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

Abstract

Coherent photoproduction of η-mesons off 3 He, 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 → γγ and η → 3π 0 → 6γ 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 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 π 0 -proton back-to-back emission in the γ 3 He → π 0 pX reaction revealed a very complicated structure of the background and could not support previous conclusions.

Coherentπ0π0photoproduction on lightest nuclei

“…[18] and [20], give for the ratio σ γ 3 He→η 3 He /σ γd→ηd in the region of S 11 (1535) the value about 20. The experimental results for the γd → ηd and γ 3 He→ η 3 He cross sections [16,21] indicate, that this ratio is likely to be essentially smaller, about 4. This difference is primarily due to larger value of the deuteron cross section in comparison to that predicted by the model of [18] in which the possible nontrivial phase between the neutron and the proton amplitudes is neglected (see discussion after Eq.…”

Search for quasi boundηmesons

“…It should be mentioned that at threshold the relative velocity between the η and the α particle is zero and hence the formation of a bound state is favoured. On the other hand at just this energy the cross section for producing a free η [87]. Note that E γ ∝ W 2 (see Eq.…”

η -nuclear bound states revisited

“…To date, however, experimental searches for such bound states have been unsuccessful, e.g. the latest negative results for 3 η He (in photoproduction on 3 He [14]) and for 4 η He (in dd → 3 He pπ − [15]). …”