Quasifree Photoproduction of $\eta$ Mesons off the Neutron

Quasifree photoproduction of $\eta$ mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The $\eta$ mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree $n(\gamma ,\eta )n$ and $p(\gamma ,\eta )p$ reactions. The excitation function for $\eta$ production off the neutron shows a pronounced bumplike structure at $W=1.68\mathrm{GeV}$ ($E_{\gamma }\approx 1\mathrm{GeV}$), which is absent for the proton.

Figure 1

Left-hand side: quasifree cross sections off proton and neutron. Curves: model predictions MAID model [14], and Shklyar et al. [36], all folded with Fermi motion. Neutron data and curves are scaled up by a factor of $3/2$. Insert: comparison of neutron cross sections from different analyses (see text). Right-hand side: neutron/proton cross section ratio. Old low-energy data from Refs. [18, 21].

Figure 2

Angular distributions: Blue triangles: quasifree proton; red circles: quasifree neutron. Black solid (dashed) lines: fit of neutron (proton) data (see text), bar charts: half difference of ${\sigma }_n$ and (${\sigma }_{np}-{\sigma }_p$). Green dotted curves: free-proton results [9] folded with Fermi motion.

Figure 3

Legendre coefficients of angular distributions. Red circles: quasifree neutron; blue triangles: quasifree proton, Curves: free-proton data [9] (black), MAID neutron [14] (red), Shklyar et al. neutron [36] (green), all folded with Fermi motion. Insert: logarithmic scale for $A_0$.

Figure 4

Proton and neutron excitation functions for $\cos ({\Theta }_{\eta }^{\star })<-0.1$ without (invariant mass $W_B$ from incident photon energy) and with (invariant mass $W_R$ from neutron-$\eta$ 4-vectors) event-by-event correction of Fermi motion. The efficiency loss due to a cut on nucleon-meson coplanarity was not corrected. Curves: solid: full fit, dash-dotted: BW-curve of $S_{11}(1535)$, dashed: BW-curve for second structure. Stars: response for a $\delta$-function due to finite energy resolution. Neutron data scaled by factor $3/2$.