Gamma-ray Spectroscopy of Hypernuclei — Recent Results and Prospect at J-PARC —

Abstract

The present status and future prospects of precision γ-ray spectroscopy of Λ hypernuclei are described. A new experiment (J-PARC E13) has been performed at the J-PARC K1.8 line employing a newly-developed germanium (Ge) detector array, Hyperball-J, via the (K − , π − ) reaction on 4 He and 19 F targets. This is the first study of s-shell and sd-shell Λ hypernuclei with Ge detectors. The 4 He target spectrum exhibited a γ-ray peak of 4 Λ He(1 + → 0 + ) at 1.406 ± 0.002 ± 0.002 MeV. Compared with the energy (1.09 ± 0.02 MeV) of the corresponding transition for the mirror hypernucleus, 4 Λ H(1 + → 0 + ), the present result clearly indicates a large charge symmetry breaking (CSB) effect. This decisive data for CSB allows a stringent test of baryon-baryon interaction models. As for the 19 F target run, a preliminary spectrum shows several γ-ray peaks assigned to 19 Λ F transitions. Future plans of γ-ray spectroscopy are also discussed.