Workshop on the Physics of Excited Nucleon – NSTAR2009, Beijing, April 19 – 22, 2009

Invited plenary talk

Invited parallel talk

Oral contribution

Authors: Igal Jaegle for CB-ELSA / TAPS collaboration

Title: Photoproduction of mesons off deuteron reactions of the quasifree neutron and coherent processes.

A systematic investigation of mesons photoproduction off the deuteron has been performed at a tagged photon beam of the Bonn ELSA accelerator with the combined Crystal Barrel - TAPS electromagnetic calorimeter for incident photon energies up to 2.5 GeV.

This investigation takes place in the current hadronic issues i.e. the search of "missing resonances" and the studies of the nature and properties of the known resonances. For example, the extraction of the isospin compostion of a resonance requires the measurement of different observables off the proton, the neutron and the deuteron.

In particular the photoproduction of etaprime-mesons off the deuteron (quasifree off the bound proton and neutron) is discussed. The etaprime-meson is a good candidate to tag resonances around 2 GeV mass location where most of the "missing resonances" are lying and resonances with properties not established yet. The etaprime photoproduction off the neutron exhibits a large difference in total cross section and shape of angular distributions around the cross section maximum for free proton.

New results for the quasifree production of pi0-eta pairs off the deuteron will be shown. This results demonstrate that proton and neutron cross sections are similar, which supports the analysis of the pi0-eta production off the free nucleon where the dominant contributions of the Delta-resonances decaying via eta-emission to the Delta(1232) have been suggested.

Furthemore, the coherent photoproduction of pi0-eta pairs off the deuteron, which is clearly identified via the detection of the recoil deuterons, is also shown. In this reaction, the eta-mesons are emitted with very small kinetic energies, so that it can be used as a tool to study the much discussed eta-nucleon, eta-deuteron interaction at small relative momenta.

To finish an update and future plan to study the resonance-like structure in the neutron excitation function, which is absent for the proton will be discussed.