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Universality of Unintegrated Gluon Distributions at small x

Description: We systematically study dijet production in various processes in the small-x limit and establish an effective kt-factorization for hard processes in a system with dilute probes scattering on a dense target. In the large-Nc limit, the unintegrated gluon distributions involved in different processes are shown to be related to two widely proposed ones: the Weizsacker-Williams gluon distribution and the dipole gluon distribution.
Date: January 4, 2011
Creator: Dominguez, Fabio; Marquet, Cyrille; Xiao, Bowen & Yuan, Feng
Partner: UNT Libraries Government Documents Department

Longitudinal double-spin asymmetry for inclusive jet production in vec p + vec p collisions at sqrt s = 200 GeV

Description: We report a new STAR measurement of the longitudinal double-spin asymmetry A{sub LL} for inclusive jet production at mid-rapidity in polarized p + p collisions at a center-of-mass energy of {radical}s = 200 GeV. The data, which cover jet transverse momenta 5 < p{sub T} < 30 GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit of polarized deep-inelastic scattering measurements.
Date: October 7, 2007
Creator: Coll, STAR
Partner: UNT Libraries Government Documents Department

Gluon density in nuclei

Description: In this talk we present our detailed study (theory and numbers) on the shadowing corrections to the gluon structure functions for nuclei. Starting from rather controversial information on the nucleon structure function which is originated by the recent HERA data, we develop the Glauber approach for the gluon density in a nucleus based on Mueller formula and estimate the value of the shadowing corrections in this case. Then we calculate the first corrections to the Glauber approach and show that these corrections are big. Based on this practical observation we suggest the new evolution equation which takes into account the shadowing corrections and solve it. We hope to convince you that the new evolution equation gives a good theoretical tool to treat the shadowing corrections for the gluons density in a nucleus and, therefore, it is able to provide the theoretically reliable initial conditions for the time evolution of the nucleus-nucleus cascade. The initial conditions should be fixed both theoretically and phenomenologically before to attack such complicated problems as the mixture of hard and soft processes in nucleus-nucleus interactions at high energy or the theoretically reliable approach to hadron or/and parton cascades for high energy nucleus-nucleus interaction. 35 refs., 24 figs., 1 tab.
Date: October 1996
Creator: Ayala, A. L.; Ducati, M. B. G. & Levin, E. M.
Partner: UNT Libraries Government Documents Department

Power corrections, renormalons and resummation

Description: I briefly review three topics of recent interest concerning power corrections, renormalons and Sudakov resummation: (a) 1/Q corrections to event shape observables in e(+)e(-) annihilation, (b) power corrections in Drell-Yan production and (c) factorial divergences that arise in resummation of large infrared (Sudakov) logarithms in moment or `real` space.
Date: August 1, 1996
Creator: Beneke, M.
Partner: UNT Libraries Government Documents Department

Strings and supersymmetry as tools for perturbative QCD

Description: We review techniques simplifying the analytic calculation of one-loop QCD amplitudes with many external legs, for use in next-to-leading-order corrections to multi-jet processes. We explain how a supersymmetry-inspired organization works well in conjunction with other tools, namely the color and helicity decompositions of amplitudes, and the constraints imposed by perturbative unitarity and collinear singularities. String theory seems most useful as a heuristic guide. Using these techniques, the complete set of one-loop five-parton QCD amplitudes, as well as certain sequences of special helicity amplitudes with an arbitrary number of external gluons, have been obtained.
Date: July 1, 1995
Creator: Dixon, L.
Partner: UNT Libraries Government Documents Department

Study of the uncertainty of the gluon distribution

Description: 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{sup 2}. The uncertainties are presented for integrated gluon-gluon and gluon-quark luminosities for both the Tevatron and LHC as a function of {radical}{tau}={radical}x{sub 1}x{sub 2}={radical}{cflx s}/s, the most relevant quantity for new particle production. The uncertainties are reasonably small, except for large x.
Date: July 1, 1998
Creator: Huston, J., FERMI
Partner: UNT Libraries Government Documents Department

Hadron mass scaling in Regge phenomenology

Description: The authors show that Regge phenomenology is consistent with the only scaling law for the hadron masses, M{sup *}/M = ({alpha}{prime}/{alpha}{prime}{sup *}){sup 1/2}, where asterisk indicates a finite-temperature quantity. Phenomenological models further suggest the following expression of the above scaling in terms of the temperature-dependent gluon condensate: M{sup *}/M = ({alpha}{prime}/{alpha}{prime}{sup *}){sup 1/2} = (<G{sub {mu}{nu}}{sup {alpha}}>{sup *}/<G{sup {alpha}{mu}{nu}}>){sup 1/4}.
Date: December 1998
Creator: Burakovsky, L.
Partner: UNT Libraries Government Documents Department

THE FIRST FERMI IN A HIGH ENERGY NUCLEAR COLLISION.

Description: At very high energies, weak coupling, non-perturbative methods can be used to study classical gluon production in nuclear collisions. One observes in numerical simulations that after an initial formation time, the produced partons are on shell, and their subsequent evolution can be studied using transport theory. At the initial formation time, a simple non-perturbative relation exists between the energy and number densities of the produced partons, and a scale determined by the saturated parton density in the nucleus.
Date: August 9, 1999
Creator: KRASNITZ,A.
Partner: UNT Libraries Government Documents Department

Next to leading order three jet production at hadron colliders

Description: I present results from a next-to-leading order event generator of purely gluonic jet production. This calculation is the first step in the construction of a full next-to-leading order calculation of three jet production at hadron colliders. Several jet algorithms commonly used in experiments are implemented and their numerical stability is investigated. A numerical instability is found in the iterative cone algorithm which makes it inappropriate for use in fixed order calculations beyond leading order.
Date: May 1, 1997
Creator: Kilgore, W.B.
Partner: UNT Libraries Government Documents Department

Naturalness and Higgs Decays in the MSSM with a Singlet

Description: The simplest extension of the supersymmetric standard model--the addition of one singlet superfield--can have a profound impact on the Higgs and its decays. We perform a general operator analysis of this scenario, focusing on the phenomenologically distinct scenarios that can arise, and not restricting the scope to the narrow framework of the NMSSM. We reexamine decays to four b quarks and four {tau}'s, finding that they are still generally viable, but at the edge of LEP limits. We find a broad set of Higgs decay modes, some new, including those with four gluon final states, as well as more general six and eight parton final states. We find the phenomenology of these scenarios is dramatically impacted by operators typically ignored, specifically those arising from D-terms in the hidden sector, and those arising from weak-scale colored fields. In addition to sensitivity of m{sub z}, there are potential tunings of other aspects of the spectrum. In spite of this, these models can be very natural, with light stops and a Higgs as light as 82 GeV. These scenarios motivate further analyses of LEP data as well as studies of the detection capabilities of future colliders to the new decay channels presented.
Date: November 21, 2005
Creator: Chang, Spencer; Fox, Patrick J. & Weiner, Neal
Partner: UNT Libraries Government Documents Department

EXPLORING THE POLARIZATION OF GLUONS IN THE NUCLEON.

Description: We give an overview of the current status of investigations of the polarization of gluons in the nucleon. We describe some of the physics of the spin-dependent gluon parton distribution and its phenomenology in high-energy polarized hadronic scattering. We also review the recent experimental results.
Date: October 22, 2007
Creator: STRATMANN,M. & VOGELSANG,W.
Partner: UNT Libraries Government Documents Department

A systematic study of J/psi suppression in cold nuclear matter

Description: Understanding the effects of cold nuclear matter on J/psi production is a key requirement in order to interpret the J/psi suppression reported in heavy-ion collisions. Based on a Glauber model, the J/psi-nucleon inelastic cross section is determined from a statistical analysis of J/psi world data on nuclear targets. The global fit of all data gives sigmaJ/psiN = 3.4 +/- 0.2 mb, significantly smaller than previous estimates, yet the chi2 of the fit is pretty large, chi2/ndf = 1.5. A similar value, sigmaJ/psiN = 3.5 +/- 0.2 +/- 1.7 mb, is obtained when the De Florian-Sassot modifications of the nuclear parton densities are included in the analysis, although we emphasize that the present uncertainties on gluon (anti)shadowing do not allow for a precise determination of sigmaJ/psiN. Finally, the possible energy dependence of the J/psi-nucleon inelastic cross section is investigated within this framework. No significant energy dependence of the J/psi-N interaction is observed.
Date: April 5, 2008
Creator: Arleo, Francois & Tram, Vi-Nham
Partner: UNT Libraries Government Documents Department

Search for squarks and gluinos using dileptons

Description: Gluinos and squarks can be produced in {ital p{anti p}} collisions at {radical}s = 1.8 TeV and decay via charginos and neutralinos to final states containing two (or more) leptons. There is no charge correlation between the leptons of charginos from gluino decays as gluinos are Majorana particles. The strategy of this analysis is to search for an excess of events containing two like-sign isolated leptons, missing energy, and jets. The analysis is based on 81 pb{sup -1} of 1994-5 data recorded at CDF.
Date: October 1, 1996
Creator: Done, J. P.
Partner: UNT Libraries Government Documents Department

The strong running coupling from an approximate gluon Dyson-Schwinger equation

Description: Using Mandelstam`s approximation to the gluon Dyson-Schwinger equation we calculate the gluon self-energy in a renormalisation group invariant fashion. We obtain a non-perturbative {Beta} function. The scaling behavior near the ultraviolet stable fixed point is in good agreement with perturbative QCD. No further fixed point for positive values of the coupling is found: {alpha}{sub S} increases without bound in the infrared.
Date: October 1, 1996
Creator: Alkofer, R.; Hauck, A. & Von Smekal, L.
Partner: UNT Libraries Government Documents Department

Elastic and transition form factors of mesons

Description: The Dyson-Schwinger equations [DSEs] provide a Poincare invariant, non-perturbative, renormalisable, continuum approach to solving gauge field theories, which also yields perturbation theory in the weak coupling limit. The elementary quantities in this approach are the Schwinger functions, which are the moments of the measure of the field theory. their analytic properties provide information about confinement and dynamical chiral symmetry breaking two-point quark and gluon Schwinger functions is a sufficient condition for confinement because it ensures the absence of quark and gluon production thresholds in color-singlet to singlet S-matrix amplitudes. These features make the approach well suited to studying hadronic observables and elucidating the observable effects of bound state substructure.
Date: July 1, 1996
Creator: Roberts, C.D.
Partner: UNT Libraries Government Documents Department

Gluonic three jet production at next to leading order

Description: I report results from a next-to-leading order event generator of purely gluonic jet production. This calculation, performed in collaboration with Walter Giele, is the first step in the construction of a full next-to-leading order calculation of three jet production at hadron colliders.
Date: August 1, 1996
Creator: Kilgore, W.B.
Partner: UNT Libraries Government Documents Department

The infrared behaviour of {alpha}{sub s} from Mandelstam`s approximation to the gluon Dyson-Schwinger equation

Description: Using Mandelstam`s approximation to the gluon Dyson-Schwinger equation we calculate the gluon self-energy in a renormalisation group invariant fashion. We obtain a non-perturbative {beta} function. The scaling behaviour near the ultraviolet stable fixed point is in good agreement with perturbative QCD. No further fixed point for positive values of the coupling is found, i.e., {alpha}{sub s} increases without bound in the infrared. The non-perturbative result for an infrared singular quark interaction can be used to relate the string tension {sigma} to the QCD scale {Lambda}. 3 refs., 1 fig.
Date: October 1, 1996
Creator: Hauck, A.; Alkofer, R. & Von Smekal, L.
Partner: UNT Libraries Government Documents Department

QCD tests at CDF

Description: We present results for the inclusive jet cross section and the dijet mass distribution. The inclusive cross section and dijet mass both exhibit significant deviations from the predictions of NLO QCD for jets with E{sub T}>200 GeV, or dijet masses > 400 GeV/c{sup 2}. We show that it is possible, within a global QCD analysis that includes the CDF inclusive jet data, to modify the gluon distribution at high x. The resulting increase in the jet cross-section predictions is 25-35%. Owing to the presence of k{sub T} smearing effects, the direct photon data does not provide as strong a constraint on the gluon distribution as previously thought. A comparison of the CDF and UA2 jet data, which have a common range in x, is plagued by theoretical and experimental uncertainties, and cannot at present confirm the CDF excess or the modified gluon distribution.
Date: February 1, 1996
Creator: Kovacs, E.
Partner: UNT Libraries Government Documents Department

Threshold resummation and the total cross section for top quark production

Description: We discuss the motivation for resummation of the effects of initial-state soft gluon radiation, to all orders in the strong coupling strength, for processes in which the near-threshold region in the partonic subenergy is important. We summarize our calculation of the total cross section for top quark production at hadron colliders. Comments are included on the differences between our treatment of subleading logarithmic terms and other methods.
Date: June 12, 1997
Creator: Berger, E.L. & Contopanagos, H.
Partner: UNT Libraries Government Documents Department

Improving the staggered quark action to reduce flavor symmetry violations

Description: The authors investigate a class of actions for lattice QCD with staggered quarks aimed at reducing the flavor symmetry violations associated with using staggered fermions. These actions replace the gauge field link fields in the quark action with covariantly smeared fields. As such they are an extension of actions considered by the MILC collaboration. They show that such actions systematically reduce flavor symmetry violations in the weak coupling limit. Using the mass splitting between Goldstone and non-Goldstone pions as a measure of flavor symmetry violations they find that these actions have considerably less flavor symmetry violations than the standard staggered action, and represent an improvement on what can be achieved with the MILC action, on quenched configurations with {beta} = 5.7.
Date: July 1, 1998
Creator: Lagaee, J.F. & Sinclair, D.K.
Partner: UNT Libraries Government Documents Department