Photoproduction of eta-mesons off nuclei for Eg < 2.2 GeV

TL;DR

This study measures eta-meson photoproduction off various nuclei at energies up to 2.2 GeV to investigate in-medium properties of the S11(1535) resonance and eta absorption in nuclear matter, finding no unexpected in-medium effects.

Contribution

It provides new experimental data on eta-meson production in nuclei and analyzes nuclear absorption, contributing to understanding in-medium resonance behavior and meson-nucleus interactions.

Findings

Nuclear absorption cross section for eta mesons is constant across eta momenta.

No unexplained in-medium modifications of the S11(1535) resonance are observed.

The data supports existing BUU-model calculations for eta photoproduction.

Abstract

Photoproduction of $\eta$ mesons off $^{12}$C, $^{40}$Ca, $^{93}$Nb, and $^{nat}$Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2 GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S$_{11}$(1535) nucleon resonance and the study of the absorption properties of nuclear matter for $\eta$ mesons. Careful consideration was given to contributions from $\eta\pi$ final states and secondary production mechanisms of $\eta$-mesons e.g. from inelastic $\pi N$ reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross section $\sigma_{N\eta}$ for $\eta$ mesons is constant over a wide range of the $\eta$ momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU-model show no unexplained in-medium modifications of the S$_{11}$(1535).