Coherent Π0 Photoproduction Research Articles

Beam-target double spin asymmetries in the reaction γd → π0d near threshold and the role of D-wave component of the deuteron wave function

We investigate coherent π0-photoproduction on the deuteron near threshold with special emphasis on beam-target double spin asymmetries. We present results for the spin asymmetries $T^{c}_{1M}$ (M = 0, 1) for circular polarized photons and vector polarized deuterons, $T^{c}_{2M}$ (M = 1, 2) for circular polarized photons and tensor polarized deuterons, $T^{l}_{1M}$ (M = 0, ± 1) for longitudinal polarized photons and vector polarized deuterons, and $T^{l}_{2M}$ (M = 0, ± 1, ± 2) for longitudinal polarized photons and tensor polarized deuterons in the γd → π0d reaction near threshold. We also discuss the sensitivity of the estimated results to the D-wave component of the deuteron wave function. The unitary isobar MAID-2007 model is used for the elementary γN → πN amplitude. This model is parameterized in terms of invariant amplitudes and allows one to give a more reliable description of the threshold region. The realistic and high-precision Bonn NN potential (full model) is used for the deuteron wave function. We have found that the D-wave component of the deuteron wave function has a little effect on the $T^{c}_{1M}$ (M = 0, 1) and $T^{l}_{2M}$ (M = 0, 1, 2) spin asymmetries. On the contrary, we have obtained a significant role of the D-wave component on the spin asymmetries $T^{c}_{2M}$ (M = 1, 2), $T^{l}_{1M}$ (M = 0, ± 1), and $T^{l}_{2M}$ (M = − 1,− 2).

Coherent π0-Photoproduction on the Deuteron near Threshold and the Role of D-Wave Component of the Deuteron Wave Function

Near-threshold coherent photoproduction of π0-meson on the deuteron is investigated. Numerical results for the unpolarized differential and total cross sections, the linear photon Σ-asymmetry, the vector T11 and tensor T2M (M = 0, 1, 2) deuteron spin asymmetries, and the spin asymmetry $$\tilde{T}_{20}$$ of the total cross section are presented. The role of D-wave component of the deuteron wave function on these physical observables is studied. The calculations are based on the impulse approximation in which we use the realistic and high-precision Bonn NN potential (full model) for the deuteron wave function and the unitary isobar MAID-2007 model for the elementary γN → πN amplitude that describes well the threshold region. We have found that the D-wave component has a negligible effect on the unpolarized cross sections and the vector deuteron T11-asymmetry at photon lab-energies close to threshold. On the contrary, we have obtained a significant role of the D-wave part of the deuteron wave function on the Σ, T2M, and $$\tilde{T}_{20}$$ spin asymmetries. The results for the unpolarized differential cross section are compared to the experimental data from TAPS and a satisfactory agreement within the uncertainties has been obtained at forward pion angles. At backward pion angles, our results underestimate the experimental data and a disagreement has been found.

Tensor Target Spin Asymmetries in Coherent π0-Photoproduction on the Deuteron Including Intermediate ηNN Interaction Within a Three-Body Approach

Motivated by the recent measurements from the VEPP-3 electron storage ring, we investigate the tensor target polarization asymmetries T2M (M = 0, 1, 2) in the reaction γd → π0d with a particular interest in the effect of the intermediate ηNN three-body approach. This approach is based on realistic separable representations of the driving two-body interaction in the πN, ηN, and NN subsystems. It is shown that the influence of rescattering effects in the intermediate state on the tensor target spin asymmetries is sizable at extreme backward pion angles. At forward angles, the contribution from the pure impulse approximation is dominated and the spin asymmetries show very little influence of rescattering effects. The sensitivity of results to the elementary pion photoproduction operator and to the NN potential model adopted for the deuteron wave function is investigated, and considerable dependences are found. The predicted spin asymmetries are also compared with available experimental data, and a satisfactory agreement with the recent data from VEPP-3 is obtained at photon energies below 400 MeV. At higher energies, the calculated spin asymmetries slightly underestimate the data.

Influence of the Choice of NN Potential Model on γd → π0d Observables near η-Threshold

Influence of the choice of the NN potential model governing the deuteron wave function on the observables for coherent π0-photoproduction on the deuteron near η-threshold is investigated by using a three-body model for the intermediate ηNN system with separable two-body interactions. Results for unpolarized differential cross section and polarization observables are predicted. It is revealed that the choice of the NN potential model has a visible effect on the differential cross section and most of the polarization observables, especially in the photon energy range of 600–800 MeV and extreme backward pion angles. We find that the deviation among results obtained by using different deuteron wave functions is quite large. The use of the CD-Bonn NN potential for deuteron wave function doubles the differential cross section in this kinematic region. Compared with the experimental data from CLAS collaboration for differential cross section, sizeable discrepancies are found.

Sensitivity of γd → π0d Observables Near η-Threshold to the Intermediate ηNN Interaction and the Choice of Elementary Pion Production Amplitude

The role of intermediate ηNN interaction is studied in coherent π0-photoproduction on the deuteron near η-threshold using a three-body model with separable two-body interactions. This work is motivated by the measurements of the CLAS Collaboration at Jefferson laboratory, where a cusp-like structure in the energy dependence of the differential cross section has been observed at extremely backward pion angles. Results for differential and total cross sections are predicted and compared with available data. We found that the results for differential cross section differ significantly from those predicted by a simple model in which only the impulse approximation and two-body process contributions are considered. In addition, the sensitivity of the results for unpolarized cross sections and polarization observables to the elementary amplitude is investigated and considerable dependences are obtained. The results revealed that the intermediate ηNN three-body calculation and the choice of the elementary amplitu...

Helicity-dependent reaction γd → π0d near the η-threshold and its contribution to the E-asymmetry and the GDH sum rule for the deuteron

The helicity-dependent coherent π0-photoproduction in the reaction γd → π0d near the η-threshold is investigated. The calculations are performed within an approach which includes the reaction amplitudes of the impulse approximation (IA), two-step process with intermediate πN- and ηN-rescattering, and the higher order terms in the multiple scattering series for the intermediate ηNN interaction. The contribution of γd → π0d to the deuteron spin asymmetry is calculated and its contribution to the Gerasimov–Drell–Hearn (GDH) integral is explicitly evaluated by integration up to a photon energy of 900 MeV. In addition, the helicity E-asymmetry is calculated. The results revealed that the doubly polarized differential cross-sections and the helicity E-asymmetry are sensitive to the interference of rescattering effects, specially at photon energies 600–800 MeV and extreme backward pion angles. The sensitivity of the obtained results for the GDH integral to the choice of NN potential model governing the deuteron wave function is discussed. We find that the deviation among results obtained for the deuteron GDH integral using different deuteron wave functions is quite large.

Influence of intermediate [formula omitted] interaction on spin asymmetries for [formula omitted] reaction near the [formula omitted]-threshold within a three-body approach

The influence of intermediate ηNN interaction on spin asymmetries for coherent π0-photoproduction on the deuteron in the energy region around the η-threshold is studied using a three-body model with separable two-body interactions. We calculate all possible spin asymmetries with polarized photons and/or oriented deuterons and compare our results with available data. The calculations are based on a theoretical approach which includes the amplitudes of the impulse approximation, two-body process with πN- and ηN-rescattering, and the higher order terms in the multiple scattering series for the intermediate ηNN interaction. We found that the results for spin asymmetries of the differential cross section differ significantly from those calculated by a simple model in which only the impulse approximation and first-order rescattering contributions are considered. The effect of intermediate ηNN interaction is significant in certain spin asymmetries, specially at extreme backward pion angles. It turns out that the inclusion of a full ηNN three-body contribution is quite essential. The spin asymmetries of the total cross section are much less affected by rescattering contributions. Compared to the experimental data from YerPhi Collaboration for the linear photon asymmetry, sizeable discrepancies are found.

Coherent [formula omitted]-photoproduction on the deuteron near the [formula omitted]-production threshold including polarization observables

Coherent π0-photoproduction on the deuteron including polarization observables is studied in the energy region near the η-production threshold at backward center-of-mass angles of the outgoing pion. This work is motivated by the measurements of the CLAS Collaboration at Jefferson Lab, where a cusp-like structure in the energy dependence of the differential cross section has been observed at extremely backward pion angles. The present approach is based on the impulse approximation and first-order rescattering diagrams with intermediate production of both π- and η-mesons. Numerical results for unpolarized cross sections, the linear photon asymmetry (Σ), the vector (T11) and tensor (T2M, M=0, 1, 2) deuteron target asymmetries, and the double polarization E-asymmetry are predicted and compared with available experimental data and other theoretical models. The effect of first-order rescattering is found to be much larger in spin asymmetries than in the unpolarized cross sections. It reaches on average about 40% in the tensor target and E asymmetries. Compared to the experimental data from CLAS Collaboration, sizable discrepancies are found. This is not the case for the linear photon asymmetry, for which a better comparison with the data from YerPhI Collaboration is obtained.

Coherent π0-photoproduction from atomic nuclei

Differential and total cross sections for coherent pi0-photoproduction from 12C, 40Ca, 93Nb and Pb targets have been measured throughout the region of the Delta(1232)-resonance. The experiments were performed with the TAPS-detector at the Mainz accelerator MAMI. The characteristic proportionality of the cross section to the square of the atomic mass number and to the nuclear mass form factor is clearly demonstrated. The data allow for the first time detailed tests of model predictions for this reaction. The comparison of the data to model predictions shows that the Delta-nucleus interaction saturates: it is described for heavy nuclei with the same potential parameters as for 4He.

Coherent π0 photoproduction from 4He

Differential cross sections and beam asymmetries for coherent π 0 photoproduction from 4He in the Δ energy-range have been measured with high statistical and systematic precision using both decay photons for identifying the process. The experiment was performed at the MAinz MIcrotron using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. The differential cross sections are in excellent agreement with predictions based on the DWIA if an appropriate parametrization of the Δ-nuclear interaction is applied. The beam asymmetries are interpreted in terms of degrees of linear polarization of collimated coherent bremsstrahlung. The expected increase of the degree of linear polarization with decreasing collimation angle is confirmed. Agreement with calculations is obtained on a few-percent level of precision in the maxima of the coherent peaks.

Coherent π0 photo-production on 4He at intermediate energies with polarized photons

The reaction 4 He( γ,π 0) 4 He has been studied with the LEGS tagged, polarized gamma-ray beam. Data are presented at laboratory angles of 31°, 45°, 72.5°, 90°, 110° and 130° for incoming photon energies of 206, 219, 232, 245, 259, 274, 289, 306 and 322 MeV. The unpolarized cross sections are compared with the Δ-hole theoretical model results available in literature. The measured photon beam asymmetry is −1 as expected from conservation of angular momentum and parity.

Shadowing in photoabsorption on nuclei

The treatment of shadowing, as due to the interference between one and two-step processes in the nuclear photoproduction amplitude, is reconsidered. Shadowing is related to theA dependence of coherent photoproduction. It is shown not to occur in the Δ region because of the features of coherent π0 photoproduction. Its onset is predicted above the ρ production threshold. It is pointed out that the longitudinal (e, e′) response function may exhibit a different shadowing behaviour.

Off-shell effects in the coherent π0 photoproduction off nuclei

The strong sensitivity of coherent π 0 photoproduction to the choice of the reaction energy ω in the elementary t πγ (ω)-matrix is demonstrated. The best agreement of the DWIA results with the experimental data is achieved when ω is chosen as an eigenvalue of the free relativistic hamiltonian of the πN system. This is in agreement with the consequences of the relativistic potential theory.

Coherent π0 photoproduction near threshold on closed-shell nuclei

Coherent π 0 photoproduction is considered on closed-shell nuclei in the DWIA frame from threshold up to pion energies of 50 MeV. The elementary photoproduction operator is taken from Blomqvist and Laget and treated in the coordinate-space representation. It includes, besides the negligible Born terms, delta formation in the s-channel and ω 0 exchange in the t-channel, two mechanisms which have comparable importance even near threshold. The emerging pion wave is distorted by optical-model potentials which give equally good fits both to pionic atom data and pion scattering data. In the considered energy range the DWIA cross section is enhanced with respect to the plane-wave case by an amount between 50% and 100% depending on the particular optical-model potential. Distortion is necessary to gain a satisfactory agreement with the available data.

Coherent pion photoproduction from deuterium at intermediate energies

We study the coherent π0 photoproduction on deuterons at energies around the (3, 3)-resonance and discuss the effects of Fermi motion, rescattering and kinematical approximations. The results are very dependent upon what kinematical approximations one adopts for the impulse approximation term, which dominates up to very large angles. Allowing for this uncertainty in the kinematics, our results are in good agreement with the most recent experimental data.

Gamma ray and π0 production from hydrogen and complex nuclei by 270–375 MeV gamma rays

Measurements are reported on the differential cross section for secondary gamma ray production on hydrogen and nuclear targets at 90° in the laboratory through the interactions of primary gamma rays in the energy range 270–375 MeV. A difference method using bremsstrahlung beams at different end point energies was employed. The gamma ray detector was a high resolution Nal(Tl) spectrometer and time of flight discrimination was used to reject neutrons. For incident gamma rays in the range 270–375 MeV the first pion nucleon resonance is strongly excited and the observed gamma ray yields are consistent with those expected due to an admixture of coherent and incoherent π0 photoproduction in the resonance region. The experiment marks the first successful use of a large Nal(Tl) crystals as a gamma ray spectrometer at a high energy electron linear accelerator, despite the low duty cycle and its accompanying difficulties for such detectors.

Coherent π0-photoproduction on deuterium in the first resonance region

Differential cross sections for coherent π0-photoproduction from deuterium have been measured in the photon energy range from 240 to 400 MeV and for pion c.m. angles between 70 ° and 160 °. The recoil deuterons were analysed in angle and momentum by a magnetic spectrometer. The cross sections obtained were higher by a factor of about 2 compared with the results from Stanford [7], the only data available up to now in the first resonance region. Below the resonance the measured cross sections give a smooth extension to the low energy data from Glasgow [5] and Orsay [6].