M. Dillig

Microscopic description of the A(p, π)A + 1 reaction

We have developed a flexible and thoroughly tested computer program for microscopic calculations of proton-induced pion production in nuclei at bombarding energies up to 500 MeV. The model used includes explicitly both the one-nucleon (pionic stripping) mechanism and the resonant p-wave rescattering part of the two-nucleon mechanism. Initial and final state interactions are included through proton-nucleus and pion-nucleus optical model distortions, with careful consideration of the multiple scattering series. The intermediate delta in the two-nucleon mechanism is treated as static with possible inclusion of a local density approximation for the delta-nucleus interaction. We explain the philosophy of the model, detail the parametrization of the microscopic dynamics, and give the formalism for the calculation of experimental observables. We also describe the testing of the computer program and present preliminary results for the 3He(p, π+)4He reaction.

Chiral symmetry and the near threshold pion-induced 2π production on the nucleon: (I). General formalism☆

Abstract The exclusive πN → ππN reaction around the Δ-isobar region provides an excellent testing ground of chiral symmetry. As a first step, a diagrammatical approach to the π − p → π + π − n reaction is formulated based on Weinbergs chiral lagrangian. Tested are the role of various dynamical elements, particularly the significance of nonlinear contributions in the pion field as a signature of chiral symmetry.

Chiral symmetry and the near-threshold pion-induced 2π production on the nucleon: II. Relativistic corrections and ππ initial- and final-state interactions☆

Abstract Some extensions of a model for the π N → ππ N reaction, presented in a preceding paper, are outlined: the leading relativistic corrections to the static treatment of the baryons are derived and the main features of the ππ interaction in the initial and final state are included via σ- and ϱ-meson exchange. Besides a detailed comparison with the total cross section, the discussion focuses on the sensitivity of exclusive ππ angular distributions on relativistic and initial- and final-state corrections.

Magnetic dipole spin-flip excitations in complex nuclei and the virtual Δ-isobar admixture amplitude in Jπ=1+ states☆

Abstract In the framework of a two-state model and a meson exchange model we estimate the strength of the virtual Δ(1232)-hole admixture in the wave function of J π =1 + states for different nuclei. Consistently, both models predict a strength of about 4% for the Δ amplitude in qualitative agreement with a recent estimate of Gaarde. We briefly comment on the discrepancy of this result to a recent analysis of inelastic pion scattering data.

Energy dependence of the 3He(p, π+)4He and 3H(n, π−)4 He reactions

Abstract We examine in detail the energy dependence of the various components of a microscopic calculation of the 3 He(p, π + ) 4 He reaction from the near-threshold region ( T lab p = 178 MeV, T c.m. π = 10.5 MeV) to an energy at which the Δ 1232 resonance should clearly dominate the reaction mechanism ( T lab p = 325 MeV, T c.m. π = 110 MeV) and demonstrate the sensitivity of the calculations to pion and proton distortions. The discrepancy in size between the calculated and measured cross sections at the lower energies ( T lab p = 178 and 200 MeV), persisting in two earlier publications, is found to disappear when Coulomb effects are properly taken into account. Our present calculations give a reasonably good description of the measured cross section and analyzing-power data at these lower energies and predict a smooth energy dependence of the cross section at the higher energies reflecting the increasing importance of the Δ-resonance. Calculations for the charge-symmetric 3 H(n, π − ) 4 He reaction are compared with existing cross-section data for the time-reversed 4 He(π − , n) 3 H reaction at the higher energies using detailed balance, while differences between the 3 He(p, π + ) 4 He and 3 H(n, π − ) 4 He reactions due to Coulomb effects are investigated.

The 3He(p, π+)4He reaction near threshold

Abstract We apply a recently developed microscopic model of nuclear pion production to the 3 He ( p , π + ) 4 He reaction at T p = 200 MeV and show the approximate equivalence for this reaction of a one-nucleon mechanism (ONM) calculation with full proton and pion distortions to a more microscopic calculation involving the coherent sum of the ONM amplitude with proton and nonresonant pion distorted waves and the pion-exchange projectile-emission part of the two-nucleon mechanism amplitude with proton and full pion distortions. A full microscopic calculation including target-emission and ρ-meson exchange contributions describes qualitatively the shapes of the differential cross section and analyzing power angular distributions but underestimates the magnitude of the measured cross sections. The sensitivity of the calculations to D-state admixtures in the bound-state wave functions is shown to be small.

Chiral symmetry and the near threshold pion-induced 2π production on the nucleon: (III). Perturbative estimate of initial and final state interactions

Abstract We investigate initial and final state interactions in the exclusive reaction πN → π πN near the 2π-threshold. Based on a diagrammatical expansion of the transition amplitude we estimate the leading loop and triangle diagrams in lowest order. We find, that for monopole cutoff masses Λ ⩽ 800 MeV (corresponding to radii of R ⩾ 0.7 fm in the cloudy bag model) at the different vertices, both contributions are at least one order of magnitude suppressed compared to the leading non-loop contributions to the total πN → π πN cross section.

Theoretical aspects of proton-induced exclusive pion and photo-production on nuclei

Abstract The status of exclusive single-nucleon transfer processes on nuclei at medium energies, with emphasis on the ( p , π ) and the (p,γ)process, is presented. Various models on the market are discussed with respect to their underlying physical ideas and, very briefly, their successes and failures. Fragmentary suggestions are added to utilize characteristic trends in the experimental data for the future model building.

Nonlinear σ,δ Couplings in a Relativistic Mean Field Theory for Neutron Star Matter

We study dense hadronic matter in a generalized relativistic mean field approach which contains nonlinear couplings of the σ, ω, ρ, δ fields and compare its predictions for properties of neutron stars with the corresponding results from different models found in the literature. Our predictions indicate a substantial modification in static global properties of nuclear matter and neutron stars with the inclusion of the δ meson into the formalism.

Naturalness of nonlinear scalar self-couplings in a relativistic mean field theory for neutron stars

We investigate the role of naturalness in effective field theory. We focus on dense hadronic matter using a generalized relativistic multi‐baryon lagrangian density mean field approach which contains nonlinear self‐couplings of the σ, δ meson fields and the fundamental baryon octet. We adjust the model parameters to describe bulk static properties of ordinary nuclear matter. Then, we show that our approach represents a natural modelling of nuclear matter under the extreme conditions of density as the ones found in the interior of neutron stars.