Photoproduction of neutral pions on hydrogen at photon energies between 200 and 440 MeV

At the Bonn 500 MeV electron synchrotron the differential cross section of the reaction γ+p → π⁰+p has been measured detecting the recoil proton by a magnetic spectrometer. Angular distributions between pion c.m. angles of 50 ° and 160 ° were taken at photon energies between 220 and 420 MeV.     

Photoproduction of neutral pions to discrete nuclear states

We study the photoproduction of neutral pions on nuclei to discrete final states at intermediate energies. Both photoproduction and distortion of the π⁰ are dominated by Δ-excitation, which requires a careful treatment of the Δ-nucleus dynamics. We use the Δ-hole approach, which has been used to describe a variety of pion- and photon-induced nuclear reactions. It is shown that π⁰ photoproduction is very sensitive to medium effects of Δ-propagation and can therefore be used to investigate the Δ-nucleus interaction. The validity of the distorted-wave impulse approximation for this reaction is examined. Results for low-lying excitations in ¹²C are compared with the available data.     

Photoproduction of positive pions from hydrogen with PHOENICS at ELSA

The differential cross section of the reaction γp→π⁺n has been measured with the PHOENICS detector at ELSA in Bonn. For the first time this cross section has been determined simultaneously over a large range of photon energies (E_γ = 220−900 MeV) and pion angles (Θ^c.m._pi = 35°−135°) with a tagged photon facility. The experimental set-up allowed a considerable kinematic overdetermination of the investigated reaction. Accordingly, the background contributions have been suppressed to below 1%. The measured differential cross section is in good agreement with existing data. The comparison with different model calculations is presented.     

Photoproduction of neutral pions in the region of the fourth nucleon resonance

We report the measurement of the differential cross section of the reaction γ +p→π⁰ +p at a photon energy of 1.4 GeV and pion c.m. angles between 60 and 175 degrees. The angular distribution confirms the simple quark model prediction of a pure magnetic excitation of theF ₃₇ (1950) resonance.     

Photoproduction of neutral pions on deuterium at forward angles in the first resonance region

At the 500 MeV electron synchrotron in Bonn differential cross sections of the reaction γ + d → π⁰ + d were measured in forward pion c.m. angles and photon energies between 340 and 420 MeV.     

Photoproduction of neutral pions off hydrogen at photonenergies between 0.55 and 2.2 GeV at a cm-angle of 120°

The differential cross section has been measured for the reaction γ+p→π^o + p at the Bonn 2.5 GeV electron synchrotron in the energy range from 0.55 to 2.2 GeV at a c.m.angle of 120 degrees.     

Photoproduction of pions at low energy

Presented in this paper is a field theory model for photoproduction of pions at low energy. In particular, it is shown that certain interaction terms, which were previously considered to be essential in order to explain the experimental situation corresponding to the spin-3/2 resonances, are really unnecessary and that the presence of these terms are not justifiable from the theoretical point of view. Further, the free parameters in the theory representing the off-mass-shell effects of the spin-3/2 particles are fixed on the basis of theoretical considerations.     

Photoproduction of pions in chiral bag models

Threshold pion photoproduction has been calculated for the nucleon, within the framework of chiral bag models. The results of low-energy theorems (LET) are recovered only by taking account of Lorentz ivariance and center-of-mass (CM) correlations of the composite system. In particular the static bag model describes only the Kroll-Ruderman term of the amplitude. The suppression of spurious CM excitations by the concept of effective charges and the use of properly boosted wave functions are necessary in order to obtain also the higher-order contributions as predicted by LET.     

Photoproduction of neutral pions at backward angles in the resonance region and the η-cusp

At the Bonn 2.5 GeV electron synchrotron we have measured the differential cross section of the reaction γp→π⁰ p at a pion CM angle of 170° and at photon energiesK _γ between 0.6 and 1.8 GeV. In comparison to previous measurements the accuracy of the data was improved substantially. For the first time in neutral pion photoproduction a cusp structure at the η-threshold has been confirmed [1].     

Photoproduction of pions and etas in nuclei

We calculate the cross sections for inclusive one-pion, two-pion and eta photoproduction in nuclei in the photon energy range from 300 to 900 MeV within the framework of a semi-classical BUU transport model. Our results are compared with existing experimental data and discussed with respect to a calculation of the total photoabsorption cross section.     

Polarization effects in pion-photoproduction on protons below 500 MeV. II

Using the same model as in the previous paper a prediction for the polarization of the recoil protons is given in case of polarized photons. The determination ofE ⁰⁺ andE ¹⁺ from measurements is discussed. Furthermore the influence of theππ-interaction on polarization is considered. In the case ofπ ⁺-production the polarization of the recoil neutrons is investigated and the dependence of the cross section on the polarization state of the target protons is discussed.     

Subthreshold production of neutral pions with Ar ions of 44 MeV/u

The production of neutral pions in nucleus nucleus collisions with 44 MeV/u Ar projectiles from the accelerator GANIL has been observed. The angular distributions indicate a collective production process, and the pion spectra reflect properties of the collision dynamics.     

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.     

Photoproduction of pions from deuterons in impulse approximation

The differential cross section for photoproduction of pions from deuterons is calculated in the relativistic impulse approximation. In the discussion of the resulting formula special emphasis is given to the following three problems: test of the impulse approximation, test of the photoproduction amplitudes, and measurement of the deuteron wave-function.     

Photoproduction of pions in the constituent quark model

Threshold photoproduction of pions has been calculated for composite systems, both for the nucleon in terms of constituent quarks and the ³H/³He isodoublet in terms of nucleons. The results of the low energy theorems are recovered only if both size effects and internal excitation of the composite system are properly taken into account. In particular, the ground state has to be non-degenerate and the excitation spectrum has to have the proper symmetries of the many-body system.