Search for signals of bound nuclear states in antiproton-4He annihilations at rest

Λp and Λd narrow peaks in the pπ−p and pπ−d invariant mass distributions are observed in antiproton-4He annihilations at rest. By considering the Λp peak as the signature of the two-body decay of the bound state, its mass, binding energy and decay width would be 2212.1±4.9 MeV, −160.9±4.9 MeV and <24.4±8.0 MeV with a statistical significance of 3.7σ. By considering the Λd peak as the signature of the two-body decay of the bound state, the same quantities should be 3190±15 MeV, −121±15 MeV and <60 MeV with a statistical significance of 2.6σ. Alternative explanations are also discussed. The data were collected with the OBELIX spectrometer at the LEAR complex at CERN.