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 mesonexchange 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 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.

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 π−production threshold using a relativistic hadronic current density that includes impulse, wave function re-orthonormalization, meson-recoil, NN creation and annihilation, ρπγ + ωπγ + ρηγ + ωηγ vector-meson decay and N∆γ(π, ρ) exchange currents. We obtain a good description of the available data. The N∆γ(π) current is shown to dominate the large two-body contributions and closed-form expressions for various non-relativistic approximations are analyzed. An experimental sensitivity to the admixture of pseudo-scalar and pseudo-vector admixture of the NNπ interaction is demonstrated. We examine the Lorentz invariance of the NN ⇀ ↽NN t-matrices and show a dominantly pseudo-vector NNπ coupling renders impulse approximation calculations without boost operators to be essentially exact. Conversely, a similar analysis of the ∆N ⇀ ↽NN transitions shows that boost operators and the two-body N ∆γ wave function re-orthonormalization meson-recoil currents are required in NN, ∆N and ∆∆ coupled channel t-matrix applications. The need for additional data is stressed.

We present pp-bremsstrahlung cross section calculations at the π-production threshold to search for kinematics that best differentiate model-dependent descriptions of the NN t-matrix and accentuate the contributions of isoscalar meson-exchange currents. Existing optimization procedures are shown to be completely unreliable due to the neglect of meson-exchange currents and phase space variations. We show that phase-space hides the most important differences in model t-matrices, but that improved data will be critical to further investigations of exchange currents.

Abstract. We describe a relativistic approach to the calculation of nucleon-nucleon Bremsstrahlung, where all meson-baryon and meson-baryon-photon interactions can be calculated consistently and microscopically. In this first relativistic approach to the problem, we present numerical results including both single-scatter and rescatter contributions with a relativistic current density, within a model where the explicit photon coupling to meson-exchange currents are small. The need for high precision (p, PY) measurements is stressed. PACS: 13.75.Cs; 25.10.+s; 27.30.+y The consistent microscopic prediction of (p, PT) observables requires the calculation of the photon coupling to an interacting meson-baryon system. For processes where the photon couples to a free nucleon before or after strong-interactions, one can express the (P, PT) invariant amplitude in the two-potential formalism of GellMann and Goldberger [1], consisting of a one-body nuclear current density and an off-shell NN T-matrix. The essential advantage of this approach is that the NN Tmatrix is defined to all orders in the strong-interaction coupling constant. All recent (p, p 7) calculations use this two-potential formalism, and obtain the NN T-matrices from the Lippmann-Schwinger [24] or BlankenbeclerSugar [5,6] equations for a non-relativistic boson-exchange potential such as Bonn [7] or Paris [8], or from inverse-scattering methods [9], and therefore include only the positive frequency components of the two-nucleon wavefunctions. Under this restriction, only the positive frequency processes that are shown in Fig. 1 are included.In all existing (p, P7) calculations, where the non-relativistic strong-interaction models neglect the negativefrequency components of the off-shell nulceons, it is necThis work is supported by COSY-KFA Jfilich (41140512) and Deutsche Forschungsgemeinschaft (Ga 153/11-4) essary to apply some kind of non-relativistic reduction to the current operator. This may take the form [2, 41 of a Foldy Wouthuysen transformation to obtain a nonrelativistic current with 'relativistic' spin corrections, which are retained to some approximate order, according to where the infinite p/m-expansion is truncated. In a more approximate scheme, it may take the form [6] of a direct Pauli reduction. These two approaches lead to differences of order 7% in the (p, pT) cross section at 280 MeV [10], and differ from the results that are obtained with a completely non-relativistic current density [3,5] by as much as 15% [-2, 6]. The corresponding differences in the spin-observables are generally even larger.This suggests the need for relativistic wave functions in (p, PT) calculations, so that the relativistic current density can be calculated without recourse to any (p/m)_ expansion, and the off-shell two-nucleon state can be properly described as including both its the positive and negative frequency contributions, as shown in Fig. 1. In the Feynman-Stfikelberg interpretation, the negative frequency contributions represent NN creation and annihilat...

The Ca(p, n) reaction cross section has been measured with tagged photons with energies ranging from 31.2 to 102 MeV, for neutron emission angles of 45', 60', 75', 90', and 135'. Exclusive difFerential cross sections for the Ca(p, n) andCa(p, p) reactions are found to be of a similar magnitude, but are not well predicted by recent random-phase-approximation and relativistic direct knock-out calculations. A quasideuteron model calculation with spin-orbit splitting of single-particle shells and an improved nucleon transparency correction is found to be consistent with the data. However, it is shown that this does not establish that the photoabsorption matrix element for the deuteron is necessarily similar to that of a correlated np-pair in nuclear matter. This result is generalized to question the predicted sensitivity to short-range correlations for photonuclear reactions calculated within the Gottfried factorization.