Coupled-channel calculation of bound and resonant spectra of ⁹Be and ¹³C hypernuclei

A multi-channel algebraic scattering (MCAS) approach has been used to analyze the spectra of two hypernuclear systems, ⁹Be and ¹³ C$. The splitting of the two odd-parity excited levels (1/2^- and 3/2^-) at 11 MeV excitation in ¹³Λ C is driven mainly by the weak Λ-nucleus spinorbit force, but the splittings of the 3/2⁺ and 5/2⁺ levels in both ⁹ΛBe and ¹³ΛC 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 ⁹ΛBe is reproduced with the addition of a weak spinspin interaction acting between the hyperon and the spin of the excited target. With no such spinspin interaction, the level ordering in ⁹ΛBe 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.