Abstract
Quasi-free photoproduction of η-mesons has been measured off nucleons bound in 3He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost 4π covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The η-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the S11(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final-state interaction effects.
Highlights
The only practical method to investigate the electromagnetic excitation spectrum of the neutron is photoproduction of mesons off nucleons bound in light nuclei
Cross sections and other observables measured off free and quasi-free protons are in good agreement, which is the basis for the extraction of such data for neutron targets from quasi-free deuteron data
The Crystal Ball (CB) and the TAPS detector were subdivided into logical sectors: The TAPS detector into 6 × 64 modules in a pizza-like geometry and the CB into 45 groups of 16 modules
Summary
The only practical method to investigate the electromagnetic excitation spectrum of the neutron is photoproduction of mesons off nucleons bound in light nuclei. The aim is to establish a reliable electromagnetic excitation scheme of the nucleon; since the electromagnetic interaction is isospin dependent, measurements with neutron targets are mandatory for a complete picture Programs to measure such reactions off neutrons bound in the deuteron are currently under way at several laboratories (see [1] for an overview). The present experiment used 3He nuclei as a quasifree neutron target This allows the study of the structure in a different nuclear environment to test whether it behaves as expected due to the different momentum distributions of the nucleons in 2H and 3He nuclei and to investigate the effects of FSI on η photoproduction from 3He nuclei. Coherent photoproduction of η-mesons from 3He has been analyzed in a separate study to provide information on the formation of eta-mesic nuclei [13]
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