The η′-carbon potential at low meson momenta
2018
The production of $\eta^{\prime}$
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 $\eta^{\prime}$
mesons have been identified via the $\eta^{\prime} \rightarrow \pi^{0} \pi^{0}\eta \rightarrow 6 \gamma$
decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF2 array at polar angles of $2^{\circ} \le \theta_{p} \le 11^{\circ}$
. Under these kinematic constraints the $\eta^{\prime}$
mesons are produced with relatively low kinetic energy (
$\approx 150$
MeV) 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 $\eta^{\prime}$
-carbon potential at low meson momenta by comparing with collision model calculations of the $\eta^{\prime}$
kinetic energy distribution and excitation function. Fitting the latter data for $\eta^{\prime}$
mesons going backwards in the center-of-mass system yields a potential depth of $V = -(44 \pm 16(stat) \pm 15(syst))$
MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average $\eta^{\prime}$
momenta of $\approx 1.1$
GeV/c. Within the experimental uncertainties, there is no indication of a momentum dependence of the $\eta^{\prime}$
-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 $\eta^{\prime}$
photoproduction off the free proton.
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