Investigation of the anomaly in eta-photoproduction off the neutron

TL;DR

This study investigates an unexpected bump in eta-photoproduction off the neutron, revealing a narrow structure near 1675 MeV that is absent in proton data, suggesting a potential new resonance or phenomenon.

Contribution

First detailed exclusive measurement of eta-photoproduction off the neutron showing a narrow structure near 1675 MeV.

Findings

A bump-like structure observed in neutron excitation function near 1675 MeV

Width of the structure estimated below 50 MeV

No similar structure observed in proton data

Abstract

Quasi-free photoproduction of eta-mesons off the neutron and off the proton has been studied using a deuterium target and bremsstrahlung photons produced by MAMI-C with incident energies up to 1.5 GeV. The eta-mesons were detected in coincidence with the recoil nucleons thus a fully exclusive measurement was performed. Preliminary results show a bump-like structure in the excitation function for the neutron close to W $\approx$ 1675 MeV which is not seen for the proton. Considering the experimental resolution and using a Breit-Wigner fit the width of this structure was approximated below 50 MeV.