Search for the He- bound states with the WASA-at-COSY facility

The existence of η -mesic nuclei in which the η meson is bound with nucleus by means of the strong interaction was postulated already in 1986, however till now no experiment confirmed it empirically. Discovering of this new kind of an exotic nuclear matter would be very important as it might allow for better understanding of the η meson structure and its interaction with nucleons. The search for η -mesic helium (4He- η ) is carried out with high statistic and high acceptance by means of the WASA detector, installed at the cooler synchrotron COSY in the Research Center Jülich. The search is conducted via the measurement of the excitation function for the chosen decay channels of the 4He- η system. In the experiment performed in November 2010 two reactions d d → ( 4 He - η ) b s → 3 He p π − and d d → ( 4 He - η ) b s → 3 He n π 0 were measured with the beam momentum ramped from 2.127 GeV/c to 2.422 GeV/c. The report includes the description of experimental method and status of the measurement.