High-resolution hypernuclear spectroscopy at Jefferson Lab, Hall A

The experiment E94-107 in Hall A at Jefferson Lab started a systematic study of high-resolution hypernuclear spectroscopy in the $0p$-shell region of nuclei such as the hypernuclei produced in electroproduction on $^{9}\mathrm{Be},\phantom{\rule{0.16em}{0ex}}^{12}\mathrm{C}$, and $^{16}\mathrm{O}$ targets. In order to increase counting rates and provide unambiguous kaon identification, two superconducting septum magnets and a ring-imaging Cherenkov detector were added to the Hall A standard equipment. The high-quality beam, the good spectrometers, and the new experimental devices allowed us to obtain very good results. For the first time, measurable strength with sub-MeV energy resolution was observed for the core-excited states of $_{\phantom{\rule{3.33333pt}{0ex}}\mathrm{\ensuremath{\Lambda}}}^{12}\mathrm{B}$. A high-quality $_{\phantom{\rule{3.33333pt}{0ex}}\mathrm{\ensuremath{\Lambda}}}^{16}\mathrm{N}$ hypernuclear spectrum was likewise obtained. A first measurement of the $\mathrm{\ensuremath{\Lambda}}$ binding energy for $_{\phantom{\rule{3.33333pt}{0ex}}\mathrm{\ensuremath{\Lambda}}}^{16}\mathrm{N}$, calibrated against the elementary reaction on hydrogen, was obtained with high precision, $13.76\ifmmode\pm\else\textpm\fi{}0.16$ MeV. Similarly, the first $_{\mathrm{\ensuremath{\Lambda}}}^{9}\mathrm{Li}$ hypernuclear spectrum shows general agreement with theory (distorted-wave impulse approximation with the SLA and BS3 electroproduction models and shell-model wave functions). Some disagreement exists with respect to the relative strength of the states making up the first multiplet. A $\mathrm{\ensuremath{\Lambda}}$ separation energy of 8.36 MeV was obtained, in agreement with previous results. It has been shown that the electroproduction of hypernuclei can provide information complementary to that obtained with hadronic probes and the $\ensuremath{\gamma}$-ray spectroscopy technique.