Determination of the η^'-nucleus potential at low momenta

The real part of the η^'-nucleus potential has been determined at low momenta by analyzing the η^' kinetic energy distribution and the excitation function in photoproduction of of η^' mesons off C in coincidence with forward going protons. The forward going protons take over most of the momentum of the incoming photon beam and thus allow a study of the η^'-nucleus interaction at very low energies. This experimental approach was previously used in the determination of the ω-nucleus potential at low momenta. The present measurement extends earlier determinations of the η^'-nucleus potential at higher average momenta towards the production threshold. A comparison of the data with calculations by E. Paryev indicates that also at low momenta deep η^'-nucleus potentials of ≥100 MeV can be excluded.