Photoproduction of charged pions on deuterium in the first resonance region

Photoproduction of π⁺ and π^? on deuterium has been measured in the photon energy range from 240 to 400 MeV and for pion c.m. angles between 15° and 180°. The pions were analysed in angle and momentum by a magnetic spectrometer. From the measured π^?/π⁺ ratio, corrected for Coulomb interactions in the final state, differential cross sections of the reaction γ+n→π^?+p were calculated. Together with the π⁺ photoproduction our data show no isotensor contribution. Comparison of our data with the recent experiments done on the inverse reaction shows no evidence of a violation of time reversal invariance. With the measured π⁺ photoproduction on deuterium, a test of the spectator model has been made. Using the closure-approximation of Chew and Lewis our data agree within a range of ±10%.     

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.     

Forward photoproduction of neutral pions on polarized protons in the third resonance region

Angular distributions of the target symmetry for the reaction γ + p → π⁰ + p have been measured at the Bonn 2.5 GeV Electron Synchrotron at pion c.m. angles between 13° and 63° and photon energies of 1.0 and 1.1 GeV. The π⁰ mesons were detected by their two decay photons with total absorption lead-glass ?erenkov counters. Butanol was used as target material in a continuous flow ³He cryostat operating at 0.5 K and 25 kG. The π⁰ counting rate from free protons in the butanol target was derived from the measurements of the differential cross section on hydrogen. The data are compared with data of other laboratories and the results of two recent partial-wave analyses.     

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].     

Evaluation of the photoproduction of negative pions on neutrons in the region of the first resonance

In this paper we extend our systematic investigation of pion photoproduction to the case of negative pion production on neutrons. In our approach two phenomenological parameters occur in the electric and magnetic dipole amplitudesE ₀₊ andM _1?. These constants have been determined by a least squares fit of the total cross section data of the ABBHHM collaboration. We obtain a very good fit, also the prediction of the asymmetry ratio for polarized photons agrees with experiment at energies below and at the resonance. Serious discrepancies between theory and experiment are found for theπ ^?/π ⁺ ratio, moreover the relationship between the electric dipole at threshold, the Panofsky ratio and the difference ofs-waveπN-scattering lengths is fulfilled only within 20%. These discrepancies are discussed thoroughly.     

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.     

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 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 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.     

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 neutral pions on hydrogen below 500 MeV

At the Bonn 500 MeV electron synchrotron differential cross sections of the reactionγ+p→Π ⁰+p were measured in the kinematical region of photon energies between 340 and 420 MeV and pionCM angles from 10° to 70° by means of detecting the two photons from the pion decay by two Cherenkov counters.     

Forward photoproduction of neutral pions on protons in the third resonance region

Four angular distributions of the differential cross section of the reaction γ + p → π⁰ + p have been measured at the Bonn 2.5 GeV Synchrotron at pion c.m. angles between 3° and 63° in the photon energy range from 0.9 GeV to 1.2 GeV. The π⁰ mesons were detected by their two decay photons. The data are compared with data of other laboratories and predictions of two recent partial-wave analyses.     

Photoproduction of pions in forward direction and Regge poles

Photoproductions of single pions for high energies and small momentum transfer is considered. The production amplitude is approximated by exchange ofρ-,ω-,?- andπ-meson in the crossed chanellγ+π→N+¯N. These mesons are treated either as “elementary” or as Regge poles. The angular distribution is caclulated in the high energy limit and discussed in detail.     

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 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.     

Scattering of pions on pions in threshold region

It is shown that the integral equations for isoscalar-scattering, obtained from the Mandelstam representation by the method of so-called differential approximation, admit the threshold behaviour of partial amplitudes which is in agreement with the quantum theory of scattering and, moreover, they also admit solutions with other possible threshold behaviours. This result does not depend on whether the conformal mapping technique is used or not.     

Electroproduction of neutral pions at energies above the resonance region

The reaction e p→e'pπ⁰ has been measured at W=2.55 GeV a fixed electron scattering angle of 10.3°. Two magnetic spectrometers and a lead glass hodoscope were used to detect all four final state particles. Electroproduction cross sections in the t range ?0.15 to ?1.4 (GeV/c)² at q² = ?0.22, ?0.55 and ?0.85 (GeV/c)² are presented. Above |t|=0.6 (GeV/c)² the cross sections are considerably smaller than those for photoproduction.