arXiv subjects

W. K. Tung

Publications and source records attributed to W. K. Tung.

29 records · Page 2Linked to original sources

arXiv1999

Large-x Parton Distributions

Reliable knowledge of parton distributions at large x is crucial for many searches for new physics signals in the next generation of collider experiments. Although these are generally well determined in the small and medium x range, it has been shown that their uncertainty grows rapidly for x > 0.1. We examine the status of the gluon and quark distributions in light of new questions that have been raised in the past two years about large-x parton distributions, as well as recent measurements which have improved the parton uncertainties. We perform several specific global fits to answer the outstanding questions, and to investigate the potential of future experiments, particularly at HERA, which may provide critical constraints on parton distributions at large x.

hep-ph↗

arXiv1999

Global QCD Analysis of Parton Structure of the Nucleon: CTEQ5 Parton Distributions

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.

hep-ph↗

arXiv1998

kt Effects in Direct-Photon Production

We discuss the phenomenology of initial-state parton-kt broadening in direct-photon production and related processes in hadron collisions. After a brief summary of the theoretical basis for a Gaussian-smearing approach, we present a systematic study of recent results on fixed-target and collider direct-photon production, using complementary data on diphoton and pion production to provide empirical guidance on the required amount of kt broadening. This approach provides a consistent description of the observed pattern of deviation of next-to-leading order QCD calculations relative to the direct-photon data, and accounts for the shape and normalization difference between fixed-order perturbative calculations and the data. We also discuss the uncertainties in this phenomenological approach, the implications of these results on the extraction of the gluon distribution of the nucleon, and the comparison of our findings to recent related work.

hep-ph↗

arXiv1998

Study of the Uncertainty of the Gluon Distribution

The uncertainty in the calculation of many important new processes at the Tevatron and LHC is dominated by that concerning the gluon distribution function. We investigate the uncertainty in the gluon distribution of the proton by systematically varying the gluon parameters in the global QCD analysis of parton distributions. The results depend critically on the parton momentum fraction x and the QCD scale Q^2. The uncertainties are presented for integrated gluon-gluon and gluon-quark luminosities for both the Tevatron and LHC as a function of sqrt(tau)=sqrt(x1x2)=sqrt(shat/s), the most relevant quantity for new particle production. The uncertainties are reasonably small, except for large x.

hep-ph↗

arXiv1997

Hera Events, Tevatron Jets, and the Uncertainty On Quarks At Large X

The recently reported excess of events at HERA compared to QCD calculations impels us to examine all possible Standard Model explanations before invoking ``new physics''. We explore the possibility of adding an unusual, but small, component of additional quarks at large x (beyond x>0.75) as a way to increase the predicted SM cross-section in the HERA kinematic region by the QCD evolution feed-down effect. We describe various scenarios under which this can be achieved while maintaining good global fits to all established data sets. This implies a much larger SM uncertainty than commonly assumed. In addition, the modified parton distributions provide another possible mechanism to account for the CDF high-pt jet excess which occurs at similar x and Q^2 values.

hep-ph↗

arXiv1997

Charm Production and Parton Distributions

Recent accurate data on F_2(x,Q) and on F_2^c(x,Q) from HERA at small-x require a more precise treatment of charm production in the global QCD analysis of parton distributions. We improve on existing global analyses by implementing the leptoproduction formalism of Aivazis et al. which represents a natural generalization of the conventional zero mass QCD parton framework to include heavy quark mass effects. We also perform analyses based on the fixed-flavor-number scheme, which is widely used in the literature, and demonstrate their uses and limitations. We discuss the implications of the improved treatment of heavy quark mass effect in practical applications of PQCD and compare our results with recent related works.

hep-ph↗

arXiv1996

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 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 $α_s$ is fully taken into account. A new generation of CTEQ parton distributions, CTEQ4, is presented. It consists of the three standard sets (\mbox{\small {$\overline {\rm MS}$}}, {\protect \small DIS} and leading order), a series that gives a range of parton distributions with corresponding $α_s$'s, and a set with a low starting value of $Q$. Previously obtained gluon distributions that are consistent with the high $E_t$ jet cross-section are also discussed in the context of this new global analysis.

hep-ph↗

arXiv1996

Large Transverse Momentum Jet Production and the Gluon Distribution Inside the Proton

The CDF experiment has reported an excess of high-$p_t$ jets compared to previous next-to-leading order QCD expectations. Before attributing this to new physics effects, we investigate whether these high-$p_t$ jets can be explained by a modified gluon distribution inside the proton. We find enough flexibility in a global QCD analysis including the CDF inclusive jet data to provide a 25-35\% increase in the jet cross sections at the highest $p_t$ of the experiment. Two possible sets of parton distributions are presented, and the effects of these on other existing data sets are presented. Further theoretical and experimental work needed to clarify unresolved issues is outlined.

hep-ph↗

arXiv1996

Comparison of CDF and D0 Inclusive Jet Cross-sections

The recently reported CDF and D0 inclusive jet cross-sections are compared, using a uniform theoretical NLO QCD calculation to account for the different kinematic coverages of the pseudo-rapidity variable in the two experiments. The two data sets are found to be in good agreement. With a 2-3\% relative overall normalization adjustment, the data sets appear to agree over the entire $E_t$ range, even without taking into account the other systematic errors.

hep-ph↗

arXiv1995

A GLOBAL QCD STUDY OF DIRECT PHOTON PRODUCTION

A global QCD analysis of the direct photon production process from both fixed target and collider experiments is presented. These data sets now completely cover the parton $x$ range from 0.01 to 0.6, thereby providing a stringent test of perturbative QCD and parton distributions. Previous detailed studies of direct photons emphasized fixed target data. We find most data sets have a steeper $p_t$ distribution than the QCD prediction. Neither global fits with new parton distributions nor improved photon fragmentation functions can resolve this problem since the deviation occurs at different $x$ values for experiments at different energies. A more likely explanation is the need for additional broadening of the $k_t$ of the initial state partons. The magnitude and the possible physical origin of this effect are investigated and discussed.

hep-ph↗

arXiv1994

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. 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.

hep-ph↗