativistic Heavy Ion Collider

Abstract: Thermalization and collective flow of charm (c) and bottom (b) quarks in ultrarelativistic heavy-ion collisions are evaluated based on elastic parton rescattering in an expanding quark-gluon plasma (QGP). We show that resonant interactions in a strongly interacting QGP (sQGP), as well as parton coalescence, can play an essential role in the interpretation of recent data from the BNL Relativistic Heavy-Ion Collider (RHIC), and thus illuminate the nature of the sQGP and its hadronization. Our main assumption, motivated by recent findings in lattice quantum chromodynamics, is the existence of D- and B-meson states in the sQGP, providing resonant cross sections for heavy quarks. Pertinent drag and diffusion coefficients are implemented into a relativistic Langevin simulation to compute transverse-momentum spectra and azimuthal asymmetries (${v}_{2}$) of b- and c-quarks in Au-Au collisions at RHIC. After hadronization into D- and B-mesons using quark coalescence and fragmentation, associated electron-decay spectra and ${v}_{2}$ are compared to recent RHIC data. Our results suggest a reevaluation of radiative and elastic quark energy-loss mechanisms in the sQGP.

Nonlinear screening effects in high energy hadronic interactions

Abstract: Nonlinear effects in hadronic interactions are treated by means of enhanced Pomeron diagrams, assuming that Pomeron-Pomeron coupling is dominated by soft partonic processes. It is shown that the approach allows to resolve a seeming contradiction between realistic parton momentum distributions, measured in deep inelastic scattering experiments, and the energy behavior of total proton-proton cross section. Also a general consistency with both soft and hard diffraction data is demonstrated. An important feature of the proposed scheme is that the contribution of semihard processes to the interaction eikonal contains a significant nonfactorizable part. On the other hand, the approach preserves the QCD factorization picture for inclusive high-${p}_{t}$ jet production.

Measurement and QCD analysis of the diffractive deep-inelastic scattering cross-section at HERA

Abstract: A detailed analysis is presented of the diffractive deep-inelastic scattering process $ep\to eXY$, where $Y$ is a proton or a low mass proton excitation carrying a fraction $1 - \xpom > 0.95$ of the incident proton longitudinal momentum and the squared four-momentum transfer at the proton vertex satisfies $|t|<1 {\rm GeV^2}$. Using data taken by the H1 experiment, the cross section is measured for photon virtualities in the range $3.5 \leq Q^2 \leq 1600 \rm GeV^2$, triple differentially in $\xpom$, $Q^2$ and $\beta = x / \xpom$, where $x$ is the Bjorken scaling variable. At low $\xpom$, the data are consistent with a factorisable $\xpom$ dependence, which can be described by the exchange of an effective pomeron trajectory with intercept $\alphapom(0)= 1.118 \pm 0.008 {\rm (exp.)} ^{+0.029}_{-0.010} {\rm (model)}$. Diffractive parton distribution functions and their uncertainties are determined from a next-to-leading order DGLAP QCD analysis of the $Q^2$ and $\beta$ dependences of the cross section. The resulting gluon distribution carries an integrated fraction of around 70% of the exchanged momentum in the $Q^2$ range studied. Total and differential cross sections are also measured for the diffractive charged current process $e^+ p \to \bar{ u}_e XY$ and are found to be well described by predictions based on the diffractive parton distributions. The ratio of the diffractive to the inclusive neutral current $ep$ cross sections is studied. Over most of the kinematic range, this ratio shows no significant dependence on $Q^2$ at fixed $\xpom$ and $x$ or on $x$ at fixed $Q^2$ and $\beta$.

Matching Parton Showers and Matrix Elements

Abstract: We compare different procedures for combining fixed-order tree-level matrix element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW scheme and one based on the so-called MLM scheme using different matrix element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.

Variable-flavor number scheme for next-to-next-to-leading order

Abstract: At NNLO it is particularly important to have a Variable-flavor Number Scheme (VFNS) to deal with heavy quarks because there are major problems with both the zero-mass variable-flavor number scheme and the fixed-flavor number scheme. I illustrate these problems and present a general formulation of a Variable-flavor Number Scheme (VFNS) for heavy quarks that is explicitly implemented up to NNLO in the strong coupling constant ${\ensuremath{\alpha}}_{S}$, and may be used in NNLO global fits for parton distributions. The procedure combines elements of the ACOT($\ensuremath{\chi}$) scheme and the Thorne-Roberts scheme. Despite the fact that at NNLO the parton distributions are discontinuous as one changes the number of active quark flavors, all physical quantities are continuous at flavor transitions and the comparison with data is successful.

Determination of the jet energy scale at the collider detector at Fermilab

Abstract: A precise determination of the energy scale of jets at the Collider Detector at Fermilab at the Tevatron p p ¯ collider is described. Jets are used in many analyses to estimate the energies of partons resulting from the underlying physics process. Several correction factors are developed to estimate the original parton energy from the observed jet energy in the calorimeter. The jet energy response is compared between data and Monte Carlo simulation for various physics processes, and systematic uncertainties on the jet energy scale are determined. For jets with transverse momenta above 50 GeV the jet energy scale is determined with a 3 % systematic uncertainty.

Infrared-safe definition of jet flavour

Abstract: It is common, in both theoretical and experimental studies, to separately discuss quark and gluon jets. However, even at parton level, widely-used jet algorithms fail to provide an infrared-safe way of making this distinction. We examine the origin of the problem and propose a solution in terms of a new “flavour-kt” algorithm. As well as being of conceptual interest this can be a powerful tool when combining fixed-order calculations with multi-jet resummations and parton showers. It also has applications to studies of heavy-quark jets.

Minijet deformation and charge-independent angular correlations on momentum subspace (eta, phi) in Au-Au collisions at root S-NN=130 GeV

Abstract: Measurements of two-particle correlations on angular difference variables η1 − η2 (pseudorapidity) and φ1 − φ2 (azimuth) are presented for all primary charged hadrons with transverse momentum 0.15 ≤ p_t ≤ 2 GeV/c and |η| ≤ 1.3 from Au-Au collisions at √s_(NN) = 130 GeV. Large-amplitude correlations are observed over a broad range in relative angles where distinct structures appear on the same-side and away-side (i.e., relative azimuth less than π/2 or greater than π/2). The principal correlation structures include that associated with elliptic flow plus a strong, same-side peak. It is hypothesized that the latter results from correlated hadrons associated with semi-hard parton scattering in the early stage of the heavy-ion collision which produces a jet-like correlation peak at small relative angles. The width of the jet-like peak on η1 − η2 increases by a factor 2.3 from peripheral to central collisions, suggesting strong coupling of semi-hard scattered partons to a longitudinally-expanding medium. The new methods of jet analysis introduced here provide access to scattered partons at low transverse momentum well below the kinematic range where perturbative quantum chromodynamics and standard fragmentation models are applicable.

Scaling tests of the cross section for deeply virtual Compton scattering.

Abstract: We present the first measurements of the e[over -->]p-->epgamma cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q(2) dependence (from 1.5 to 2.3 GeV(2)) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q(2). The helicity-independent cross section is also measured at Q(2)=2.3 GeV(2). We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.

Single transverse-spin asymmetry in high transverse momentum pion production in pp collisions

Abstract: We study the single-spin (left-right) asymmetry in single-inclusive pion production in hadronic scattering. This asymmetry is power-suppressed in the transverse momentum of the produced pion and can be analyzed in terms of twist-three parton correlation functions in the proton. We present new calculations of the corresponding partonic hard-scattering functions that include the so-called ``nonderivative'' contributions not previously considered in the literature. We find a remarkably simple structure of the results. We also present a brief phenomenological study of the spin asymmetry, taking into account data from fixed-target scattering and also the latest information available from Relativistic Heavy Ion Collider (RHIC). We make additional predictions that may be tested experimentally at RHIC.

fastNLO: Fast pQCD Calculations for PDF Fits

Abstract: We present a method for very fast repeated computations of higher-order cross sections in hadron-induced processes for arbitrary parton density functions. A full implementation of the method for computations of jet cross sections in Deep-Inelastic Scattering and in Hadron-Hadron Collisions is offered by the "fastNLO" project. A web-interface for online calculations and user code can be found at this http URL .

Deep-inelastic scattering and the operator product expansion in lattice QCD

Abstract: We discuss the determination of deep-inelastic hadron structure in lattice QCD. By using a fictitious heavy-quark, direct calculations of the Compton scattering tensor can be performed in Euclidean space that allow the extraction of the moments of structure functions. This overcomes issues of operator mixing and renormalization that have so far prohibited lattice computations of higher moments. This approach is especially suitable for the study of the twist-two contributions to isovector quark distributions, which is practical with current computing resources. While we focus on the isovector unpolarized distribution, our method is equally applicable to other quark distributions and to generalized parton distributions. By looking at matrix elements such as (where V{sup {mu}} and A{sup {nu}} are vector and axial-vector heavy-light currents) within the same formalism, moments of meson distribution amplitudes can also be extracted.

Parton distributions and the strong coupling: CTEQ6AB PDFs

Abstract: We study the global analysis for parton distributions as a function of the QCD strong coupling strength αs, and present a new series of distributions that span the range 0.110 < αs(mZ) < 0.128. We use these distributions to explore the correlation between αs and the gluon distribution; the viability of global analysis as a method to measure αs; and the dependence on αs of predictions for W, Z, inclusive jet, and Higgs boson production (b→H and gg→H) cross sections at the Tevatron and the LHC. We find that the uncertainty in αs is the dominant source of uncertainty for inclusive jet production at moderately small pT and for Higgs production at intermediate masses in the Standard Model.

Next-to-leading order QCD corrections to W+W− production via vector-boson fusion

Abstract: Vector-boson fusion processes constitute an important class of reactions at hadron colliders, both for signals and backgrounds of new physics in the electroweak interactions. We consider what is commonly referred to as W+W− production via vector-boson fusion (with subsequent leptonic decay of the Ws), or, more precisely, e+νe μ−μ + 2 jets production in proton-proton scattering, with all resonant and non-resonant Feynman diagrams and spin correlations of the final-state leptons included, in the phase-space regions which are dominated by t-channel electroweak-boson exchange. We compute the next-to-leading order QCD corrections to this process, at order α6αs. The QCD corrections are modest, changing total cross sections by less than 10%. Remaining scale uncertainties are below 2%. A fully-flexible next-to-leading order partonic Monte Carlo program allows to demonstrate these features for cross sections within typical vector-boson-fusion acceptance cuts. Modest corrections are also found for distributions.

Sivers effect in semiinclusive deeply inelastic scattering

Abstract: The Sivers function is extracted from HERMES data on single spin asymmetries in semi-inclusive deeply inelastic scattering. Our analysis use a simple Gaussian model for the distribution of transverse parton momenta, together with the flavor dependence given by the leading 1/N{sub c} approximation and a neglect of the Sivers antiquark distribution. We find that within the errors of the data these approximations are sufficient.

Soft gluons at large angles in hadron collisions

Abstract: A general discussion is presented of the single logarithmic soft factor that appears in two scale QCD observables in processes involving four partons. We treat it as the ``fifth form factor'', accompanying the four collinear singular Sudakov form factors attached to colliding and outgoing hard partons. The fifth form factor is expressed in terms of the Casimir operators (squared colour charges) of irreducible representations in the crossing t- and u-channels. As an application we revisit the problem of large angle radiation in gg→gg and give a relatively simple solution and interpretation of the results. We found an unexpected symmetry of the soft anomalous dimension under exchange of internal and external variables of the problem whose existence calls for explanation.

Chiral perturbation theory for nucleon generalized parton distributions

Abstract: We analyze the moments of the isosinglet generalized parton distributions H, E, $ \tilde{{H}}$ , $ \tilde{{E}}$ of the nucleon in one-loop order of heavy-baryon chiral perturbation theory. We discuss in detail the construction of the operators in the effective theory that are required to obtain all corrections to a given order in the chiral power counting. The results will serve to improve the extrapolation of lattice results to the chiral limit.

Electroweak corrections to top-quark pair production in quark-antiquark annihilation

Abstract: Top-quark physics plays an important role at hadron colliders such as the Tevatron at Fermilab or the LHC at CERN. Given the planned precision at these colliders, precise theoretical predictions are required. In this paper we present the complete electroweak corrections to QCD-induced top-quark pair production in quark-antiquark annihilation. In particular we provide compact analytic expressions for the differential partonic cross section, which will be useful for further theoretical investigations.

Tevatron-for-LHC Report of the QCD Working Group

Abstract: The experiments at Run 2 of the Tevatron have each accumulated over 1 fb{sup -1} of high-transverse momentum data. Such a dataset allows for the first precision (i.e. comparisons between theory and experiment at the few percent level) tests of QCD at a hadron collider. While the Large Hadron Collider has been designed as a discovery machine, basic QCD analyses will still need to be performed to understand the working environment. The Tevatron-for-LHC workshop was conceived as a communication link to pass on the expertise of the Tevatron and to test new analysis ideas coming from the LHC community. The TeV4LHC QCD Working Group focused on important aspects of QCD at hadron colliders: jet definitions, extraction and use of Parton Distribution Functions, the underlying event, Monte Carlo tunes, and diffractive physics. This report summarizes some of the results achieved during this workshop.