The {\eta}'-carbon potential at low meson momenta

TL;DR

This study measures the low-momentum $ ext{ exteta}^ extprime$-carbon potential by analyzing $ ext{ exteta}^ extprime$ meson production in photon-induced reactions, finding a potential depth of about -44 MeV with no clear momentum dependence.

Contribution

First direct measurement of the $ ext{ exteta}^ extprime$-carbon potential at low meson momenta using coincidence detection and collision model comparison.

Findings

Potential depth V = -44 ± 16 (stat) ± 15 (syst) MeV.

No evidence of momentum dependence of the $ ext{ exteta}^ extprime$-carbon potential.

Good agreement of $ ext{ exteta}^ extprime$ photoproduction cross sections with previous data.

Abstract

The production of $\eta^\prime$ mesons in coincidence with forward-going protons has been studied in photon-induced reactions on $^{12}$C and on a liquid hydrogen (LH$_2$) 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 BaF$_2$ 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 LH$_2$ 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.