The eta'-carbon potential at low meson momenta

The production of backslasheta^backslashprime$mesons in coincidence with forward-going protons has been studied in photon-induced reactions on 12C and on a liquid hydrogen (LH2) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The backslasheta^backslashprime$mesons have been identified via the backslasheta^backslashprime backslashrightarrow backslashpi^0 backslashpi^0backslasheta backslashrightarrow 6 backslashgamma$textrightarrow$0$0$textrightarrow6$decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF2 array at polar angles of 2^backslashcirc backslashle backslashthetap backslashle 11^backslashcirc2∘≤$p≤11∘. Under these kinematic constraints the backslasheta^backslashprime$mesons are produced with relatively low kinetic energy (backslashapprox 150≈150MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the backslasheta^backslashprime$-carbon potential at low meson momenta by comparing with collision model calculations of the backslasheta^backslashprime$kinetic energy distribution and excitation function. Fitting the latter data for backslasheta^backslashprime$mesons going backwards in the center-of-mass system yields a potential depth of V = -(44 backslashpm 16(stat) backslashpm 15(syst))V=-(44textpm16(stat)textpm15(syst))MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average backslasheta^backslashprime$momenta of backslashapprox 1.1≈1.1GeV/c. Within the experimental uncertainties, there is no indication of a momentum dependence of the backslasheta^backslashprime$-carbon potential. The LH2 data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for backslasheta^backslashprime$photoproduction off the free proton.