1,417 research outputs found

    Nuclear parton distribution functions and their uncertainties

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    We analyze experimental data of nuclear structure-function ratios F_2^A/F_2^{A'} and Drell-Yan cross section ratios for obtaining optimum parton distribution functions (PDFs) in nuclei. Then, uncertainties of the nuclear PDFs are estimated by the Hessian method. Valence-quark distributions are determined by the F_2 data at large x; however, the small-x part is not obvious from the data. On the other hand, the antiquark distributions are determined well at x~0.01 from the F_2 data and at x~0.1 by the Drell-Yan data; however, the large-x behavior is not clear. Gluon distributions cannot be fixed by the present data and they have large uncertainties in the whole x region. Parametrization results are shown in comparison with the data. We provide a useful code for calculating nuclear PDFs at given x and Q^2.Comment: 9 pages, REVTeX, 23 eps files, Phys. Rev. C in press. Nuclear PDF library is available at http://hs.phys.saga-u.ac.jp/nuclp.htm

    Production of J/ψJ/\psi-pairs at HERA-N\vec{{\rm N}}

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    The production of J/ψJ/\psi-pairs as a possible measure of the polarized gluon distribution ΔG(x)\Delta G(x) is studied for proton--nucleon collisions at \sqrt{s} =40\;\mbox{GeV}^2 (HERA-N\vec{{\rm N}}). Possibilities of reconstructing the helicity state of at least one of the J/ψJ/\psi's are critically reviewed. The observation of production asymmetries in the single polarized mode of HERA-N\vec{{\rm N}} is found to be not feasible.Comment: 8 pages, LATeX, 3 figures availabe as .uu-fil

    Extracting the Proton ubar content from pp->Direct Photon plus Jet Cross Sections

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    An analysis procedure is proposed to measure the antiquark distributions in the proton over the region 0.01 < x < 0.1. The procedure involves the measurement of high p_t asymmetric direct photon and jet final states in pp interactions. This measurement can be made at the RHIC collider running in pp mode at an energy of sqrt(s)=500 GeV/c. This analysis identifies a region of phase space where the contribution from quark-antiquark annihilation uncharacteristically approaches the magnitude of the contribution from the leading process, quark-gluon Compton scattering. The forward-backward angular asymmetry in the parton center of mass is sensitive to the antiquark content of the proton and the ubar parton density function can be extracted.Comment: 21 pages, 7 figure

    Experimental Evidence for Simple Relations between Unpolarized and Polarized Parton Distributions

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    The Pauli exclusion principle is advocated for constructing the proton and neutron deep inelastic structure functions in terms of Fermi-Dirac distributions that we parametrize with very few parameters. It allows a fair description of the recent NMC data on F2p(x,Q2)F^p_2(x,Q^2) and F2n(x,Q2)F^n_2(x,Q^2) at Q2=4GeV2Q^2=4 GeV^2, as well as the CCFR neutrino data at Q2=3Q^2=3 and 5GeV25 GeV^2. We also make some reasonable and simple assumptions to relate unpolarized and polarized quark parton distributions and we obtain, with no additional free parameters, the spin dependent structure functions xg1p(x,Q2)xg^p_1(x,Q^2) and xg1n(x,Q2)xg^n_1(x,Q^2). Using the correct Q2Q^2 evolution, we have checked that they are in excellent agreement with the very recent SMC proton data at Q2=10GeV2Q^2=10 GeV^2 and the SLAC neutron data at Q2=2GeV2Q^2=2 GeV^2.Comment: 17 pages,CPT-94/P.3032,latex,6 fig available on cpt.univ-mrs.fr directory pub/preprints/94/fundamental-interactions /94-P.303

    Scheme Independence of g1p(x,Q2)g_1^p (x, Q^2)

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    We work with two general factorization schemes in order to explore the consequences of imposing scheme independence on g1p(x,Q2)g_1^p (x, Q^2). We see that although the light quark sector is indifferent to the choice of a particular scheme, the extension of the calculations to the heavy quark sector indicates that a scheme like the MSˉ\bar{MS} is preferable.Comment: 11 pages, 2 figures. To appear in the Brief Reports of Phys. Rev.

    Small Momentum Evolution of the Extended Drell--Hearn--Gerasimov Sum Rule

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    We investigate the momentum dependence of the extended Drell-Hearn-Gerasimov sum rule. An economical formalism is developed which allows to express the extended DHG sum rule in terms of a single virtual Compton amplitude in forward direction. Rigorous results for the small momentum evolution are derived from chiral perturbation theory within the one-loop approximation. Furthermore, we evaluate some higher order contributions arising from Δ(1232)\Delta(1232) intermediate states and relativistic corrections. (2 figures available upon request).Comment: 12 pages, TeX, BUTP-92/51 and CRN-92-5

    Gamma ray astronomy and baryonic dark matter

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    Recently, Dixon et al. have re-analyzed the EGRET data, finding a statistically significant diffuse γ\gamma-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which γ\gamma-rays are produced through the interaction of high-energy cosmic-ray protons with cold H2H_2 clouds clumped into dark clusters - these dark clusters supposedly populate the outer galactic halo and can show up in microlensing observations. Our estimate for the halo γ\gamma-ray flux turns out to be in remarkably good agreement with the discovery by Dixon et al. We also address future prospects to test our predictions.Comment: 9 pages, 1 figure included, to appear in ApJ 510, L103 (1999

    Polarized and Unpolarized Nucleon Structure Functions from Lattice QCD

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    We report on a high statistics quenched lattice QCD calculation of the deep-inelastic structure functions F1F_1, F2F_2, g1g_1 and g2g_2 of the proton and neutron. The theoretical basis for the calculation is the operator product expansion. We consider the moments of the leading twist operators up to spin four. Using Wilson fermions the calculation is done for three values of κ\kappa, and we perform the extrapolation to the chiral limit. The renormalization constants, which lead us from lattice to continuum operators, are calculated in perturbation theory to one loop order.Comment: 17 pages, uuencoded postscript file. Renormalization constant of now include

    The Nucleon ``Tensor Charges'' and the Skyrme Model

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    The lowest moment of the twist-two, chiral-odd parton distribution h1(x)h_1(x) of the nucleon can be related to the so-called ``tensor charges'' of the nucleon. We consider the tensor charges in the Skyrme model, and find that in the large-NcN_c, SU(3)-symmetric limit, the model predicts that the octet isosinglet tensor charge, gT8g^8_T, is of order 1/Nc1/N_c with respect to the octet isovector tensor charge, gT3g^3_T. The predicted F/DF/D ratio is then 1/3, in the large-NcN_c limit. These predictions coincide with the Skyrme model predictions for the octet axial{\it axial} charges, gA8g^8_A and gA3g^3_A. (The prediction F/D=1/3F/D=1/3 for the axial charges differs from the commonly quoted prediction of 5/9, which is based on an inconsistent treatment of the large-NcN_c limit.) The model also predicts that the singlet tensor charge, gT0g^0_T, is of order 1/Nc1/N_c with respect to gT3g^3_T.Comment: 9 single-spaced pages, no figures, MIT-CTP-212

    A Self-Consistent Approach to Neutral-Current Processes in Supernova Cores

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    The problem of neutral-current processes (neutrino scattering, pair emission, pair absorption, axion emission, \etc) in a nuclear medium can be separated into an expression representing the phase space of the weakly interacting probe, and a set of dynamic structure functions of the medium. For a non-relativistic medium we reduce the description to two structure functions S_A(\o) and S_V(\o) of the energy transfer, representing the axial-vector and vector interactions. SVS_V is well determined by the single-nucleon approximation while SAS_A may be dominated by multiply interacting nucleons. Unless the shape of S_A(\o) changes dramatically at high densities, scattering processes always dominate over pair processes for neutrino transport or the emission of right-handed states. Because the emission of right-handed neutrinos and axions is controlled by the same medium response functions, a consistent constraint on their properties from consideration of supernova cooling should use the same structure functions for both neutrino transport and exotic cooling mechanisms.Comment: 33 pages, Te
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