Coherent photoproduction of eta-mesons off 3He - search for eta-mesic nuclei

TL;DR

This study investigates the coherent photoproduction of eta-mesons off helium-3 near the threshold, providing evidence for a strong final state interaction and a possible eta-mesic nucleus, using advanced detection techniques.

Contribution

It presents new experimental data on eta-meson production off helium-3, indicating a potential eta-mesic nucleus and analyzing the reaction's angular distributions and cross sections.

Findings

Sharp rise in cross section at threshold suggests strong final state interaction.

Evidence points to a possible resonant state related to eta-mesic nuclei.

Complex background structures hinder conclusive evidence in related pion production channels.

Abstract

Coherent photoproduction of $\eta$-mesons off $^3$He, i.e. the reaction $\gamma ^3{He}\rightarrow \eta ^3{He}$, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the $\eta\rightarrow \gamma\gamma$ and $\eta\rightarrow 3\pi^0\rightarrow 6\gamma$ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong $\eta {^3{He}}$ final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of $\pi^0$-proton back-to-back emission in the $\gamma ^3{He}\rightarrow \pi^0 pX$ reaction revealed a very complicated structure of the background and could not support previous conclusions.