An experiment with a newly developed high-resolution kaon spectrometer and a scattered electron spectrometer with a novel configuration was performed in Hall C at Jefferson Lab. The ground state of a neutron-rich hypernucleus, HeΛ7, was observed for the first time with the (e, e ′K+) reaction with an energy resolution of ∼0.6 MeV. This resolution is the best reported to date for hypernuclear reaction spectroscopy. The HeΛ7 binding energy supplies the last missing information of the A=7, T=1 hypernuclear isotriplet, providing a new input for the charge symmetry breaking effect of the ΛN potential.
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