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The search for 4He-eta bound states was performed with the WASA-at-COSYfacility via the measurement of the excitation function for the dd->3He n pi0and dd->3He p pi- processes. The beam momentum was varied continuously between2.127 GeV/c and 2.422 GeV/c, corresponding to the excess energy for the dd->4Heeta reaction ranging from Q=-70 MeV to Q=30 MeV. The luminosity was determinedbased on the dd->3He n reaction and quasi-free proton-proton scattering viadd->pp n_spectator n_spectator reactions. The excitation functions determinedindependently for the measured reactions do not reveal a structure which couldbe interpreted as a narrow mesic nucleus. Therefore, the upper limits of thetotal cross sections for the bound state production and decay indd->(4He-eta)_bound->3He n pi0 and dd->(4He-eta)_bound->3He p pi- processeswere determined taking into account the isospin relation between both thechannels considered. The results of the analysis depend on the assumptions ofthe N* momentum distribution in the anticipated mesic-4He. Assuming as in theprevious works, that this is identical with the distribution of nucleons boundwith 20 MeV in 4He, we determined that (for the mesic bound state width in therange from 5 MeV to 50 MeV) the upper limits at 90% confidence level are about3 nb and about 6 nb for npi0 and ppi- channels, respectively. However, based onthe recent theoretical findings of the N*(1535) momentum distribution in theN*-3He nucleus bound by 3.6 MeV, we find that the WASA-at-COSY detectoracceptance decreases and hence the corresponding upper limits are 5 nb and 10nb for npi0 and ppi- channels respectively.

Search for eta-mesic 4He in the dd->3He n pi0 and dd->3He p pi- reactions with the WASA-at-COSY facility