877 research outputs found

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

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

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 $Q_T$ of the pair. When $Q_T$ is much larger than the
pair's invariant mass $Q$, this factorization formula systematically resums the
logarithmic contributions of the type $\alpha_s^m \ln^m(Q_T^2/Q^2)$ to all
orders in the strong coupling $\alpha_s$. When $Q_T\sim Q$, 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
$\sqrt{S}$ is large enough and $Q_T\gg Q$, 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

The hadroproduction of lepton pairs with mass $Q$ and transverse momentum
$Q_T$ 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
$Q_T>Q/2$. 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

We introduce operator definitions for virtual photon fragmentation functions,
which are needed for reliable calculations of Drell-Yan transverse momentum
($Q_T$) distributions when $Q_T$ is much larger than the invariant mass $Q$. 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 $z$, and
the fragmentation functions to a transversely polarized virtual photon dominate
the large $z$ region. We discuss the implications of this finding to the
J/$\psi$ 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

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 $e^+ e^-$ 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 $m_{\tilde{b}_2} \le 120$ GeV.Comment: 22 pages, latex, 6 figure

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

We analise the implications of the measurement of $B$ and $J/\psi$ inclusive
\pt\ distributions performed in $p\bar p$ 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

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

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### Petrologic and oxygen-isotopic investigations of eucritic and anomalous mafic achondrites

The most common asteroidal igneous meteorites are eucrite-type basalts and gabbros rocks composed of ferroan pigeonite and augite, calcic plagioclase, silica, ilmenite, troilite, Ca-phosphate, chromite and Fe-metal. These rocks are thought to have formed on a single asteroid along with howardites and diogenites (HEDs). However, Northwest Africa (NWA) 011 is mineralogically identical to eucrites, but has an O-isotopic composition distinct from them and was derived from a different asteroid. Modern analyses with higher precision have shown that some eucrites have smaller O-isotopic differences that are nevertheless well-resolved from the group mean

### Spin Asymmetries In Diffractive $J/\Psi$ Leptoproduction

In this report we calculate the cross section and $A_{ll}$ asymmetry for the
diffractive $J/\Psi$ 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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