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We report on the experimental search for the bound state of an $\eta$ mesonand $^{3}\hspace{-0.03cm}\mbox{He}$ nucleus performed using the WASA-at-COSYdetector setup. In order to search for the $\eta$-mesic nucleus decay, the$pd\rightarrow$ $^{3}\hspace{-0.03cm}\mbox{He} 2\gamma$ and $pd\rightarrow$$^{3}\hspace{-0.03cm}\mbox{He} 6\gamma$ channels have been analysed. Thesereactions manifest the direct decay of $\eta$ meson bound in$^{3}\hspace{-0.03cm}\mbox{He}$ nucleus. This non-mesonic decay channel hasbeen considered for the first time. When taking into account only statisticalerrors, the obtained excitation functions reveal a slight indication for apossible bound state signal corresponding to an $^3$He-$\eta$ nucleus width$\Gamma$ above 20 MeV and binding energy $B_s$ between 0 and 15 MeV. However,the determined cross sections are consistent with zero in the range of thesystematic uncertainty. Therefore, as final result we estimate only the upperlimit for the cross section of the $\eta$-mesic $^{3}\hspace{-0.03cm}\mbox{He}$nucleus formation followed by the $\eta$ meson decay which varies between $2$nb and $15$ nb depending on possible bound state parameters.

Search for eta mesic 3He with the WASA-at-COSY facility in the pd-> 3He 2gamma and pd-> 3He 6gamma reactions