Wu-Ki Tung

Parton distributions from a global QCD analysis of deep inelastic scattering and lepton pair production

Parton distribution functions consistent with neutrino and muon deep inelastic scattering as well as Drell-Yan pair production results have been extracted. This analysis incorporates experimental systematic errors which are the dominant errors in recent deep inelastic scattering experiments. The dependence of the results on factors such as kinematic cuts in the data, heavy target corrections, and choice of initial functional form are also explored. The form adopted is motivated by perturbative QCD and particularly useful in exploring the small-x extrapolation of the distributions. This is crucial for studying the range of predictions for Collider, HERA, and SSC/LHC cross sections. Representative distribution function sets are presented in a very compact parametrized form both in the DIS and MS-bar renormalization schemes.

Neutral and charged Higgs detection: Heavy quark fusion, top quark mass-dependence and rare decays

Abstract We examine the relevance of heavy quark fusion processes for production of both neutral and charged Higgs bosons. Top quark mass-dependence of these mechanisms as well as of gg fusion production of the neutral Higgs is explored. We find that gg fusion is generally larger than t t fusion for the neutral Higgs and even dominates WW/ZZ fusion in the region m H 0 ≤ 1 TeV for top masses above m t = 150 GeV. For charged Higgs boson production via t b fusion, the rate may be sufficiently large to permit its detection by making use of the H + → τ + ν τ decay mode; other distinctive final states, available in supersymmetric models, are also considered. Finally, we show that backgrounds to heavy quark fusion subprocesses can be reduced by tagging on a lepton emitted from the accompanying spectator t-quark.

Calculating heavy quark distributions

A systematic calculation of the evolution of parton distribution functions including the effects of heavy-quark masses is presented. The method involves the use of a special renormalization scheme which ensures ordinary massless evolution with the correct number of active quark flavors at all stages, and specifies appropriate matching conditions at thresholds. This method is applicable to all orders of the perturbation expansion in principle, and it is simple to implement in practice. Results of this calculation using known distributions at low energies as input are examined and compared with published results. The heavy-quark distribution functions are found to be about a factor of two larger than the well-known EHLQ results.

When Is a Heavy Quark Not a Parton? Charged Higgs Production and Heavy Quark Mass Effects in the QCD Based Parton Model

Abstract Several issues pertaining to the application of the QCD-based parton model to new physics processes involving heavy partons are described and quantitatively studied using charged Higgs boson production as a prime example. The naive parton model predictions are found to over-estimate the actual cross section by a factor of 2 to 5, depending on the top-quark and Higgs masses. The role of the top quark as a “parton” is examined by a detailed study of the cancellation between the straight parton model contribution and a subtraction term required by QCD corrections. The accuracy of the zero-mass method for evaluating the first-order QCD correction is assessed (in light of the potentially large mass of the top quark) by a quantitative analysis of the cancellation of mass singularities between the correction terms. A pragmatic procedure for calculation based on a renormalization scheme without the heavy quark-parton is formulated and compared with the usual perturbative QCD formalism. The energy ranges over which heavy quarks (or other particles) should or should not be naturally treated as “partons” are delineated. Properly evolved parton distribution functions relevant to the specific renormalization schemes considered are employed for all the numerical studies in order to ensure consistency in the QCD framework.

Structure function relations at large transverse momenta in Lepton-pair production processes

Abstract Features of the lepton angular distributions due to hard parton sub-processes in lepton-pair production at relatively large q⊥ are studied. Structure function relations which characterize these mechanisms are derived. Comparisons of these features with those derived from the Drell-Yan mechanism are made.

Small-x behavior of parton distribution functions in the next-to-leading order QCD parton model

Abstract The systematic application of the QCD-based parton model to the quantitative analysis of high energy processes requires reliable parton distributions including second-order QCD evolution effects — both for consistency and for accuracy, especially in exceptional kinematic regimes such as the small-x region. We discuss qualitative expectations and present quantitative results on the behavior of second-order evolved parton distribution in this region which is particularly important for studying physical processes at future accelerators. Direct comparisons with recently available second-order parton distributions are made in order to check consistency for future studies of high energy processes. Also presented are quantitative results on the scheme-dependence of parton distributions in the next-to-loading order approximation.

QCD asymptotics and kinematic thresholds in deep inelastic scattering

Abstract The Nachtmann moments of deep inelastic scattering structure functions are required by kinematics to contain a so far neglected threshold factor which is dependent on both n and q 2 . Its presence significantly affects the “moment analysis” in the usual QCD phenomenology and it resolves the difficulties connected with improper threshold behavior of the “ξ-scaling” analysis of structure functions.

Comparison of asymptotically free theories with high-energy deep inelastic scattering data

Abstract Comparison of the predictionsof asymptotically free gauge theories on ϑ 1n vW2(x, q2)|ϑ 1n q2 in the small x region with recent data on high-energy muon-hadron scattering allows a fairly direct determination of the effective coupling constant. The result is α(q2)≲0.21/[1 + 0.14 1n q2 (GeV2)]. This is much smaller than previous indirect estimates.

Probing the tri-boson coupling in high-energy vector-boson pair production

Abstract A method is presented for probing the effects of the tri-boson gauge coupling in order to test the standard model and to discover new physics. For vector-boson pair-production in electron-positron annihilation, appropriate helicity amplitudes are defined which confine the contributions of the t -channel fermion-exchange process (with known couplings) to a limited set with simple behaviour. The remaining amplitudes depend entirely on the s -channel vector-boson-exchange process with the associated tri-boson coupling. Angular correlation coefficients and asymmetry parameters in this helicity basis are more directly linked to the tri-boson couplings than in the conventional formalism.

Small x Physics at the SSC and the Tevatron

We examine the integrated rates and differential cross-sections for b-quark pair production and Drell-Yan processes at the SSC and the Tevatron colliders, and we determine their sensitivity to a range of possible small x behaviors for the parton distribution functions (PDF’s). We find that both the integrated rates and the differential distributions for the Drell-Yan process are very sensitive to the unknown small x behavior of the PDF’s; whereas, only the integrated rates can distinguish various input PDF’s in the b-pair production process. Experiments at the Tevatron can provide very useful information on small x physics if these two processes, especially Drell-Yan, can be measured.