Manfred F. Gari

Meson exchange currents in deuteron electro-disintegration

Abstract It is shown that the impulse approximation cross section for backward ed → enp at large momentum transfer near threshold is very much smaller than the empirical data. It is found that the conventional “Jankus theory” is inadequate at large momentum transfer because of the neglect of the3D1 component of the deuteron state. By taking into account the simplest meson exchange currents of pionic range the theoretical prediction of the cross section comes into reasonable agreement with the existing data.

Semiphenomenological synthesis of meson and quark dynamics and the E.M. structure of the nucleon

We show that an extended vector dominance model which incorporates quark dynamics at largeQ2 via perturbative QCD gives an excellent description of existing data on elastic electron-proton/neutron cross-sections. Results of a simultaneous fit of the nucleon form factorsGM,Ep,n to the cross-sections are given. Information is obtained about the QCD-scale parameterΛQCD, as well as on the range of applicability of perturbative QCD calculations. Constraints on the hadronic wavefunctions are also obtained.

The electromagnetic form factor of the neutron

Abstract It is shown that for a momentum transfer of Q 2 > 4 M 2 , the neutron electric form factor G n E dominates the magnetic one ( G n M ). Therefore G n E can be measured reliably at large momentum transfer. While the proton form factor is dominated by the Dirac-, the neutron form factor in contrast is dominated by the Pauli contribution. Therefore high-momentum measurements can separate both effects. In view of the importance of the nucleon form factors for the underlying strong interaction theory additional precise measurements in the region of momentum transfer: 1 GeV 2 Q 2 2 , are of the greatest importance.

The electric neutron form factor and the strange quark content of the nucleon

Abstract We have reanalyzed the electromagnetic form factor data of the nucleon within the QCD-VM model with special emphasis on the electric neutron form factor. We show that the strange quark content of the nucleon is of especial importance. Although, due to the Zweig rule, the φ does not couple to the nucleon directly and does not contribute to the asymptotic behaviour of the form factors, the strange quarks can give important contributions via two-step processes at intermediate momentum transfers. The analysis shows that the strange quark contributions can reduce the electric form factor of the neutron at low Q 2 , but have little effect on the asymptotic behaviour or on the other form factors.

Electromagnetic form factors of the nucleon from perturbative QCD and QCD sum rules

Abstract Perturbative quantum chromodynamics (QCD) and QCD sum rules are combined to calculate nucleon wave functions in the light-cone framework. The use of these wave functions leads to predictions for the Dirac form factors of the proton and neutron which are in remarkable agreement with experiment.

A first step towards realistic calculations of parity-mixing effects in heavy nuclei

Abstract As an example the circular polarization of the 482 keV line in 181Ta is calculated using semi-microscopic description of the nucleus (independent particle model with the effective residual interactions of the pairing plus quadrupole and octupole types) and taking into account short-range correlations via the generalized Bethe-Goldstone equation. The results obtained for various strong and weak nucleon-nucleon potentials are far away from agreement with other calculations done up to now. No weak interaction theory seems to be able to explain the experimental values.

Does the 3{ital N} force have a hard core?

The meson-nucleon dynamics that generates the hard core of the RuhrPot two-nucleon interaction is shown to vanish in the irreducible 3N force. This result indicates a small 3N force dominated by conventional light meson-exchange dynamics and holds for an arbitrary meson-theoretic Lagrangian. The resulting RuhrPot 3N force is defined in the appendix. A completely different result is expected when the Tamm-Dancoff/Bloch-Horowitz procedure is used to define the NN and 3N potentials. In that approach, (e.g. full Bonn potential) both the NN {\it and} 3N potentials contain non-vanishing contributions from the coherent sum of meson-recoil dynamics and the possibility of a large hard core requiring explicit calculation cannot be ruled out.

Short-range correlations and parity impurities

Abstract Two problems arising in calculating parity-violating effects in nuclei are discussed: the lack of reliable information on (i) weak parity-violating NN potentials and (ii) strong NN potentials at short relative distances and their influence on nuclear structure (short-range correlations). It is shown how to compute the latter reliably with the result that the parity admixtures depend strongly on the nucleon-nucleon potentials used. From this we conclude that the determination of the weak NN forces cannot be separated from the determination of the short-range behaviour of strong NN potentials.

Consistent meson-field-theoretical description of pp bremsstrahlung

A parameter-free and relativistic extension of the RuhrPot meson-baryon model is used to define the dominant isoscalar meson-exchange currents. We compute pp-bremsstrahlung observables below the

Sensitivity of pp-bremsstrahlung to meson-exchange currents☆

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