Determination of the η′-nucleus optical potential

The excitation function and momentum distribution of η′ mesons have been measured in photon induced reactions on 12C in the energy range of 1250-2600 MeV. The experiment was performed with tagged photon beams from the ELSA electron accelerator using the Crystal Barrel and TAPS detectors. The data are compared to model calculations to extract information on the sign and magnitude of the real part of the η′-nucleus potential. Within the model, the comparison indicates an attractive potential of −(37±10(stat)±10(syst))MeV depth at normal nuclear matter density. Since the modulus of this depth is larger than the modulus of the imaginary part of the η′-nucleus potential of −(10±2.5)MeV, determined by transparency ratio measurements, a search for resolved η′-bound states appears promising.