Spectroscopy of Λ hypernuclei

Spectroscopy of Λ hypernuclei has recently become one of the most valuable tools for the experimental investigation of strangeness nuclear physics. Following the pioneering (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 Λ hypernuclei with the superconducting kaon spectrometer (SKS) at the KEK 12 GeV PS. More recently, Λ hypernuclear spectroscopy was carried out for the first time using the (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 γ-ray spectroscopy with a germanium detector array (Hyperball) has been successfully performed for p-shell Λ hypernuclei at the KEK 12 GeV PS and at the BNL AGS. Quantitative information on Λ hypernuclear structure was obtained and the strengths of the spin-dependent Λ-nucleon interaction in the p-shell region were derived. In this review article, the progress of Λ hypernuclear spectroscopy is described and future prospects are presented.