Search for η -mesic 4He in the dd → 3Henπ0 and dd → 3Hepπ - reactions with the WASA-at-COSY facility

The search for 4He-ηbound states was performed with the WASA-at-COSY facility via the mea-surement of the excitation function for the dd→3Henπ0and dd→3Hepπ−processes. The deuteron beam momentum was varied continuously between 2.127GeV/c and 2.422GeV/c, corresponding to the excess energy for the dd→4Heηreaction ranging from Q =−70MeVto Q =30MeV. The luminosity was determined based on the dd→3Henreaction and the quasi-free proton–proton scat-tering via dd→ppnspectatornspectatorreactions. The excitation functions, determined independently for the measured reactions, do not reveal a structure which could be interpreted as a narrow mesic nu-cleus. Therefore, the upper limits of the total cross sections for the bound state production and decay in dd→(4He-η)bound→3Henπ0and dd→(4He-η)bound→3Hepπ−processes were determined taking into account the isospin relation between the both of the considered channels. The results of the analysis depend on the assumptions of the N∗(1535) momentum distribution in the anticipated mesic-4He. Assum-ing, as in the previous works, that this is identical with the distribution of nucleons bound with 20MeV in 4He, we determined that (for the mesic bound state width in the range from 5MeV to 50MeV) the upper limits at 90% confidence level are about 3nb and about 6nb for nπ0and pπ−channels, respectively. However, based on the recent theoretical findings of the N∗(1535) momentum distribution in the N∗-3Henucleus bound by 3.6MeV, we find that the WASA-at-COSY detector acceptance decreases and hence the corresponding upper limits are 5nb and 10nb for nπ0and pπ−channels respectively