An exclusive measurement of the excitation function for the $dd\to {}^3\mathrm{He}p{\pi }^{-}$ reaction was performed at the Cooler Synchrotron COSY-Jülich with the WASA-at-COSY detection system. The data were taken during a slow acceleration of the beam from 2.185 to 2.400 GeV/$c$ crossing the kinematic threshold for the $\eta$-meson production in the $dd\to {}^4\mathrm{He}\eta$ reaction at 2.336 GeV/$c$. The corresponding excess energy with respect to the ${}^4$He-$\eta$ system varied from $-$51.4 to 22 MeV. The integrated luminosity in the experiment was determined using the $dd\to {}^3\mathrm{He}n$ reaction. The shape of the excitation function for the $dd\to {}^3\mathrm{He}p{\pi }^{-}$ reaction was examined. No signal of the ${}^4$He-$\eta$ bound state was observed. An upper limit for the cross section for the bound state formation and decay in the process $dd\to {({}^4\mathrm{He}-\eta )}_{\mathrm{bound}}\to {}^3\mathrm{He}p{\pi }^{-}$ was determined on the 90$\%$ confidence level and it varies from 20 to 27 nb for the bound state width ranging from 5 to 35 MeV, respectively.

• Received 4 January 2013

DOI:https://doi.org/10.1103/PhysRevC.87.035204