Shadowing and Antishadowing in Neutrino Deep Inelastic Scattering

The coherence of multiscattering quark nuclear processes leads to shadowing and antishadowing of the electromagnetic nuclear structure functions in agreement with measurements. This picture leads to substantially different antishadowing for charged and neutral current processes, particularly in anti-neutrino reactions, thus affecting the extraction of the weak-mixing angle $\sin^2\theta_W$.Comment: 4 pages, 1 figure, Prepared for the International Conference on QCD and Hadronic Physics, Beijing, China, June 16-20 200

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

Statistical Model for the Nucleon Structure Functions

A phenomenological model for the nucleon structure functions is presented. Visualising the nucleon as a cavity filled with parton gas in equilibrium and parametrizing the effects due to the finiteness of the nucleon volume, we obtain a good fit to the data on the structure function $F_2^p$. The model then successfully predicts other unpolarized structure function data.Comment: 11 pages, Latex, 3 ps figures. typographical errors in the values of a and b on page 6, and in the footnote were corrected. No other change

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

\bar u - \bar d asymmetry - a few remarks

We make a few remarks on possible sources of uncertainties of the $\bar d - \bar u$ asymmetry obtained by different methods and comment on its possible verification in the future. In addition we comment on its present understanding.Comment: 3 pages, a talk at the 7th International Workshop on Deep Inelastic Scattering and QCD (DIS99), Zeuthen, April 199

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]

Charge Symmetry Violating Contributions to Neutrino Reactions

The NuTeV group has measured charged and neutral current reactions for neutrinos on iron targets. Ratios of these cross sections provide an independent measurement of the Weinberg angle. The NuTeV value for sin^2 theta_W is three standard deviations larger than the value measured in other electroweak processes. By reviewing theoretical estimates of parton charge symmetry violation (CSV), we study CSV contributions to the NuTeV measurement. We conclude that charge symmetry violating effects should remove roughly 30% of the discrepancy between the NuTeV result and other determinations of sin^2 theta_W.Comment: 15 pages, 2 figures; Fig 2 replaced [CSV distribution calculated at low Q^2, evolved to 20 GeV^2]; table II change

OPEN CHARM PRODUCTION IN HADRONIC AND HEAVY-ION COLLISIONS AT RHIC AND LHC ENERGIES TO O(αs3)O(\alpha_s^3)

We present results on rapidity and transverse momentum distributions of inclusive charm quark production in hadronic and heavy-ion collisions at RHIC and LHC energies, including the next-to-leading order, $O(\alpha_s^3)$, radiative corrections and the nuclear shadowing effect. We determine the hadronic and the {\it effective} (in-medium) K-factor for the differential and total inclusive charm cross sections. We show that these K-factors have strong $p_T$ dependence. We discuss how measurements of charm production at RHIC and LHC can provide valuable information about the gluon density in a nucleus.Comment: LaTex, 4 pages, 2 figs (available on request) Talk presented at Eleventh International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions -- Quark Matter '95, Monterey, CA, January 9-13, 1995

The role of five-quark components in gamma decay of the Δ(1232)\Delta(1232)

An admixture of 10-20 % of qqqq\bar q components in the Delta(1232) resonance is shown to reduce the well known underprediction for the decay width for Delta(1232)->N gamma decay by about half and that of the corresponding helicity amplitudes from a factor ~ 1.7 to ~ 1.5. The main effect is due to the quark-antiquark annihilation transitions qqqq\bar q -> qqq gamma, the consideration of which brings the ratio A_{3/2}/A_{1/2} and consequently the E2/M1 ratio R_{EM} into agreement with the empirical value. Transitions between qqqq\bar q components in the resonance and the nucleon qqqq\bar q->qqqq\bar q gamma are shown to enhance the calculated width by only a few percent, as long as the probability of the qqqq\bar q component of the Delta(1232) and the proton is at most ~ 20 %. The transitions qqqq\bar q->qqqq\bar q gamma between the qqqq\bar q components in the Delta(1232) and the proton do not lead to a nonzero value for R_{EM}

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