Coupled-channel calculation of bound and resonant spectra of _Λ⁹Be and _Λ¹³C hypernuclei

A multi-channel algebraic scattering (MCAS) approach has been used to analyze the spectra of two hypernuclear systems, _Λ⁹Be and _Λ¹³C . The splitting of the two odd-parity excited levels (1^–/2 and 3^–/2) at 11 MeV excitation in _Λ¹³C is driven mainly by the weak Λ-nucleus spin–orbit force, but the splittings of the 3⁺/2 and 5⁺/2 levels in both _Λ⁹Be and _Λ¹³C have a different origin. These cases appear to be dominated by coupling to the collective 2⁺ states of the core nuclei. Using simple phenomenological potentials as input to the MCAS method, the observed splitting and level ordering in _Λ⁹Be is reproduced with the addition of a weak spin–spin interaction acting between the hyperon and the spin of the excited target. With no such spin–spin interaction, the level ordering in _Λ⁹Be is inverted with respect to that currently observed. In both hypernuclei, our calculations suggest that there are additional low-lying resonant states in the Λ-nucleus continua. (author)