W. K. Tung

New Generation of Parton Distributions with Uncertainties from Global QCD Analysis

A new generation of parton distribution functions with increased precision and quantitative estimates of uncertainties is presented. This work signiflcantly extends previous CTEQ and other global analyses on two fronts: (i) a full treatment of available experimental correlated systematic errorsforbothnewandolddata sets; (ii) asystematic and pragmatic treatment of uncertainties of the parton distributions and their physical predictions, using a recently developed eigenvector-basis approach to the hessian method. The new gluon distribution is considerably harder than that of previous standard flts. A numberofphysicsissues,particularlyrelatingtothebehaviorofthegluondistribution,are addressedinmorequantitativetermsthanbefore. Extensiveresultsontheuncertaintiesof parton distributions at various scales, and on parton luminosity functions at the Tevatron RunII and the LHC, are presented. The latter provide the means to quickly estimate the uncertainties of a wide range of physical processes at these high-energy hadron colliders, basedoncurrentknowledgeofthepartondistributions. Inparticular, theuncertaintieson the production cross sections of the W, Z at the Tevatron and the LHC are estimated to be§4% and§5%, respectively, and that of a light Higgs at the LHC to be§5%.

Implications of CTEQ global analysis for collider observables

The latest CTEQ6.6 parton distributions, obtained by global analysis of hard-scattering data in the framework of general-mass perturbative QCD, are employed to study theoretical predictions and their uncertainties for significant processes at the Fermilab Tevatron and CERN Large Hadron Collider. The previously observed increase in predicted cross sections for the standard-candle W and Z boson production processes in the general-mass scheme (compared to those in the zero-mass scheme) is further investigated and quantified. A novel method to constrain parton distribution function (PDF) uncertainties in LHC observables, by effectively exploiting PDF-induced correlations with benchmark standard model cross sections, is presented. Using this method, we show that the tt cross section can potentially serve as a standard-candle observable for the LHC processes dominated by initial-state gluon scattering. Among other benefits, precise measurements of tt cross sections would reduce PDF uncertainties in predictions for single top-quark and Higgs boson production in the standard model and minimal supersymmetric standard model.

Global QCD analysis of parton structure of the nucleon: CTEQ5 parton distributions

Abstract. An up-to-date global QCD analysis of high energy lepton-hadron and hadron-hadron interactions is performed to better determine the gluon and quark parton distributions in the nucleon. Improved experimental data on inclusive jet production, in conjunction with precise deep inelastic scattering data, place good constraints on the gluon over a wide range of x; while new data on asymmetries in Drell-Yan processes contribute to better determine the d/u ratio. Comparisons with results of other recent global analyses are made, and the differences are described. Open issues and the general problem of determining the uncertainties of parton distributions are discussed.

Improved parton distributions from global analysis of recent deep inelastic scattering and inclusive jet data

The impact of recent precision measurements of DIS structure functions and inclusive jet production at the Fermilab Tevatron on the global QCD analysis of parton distribution functions is studied in detail. Particular emphasis is placed on exploring the range of variation of the gluon distribution G(x,Q) allowed by these new data. The strong coupling of G(x,Q) with {alpha}{sub s} is fully taken into account. A new generation of CTEQ parton distributions, CTEQ4, is presented. It consists of the three standard sets [modified minimal subtraction ({ovr MS}), deep inelastic scattering (DIS), and leading order (LO)], a series that gives a range of parton distributions with corresponding {alpha}{sub s}`s, and a set with a low starting value of Q. Previously obtained gluon distributions that are consistent with the high E{sub t} jet cross section are also discussed in the context of this new global analysis. {copyright} {ital 1997} {ital The American Physical Society}

Inclusive Jet Production, Parton Distributions, and the Search for New Physics

Jet production at the Tevatron probes some of the smallest distance scales currently accessible. A gluon distribution that is enhanced at large x compared to previous determinations provides a better description of the Run 1b jet data from both CDF and D?. However, considerable uncertainty still remains regarding the gluon distribution at high x. In this paper, we examine the effects of this uncertainty, and of the remaining uncertainties in the NLO QCD theory, on jet cross section comparisons to Run 1b data. We also calculate the range of contributions still possible from any new physics. Predictions are also made for the expanded kinematic range expected for the ongoing Run 2 at the Tevatron and for the LHC.

Global QCD analysis and the CTEQ parton distributions

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.

CTEQ parton distributions and flavor dependence of sea quarks

Abstract This paper describes salient features of new sets of parton distributions obtained by the CTEQ Collaboration based on a comprehensive QCD global analysis of all available data. The accuracy of the new data on deep inelastic scattering structure functions obtained by the very high statistics NMC and CCFR experiments provides unprecedented sensitivity to the flavor dependence of the sea-quark distributions. In addition to much better determination of the small-x dependence of all parton distributions, we found: (i) the strange quark distribution is much softer than the non-strange sea quarks and rises above the latter at small-x; and (ii) the difference d - u changes sign as a function of x. A few alternative sets of viable distributions with conventional assumptions are also discussed.

Heavy Quark Mass Effects in Deep Inelastic Scattering and Global QCD Analysis

A new implementation of the general PQCD formalism of Collins, including heavy quark mass effects, is described. Important features that contribute to the accuracy and efficiency of the calculation of both neutral current (NC) and charged current (CC) processess are explicitly discussed. This new implementation is applied to the global analysis of the full HERA I data sets on NC and CC cross sections, with correlated systematic errors, in conjunction with the usual fixed-target and hadron collider data sets. By using a variety of parametrizations to explore the parton parameter space, robust new parton distribution function (PDF) sets (CTEQ6.5) are obtained. The new quark distributions are consistently higher in the region x ~ 10−3 than previous ones, with important implications on hadron collider phenomenology, especially at the LHC. The uncertainties of the parton distributions are reassessed and are compared to the previous ones. A new set of CTEQ6.5 eigenvector PDFs that encapsulates these uncertainties is also presented.

Charm parton content of the nucleon

We investigate the charm sector of the nucleon structure phenomenologically, using the most up-to-date global QCD analysis. Going beyond the common assumption of purely radiatively generated charm, we explore possible degrees of freedom in the parton parameter space associated with nonperturbative (intrinsic) charm in the nucleon. Specifically, we explore the limits that can be placed on the intrinsic charm (IC) component, using all relevant hard-scattering data, according to scenarios in which the IC has a form predicted by light-cone wave function models; or a form similar to the light sea-quark distributions. We find that the range of IC is constrained to be from zero (no IC) to a level 2-3 times larger than previous model estimates. The behaviors of typical charm distributions within this range are described, and their implications for hadron collider phenomenology are briefly discussed.

Parton distributions and the strong coupling: CTEQ6AB PDFs

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.