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

摘要

Coherent photoproduction of η-mesons off ~3He, i.e. the reaction γ ~3 He → η ~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 η → γ γ and η → 3 π ~0 → 6 γ 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 η He ~3 final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of π ~0 -proton back-to-back emission in the γ ~3 He → π ~0 p X reaction revealed a very complicated structure of the background and could not support previous conclusions.