Core-Excited States ofCΛ12Hypernuclei Formed in the (π+, K+) reaction

A spectroscopic study of the ${}_{\Lambda }{}^{12}{}_{}{}^{}\mathrm{C}$ hypernucleus by the (${\pi }^{+}, K^{+}$) reaction has been performed using a new superconducting kaon spectrometer (INS-SKS) at the KEK 12 GeV Proton Synchrotron with an energy resolution of 2 MeV (FWHM). In addition to two prominent peaks which correspond to the $s$ and $p$ orbitals of a $\Lambda$ hyperon, for the first time two smaller peaks were clearly observed at excitation energies of 2.6 and 6.9 MeV. These two peaks are interpreted as states where the ${}_{}{}^{11}{}_{}{}^{}\mathrm{C}$ excited core and a $\Lambda$ hyperon in the $s$ orbit are weakly coupled. The excitation energies and the cross sections of these peaks provide information on the $\Lambda N$ interaction.