1,778 research outputs found

    Chiral extrapolation of lattice moments of proton quark distributions

    Get PDF
    We present the resolution of a long-standing discrepancy between the moments of parton distributions calculated from lattice QCD and their experimental values. We propose a simple extrapolation formula for the moments of the nonsinglet quark distribution u-d, as a function of quark mass, which embodies the general constraints imposed by the chiral symmetry of QCD. The inclusion of the leading nonanalytic behavior leads to an excellent description of both the lattice data and the experimental values of the moments.Comment: 9 pages, 1 figure, to appear in Physical Review Letter

    Paying clinicians to join clinical trials : a review of guidelines and interview study of trialists

    Get PDF
    Background: The motivations of clinicians to participate in clinical trials have been little studied. This project explored the potential role of payment for participation in publicly funded clinical trials in the UK. The aims were to review relevant guidelines and to collate and analyse views of clinical trialists on the role of payments and other factors that motivated clinicians to join clinical trials. Methods: Review of guidelines governing payments to clinicians for recruitment to trials. Semistructured interviews with a range of NHS clinical trial leaders, analysed using qualititative methods. Results: While UK guidelines had little to say specifically on payments linked to recruitment, all payments have become highly regulated and increasingly transparent. Interview participants believed that expenses arising from research should be covered. Payments in excess of expenses were seen as likely to increase participation but with the risk of reducing quality. Motivations such as interest in the topic, the scope for patients to benefit and intellectual curiosity were considered more important. Barriers to involvement included bureaucracy and lack of time. Discussion: Limited scope exists for paying clinicians over-and-above the cost of their time to be involved in research. Most trialists favour full payment of all expenses related to research. Conclusion: Payment of clinicians beyond expenses is perceived to be a less important motivating factor than researching important, salient questions, and facilitating research by reducing bureaucracy and delay

    An Alternative Method to Obtain the Quark Polarization of the Nucleon

    Get PDF
    An alternate method is described to extract the quark contribution to the spin of the nucleon directly from the first moment of the deuteron structure function, g1dg^d_1. It is obtained without recourse to the use of input on the nucleon wave function from hyperon decays involving the flavor symmetry parameters, F and D. The result for the quark polarization of the nucleon, ΔΣN,\Delta\Sigma_ N, is in good agreement with the values of the singlet axial current matrix element, a0a_0, obtained from recent next-to-leading order analyses of current proton, neutron and deuteron data.Comment: 7 pages, 1 figur

    Gluon distributions in nucleons and pions at a low resolution scale

    Full text link
    In this paper we study the gluon distribution functions in nucleons and pions at a low resolution Q2Q^2 scale. This is an important issue since parton densities at low Q2Q^2 have always been taken as an external input which is adjusted through DGLAP evolution to fit the experimental data at higher scales. Here, in the framework of a model recently developed, it is shown that the hypothetical cloud of {\it neutral} pions surrounding nucleons and pions appears to be responsible for the characteristic valence-like gluon distributions needed at the inital low scale. As an additional result, we get the remarkable prediction that neutral and charged pions have different intrinsic sea flavor contents.Comment: final version to appear in Phys. Rev. D. Discussion on several points enlarge

    dbar/ubar Asymmetry and the Origin of the Nucleon Sea

    Full text link
    The Drell-Yan cross section ratios, σ(p+d)/σ(p+p)\sigma(p+d)/\sigma(p+p), measured in Fermilab E866, have led to the first determination of dˉ(x)/uˉ(x)\bar d(x) / \bar u(x), dˉ(x)uˉ(x)\bar d(x) - \bar u(x), and the integral of dˉ(x)uˉ(x)\bar d(x) - \bar u(x) for the proton over the range 0.02x0.3450.02 \le x \le 0.345. The E866 results are compared with predictions based on parton distribution functions and various theoretical models. The relationship between the E866 results and the NMC measurement of the Gottfried integral is discussed. The agreement between the E866 results and models employing virtual mesons indicates these non-perturbative processes play an important role in the origin of the dˉ\bar d, uˉ\bar u asymmetry in the nucleon sea.Comment: 5 pages, 3 figures, ReVTe

    Physics at a Neutrino Factory

    Full text link
    In response to the growing interest in building a Neutrino Factory to produce high intensity beams of electron- and muon-neutrinos and antineutrinos, in October 1999 the Fermilab Directorate initiated two six-month studies. The first study, organized by N. Holtkamp and D. Finley, was to investigate the technical feasibility of an intense neutrino source based on a muon storage ring. This design study has produced a report in which the basic conclusion is that a Neutrino Factory is technically feasible, although it requires an aggressive R&D program. The second study, which is the subject of this report, was to explore the physics potential of a Neutrino Factory as a function of the muon beam energy and intensity, and for oscillation physics, the potential as a function of baseline.Comment: 133 pages, 64 figures. Report to the Fermilab Directorate. Available from http://www.fnal.gov/projects/muon_collider/ This version fixes some printing problem

    Models for the Polarized Parton Distributions of the Nucleon

    Get PDF
    Polarized deep inelastic scattering (DIS) data are analyzed in leading and next-to-leading order of QCD within the common `standard' scenario of polarized parton distributions with a flavor-symmetric light sea (antiquark) distribution δqˉ\delta\bar{q}, and a completely SU(3)f_f broken `valence' scenario with totally flavor-asymmetric light sea densities (δuˉδdˉδsˉ)(\delta\bar{u}\neq\delta\bar{d}\neq\delta\bar{s}). The latter flavor-broken light sea distributions are modelled with the help of a Pauli-blocking ansatz at the low radiative/dynamical input scales of μLO(NLO)2=0.26\mu_{\rm LO(NLO)}^2=0.26 (0.40) GeV2^2 which complies with predictions of the chiral quark-soliton model and expectations based on the statistical parton model as well as with the corresponding, well established, flavor-broken unpolarized sea (dˉ>uˉ\bar{d}>\bar{u}). Present semi-inclusive DIS data cannot yet uniquely discriminate between those two flavor-symmetric and flavor-broken polarized light sea scenarios.Comment: 39 pages, LaTe

    Parton energy loss limits and shadowing in Drell-Yan dimuon production

    Get PDF
    A precise measurement of the ratios of the Drell-Yan cross section per nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is reported. The behavior of the Drell-Yan ratios at small target parton momentum fraction is well described by an existing fit to the shadowing observed in deep-inelastic scattering. The cross section ratios as a function of the incident parton momentum fraction set tight limits on the energy loss of quarks passing through a cold nucleus
    corecore