COUPLED-CHANNEL CALCULATION OF BOUND AND RESONANT SPECTRA OF $^9_\Lambda{\rm Be}$ AND $^{13}_\Lambda\rm C$ HYPERNUCLEI

A multi-channel algebraic scattering (MCAS) approach has been used to analyze the spectra of two hypernuclear systems, $^9_\Lambda{\rm Be}$ and $^{13}_\Lambda\rm C$. The splitting of the two odd-parity excited levels ($\frac{1}{2}^-$ and $\frac{3}{2}^-$) at 11 MeV excitation in $^{13}_\Lambda\rm C$ is driven mainly by the weak Λ-nucleus spin–orbit force, but the splittings of the $\frac{3}{2}^+$ and $\frac{5}{2}^+$ levels in both $^{9}_\Lambda{\rm Be}$ and $^{13}_\Lambda\rm C$ 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 $^{9}_\Lambda{\rm Be}$ 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 $^{9}_\Lambda{\rm Be}$ is inverted with respect to that currently observed. In both hypernuclei...