120073991
Role of the U(1) ghost beyond leading order in a large-Nc expansion
The 1/Nc expansion is one of the very few methods we have for generating a systematic expansion of QCD at the energy scale relevant to hadron structure. The present formulation of this theory relies on the double-line notation for calculating the leading order of a diagram in the 1/Nc expansion, where the local SU(Nc) gauge symmetry is substituted by a U(Nc) symmetry and the associated U(1) ghost field is ignored. In the current work we demonstrate the insufficiency of this formulation for describing certain non-planar diagrams of interest in current attempts to model QCD. We derive a more complete set of Feynman rules that include the U(1) ghost field and provide a useful tool for calculating both color factors and 1/Nc orders of all color-singlet diagrams.
16608552
Kaon-nucleus Drell-Yan processes and kaon structure functions.
Abstract We investigate the information which could be obtained from Drell-Yan processes with sufficiently intense beams of charged kaons on isoscalar targets. It is found that combinations of K + and K − Drell-Yan measurements on isoscalar nuclear targets would allow one to extract the kaon sea quark distributions. These cross sections are also sensitive to the strange valence quark distribution in the kaon, although one would need a significant increase over the presently available kaon fluxes in order to extract this quantity with sufficient accuracy.
122078954
Proton pole contribution in proton decay
We show that nucleon pole terms contribute significantly to proton decay, increasing the branching ratio for e+π0 and νRπ+ modes. The total lifetime is decreased to the point where keeping the SU(5) value of the lifetime below the experimental upper bound forces us to the choice of QCD scale parameter and proton radius parameter which are near the edge of the allowed region.
18714425
Near-threshold Isospin Violation in the Pion Form Factor from Chiral Perturbation Theory
We examine the isospin violation in the timelike pion form factor near threshold at next-to-leading order in the chiral expansion using the techniques of Chiral Perturbation Theory. This next-to-leading order contribution contains the first nonvanishing isospin violation. This isospin violation is found to be very small near threshold. In particular, the isospin violation at threshold is found to be of order 10 4 , which should be compared with the few percent level seen in the vector
120110530
Deep inelastic scattering as a probe of nucleon and nuclear structure
Abstract We briefly review recent thinking about the meaning of various phenomenologlcal models of hadronic structure in the light of modern deep-inelastic scattering data. As a particularly topical example, we argue that a widely accepted explanation of the EMC effect, in terns of a change of confinement scale, is unlikely to be correct.
124728025
A microscopic understanding of the structure functions of finite nuclei
Abstract A number of relatively successful quark model descriptions of nuclear matter have recently been developed, based on a mean-field description of non-overlapping nucleon bags bound by the self-consistent exchange of scalar (σ) and vector (ω) mesons. The meson fields modify the internal quark motion in the nucleon bag in medium and this induces a saturation mechanism for nuclear matter. The model gives a nuclear compressibility around 220 MeV and provides a rather satisfactory interpretation of nuclear matter properties. Furthermore, it has recently been shown that the twist-2 valence quark distributions of the free nucleon calculated in the MIT bag model are in reasonable agreement with world data. By using a local density approximation we combine these developments to microscopically investigate the structure functions of finite nuclei. The effect of nuclear shadowing and antishadowing is taken into account phenomenologically. The model can quantitatively reproduce the existing experimental data - the so-called European Muon Collaboration (EMC) effect. We show that the quark degrees of freedom in nuclei and the final state interaction between the debris of the struck nucleon and the residual nucleus both play important roles in the nuclear structure functions. Thus the conventional treatment based on nucleon binding within the impulse approximation is quantitatively unreliable. We also study the effects of the self-coupling terms of the σ field and nucleon-nucleon correlations on some nuclear matter properties and the nuclear structure functions.
59806645
After Leibniz...: discussions on philosophy and artificial intelligence.
This is an edited transcript of informal conversations which we have had over recent months, in which we looked at some of the issues which seem to arise when artificial intelligence and philosophy meet. Our aim was to see what might be some of the fundamental principles of attempts to build intelligent machines. The major topics covered are the relationship of AI and philosophy and what help they might be to each other: the mechanisms of natural inference and deduction; the question of what kind of theory of meaning would be involved in a successful natural language understanding program, and the nature of models in AI research.
60200548
Information Processing Analysis of Visual Perception: A Review
Abstract : It is suggested that recent advances in the construction of artificial vision systems provide the beginnings of a framework for an information processing analysis of human visual perception. A review is made of some pertinent investigations which have appeared in the psychological literature, along with some of the salient and potentially useful theoretical concepts which have resulted from the attempts to build computer vision systems. An attempt is made to integrate these two sources of ideas to suggest some desirable structural and behavioral concepts which apply to both the natural and the artificial systems.
43984541
Performance of PWO-II Prototype Arrays for the EMC of PANDA
The response function of a 3 times 3 matrix of 200-mm-long PWO-II crystals has been measured with energy-marked photons up to 675 MeV energy. For the first time the crystals were readout individually with one large area avalanche photo diode of the second generation operating at a temperature of 0degC. The reconstructed electromagnetic shower distributions show excellent energy resolutions down to 40 MeV. Reducing the temperature and increasing the light collection by an additional large-area avalanche photo diode can achieve further improvement.
123483800
The Gerasimov-Drell-Hearn sum rule at MAMI
Abstract.An extended experimental program to investigate the Gerasimov-Drell-Hearn (GDH) sum rule and related partial reaction cross sections on proton and neutron has been carried out by the GDH collaboration at the electron accelerators MAMI (Mainz) and ELSA (Bonn). The GDH sum rule connects the helicity-dependent photoabsorption cross section with the anomalous magnetic moment of the nucleon. The GDH collaboration has measured the total cross section of circularly polarised photons with longitudinally polarised protons at MAMI and ELSA to check this sum rule experimentally. In addition partial reaction channels like pion, double pion and eta production were determined. This provides new information on the helicity-dependent excitation spectrum of the nucleon. With the help of partial wave analyses it is possible to extract new, complementary information on the broad, overlapping resonances in this energy region. The double polarisation observable E measured in this experiment enhances the smaller multipoles via interference terms. The analysis of our data provides a new possibility to study the photon couplings to the nucleon resonances, especially above the Δ resonance where many properties of the observed states (e.g., coupling constants, branching ratios, helicity amplitudes) are only poorly known. In this paper we present several new results from our measurements on polarised proton and deuteron targets.
121573165
Polarization reversal of proton spins in a solid-state target by superradiance
The new Bonn frozen spin target developed for spin physics experiments with tagged photons from the Bonn electron stretcher accelerator ELSA has successfully been used in pion photoproduction experiments. Proton polarization values of −99% and +94% in ammonia, −96% and +90% in butanol have been achieved in a dilution refrigerator at a magnetic field of 3.53 T. At a temperature below 100 mK and a magnetic field of 0.35 T a very fast spontaneous polarization reversal within a time interval of 800 μs has been observed. This effect occurred only at negative polarization and has been identified as a collective spontaneous emission — i.e. superradiance — in the two level proton spin system. Detailed investigations have been made in order to study whether this effect is useful for a fast polarization reversal of the entire sample. Up to now the highest polarization reversal from −90% to +60% has been obtained in a target volume of 6.5 cm3.