Spectroscopy of ΛΛ hypernuclei

Spectroscopy of ΛΛ hypernuclei has recently become one of the most valuable tools for the experimental investigation of strangeness nuclear physics. Following the pioneering (π+,K+)(π+,K+) reaction spectroscopy experiments at the BNL AGS in the 1970’s, excitation spectra have been measured using this reaction for a wide variety of ΛΛ hypernuclei with the superconducting kaon spectrometer (SKS) at the KEK 12 GeV PS. More recently, ΛΛ hypernuclear spectroscopy was carried out for the first time using the (e,e′K+)(e,e′K+) reaction and the continuous electron beam at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). This reaction will be further explored using a new high-resolution kaon spectrometer (HKS) installed at Jefferson Lab. At the same time, precision γγ-ray spectroscopy with a germanium detector array (Hyperball) has been successfully performed for pp-shell ΛΛ hypernuclei at the KEK 12 GeV PS and at the BNL AGS. Quantitative information on ΛΛ hypernuclear structure was obtained and the strengths of the spin-dependent ΛΛ–nucleon interaction in the pp-shell region were derived. In this review article, the progress of ΛΛ hypernuclear spectroscopy is described and future prospects are presented.