877 research outputs found

    A jackknife variance estimator for self-weighted two-stage samples

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    Self-weighted two-stage sampling designs are popular in practice as they simplify field-work. It is common in practice to compute variance estimates only from the first sampling stage, neglecting the second stage. This omission may induce a bias in variance estimation; especially in situations where there is low variability between clusters or when sampling fractions are non-negligible. We propose a design-consistent jackknife variance estimator that takes account of all stages via deletion of clusters and observations within clusters. The proposed jackknife can be used for a wide class of point estimators. It does not need joint-inclusion probabilities and naturally includes finite population corrections. A simulation study shows that the proposed estimator can be more accurate than standard jackknifes (Rao, Wu, and Yue (1992)) for self-weighted two-stage sampling designs

    QCD Factorized Drell-Yan Cross Section at Large Transverse Momentum

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    We derive a new factorization formula in perturbative quantum chromodynamics for the Drell-Yan massive lepton-pair cross section as a function of the transverse momentum QTQ_T of the pair. When QTQ_T is much larger than the pair's invariant mass QQ, this factorization formula systematically resums the logarithmic contributions of the type αsmlnm(QT2/Q2)\alpha_s^m \ln^m(Q_T^2/Q^2) to all orders in the strong coupling αs\alpha_s. When QTQQ_T\sim Q, our formula yields the same Drell-Yan cross section as conventional fixed order QCD perturbation theory. We show that resummation is important when the collision energy S\sqrt{S} is large enough and QTQQ_T\gg Q, and we argue that perturbative expansions are more stable and reliable in terms of the modified factorization formula.Comment: 36 pages, latex, including 16 figure

    Low mass lepton pair production in hadron collisions

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    The hadroproduction of lepton pairs with mass QQ and transverse momentum QTQ_T can be described in perturbative QCD by the same partonic subprocesses as prompt photon production. We demonstrate that, like prompt photon production, lepton pair production is dominated by quark-gluon scattering in the region QT>Q/2Q_T>Q/2. This leads to sensitivity to the gluon density in kinematical regimes that are accessible both at collider and fixed target experiments while eliminating the theoretical and experimental uncertainties present in prompt photon production.Comment: Talk given by M. Klasen at the International Conference on the Structure and Interactions of the Photon, PHOTON 99, Freiburg i. Brsg., Germany, May 23-27, 1999. To be published in the proceedings. 6 pages, 6 postscript figure

    Virtual photon fragmentation functions

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    We introduce operator definitions for virtual photon fragmentation functions, which are needed for reliable calculations of Drell-Yan transverse momentum (QTQ_T) distributions when QTQ_T is much larger than the invariant mass QQ. We derive the evolution equations for these fragmentation functions. We calculate the leading order evolution kernels for partons to fragment into a unpolarized as well as a polarized virtual photon. We find that fragmentation functions to a longitudinally polarized virtual photon are most important at small zz, and the fragmentation functions to a transversely polarized virtual photon dominate the large zz region. We discuss the implications of this finding to the J/ψ\psi mesons' polarization at large transverse momentum.Comment: Latex, 19 pages including 6 figures. An error in the first version has been corrected, and references update

    Squark Mixing in Electron-Positron Reactions

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    Squark mixing plays a large role in the phenomenology of the minimal supersymmetric standard model, determining the mass of the lightest Higgs boson and the electroweak interactions of the squarks themselves. We examine how mixing may be investigated in high energy e+ee^+ e^- reactions, both at LEP-II and the proposed linear collider. In particular, off-diagonal production of one lighter and one heavier squark allows one to measure the squark mixing angle, and would allow one to test the mass relations for the light Higgs boson. In some cases off-diagonal production may provide the best prospects to discover supersymmetry. In the context of the light bottom squark scenario, we show that existing data from LEP-II should show definitive evidence for the heavier bottom squark provided that its mass mb~2120m_{\tilde{b}_2} \le 120 GeV.Comment: 22 pages, latex, 6 figure

    On the B and J/Psi Cross Section Measurements at Ua1 and CDF

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    We analise the implications of the measurement of BB and J/ψJ/\psi inclusive \pt\ distributions performed in ppˉp\bar p collisions by the UA1 and CDF experiments.Comment: 16 pages + 8 topdrawer figs included at the end, Latex, IFUP-TH 2/9

    Extraction of the pion distribution amplitude from polarized muon pair production

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    We consider the production of muon pairs from the scattering of pions on longitudinally polarized protons. We calculate the cross section and the single spin asymmetry for this process, taking into account pion bound state effects. We work in the kinematic region where the photon has a large longitudinal momentum fraction, which allows us to treat the bound state problem perturbatively. Our predictions are directly proportional to the pion distribution amplitude. A measurement of the polarized Drell-Yan cross section thus allows the determination of the shape of the pion distribution amplitude.Comment: 13 pages, using revtex, two figures added separately as one uuencoded Z-compressed fil

    Spin Asymmetries In Diffractive J/ΨJ/\Psi Leptoproduction

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    In this report we calculate the cross section and AllA_{ll} asymmetry for the diffractive J/ΨJ/\Psi leptoproduction. We study dependences of the asymmetry on the structure of the Pomeron-proton couplingComment: 4 pages, latex, two PS figures, presented at the International Workshop "Symmetry and Spin" PRAHA'9
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