Relativistic Calculations for the Exclusive Photoproduction of Eta Mesons from Complex Nuclei

A relativistic model for the quasifree photoproduction of eta meson from complex nuclei is developed. The interactions between fields are introduced through effective Lagrangians. Contributions from several nucleon resonances as well as nucleon Born terms and vector meson exchange diagrams are included. Nucleon and eta wavefunctions are solutions of Dirac and Klein-Gordon equations, respectively. Final state interactions of the outgoing particles are included via optical potentials. The effects of these interactions on the cross sections and photon asymmetries are studied and are found to be large. Calculations indicate that at energies near threshold the exclusive reaction takes place mainly through formation of the S_{11}(1535) resonance. Comparisons with the non-relativistic calculations show differences between the two approaches both for the cross sections and photon asymmetries. We give some detailed predictions for the reaction observables for exclusive photoproduction on ^{12}C, ^{16}O and ^{40}Ca.Comment: Latex, 27 pages, 13 uuencoded postscript figures. Accepted for publication in Phys. Rev.

The energy dependence of the in-medium ηN\eta N cross section evaluated from η\eta-photoproduction

Within the Glauber formalism and a BUU transport model we analyze the $\eta$-photoproduction data from nuclei and evaluate the in-medium $\eta N$ cross section. Our results indicate that the $\eta N$ cross section is almost independent of the $\eta$ energy up to 200 MeV.Comment: 11 pages LaTeX including 6 postscript figure

In-medium Properties of Hadrons -- Observables

We first briefly review the theoretical basis for calculations of changes of hadronic properties in dense nuclear matter. These changes have usually been investigated by means of relativistic heavy-ion reactions. Here we discuss that observable consequences of such changes can also be seen in more elementary reactions on nuclei. Particular emphasis is put on a discussion of actual observables in photonuclear reactions; we discuss in detail $\eta$- and vector-meson production. We show that photoproduction of $\eta$'s can yield essential information on in-medium properties of the $S_{11}(1535)$ resonance while the $\phi$ meson properties will probably not be accessible through the $K^+K^-$ decay channel. However, for $\omega$ mesons the $\pi^0\gamma$ decay channel, due to its reduced final state interaction, looks more promising in this respect. Completely free of final state interactions is dilepton production in the few GeV range. We show that the sensitivity of this decay channel to changes of hadronic properties in medium in photonuclear reactions on nuclei is as large as in ultrarelativistic heavy ion collisions. Finally we discuss that hadron production in nuclei at 10 -- 20 GeV photon energies can give important information on the hadronization process.Comment: Invited Lecture by U. Mosel at Erice International School on Nuclear Physics 200

Study of N* resonances by eta photoproduction

Photoproduction of neutral mesons is very well suited to investigate the excitation of baryon resonances on free and bound nucleons. The isoscalar eta-meson does not couple to Delta-resonances and therefore provides a very selective tool for the study os isospin T=1/2 N* resonances. The eta photoproduction from protons in the threshold region is strongly dominated by the excitations of the S11(1535) resonance. The electromagnetic coupling gamma p-> S11 as well as contributions from other resonances (P11, Roper)are of fundamental interest for hadron models. We have studied eta photoproduction form the proton, deuteron, and some complex nuclei (C, Ca,Nb,Pb) at the Mainz microtron using the TAPS detector system

Study of N* resonances by eta photoproduction

Photoproduction of neutral mesons is very well suited to investigate the excitation of baryon resonances on free and bound nucleons. The isoscalar eta-meson does not couple to Delta-resonances and therefore provides a very selective tool for the study os isospin T=1/2 N* resonances. The eta photoproduction from protons in the threshold region is strongly dominated by the excitations of the S11(1535) resonance. The electromagnetic coupling gamma p-> S11 as well as contributions from other resonances (P11, Roper)are of fundamental interest for hadron models. We have studied eta photoproduction form the proton, deuteron, and some complex nuclei (C, Ca,Nb,Pb) at the Mainz microtron using the TAPS detector system

Near threshold photoproduction of η-mesons from complex nuclei

Total and differential cross sections for photoproduction of eta mesons from 12C, 4OCa, 93Nb, and natPb have been obtained up to 790 MeV incident photon energy at the Mainz Microtron (MAMI) with the TAPS spectrometer. The absorption cross section sigma_etaN = (30 ± 2.5 ± 6)mb of eta mesons in nuclear matter and the absorption length lambda = (2.0 ± 0.2 ± 0.4) fm are extracted. No significant depletion of the Sll(1535) strength in the eta photoproduction on nuclei is observed