In-medium Properties of the η′-Meson from Photonuclear Reactions

Investigations of in-medium properties of the η′-meson are presented. The photoproduction of η′-mesons off 12C, 40Ca, 93Nb and 208Pb nuclei has been measured with the Crystal Barrel/TAPS detector system at the ELSA accelerator in Bonn. Comparing the meson yield per nucleon within a nucleus to the yield on a free nucleon, information on the absorption of the meson through inelastic channels in the nuclear medium and the related in-medium width of the meson is provided. From this so called transparency ratio we found the in-medium width of the η′-meson to be of the order of 20 MeV. The measured width is sufficently narrow to encourage a search for η′ mesic states. Recent results on the momentum distribution of the transparency ratio and in-medium width of the η′-meson are presented and discussed