Search for $η$-mesic helium via $dd\to {}^3{\rm He}\,nπ^0$ reaction by means of the WASA-at-COSY facility

In November 2010, we performed a search for a $^{4}\mbox{He}$-$\eta$ bound state by measuring the excitation function for the $dd\rightarrow$ $^{3}\mbox{He} n \pi{}^{0}$ and $dd\rightarrow$ $^{3}\mbox{He} p \pi{}^{-}$ reactions in the vicinity of the $\eta$ production threshold with the WASA detector.~The experiment was carried out using a deuteron COSY beam and deuteron pellet target. The beam momentum varied continuously in each of acceleration cycle from 2.127 GeV/c to 2.422 GeV/c, which corresponds to a range of excess energy $Q$ $\in$(-70,30)MeV. This dissertation is about the search for $^{4}\mbox{He}$-$\eta$ bound state in $dd\rightarrow$ $^{3}\mbox{He} n \pi{}^{0}$ reaction. The excitation function for the process was determined after identification of all outgoing particles and the application of the selection conditions based on Monte Carlo simulations of $\eta$-mesic helium production and its decay via excitation of the $N^{*}$ resonance. No narrow structure of the $\eta$-mesic helium was observed in the excitation function. The upper limit of the total cross section for the bound state formation and its decay in $dd\rightarrow({}^{4}\mbox{He}$-$\eta)_{bound} \rightarrow$ $^{3}\mbox{He} n \pi{}^{0}$ process was determined on the 90% confidence level. It varies from 21 to 36 nb for the bound state width ranging from 5 MeV to 50 MeV, respectively. However, an indication for a broad state was observed. The kinematic region, where we expect the evidence of the signal from the bound state, cannot be fully described only by the combination of the considered background processes. In contrast, the experimental excitation function is very well fitted by the background contributions for the region where the signal is not expected.