Quasifree photoproduction of $\eta$ mesons off protons and neutrons

TL;DR

This study measures the differential and total cross sections for quasifree eta meson photoproduction off protons and neutrons using the MAMI-C accelerator, providing improved data and confirming a neutron-specific resonance peak.

Contribution

The paper presents new high-statistics measurements of eta photoproduction cross sections off protons and neutrons, including a detailed comparison with free and quasifree data to investigate nuclear effects.

Findings

Confirmed the neutron-to-proton cross-section ratio in the $S_{11}(1535)$ resonance region.

Observed a resonance peak at $W = 1670$ MeV in neutron cross sections, absent in proton data.

Provided improved statistical accuracy over previous measurements.

Abstract

Differential and total cross sections for the quasifree reactions $\gamma p\rightarrow\eta p$ and $\gamma n\rightarrow\eta n$ have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0 \rightarrow 6\gamma$ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The $\eta$-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the $S_{11}(1535)$ resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy $W = (1670\pm 5)$ MeV with an intrinsic width of $\Gamma\approx 30$ MeV.