Search for η-mesic 4He with the WASA-at-COSY detector

Abstract

An exclusive measurement of the excitation function for the dd→3Hepπ- 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 η-meson production in the dd→4Heη reaction at 2.336 GeV/c. The corresponding excess energy with respect to the 4He-η system varied from -51.4 to 22 MeV. The integrated luminosity in the experiment was determined using the dd→3Hen reaction. The shape of the excitation function for the dd→3Hepπ- reaction was examined. No signal of the 4He-η bound state was observed. An upper limit for the cross section for the bound state formation and decay in the process dd→(4He-η)bound→3Hepπ- 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. © 2013 American Physical Society.