Search Results (52 total)

We study nuclear effects in charged current deep inelastic neutrino-iron scattering in the framework of a χ² analysis of parton distribution functions (PDFs). We extract a set of iron PDFs and show that under reasonable assumptions it is possible to constrain the valence, light sea, and strange quark distributions. Our iron PDFs are used to compute x_Bj-dependent and Q²-dependent nuclear correction factors for iron structure functions which are required in global analyses of free nucleon PDFs. We compare our results with nuclear correction factors from neutrino-nucleus scattering models and correction factors for ℓ^±-iron scattering. We find that, except for very high x_Bj, our correction factors differ in both shape and magnitude from the correction factors of the models and charged-lepton scattering.

Dihadron spectra in high-energy heavy-ion collisions are studied within the next-to-leading order perturbative QCD parton model with modified jet fragmentation functions due to jet quenching. High-p_T back-to-back dihadrons are found to originate mainly from jet pairs produced close and tangential to the surface of the dense matter. However, a substantial fraction also comes from jets produced at the center with finite energy loss. Consequently, high-p_T dihadron spectra are found to be more sensitive to the initial gluon density than the single hadron spectra that are more dominated by surface emission. A simultaneous χ² fit to both the single and dihadron spectra can be achieved within a range of the energy loss parameter ϵ₀=1.6–2.1  GeV/fm. Because of the flattening of the initial jet production spectra at sqrt[s]=5.5  TeV, high p_T dihadrons are found to be more robust as probes of the dense medium.

New data sets have recently become available for neutrino and antineutrino deep inelastic scattering on nuclear targets and for inclusive dimuon production in pp and pd interactions. These data sets are sensitive to different combinations of parton distribution functions in the large-x region and, therefore, provide different constraints when incorporated into global parton distribution function fits. We compare and contrast the effects of these new data on parton distribution fits, with special emphasis on the effects at large x. The effects of the use of nuclear targets in the neutrino and antineutrino data sets are also investigated.

The phase space slicing method of two cutoffs for next-to-leading-order Monte Carlo style QCD corrections has been applied to many physics processes. The method is intuitive, simple to implement, and relies on a minimum of process dependent information. Although results for specific applications exist in the literature, there is not a full and detailed description of the method. Herein such a description is provided, along with illustrative examples; details, which have not previously been published, are included so that the method may be applied to additional hard scattering processes.

The production of pairs of hadrons in hadronic collisions is studied using a next-to-leading-order Monte Carlo program based on the phase space slicing technique. Up-to-date fragmentation functions based on fits to CERN LEP data are employed, together with several versions of current parton distribution functions. Good agreement is found with data for the dihadron mass distribution. A comparison is also made with data for the dihadron angular distribution. The scale dependence of the predictions and the dependence on the choices made for the fragmentation and parton distribution functions are also presented. The good agreement between theory and experiment is contrasted with the case for single π⁰ production where significant deviations between theory and experiment have been observed.

Results for higher-order threshold enhancements in high-E_T jet production in hadron-hadron collisions are presented. Expressions are given for the next-to-next-to-leading order (NNLO) threshold corrections to the single-jet inclusive cross section at next-to-leading logarithmic (NLL) accuracy. The corrections are found to be small for the specific choice of E_T/2 for the factorization and renormalization scales, and the corrected cross section shows a substantial reduction of the scale dependence. A comparison to experimental results from the Fermilab Tevatron is presented.

The resummation of threshold logarithms for direct photon production cross sections in hadronic collisions is presented. The resummation is based on the factorization properties of the cross section and is formulated at next-to-leading logarithmic or higher accuracy. Full analytical and numerical results for the next-to-next-to-leading order expansion of the resummed cross section are given. A substantial reduction of factorization scale dependence is observed. A comparison to experimental results from the E-706 and UA-6 experiments is presented.

We discuss the phenomenology of initial-state parton-k_T 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 that provide empirical guidance on the required amount of k_T 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.

The uncertainty in the calculation of many important new processes at the Fermilab Tevatron and CERN 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 Q². The uncertainties are presented for integrated gluon-gluon and gluon-quark luminosities for both the Tevatron and LHC as a function of sqrt[τ]=sqrt[x₁x₂]=sqrt[ŝ/s], the most relevant quantity for new particle production. The uncertainties are reasonably small, except for large x.

Various jet observables in photoproduction are studied and compared to data from DESY HERA. The feasibility of using a dijet sample for constraining the parton distributions in the photon is then studied. For the current data the experimental and theoretical uncertainties are comparable to the variation due to changing the photon parton distribution set.

A new next-to-leading order Monte Carlo program for the calculation of jet cross sections in photoproduction is described. The contributions from both resolved and direct components are included to O(αα_s²). Properties of the predictions for various inclusive jet and dijet observables are discussed and comparisons with DESY HERA data are presented.

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 α_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 (MS̅ ), deep inelastic scattering (DIS), and leading order (LO)], 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.

Fully differential heavy quark contributions to the photon structure functions in deep inelastic scattering are computed in next-to-leading order QCD, including both the direct and resolved contributions. A variety of distributions are presented and discussed. Several of the distributions show marked differences between the resolved and direct cases due primarily to the presence of the gluon distribution in the former and lack thereof in the latter.

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.

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.

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.

The elements, theoretical basis, and experimental status of perturbative quantum chromodynamics are presented. Relevant field-theoretic methods are introduced at a nonspecialist level, along with a review of the basic ideas and methods of the parton model. This is followed by an account of the fundamental theorems of quantum chromodynamics, which generalize the parton model. Summaries of the theoretical and experimental status of the most important hard-scattering processes are then given, including electron-positron annihilation, deeply inelastic scattering, and hard hadron-hadron scattering, as induced both by electoweak interactions and by quantum chromodynamics directly. In addition, a discussion is presented of the global fitting approach to the determination of parton distributions in nucleons.

Little information is known about the polarization of gluons inside a longitudinally polarized proton. We investigate the sensitivity of photoproduction experiments with both beam and target longitudinally polarized to the polarization of the gluon distribution in the proton. We study the photoproduction of jets and heavy quarks and conclude that they are both sensitive to the gluon polarization.

Using an order-α_s calculation of the hadronic two-photon cross section a background study in the intermediate mass range of the Higgs boson is done for sqrt[s]=40 and 16 TeV. The effects of realistic cuts, including photon isolation, are examined.

We calculate the O(α_s) corrections to the process pp→W^±HX→lνγγX using a Monte Carlo approach. Complete spin correlations are included. We examine the size of QCD corrections before and after minimal cuts on the final state, for both the Superconducting Super Collider and the CERN Large Hadron Collider. By comparing the WH cross section to the single-W cross section, some uncertainties due to parton distributions may also be eliminated. Comparison of the theoretical cross section with an observed event rate may allow one to distinguish between the presence of a standard model or minimal supersymmetric model intermediate mass Higgs boson.

An order-α_s calculation of hadronic double-photon production is presented. The results are compared with data from both colliding beam and fixed target experiments. The calculation utilizes a combination of analytic and Monte Carlo integration methods which make it easy to calculate a variety of observables and impose experimental cuts.

An order-α_s calculation of pp^(-)→ZZ+X is presented. Results are given for the total cross section and differential distributions for Fermilab Tevatron, CERN Large Hadron and Collider, and Superconducting Super Collider energies. The calculation utilizes a combination of analytic and Monte Carlo integration methods which makes it easy to calculate a variety of observables and to impose experimental cuts.

A method for calculating the production of direct photons beyond the leading-logarithm approximation has been developed, utilizing a combination of analytic and Monte Carlo integration methods. The method is described and examples are given, including a comparison with experimental results for the inclusive single-photon invariant cross section and the photon-plus-jet cross section. The flexibility of the Monte Carlo technique makes it straightforward to calculate a variety of observables and to take into account experimental cuts while still retaining the next-to-leading-logarithm terms.

A Monte Carlo program containing both leading-logarithm and next-to-leading-logarithm contributions has been developed and used to study the photoproduction of large-transverse-momentum hadronic jets. Predictions are presented and discussed for a variety of observables including singlejet invariant cross sections, dijet cross sections, and angular distributions. For some observables the inclusion of the next-to-leading-logarithm terms dramatically lessens the dependency on the choice of the renormalization and factorization scales entering the calculation, thereby increasing the precision of the calculation. The flexibility of the Monte Carlo technique allows predictions for additional observables to be generated easily.

A common fit is performed to lepton-hadron deep-inelastic scattering and large-p_T direct-photon cross sections, using complete beyond-leading-logarithm QCD expressions. For direct-photon cross sections, theoretical uncertainties coming from the renormalization and factorization schemes are removed by applying the principle of minimal sensitivity. We find that the direct-photon data, especially from the recent fixed-target experiments, strongly constrain the gluon distribution function, whereas the deep-inelastic data determine the quark distribution functions and constrain the value of Λ_MS¯ (MS¯ denotes the modified minimal-subtraction scheme). Our analysis is a successful test of perturbative QCD, which provides a coherent and quantitative description of two very different reactions.