DWIA in the momentum space for (γ, π0) reaction near threshold

The differential and total cross-sections of neutral pion photoproduction off⁴He,¹²C and⁴⁰Ca nuclei are calculated in the framework of the DWIA in the momentum space. It is shown that the inclusion of pion wave distortion gives a less than 25% increase of the total cross sections. A good agreement with experimental data is achieved by using the full elementary amplitude that includes the pion-momentum nonlinear terms and the terms depending on nucleon momenta.     

On the use of realistic wave functions in the coherent pion photoproduction on 3He nuclei

We analyze the influence of the nuclear wave functions generated from a set of different realistic soft-core two-body potentials on the coherent charged pion photoproduction on ³He nuclei at threshold and for small momentum transfers. This process is calculated in the framework of the impulse approximation including the Fermi motion. At threshold and below the resonance, our results reproduce fairly well the experimental data. At the first γN resonance region, the agreement with experiments varies with the momentum transfer.     

Photoproduction of charged pions to discrete nuclear states

We study the nuclear photoproduction of charged pions to discrete final states at intermediate energies. The model for the nonresonant production amplitude uses the experimental single nucleon multipoles. The resonant production mechanism includes the medium effects of delta-propagation in the framework of the Δ-hole model. This goes beyond the standard DWIA approach and we discuss the importance of these medium modifications. Applications are presented for p-shell nuclei and the results discussed in detail.     

Correlation and polarization characteristics of the 40Ca(p, dπ+)39K reaction

The ⁴⁰Ca(p, dπ⁺)³⁹K reaction was studied theoretically at a proton energy of 650 MeV within the distorted wave impulse approximation (DWIA) and under the assumption of a quasi-free mechanism of the elementary process pp → dπ⁺ of pion production. The peripheral nature of the reaction on the nucleus was shown. The influence of the effects of distortions in the initial and final states of the reaction on the differential cross section and other spin-dependent characteristics are discussed.     

Coherent photoproduction of pions on spin-zero nuclei in a relativistic,non-local model

A relativistic and non-local model for the coherent photoproduction of pions on spin-zero nuclei is presented. The production operator is derived from an effective Lagrangian model that contains all the degrees of freedom known to contribute to the elementary process. Using the framework of DWIA, the matrix elements of this operator are evaluated using relativistic bound state wave functions. Final state interactions are accounted for via a pion-nucleus optical potential. The effects of in-medium modifications of the production operator are discussed. We give results for ¹²C and ⁴⁰Ca, and compare our calculations to the data of the A2 collaboration at MAMI.     

Charged pion photoproduction from 10B and 13C

Positive and negative pion photoproduction cross sections from ¹⁰B and ¹³C leading to the ground state of the residual nucleus have been calculated in the plane-wave and distorted-wave impulse approximations. The results are compared with experiment and with other theoretical calculations. While a large discrepancy is observed between experiment and theory in the case of negative pion photoproduction from ¹³C, fair agreement is obtained between experiment and the plane-wave calculations in all other reactions. It is found that in all reactions studied, except in the case of positive pion production from ¹³C, the inclusion of the pion-wave distortions moves the theory away from the experimental points. It is also found that the photopion differential-cross-section calculations show a sensitivity to the nuclear wave function different from that predicted by the electron scattering form-factor calculations.     

Measurements of the Mean Free Paths of the Proton and Pions in the Nuclear Matter

The results of measuring the yields of neutral and charged pions in the (, p) reaction on Li, C, and Al nuclei are presented as functions of the proton energy and azimuth angle of pion emission. The experiment was performed in the second resonance region of the photon energy. The measurements are analyzed within the model of single and double quasi-free pion photoproduction. The energy dependences of the mean free path of the proton and neutral and charged pions are obtained based on the dependence of the reaction yield on the atomic number of the target nucleus.     

High energy pion photoproduction amplitudes from fixed-t dispersion relations and duality

Contributions to the high energy imaginary parts of the charged and neutral pion photoproduction amplitudes from degenerate ? and A₂ exchanges and ω and B exchanges are parametrized similarly to the dual absorptive model. These contributions together with the imaginary parts of the amplitudes found at low energies from partial-wave analyses are then used to evaluate the high energy real parts of the photoproduction amplitudes from fixed-t dispersion relations. A fit is made to data on pion photoproduction at 3.4 and 16 GeV incident photon energies, for momentum transfers up to ?1(GeV/c)². It is shown that much of the data is reproduced both qualitatively and quantitatively by our model. The high energy imaginary parts of the photoproduction amplitudes given by the fit are shown to be in accord with their average low energy behaviour.     

Elastic and inelastic scattering of π mesons on 7Li nuclei

Within the diffraction theory of multiple scattering, the differential cross sections for elastic and inelastic pion scattering on a ⁷Li nucleus are calculated for the case where the final nucleus is either in the ground or in the first excited state. The nuclear wave function is set to that in the αt cluster model. The sensitivity of the calculated observables to variations in the type of the wave functions for the alpha-particle and the triton cluster and for their relative motion is investigated. Various multiplicities of scattering and rescattering on the clusters constituting the ⁷Li nucleus are taken into account, and their contributions to the cross section are revealed. The results of the calculations are compared with experimental data at E _π=143, 164, and 194 MeV.     

Role of two-nucleon mechanisms in pion photoproduction on nuclei in the region of high momentum transfers

The role of two-nucleon mechanisms in pion photoproduction on nuclei was studied in the region of high momentum transfers to the residual nucleus. The process in which the photoproduction of negative pions on a ¹²C nucleus is accompanied by proton emission was considered by way of example. The results of the calculations were compared with available experimental data.     

Formation of two pions on nuclei in A(γ, ππN)B reactions

A model is proposed for double pion photoproduction on nuclei that is accompanied by nucleon emission. Simple models that faithfully reproduce single-particle differential cross sections are used to describe photon interactions with intranuclear nucleons. The calculated cross sections for pion photoproduction on ¹²C nuclei are compared with inclusive pion spectra measured in the second resonance region of photon energies.     

Electron scattering and charged pion photoproduction on12C and13C

Electron scattering and charged pion photoproduction at 200 MeV on¹²C and¹³C have been studied by the nuclear shell model to investigateM1 andE2 nuclear form factors of the isovector type. Starting with the Op-shell model of Hauge-Maripuu, the effects of core polarization by theM 3Y force and the one-pion-exchange current are taken into account. The effect of core polarization is found to be essential in both electron scattering and pion photoproduction, while the meson-exchange current, which also improves the calculated results, gives contributions of about 10%–15%.     

Contributions of γd → πNN + π0 d channels to the spin asymmetry and the Gerasimov-Drell-Hearn integral for the deuteron

Contributions from the semi-exclusive channels γd → π^± NN + π⁰ d and γd → π⁰ X (X=pn or d) to the deuteron spin asymmetry and the Gerasimov-Drell-Hearn (GDH) integral are explicitly evaluated using an enhanced elementary pion photoproduction operator and a realistic, high-precision potential model for the deuteron wave function. The sensitivity of the results to the elementary pion photoproduction operator is also investigated and considerable dependence is found. Results for the deuteron GDH integral are compared with the measurements from A2 and GDH@MAMI Collaborations.     

The (π, 2π) reaction and spin-isospin strength distribution in nuclei

We study pion production on nuclei within the distorted wave impulse approximation (DWIA) and using the local density approximation (LDA) for the treatment of possible nuclear spin-isospin mode enhancements. The scattering amplitude input is based on a one-nucleon mechanism since two-nucleon processes are here estimatedito be relatively unimportant. For discrete nuclear levels, we study differential and total (angle-integrated) cross sections for various pion-like levels (J^P = 0^?, 1⁺, 2^?,…; T = 1); these are also compared with non-pion-like excitations. The cross sections for discrete excitations are quite small. The inclusive cross sections, achieved by summing over all nuclear states, are considerably larger, of the order of 0.1–10 mb. We use the Fermi gas model to test the importance of the quasi-free peak for various incident pion momenta k, and to study the closure approximation for summing over nuclear levels when calculating inclusive cross sections. It is found that for k of the order of about twice the Fermi momentum most of the quasi-free peak region contributes to the nuclear excitations, and the use of closure is justified and practical. All cases are studied as a function of g', the Migdal spin-isospin parameter, and it is found that the dependence upon this parameter may range from changes of an order of magnitude to a factor of two over the range 0.4 ? g' ? 0.7.     

The DWIA analysis of the pion photoproduction on carbon

The inclusive pion spectra in the¹²C(γ,π) reaction are calculated in the distorted wave impulse approximation (DWIA). The final state pion-nucleus interaction was constructed within the framework of the multiple scattering theory. The sensitivity of the cross section to the choice of different optical potential variants is investigated.     

Five-nucleon spectroscopic amplitudes for 1p-shell nuclei

Spectroscopic amplitudes of the ⁵He or ⁵Li cluster in the ground and first excited states are given for lp-shell nuclei. These Spectroscopic amplitudes are calculated with the aid of intermediate-coupling wave functions of initial and final nuclei. Sum rules for pickup and stripping reactions are presented. The selectivity in excitation of the final states in pickup and stripping reactions of the ⁵He or ⁵Li cluster for some targets are discussed.     

Giant magnetic dipole analog excitations by the allowed muon capture in 1p-shell nuclei

The distributions of the allowed muon capture strengths in 1p-shell nuclei are shown to exhibit a tendency to concentrate in a few low-lying daughter states. These strong excitations are analogs of the giant magnetic dipole transitions, predicted by Kurath, and observed in inelastic electron scattering. Similar excitations are expected in 1S radiative pion capture and low-energy charged pion photoproduction processes.     

A non-relativistic operator convenient for analysis of pion photoproduction on nuclei in the Δ(1236) region

In view of its application to the analysis of medium energy photonuclear reactions we have obtained an operator valid, to order $\text{(}\text{p}\text{m}\text{)}{}^2$, in any frame of reference. We have started from a model where the pion photoproduction on a nucleon is described by the Born terms and the s-channel Δ(1236) formation amplitude and we have deduced the non-relativistic limit of the corresponding matrix elements. Both pseudo-scalar and pseudo-vector pion-nucleon couplings are compared. The Δ(1236) parameters are chosen so as to lead to a good account of pion-nucleon reaction cross sections. The agreement with the charged photoproduction data is very good from threshold through the Δ(1236) energy region, whereas the addition of ω⁰ exchange in the t-channel is necessary to reproduce the neutral pion photoproduction cross sections. As an example this operator is used to derive the cross section of the γD → ppπ^? reaction in the spectator nucleon model when the undiscernability of the two outgoing protons is taken into account.     

Nucleon-nucleon correlations in the (γ, π<Superscript>−</Superscript><Emphasis Type="Italic">p</Emphasis>) and (γ, π<Superscript>−</Superscript><Emphasis Type="Italic">pp</Emphasis>) reactions on the <Superscript>12</Superscript>C nucleus

The photoproduction of negative pions on the carbon nucleus in the (γ, π^?p) and (γ, π^?pp) reactions has been studied experimentally. The measurements are interpreted using a model of the formation of an intermediate Δ nuclear state (Δ nucleus) that decays with the emission of a pion and a nucleon.