A. Accardi
Hampton University, Hampton, VA, USA

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Publications (10)0 Total impact

  • Article: Global parton distributions with nuclear and finite-Q^2 corrections
    J. F. Owens, A. Accardi, W. Melnitchouk
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    ABSTRACT: We present three new sets of next-to-leading order parton distribution functions (PDFs) determined by global fits to a wide variety of data for hard scattering processes. The analysis includes target mass and higher twist corrections needed for the description of deep inelastic scattering data at large x and low Q^2, and nuclear corrections for deuterium targets. The PDF sets correspond to three different models for the nuclear effects, and provide a more realistic uncertainty range for the d quark PDF, in particular, compared with previous fits. We describe the PDF error sets for each choice of the nuclear corrections, and provide a user interface for utilizing the distributions.
    12/2012;
  • Article: Moments of the longitudinal proton structure function F_L from global data in the Q^2 range 0.75-45.0 (GeV/c)^2
    P. Monaghan, A. Accardi, M. E. Christy, C. E. Keppel, W. Melnitchouk, L. Zhu
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    ABSTRACT: We present an extraction of the lowest three moments of the proton longitudinal structure function F_L from world data between Q^2 = 0.75 and 45 (GeV/c)^2. The availability of new F_L data at low Bjorken x from HERA and at large x from Jefferson Lab allows the first determination of these moments over a large Q^2 range, relatively free from uncertainties associated with extrapolations into unmeasured regions. The moments are found to be underestimated by leading twist structure function parameterizations, especially for the higher moments, suggesting either the presence of significant higher twist effects in F_L and/or a larger gluon distribution at high x.
    09/2012;
  • Article: Impact of nuclear dependence of R=\sigma_L/\sigma_T on antishadowing in nuclear structure functions
    V. Guzey, L. Zhu, C. Keppel, M. Eric Christy, D. Gaskell, P. Solvignon, A. Accardi
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    ABSTRACT: We study the impact of the nuclear dependence of R=\sigma_L/\sigma_T on the extraction of the F_2^A/F_2^D and F_1^A/F_1^D structure function ratios from the data on the \sigma^A/\sigma^D cross section ratios. Guided by indications of the nuclear dependence of R from the world data, we examine selected sets of EMC, BCDMS, NMC and SLAC data and find that F_1^A/F_1^D < \sigma^A/\sigma^D \leq F_2^A/F_2^D. In particular, we observe that the nuclear enhancement (antishadowing) for F_1^A/F_1^D in the interval 0.1 < x < 0.3 becomes significantly reduced or even disappears, which indicates that antishadowing is dominated by the longitudinal structure function F_L. We also argue that precise measurements of nuclear modifications of R and F_L^A have the potential to constrain the poorly known gluon distribution in nuclei over a wide range of x.
    06/2012;
  • Source
    Article: Impact of PDF uncertainties at large x on heavy boson production
    L. T. Brady, A. Accardi, W. Melnitchouk, J. F. Owens
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    ABSTRACT: We explore the sensitivity of W and Z boson production in hadronic collisions to uncertainties in parton distribution functions (PDFs) at large x arising from uncertainties in nuclear corrections when using deuterium data in global QCD fits. The W and Z differential cross sections show increasing influence of nuclear corrections at high boson rapidities, particularly for the d quark, which is diluted somewhat in the decay lepton rapidity distributions. The effects of PDF uncertainties on heavy W' and Z' bosons beyond the Standard Model become progressively more important for larger boson masses or rapidities, both in p-p collisions at the LHC and in p-pbar scattering at the Tevatron.
    10/2011;
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    Article: Constraints on the large-x d/u ratio from electron-nucleus scattering at x>1
    O. Hen, A. Accardi, W. Melnitchouk, E. Piasetzky
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    ABSTRACT: Recently the ratio of neutron to proton structure functions F_2n/F_2p was extracted from a phenomenological correlation between the strength of the nuclear EMC effect and inclusive electron-nucleus cross section ratios at x>1. Within conventional models of nuclear smearing, this "in-medium correction" (IMC) extraction constrains the size of nuclear effects in the deuteron structure functions, from which the neutron structure function F_2n is usually extracted. The IMC data determine the resulting proton d/u quark distribution ratio, extrapolated to x=1, to be 0.23 +- 0.09 with a 90% confidence level. This is well below the SU(6) symmetry limit of 1/2 and significantly above the scalar diquark dominance limit of 0.
    10/2011;
  • Source
    Article: Next-to leading order analysis of target mass corrections to structure functions and asymmetries
    L. T. Brady, A. Accardi, T. J. Hobbs, W. Melnitchouk
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    ABSTRACT: We perform a comprehensive analysis of target mass corrections (TMCs) to spin-averaged structure functions and asymmetries at next-to-leading order. Several different prescriptions for TMCs are considered, including the operator product expansion, and various approximations to it, collinear factorization, and xi-scaling. We assess the impact of each of these on a number of observables, such as the neutron to proton F_2 structure function ratio, and parity-violating electron scattering asymmetries for protons and deuterons which are sensitive to gamma-Z interference effects. The corrections from higher order radiative and nuclear effects on the parity-violating deuteron asymmetry are also quantified.
    08/2011;
  • Source
    Article: Uncertainties in determining parton distributions at large x
    A. Accardi, W. Melnitchouk, J. F. Owens, M. E. Christy, C. E. Keppel, L. Zhu, J. G. Morfin
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    ABSTRACT: We critically examine uncertainties in parton distribution functions (PDFs) at large x arising from nuclear effects in deuterium F2 structure function data. Within a global PDF analysis, we assess the impact on the PDFs from uncertainties in the deuteron wave function at short distances and nucleon off-shell effects, the use of relativistic kinematics, as well as the use of less a restrictive parametrization of the d/u ratio. We find that in particular the d-quark and gluon PDFs vary significantly with the choice of nuclear model. We highlight the impact of these uncertainties on the determination of the neutron structure function, and on W boson production and parton luminosity at the Tevatron and the LHC. Finally, we discuss prospects for new measurements sensitive to the d-quark and gluon distributions but insensitive to nuclear corrections.
    02/2011;
  • Source
    Article: New parton distributions from large-x and low-Q^2 data
    A. Accardi, M. E. Christy, C. E. Keppel, W. Melnitchouk, P Monaghan, J. G. Morfin, J. F. Owens
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    ABSTRACT: We report results of a new global next-to-leading order fit of parton distribution functions in which cuts on W and Q are relaxed, thereby including more data at high values of x. Effects of target mass corrections (TMCs), higher twist contributions, and nuclear corrections for deuterium data are significant in the large-x region. The leading twist parton distributions are found to be stable to TMC model variations as long as higher twist contributions are also included. The behavior of the d quark as x-->1 is particularly sensitive to the deuterium corrections, and using realistic nuclear smearing models the d-quark distribution at large x is found to be softer than in previous fits performed with more restrictive cuts. Comment: 31 pages, 8 figures. Minor corrections. References added. To appear in Phys.Rev.D
    11/2009;
  • Source
    Article: Hadron mass corrections in semi-inclusive deep inelastic scattering
    A. Accardi, T. Hobbs, W. Melnitchouk
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    ABSTRACT: We derive mass corrections for semi-inclusive deep inelastic scattering of leptons from nucleons using a collinear factorization framework which incorporates the initial state mass of the target nucleon and the final state mass of the produced hadron. The formalism is constructed specifically to ensure that physical kinematic thresholds for the semi-inclusive process are explicitly respected. A systematic study of the kinematic dependencies of the mass corrections to semi-inclusive cross sections reveals that these are even larger than for inclusive structure functions, especially at very small and very large hadron momentum fractions. The hadron mass corrections compete with the experimental uncertainties at kinematics typical of current facilities, and will be important to efforts at extracting parton distributions or fragmentation functions from semi-inclusive processes at intermediate energies. Comment: 22 pages, 6 figures; expanded discussion of kinematics and new scaling variable; appendix comparing collinear frames included; version to appear in JHEP
    07/2009;
  • Source
    Article: Target mass corrections for spin-dependent structure functions in collinear factorization
    A. Accardi, W. Melnitchouk
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    ABSTRACT: We derive target mass corrections (TMC) for the spin-dependent nucleon structure function g1 and polarization asymmetry A1 in collinear factorization at leading twist. The TMCs are found to be significant for g1 at large xB, even at relatively high Q^2 values, but largely cancel in A1. A comparison of TMCs obtained from collinear factorization and from the operator product expansion shows that at low Q^2 the corrections drive the proton A1 in opposite directions.
    09/2008;

Institutions

  • 2009
    • Hampton University
      Hampton, VA, USA
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