Determination of the real part of the η′ -Nb optical potential

The excitation function and momentum distribution of η′ mesons have been measured in photoproduction off Nb93 in the energy range of 1.2-2.9 GeV. The experiment has been performed with the combined Crystal Barrel and MiniTAPS detector system, using tagged photon beams from the ELSA electron accelerator. Information on the sign and magnitude of the real part of the η′-Nb potential has been extracted from a comparison of the data with model calculations. An attractive potential of -(41±10(stat)±15(syst)) MeV depth at normal nuclear matter density is deduced within model uncertainties. This value is consistent with the potential depth of -(37±10(stat)±10(syst)) MeV obtained in an earlier measurement for a light nucleus (carbon). This relatively shallow η′-nucleus potential will make the search for η′-nucleus bound states more difficult.