On parton distributions beyond the leading order

The importance of properly taking into account the factorization scheme dependence of parton distribution functions is emphasized. A serious error in the usual handling of this topic is pointed out and the correct procedure for transforming parton distribution functions from one factorisation scheme to another recalled. It is shown that the conventional $\overline{\rm {MS}}$ and DIS definitions thereof are ill-defined due to the lack of distinction between the factorisation scheme dependence of parton distribution functions and renormalisation scheme dependence of the strong coupling constant $\alpha_s$. A novel definition of parton distribution functions is suggested and its role in the construction of consistent next-to-leading order event generators briefly outlined.Comment: PRA-HEP-93/05, Latex, 10 pages and 2 Postscript figures appended at the end of this fil

Liquid Xenon Detectors for Positron Emission Tomography

PET is a functional imaging technique based on detection of annihilation photons following beta decay producing positrons. In this paper, we present the concept of a new PET system for preclinical applications consisting of a ring of twelve time projection chambers filled with liquid xenon viewed by avalanche photodiodes. Simultaneous measurement of ionization charge and scintillation light leads to a significant improvement to spatial resolution, image quality, and sensitivity. Simulated performance shows that an energy resolution of <10% (FWHM) and a sensitivity of 15% are achievable. First tests with a prototype TPC indicate position resolution <1 mm (FWHM).Comment: Paper presented at the International Nuclear Physics Conference, Vancouver, Canada, 201

Decisive test of color transparency in exclusive electroproduction of vector mesons

The exclusive production of vector mesons in deep inelastic scattering is a hard scattering process with the well controlled size of quark configurations which dominate the production amplitude. This allows an unambiguous prediction of color transparency effects in the coherent and incoherent production of vector mesons on nuclei. We demonstrate how the very mechanism of color transparency leads to a belated onset of color transparency effects as a function of $Q^{2}$. We conclude that the $Q^{2}$ dependence of the exclusive $\rho^{0}$-meson production on nuclei and nucleons observed in the Fermilab E665 experiment gives a solid evidence for the onset of color transparency. We propose the scaling relation between the $\rho^{0}$ and the $J/\Psi$ production, which further tests the mechanism of color transparency in exclusive (virtual) photoproduction.Comment: 11 pages, 4 figures on the request from [email protected], Juelich preprint KFA-IKP(Th)-1993-27. \phantom{.}\hspace{9cm}{\sl 8 November 1993

The Quark/Antiquark Asymmetry of the Nucleon Sea

Although the distributions of sea quarks and antiquarks generated by leading-twist QCD evolution through gluon splitting $g \rightarrow \bar q q$ are necessarily CP symmetric, the distributions of nonvalence quarks and antiquarks which are intrinsic to the nucleon's bound state wavefunction need not be identical. In this paper we investigate the sea quark/antiquark asymmetries in the nucleon wavefunction which are generated by a light-cone model of energetically-favored meson-baryon fluctuations. The model predicts striking quark/antiquark asymmetries in the momentum and helicity distributions for the down and strange contributions to the proton structure function: the intrinsic $d$ and $s$ quarks in the proton sea are predicted to be negatively polarized, whereas the intrinsic $\bar d$ and $\bar s$ antiquarks give zero contributions to the proton spin. Such a picture is supported by experimental phenomena related to the proton spin problem and the violation of the Ellis-Jaffe sum rule. The light-cone meson-baryon fluctuation model also suggests a structured momentum distribution asymmetry for strange quarks and antiquarks which could be relevant to an outstanding conflict between two different determinations of the strange quark sea in the nucleon. The model predicts an excess of intrinsic $d \bar d$ pairs over $u \bar u$ pairs, as supported by the Gottfried sum rule violation. We also predict that the intrinsic charm and anticharm helicity and momentum distributions are not identical.Comment: LaTex 18 pages, 4 figures. To obtain a copy, send e-mail to [email protected]

EPS09 - Nuclear PDFs and Their Uncertainties at NLO

In this talk, we present our recent next-to-leading order (NLO) nuclear parton distribution functions (nPDFs), which we call EPS09. As an extension to earlier NLO analyses, we supplement the deep inelastic scattering and Drell-Yan dilepton data by inclusive midrapidity pion measurements from RHIC in order to reduce the otherwize large freedom in the nuclear gluon densities. Our Hessian-type error analysis leading to a collection of nPDF error sets, is the first of its kind among the nPDF analyses.Comment: 4 pages, 4 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Flavor and Charge Symmetry in the Parton Distributions of the Nucleon

Recent calculations of charge symmetry violation(CSV) in the valence quark distributions of the nucleon have revealed that the dominant symmetry breaking contribution comes from the mass associated with the spectator quark system.Assuming that the change in the spectator mass can be treated perturbatively, we derive a model independent expression for the shift in the parton distributions of the nucleon. This result is used to derive a relation between the charge and flavor asymmetric contributions to the valence quark distributions in the proton, and to calculate CSV contributions to the nucleon sea. The CSV contribution to the Gottfried sum rule is also estimated, and found to be small

The sigma term and the quark number operator in QCD

We discuss the relationship of the forward matrix element of the operator $\bar\psi\psi$, related to the so-called sigma term, to the quark number. We show that in the naive quark model in the canonical formalism these quantities coincide in the limit of small average quark momenta. In the QCD parton model defined through light-front quantization this result is preserved at leading perturbative order but it receives radiative corrections. We analyze the theoretical and phenomenological consequences of this result, which provides a bridge between a current algebra quantity, the sigma term, and a deep-inelastic quantity, the parton number.Comment: 30 pages, 1 figure, DFTT-92-6 (April 1993

Flavor Asymmetry of the Nucleon Sea: Consequences for Dilepton Production

Parton distributions derived from a chiral quark model that generates an excess of down quarks and antiquarks in the proton's sea satisfactorily describe the measured yields of muon pairs produced in proton-nucleus collisions. Comparison of dilepton yields from hydrogen and deuterium targets promises greater sensitivity to the predicted flavor asymmetry.Comment: 11 pages, REVTEX, (Three PostScript figures available by anonymous ftp from fnth06.fnal.gov in directory /pub/Fermilab-Pub/92.264.) FERMILAB-PUB-92/264--T LBL-3298

Testing the meson cloud in the nucleon in Drell-Yan processes

We discuss the present status of the \bar u-\bar d asymmetry in the nucleon and analize the quantities which are best suited to verify the asymmetry. We find that the Drell-Yan asymmetry is the quantity insensitive to the valence quark distributions and very sensitive to the flavour asymmetry of the sea. We compare the prediction of the meson cloud model with different experimental data including the Fermilab E772 data and recent data of the NA51 Collaboration at CERN and make predictions for the planned Drell-Yan experiments.Comment: written in ReVTeX, 26 pages + 10 PS-figure