Proton-η′ interactions at threshold

Recent data on photoproduction of η′ mesons off protons have been included in the data base for the Bonn–Gatchina partial wave analysis. The real and imaginary parts of the S-wave η′p→η′p scattering amplitude in the threshold region were fit to yield the η′p scattering length and the interaction range. This new analysis found |aη′p|=(0.403±0.015±0.060) fm and a phase ϕ=(87±2)∘, while the range parameter is not well-constrained. The striking behavior of the GRAAL data on the beam asymmetry in the threshold region suggests that a narrow proton-η′ resonance might exist. However, the scattering length was found to be relatively insensitive to the possible existence of this narrow resonance.