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E-Verhandlungen 1998
Programm und Abstacts der Sitzung HK 26

•S. Kamalov, D. Drechsel, O. Hanstein und L. Tiator
Institut für Kernphysik, Universität Mainz, Becherweg 45, D-55099 Mainz

A new operator for pion photo- and electroproduction has been developed for the application to the reactions on nuclei at equivalent photon 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 the resonance electromagnetic vertices is described with appropriate form factors in the electromagnetic helicity amplitudes. Within this model we have obtained good agreement with experimental data for pion photo- and electroproduction on nucleon (for the differential cross sections and polarization observables). Furthermore we give predictions for future experiments on pion electroproduction to be performed at Mainz, Bonn and TJNAF.

In a first application to reactions on few-body systems we will present calculations for coherent photo- and electroproduction on the deuteron and ³He at threshold and in the resonance region. Possible effects of medium modification for the nucleon resonances will be discussed.

•Ralf Plötzke für die SAPHIR-Kollaboration
Institut für Strahlen- und Kernphysik, Nußallee 14-16, 53155 Bonn, Germany

We present a study of photoproduction of h^¢ mesons with the SAPHIR detector at the Electron Stretcher Ring ELSA at Bonn. The data on this reaction taken in January and April 1997 with the SAPHIR detector suggest the existence of a resonant threshold enhancement in the ph^¢ invariant mass. The mass and width of the resonance is reported in this talk.

•Jürgen Link für die SAPHIR-Kollaboration
Institut für Strahlen- und Kernphysik der Universität Bonn

Total and differential cross sections for h-photoproduction in a liquid hydrogen target have been measured using a tagged photon beam at the ELSA accelerator facility. The SAPHIR detector was employed to observe the h meson through its decay h® p⁺p^-p⁰. The nucleon resonance S₁₁(1535) is known to decay into hp, but precise data exist only up to the S₁₁(1535) peak. The S₁₁(1650) seems not to decay into hp, but no photoproduction data exist for this mass region. Our present data extend the available energy range up to 1900 MeV and will allow the investigation of this region.

V. Hejny und J. Weiß für die A2-Kollaboration
II. Physikalisches Institut, Universität Gieß en

The h-meson is a very selective probe for the S₁₁(1535) nucleon resonance. A consistent theoretical description of the large hN branching ratio is, however, still missing. In order to gain new insight, additional experimental results are urgently needed.
In 1995/96 the reactions g + d ® h + X and g + ⁴He ® h + X have been measured using the photon spectrometer TAPS at the Mainz Microtron (MAMI). In order to explore the quasifree h production from bound nucleons an additional forward wall was built to detect the recoil nucleon. Using d and ⁴He targets the quasifree proton cross section from a weakly and a strongly bound system can be compared with the reaction on a free proton. The results of this analysis can be used to extract the free neutron cross section from the quasifree data. Together with the measurement of the coherent cross section from the deuteron the isospin structure of the S₁₁ resonance can be deduced.
In case of the coherent production from ⁴He the S₁₁ does not contribute, which makes this reaction very sensitive to the much smaller non-S₁₁ contributions. Consequently, the observed yield is close to the detection limit. The current status of the analysis will be presented.

[1] supported by BMBF and SFB 201

•B. Krusche für die A2-Kollaboration
II. Physikalisches Institut, Universität Gieß en

In a series of experiments carried out with the TAPS detector at the MAMI accelerator p^o-photoproduction from complex nuclei was studied. Data were taken for ¹²C, ⁴⁰Ca, ⁹³Nb and ^natPb targets for incident photon energies ranging from 200 - 790 MeV. The data allow the extraction of total and differential cross sections for inclusive and coherent single p^o-photoproduction as well as total cross sections for quasifree double p^o-photoproduction. Preliminary results will be discussed and compared to models. The cross sections allow stringent tests of models predicting the quasifree p^o-photoproduction like BUU-models and models for coherent p^o-photoproduction like e.g. D-hole models. The mass number dependence of the cross sections can be used to study the final state interaction of pions with nuclear matter. Comparison to the models furthermore allows to extract information about possible in-medium modifications of baryon resonances.

[1] supported by BMBF and SFB 201

•A. Schmidt für die A2- und TAPS-Kollaboration
Institut für Kernphysik, Universität Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz

Photoproduction of neutral pions on the proton using linearly polarized photons has been measured in the energy range close to threshold at the Mainz Microtron MAMI as a further test of Chiral Pertubation Theory (ChPT). Linearly polarized photons were obtained by using the Glasgow-Mainz tagger with a diamond crystal as a radiator. Neutral pions were identified via their 2g-decay with TAPS, a detector system consisting of 504 individual BaF₂- scintillators.
The measurement of the photon asymmetry for the production of neutral pions in the threshold region allows to separate the s-wave amplitude E₀₊ and all p-wave amplitudes. The ChPT Low-Energy-Theorem for the p-waves will be compared to first experimental results.

^* supported by Deutsche Forschungsgemeinschaft (SFB 201)

•R. Leukel für die A2- und TAPS-Kollaboration
Institut für Kernphysik, Universität Mainz, Johann-Joachim-Becher-Weg 45, D-55099 Mainz

The photon asymmetry for the reaction [(g)\vec] + p ® p⁰ + p in the D-resonance-region allows to investigate the interference term between the small electric quadrupole amplitude (E2) and the dominant magnetic dipole amplitude (M1) of the N ® D transition. A precise experimental value for the E2/M1-ratio is an important test for different nucleon-models, since it can be interpreted as a ``deformation'' of the nucleon, the D(1232)-resonance or both.
The measurement has been performed with the tagged photon beam (Glasgow-Tagger) of the 855 MeV cw electron accelerator MAMI (Mainz Microtron). Coherent bremsstrahlung from a diamond radiator has been exploited with a degree of linear photon polarisation of » 50% in the D-region. Using the photon spectrometer TAPS, comprising 504 individual BaF₂-detectors, the neutral pion was identified by its 2g-decay.
First results will be presented.

^* supported by Deutsche Forschungsgemeinschaft (SFB 201)

•Matthias Wolf für die A2-Kollaboration
II. Physikalisches Institut, Universität Gießen

First measurements of the reaction p(g,p^op^o)p have been performed in 1992 with the photon spectrometer TAPS at the Mainz microtron MAMI [1]. This experiment revealed a dominant two-step production mechanism near the peak cross section (E_g » 720 MeV) involving the excitation of the D₁₃(1520) baryon resonance and the D(1232) as intermediate state. Due to the limited statistics the production cross section close to threshold (E_g < 450 MeV) could not be extracted unambiguously. However, it is only in this energy regime that predictions of Chiral Pertubation Theory can be tested.

Therefore the experiment was redone in 1995 with significantly increased statistics. The new data allow to determine the total cross section from threshold up to the highest photon energies (309 - 820 MeV) with a reduced systematic error near threshold. Dalitz plots ranging to smaller cm-energies may provide further information of the production mechanism. Theories expect that the cross section near threshold is no longer dominated by a two-step mechanism but by the emission of correlated pion pairs from the P₁₁(1440) resonance.

Preliminary results of the new experiment will be discussed.

[1] F. Härter et al., Physics Letters B 401 (1997) 229

[2] supported by BMBF and DFG (SFB 201)

•G. Penner, T. Feuster und U. Mosel für die A2-Kollaboration
Institut für Theoretische Physik, Universität Giessen

By introducing electromagnetic formfactors in the spacelike region we extend an effective Lagrangian model to describe electroproduction of p-mesons. The model considers Born terms, baryon resonances up to 1.7 GeV and vectormeson contributions (r, w) [1]. Since the implemented formfactors are constructed in such a way that their analytic continuation in the timelike region is possible the model is also used for predictions of pion induced dilepton production in the VMD dominated dilepton mass region.

[1] T. Feuster and U. Mosel, Nucl. Phys. A 612, (1997) 375.

Supported by BMBF, DFG and GSI Darmstadt

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