η photoproduction off 3He : search for η mesic nuclei

Photoproduction of �η-mesons off 3He has been studied via the η -> γγ and 3π° -> 6γ decaymodes at the tagged photon beamof theMainzMAMI ac-
celerator using the combined 4�γ Crystal Ball/TAPS calorimeter. In a previous
experiment, Pfeiffer et al. had reported evidence (although at low statistical
significance) for the formation of a quasi-bound �-nucleus state. The present
experiment aimed at an improved statistical quality for the structure reported
for coherent η�-photoproduction.
Coherent cross-section is calculated from the integral of differential excitation
function up to 700MeV, the next step of the analysis will be to extend the en-
ergy range to 1.4GeV. At threshold, angular distribution shows a flat behavior,
sign of a strong final state interaction between �η and nucleus due to the ex-
istence of a virtual pole near threshold resulting in a strong threshold cusp
effect. η coherent cross section show a very rapid rise in the threshold region
similar to COSY-ANKE data, this give rise to the assumption that there is a
quasi bound or virtual state of the �η-3He system .
Moreover the �
π0-p back-to-back channel is a candidate to emphasize the ex-
istence of the �η-mesic nuclei. The new experiment set allow a more precise
measurement at coherent channel in order to confirm the enhancement in the
excitation function for opening angles of 170-180 observed by Pfeiffer et al.
Others tracks of investigation such has the double�
π0-p back-to-back channel
will be discuss.