Search Results (52 total)

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.

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.

Expectations for the momentum distribution of nonperturbative charm and bottom quarks in the proton are derived from a variety of models for the Fock space wave function on the light cone.

Light strongly interacting supersymmetric particles may be treated as partonic constituents of nucleons in high energy scattering processes. We construct parton distribution functions for protons in which a light gluino is included along with standard model quark, antiquark, and gluon constituents. A global analysis is performed of a large set of data from deep-inelastic lepton scattering, massive lepton pair and vector boson production, and hadron jet production at large values of transverse momentum. Constraints are obtained on the allowed range of gluino mass as a function of the value of the strong coupling strength α_s(M_Z) determined at the scale of the Z boson mass. We find that gluino masses as small as 10 GeV are admissible provided that α_s(M_Z)≥0.12. Current hadron scattering data are insensitive to the presence of gluinos heavier than ∼100–150  GeV.

A next-to-leading-order (NLO) calculation of neutrino cross sections, including power-suppressed mass terms, is used to evaluate the Paschos-Wolfenstein ratio, in order to better assess the validity and significance of the NuTeV anomaly. We study the shift of sin⁡²θ_W obtained in calculations with parton distribution function sets that allow s(x)≠s̄(x), enabled by recent neutrino dimuon data from CCFR and NuTeV. The extracted value of sin⁡²θ_W is closely correlated with the strangeness asymmetry. Taken together with recent developments of possible isospin violation and electroweak effects, our results suggest that the new dimuon data, the Weinberg angle measurement, and other data sets used in global QCD parton structure analysis can all be consistent within the standard model. A full NLO analysis of the actual experimental measurement will help to clarify this issue further.

We develop a general method to quantify the uncertainties of parton distribution functions and their physical predictions, with emphasis on incorporating all relevant experimental constraints. The method uses the Hessian formalism to study an effective chi-squared function that quantifies the fit between theory and experiment. Key ingredients are a recently developed iterative procedure to calculate the Hessian matrix in the difficult global analysis environment, and the use of parameters defined as components along appropriately normalized eigenvectors. The result is a set of 2D eigenvector basis parton distributions (where d≈16 is the number of parton parameters) from which the uncertainty on any physical quantity due to the uncertainty in parton distributions can be calculated. We illustrate the method by applying it to calculate uncertainties of gluon and quark distribution functions, W boson rapidity distributions, and the correlation between W and Z production cross sections.

We apply the Lagrange multiplier method to study the uncertainties of physical predictions due to the uncertainties of parton distribution functions (PDF’s), using the cross section σ_W for W production at a hadron collider as an archetypal example. An effective χ² function based on the CTEQ global QCD analysis is used to generate a series of PDF’s, each of which represents the best fit to the global data for some specified value of σ_W. By analyzing the likelihood of these “alterative hypotheses,” using available information on errors from the individual experiments, we estimate that the fractional uncertainty of σ_W due to current experimental input to the PDF analysis is approximately ±4% at the Fermilab Tevatron, and ±8–10% at the CERN Large Hadron Collider. We give sets of PDF’s corresponding to these up and down variations of σ_W. We also present similar results on Z production at the colliders. Our method can be applied to any combination of physical variables in precision QCD phenomenology, and it can be used to generate benchmarks for testing the accuracy of approximate methods based on the error matrix.

When a large body of data from diverse experiments is analyzed using a theoretical model with many parameters, the standard error-matrix method and the general tools for evaluating errors may become inadequate. We present an iterative method that significantly improves the reliability of the error matrix calculation. To obtain even better estimates of the uncertainties on predictions of physical observables, we also present a Lagrange multiplier method that explores the entire parameter space and avoids the linear approximations assumed in conventional error propagation calculations. These methods are illustrated by an example from the global analysis of parton distribution functions.

Jets observed in hadron-hadron scattering contain a contribution from the “underlying event” that is produced by spectator interactions taking place incoherently with the major parton-parton collision, due to the extended composite structure of the colliding hadrons. Using a recent measurement of the double parton interaction rate, we calculate that the underlying event may be 2–3 times stronger than generally assumed, as a result of semi-hard perturbative multiple-parton interactions. This can have an important influence on the inclusive jet cross section at moderate values of E_T, persisting at the 5–10 % level to the largest observable E_T. We show how the underlying event can be measured accurately using a generalization of a method first proposed by Marchesini and Webber.

QCD predicts soft radiation patterns that are particularly simple for W+ jet production. We demonstrate how these patterns can be used to distinguish between the parton-level subprocesses probabilistically on an event-by-event basis. As a test of our method we demonstrate correlations between the soft radiation and the radiation inside the outgoing jet.

Mass measurements of objects that decay into hadronic jets, such as the top quark, are shown to be improved by using a variant of the k_t jet algorithm in place of standard cone algorithms. The possibility and importance of better estimating the neutrino component in tagged b jets is demonstrated. These techniques will also be useful in the search for Higgs boson →bb̅ .

Two possible ways to improve the mass resolution for observing hadronic top quark decay t→bW→3 jets are studied: (1) using fixed cones in the rest frames of the t and W to define the decay jets, instead of the traditional cones in the rest frame of the detector; and (2) using the jet angles in the top rest frame to measure m_t/m_W. By Monte Carlo simulation, the second method is found to give a useful improvement in the mass resolution. It can be combined with the usual invariant mass method to get even better mass resolution. The improved resolution can be used to make a more accurate determination of the top quark mass, and to improve the discrimination between tt¯ events and background for studies of the production mechanism. © 1996 The American Physical Society.

We present an optimized and physically motivated method for separating top quark signal events from background events at the Fermilab Tevatron. For the top quark signal tt¯→e/μ+4 jets, we show how to reject all but 25% of the background in a data sample while retaining 83% of the signal, without introducing bias into the subsequent mass measurement. The technique used is the binary decision tree. Combining this highly efficient procedure for signal identification with a novel algorithm for top quark reconstruction, we propose a powerful new way to measure the top quark mass.

I extend the two-gluon exchange picture of elastic scattering, known as the Low-Nussinov or subtractive quark model, to predict cross sections for double Pomeron exchange processes. In particular, I calculate pp¯→pp¯qq¯ where the qq¯ partons will appear as jets separated from the final p and p¯ by large gaps in rapidity. The predicted cross section is large enough that this process should be observable at the Fermilab Tevatron and at the CERN Large Hadron Collider. It can be distinguished from the background of ordinary jet production by an absence of particles produced in the gap regions.

We present a program to measure the parton densities in the Pomeron using diffractive deep inelastic scattering and diffractive photoproduction, and to test the resulting parton densities by applying them to other processes such as the diffractive production of jets in hadron-hadron collisions. Since QCD factorization has been predicted not to apply to hard diffractive scattering, this program of fitting and using parton densities might be expected to fail. Its success or failure will provide useful information on the space-time structure of the Pomeron.

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.

We examine the phenomenology of particle multiplicity distributions, with special emphasis on the low multiplicities that are a background to the study of rapidity gaps. In particular, we analyze the multiplicity distribution in a rapidity interval between two jets, using the herwig QCD simulation with some necessary modifications. The distribution is not of the ‘‘negative binomial’’ form, and displays an anomalous enhancement at zero multiplicity. Some useful mathematical tools for working with multiplicity distributions are presented. It is demonstrated that ignoring particles with p_⊥<0.2 GeV/c has theoretical advantages, in addition to being convenient experimentally.

A new method to identify the gluon jet in 3-jet "Y" decays of Z⁰ is presented. The method is based on differences in particle multiplicity between quark jets and gluon jets, and is more effective than tagging by leptonic decay. An experimental test of the method and its application to a study of the "string effect" are proposed. Various jet-finding schemes for 3-jet events are compared.

I describe possible ways to discover hard double Pomeron exchange (HDPE) with the existing detectors at the Fermilab Tevatron, by using the small-angle "luminosity" counters as a veto. Estimates of the cross sections and backgrounds are made. In addition to the intrinsic importance of HDPE, its observation would be useful for calibrating the detectors, and for estimating the "survival probability" of rapidity gaps.

A method is described to observe two-jet hadronic decays of W^± and Z⁰ at hadron colliders such as the Fermilab Tevatron. The method will allow the rather accurately known masses of W and Z to be used as benchmarks to calibrate both the detector and the QCD Monte Carlo model of hadron shower development. It will also provide an independent, though not precise, measurement of M_W-M_Z. The method is based on suppressing the large QCD background by means of a cut on the decay angle of the W or Z, a cut on the p_T of the two-jet system, and a cut on the form of the jets observed in a calorimeter detector, to remove gluons. Along with reducing the background, these cuts significantly improve the jet+jet mass resolution. A simple trigger signature is described that can be used to avoid recording unnecessarily many background events.

A method to distinguish between light-quark jets and gluon jets in a calorimeter detector is presented. The method could be used to refine the experimental test of QCD based on the single-jet cross section, by comparing the quark and gluon contributions separately. It could also be used to enhance the signal/background ratio in searching for the top quark in W^±+jets channels. Optimistically, it might even make it possible to observe the hadronic decays W^±→jj and Z⁰→jj, which would allow the W and Z masses to be used as benchmarks for calibrating the calorimeter, and perhaps even provide a useful measurement of M_W/M_Z. The method has been developed using Monte Carlo simulations of jet events. Ways to compare it with experiment are discussed. The comparison would provide an important test for QCD shower simulation models.

Contributions to neutrino scattering which are proportional to 1/Q² are estimated using vector and axial-vector meson dominance. They are found to be potentially larger than previous estimates of nonleading twist. They lead to an additional theoretical uncertainty, on the order of _-0.00^+0.01, in the value of sin²θ_W which is extracted from the neutrino cross-section ratio σ^NC/σ^CC. This suggests that the mass of the top quark is likely to lie near the lower end of its currently allowed range in the minimal standard model.

It has been suggested that the lifetime of the τ lepton can be calculated using perturbative QCD, with sufficient accuracy that comparison with the measured lifetime can be used to determine the QCD scale parameter Λ. Contrary to this suggestion, nonperturbative effects are shown here to introduce uncertainties in the calculated lifetime which are so large as to make the determination of Λ unreliable. These nonperturbative effects are associated with hadronic thresholds and resonances, and are much larger than previous estimates based on QCD sum rules. Independent experimental tests, which make use of the hadronic mass spectrum in τ decay, are proposed.

A Comment on the Letter by E. Braaten, Phys. Rev. Lett. 60, 1606 (1988).

When vector bosons W^± or Z⁰ are produced in hadronic interactions, the quarks from their decay can interact via gluon exchange with spectator quarks or gluons from the original hadrons. The size of this effect is calculated in a specific model and found to be small. Arguments are presented that actual rescattering may be larger than the prediction of this model, and experimental signatures of rescattering are discussed.