A unitary isobar model for pion photo- and electroproduction on the proton up to 1 GeV

A new operator for pion photo- and electroproduction has been developed for nuclear applications at photon equivalent energies up to 1 GeV. The model contains Born terms, vector mesons and nucleon resonances (P₃₃(1232), P₁₁(1440), D₁₃(1520), S₁₁(1535), F₁₅(1680), and D₃₃(1700)). The resonance contributions are included taking into account unitarity to provide the correct phases of the pion photoproduction multipoles. The Q² dependence of electromagnetic resonance vertices is described with appropriate form factors in the electromagnetic helicity amplitudes. Within this model we have obtained good agreement with the experimental data for pion photo- and electroproduction on the nucleon for both differential cross sections and polarization observables. The model can be used as a starting point to predict and analyze forthcoming data.     

The isobar model of the γN → ηN processes in the energy range from the threshold to 1200 MeV

The isobar model of the γN → γN processes is constructed on the basis of the precision measurements of the cross section for the γp → ηp process near the threshold in Mainz (Germany) and the sensational results of recent measurements of the Σ beam asymmetry and dσ/dΩ for this process in Grenoble (France). The model involves six nucleon resonances (MeV): S ₁₁(1535), S ₁₁(1650), P ₁₁(1440), P ₁₃(1720), D ₁₃(1520), and F ₁₅(1680). The properties of these resonances are discussed. The P ₁₃(1720) and F ₁₅(1680) resonances are responsible for large positive values of the Σ beam asymmetry for the γp → ηp reaction at small angles. The contribution of the S ₁₁(1650) resonance must be taken into account in addition to the contribution of the S ₁₁(1535) resonance in order to describe the experimental dependence of the total cross section on the photon energy. The values characterizing the γn → ηn reaction are calculated on the basis of the available data on the amplitudes of electromagnetic excitation of these resonances on protons and neutrons.     

Determination of Breit-Wigner features of the S 11(1535), S 11(1650), and P 11(1710) nucleon resonances

Basic Breit-Wigner features of the S ₁₁(1535), S ₁₁(1650), and P ₁₁(1710) nucleon resonances are evaluated in a model-independent way on the basis of the results obtained previously from a partial-wave analysis of eta-meson photoproduction on protons.     

Electromagnetic excitation of nucleon resonances

Recent progress on the extraction of electromagnetic properties of nucleon resonance excitation through pion photo- and electroproduction is reviewed. Cross section data measured at MAMI, ELSA, and CEBAF are analyzed and compared to the analysis of other groups. On this basis, we derive longitudinal and transverse transition form factors for most of the four-star nucleon resonances. Furthermore, we discuss how the transition form factors can be used to obtain empirical transverse charge densities. Contour plots of the thus derived densities are shown for the Delta, Roper, S ₁₁, and D ₁₃ nucleon resonances.     

Probabilistic quark-model approach to proton fragmentation

A probabilistic approach is formulated to study the behaviour of the incident hadron quark flavours in the fragmentation process of high energy protons in lowp _T reactions. Analysis of available data onpp collisions, mainly on hyperon and antibaryon multiplicities, leads to estimates of the probabilities for the different ways in which the incident valence quarks recombine into final hadrons. We find that all three incident quarks emerge in one and the same outgoing nucleon (or nucleon resonance) with probabilityA ₃=0.35–0.4, that two of them emerge in one baryon and the third in another hadron (mostly a meson or meson resonance) with probabilityA ₂=0.6–0.5, and that they emerge in three distinct hadrons (mostly mesons or meson resonances) with probabilityA ₁=0.05–0.1. We find good support for a very simple probabilistic picture of the fragmentation process.     

Pion-nucleon vertex function with an off-shell nucleon

We present a model calculation for the π-N vertex function in the case in which there is a single off-mass-shell nucleon and a (nearly) on-mass-shell pion. We find very strong effects due to the P₁₁ resonance at 1470 MeV. We provide a simple parametrization of the vertex function in the case that at least one nucleon is on its mass shell.     

Photo- and electroproduction of eta mesons

Eta photo- and electroproduction off the nucleon is investigated in an effective lagrangian approach that contains Born terms and both vector meson and nucleon resonance contributions. In particular, we review and develop the formalism for coincidence experiments with polarization degrees of freedom. The different response functions appearing in single and double polarization experiments have been studied. We will present calculations for structure functions and kinematical conditions that are most sensitive to details of the lagrangian, in particular with regard to contributions of nucleon resonances beyond the dominantS ₁₁(1535) resonance.This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 201)     

Investigation of giant E3 resonances by (N, γ) reactions

Effects related to the presence of giant E3 resonances are investigated by nucleon radiative capture according to the direct-semidirect model. The γ-ray angular distributions from the ²⁰⁸Pb(N, γ₀) reactions are calculated in the energy region above the giant dipole resonance and the influence of the E1–E3 and E2–E3 interferences is discussed. The results provide indications of an appreciable effect on the 90° photon emission when a collective isovector E3 excitation is present.     

Helicity components of the cross section for double charged-pion production by real photons on protons

The helicity components σ _1/2 and σ _3/2 of the cross section for double charged-pion production by real photons on a nucleon are calculated within a phenomenological approach developed previously. A high sensitivity of the σ _1/2–σ _3/2 asymmetry to the contribution of nucleon resonances having strongly different electromagnetic helicity amplitudes A _1/2 and A _3/2 is demonstrated. This feature is of importance for seeking “missing” baryon states.     

In-medium properties of nucleon resonances

Recent experimental results for the in-medium properties of nucleon resonances are discussed. The experiments were done with the TAPS detector at the tagged photon beam of the MAMI accelerator in Mainz. Measured was the photoproduction of mesons (final states π⁰X, ηX, 2π⁰X and π⁰π^±X) from the nuclei ¹²C, ⁴⁰Ca, ⁹³Nb and ²⁰⁸Pb up to the second resonance region. The results were analyzed in view of the in-medium properties of the P₃₃(1232), the D₁₃(1520), and the S₁₁(1535) resonances.     

Compton polarimetry detection of small circularly and linearly polarized impurities in Mössbauer 8.4 keV (3/2-1/2) M1 γ-transition of 169Tm

The arrangement of an experiment to detect the P?odd and P, T?odd polarized part of the Mössbauer (⁺3/2– ⁺1/2) gamma transition of a deformed ¹⁶⁹Tm nucleus with an energy of 8.4 keV by Compton polarimetry is discussed. Tm ₂O₃ single crystal with a quadrupolarly split Mössbauer spectrum is proposed as a resonance polarizer. A Be-scatterer-based Compton polarimeter and a synchronously detecting system will be used to measure the P-odd circular polarization P_Cand P, T-odd linear polarization P_L.The expected accuracy of measuring the relative magnitude of the P, T-odd contribution is about 1% of the magnitude of usual weak nucleon–nucleon interaction.     

Description of initial-and final-state-interaction effects for the reaction γp → π−Δ++ in the region of nucleon-resonance excitation

A model has been developed for describing the cross section for the reaction γ_r,v p → π^?Δ⁺⁺ in the energy region of nucleon-resonance excitation, and relevant calculations have been performed. The model takes into account the contribution of all reliably established resonances with masses less than 2 GeV and the contributions of nonresonance mechanisms in the approximation of Born diagrams. On the basis of data on the amplitudes of pion-nucleon interactions, a method has been evolved for phenomenologically describing the coupling of initial and final states with open inelastic channels. Calculations performed in this study represent a first step in developing a method for determining the electromagnetic form factors for nucleon resonances in exclusive reactions of pion-pair production on a nucleon by real and virtual photons.     

Photoexcitation of baryon resonances

The study of baryon resonances via meson photoproduction reactions on the free proton, nucleons bound in light nuclei, and in the nuclear medium is discussed. Special emphasis is laid on the production of neutral mesons which due to the suppression of non-resonant backgrounds are particularly well suited for the study of excited states of the nucleon. Experiments carried out during the last ten years with the TAPS-detector at the Mainz MAMI accelerator have contributed very significantly to the detailed investigation of the four lowest lying baryon resonances the P₃₃ (1232), the P₁₁ (1440), the D₁₃(1520) and the S₁₁(1535). Future experiments with TAPS at the ELSA accelerator have a large potential for the investigation of higher lying resonances via many different decay channels.     

Construction of a unitary analytic scattering amplitude (V). π+ p scattering

Starting from the 1974 Saclay phase-shift analysis, we construct π⁺ p continuum ambiguities that leave unchanged the total cross section, the differential cross section, and the polarization. We find that most of the resonant structures are stable, but that alternative solutions are possible that lack the second S₃₁ or the G₃₉ resonances. The second P₃₃ resonance may be moved considerably, but not killed, while the mass and width of the three-star P₃₁ resonance are also subject to appreciable changes. We suggest that disagreements between different groups concerning the existence of weak resonances, or concerning the masses and widths of stronger ones, may be caused by the existence of the continuum ambiguity.     

Propagation and decay of resonances produced coherently on nuclei

We study the propagation and decay of resonances, produced coherently on nuclei and decaying strongly into typically three-body final states. The A-dependence of the total production cross section is analysed in terms of the effective total cross section, σ₂, for the interaction between the produced system and a nucleon. We find that σ₂ becomes mass dependent, since energy conservation is not instantaneously established. This feature is qualitatively supported by a large amount of experimental data. However, this in no way proves that resonance production is the dominant production mechanism, but rather that whatever the mechanism is, energy conservation is not fully established until the produced system has left the nucleus.     

Erratum to: Dilepton production in proton-induced reactions at SIS energies with the GiBUU transport model

We present dilepton spectra from p + p , d + p and p + Nb reactions at SIS energies, which were simulated with the GiBUU transport model in a resonance model approach. These spectra are compared to the data published by the HADES and DLS Collaborations. It is shown that the ?? spectral function includes non-trivial effects already in elementary reactions, due to production via baryon resonances, which can yield large contributions to the dilepton spectrum. Dilepton spectra from nuclear reactions in the energy range of the HADES experiment are thus found to be sensitive also to properties of nucleon resonances in the nuclear medium.     

A one-channelN/D calculation of the πN partial waves incorporating inelastic unitarity

The present work is a re-analysis of theJ≦3/2πN partial waves in a single channelN/D scheme. The new features are inclusion of (i) inelastic unitarity, and (ii) the forces generated by the exchange of a scalarI=0 meson. The calculation shares the same difficulties with other one-channel treatments in that (a) the D₁₃ resonance is absent, and (b) it is not possible to produce the nucleon as a dynamical bound state and at the same time obtain aP ₁₁ resonance. Apart from theseS ₁₁,P ₁₁ andD ₁₃ waves, one can secure good agreement between theory and experiments in other partial waves.