$\gamma$-ray spectroscopy of $^{11}_{\Lambda}$B and $^{12}_{\Lambda}$C: Results of the KEK E566 experiment

Bound excited states of $^{11}_{\Lambda}$B and $^{12}_{\Lambda}$C were populated following the ($\pi^+$, $K^+$)$^{12}_{\Lambda}$C/ $^{11}_{\Lambda}$B+p reaction. The 12-GeV/c primary proton beam was provided by KEK-PS and the secondary $\pi^+$ beams of 1.05 GeV/c were produced and momentum analyzed by the K6 beam line bombarding a 18.6-g/cm$^2$ Carbon target. A momentum of the scattered K$^+$ was subsequently tracked by the SKS spectrometer system. $\gamma$ rays from hypernuclei produced were detected by the Ge detector array, Hyperbal2, which consisted of 14 standard closed end-type and 6 Clover-type detectors each surrounded by BGO background suppressing counters. Construction of missing mass spectrum identified the bound state as well as the proton emitting unbound excited states of $^{12}_{\Lambda}$C, the latter leading to $^{11}_{\Lambda}$B. $\gamma$ rays in coincidence with these regions were associated with the respective hypernucleus. Six $\gamma$-ray transitions, three of each belonging to $^{11}_{\Lambda}$B and $^{12}_{\Lambda}$C, were observed. Cascade decays of $^{11}_{\Lambda}$B($3/2^+ \rightarrow 1/2^+ \rightarrow 5/2^+$) and the ground state doublet spacing of $^{12}_{\Lambda}$C were newly identified from the present analysis. In addition, a 157-keV $\gamma$ ray from a hyperfragment of $^{12}_{\Lambda}$C was observed. From the energy level spacings we check the consistency of the strength of spin-spin ($\Delta$) and nuclear-spin orbit (S$_N$) in the effective $\Lambda$N interaction of the $p$-shell hypernuclei. Results of the analysis and the discussion will be presented.