Observation of the ${}_{\Lambda }{}^7\mathrm{He}$ Hypernucleus by the ($e$, $e^{\prime }{K}^{+}$) Reaction

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, ${}_{\Lambda }{}^7\mathrm{He}$, was observed for the first time with the ($e$, $e^{\prime }{K}^{+}$) reaction with an energy resolution of $\sim 0.6\mathrm{MeV}$. This resolution is the best reported to date for hypernuclear reaction spectroscopy. The ${}_{\Lambda }{}^7\mathrm{He}$ 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 $\Lambda N$ potential.

Figure 1

Schematic figure of the E01-011 experimental setup. A newly constructed HKS and an ENGE-type split-pole spectrometer that was also used in E89-009 were used as the kaon and scattered electron spectrometers. The HKS detector package consists of drift chambers (DC), time-of-flight counters (TOF), aerogel Čerenkov (AČ) and water Čerenkov (WČ) counters. The ENGE was vertically tilted to suppress background originating from bremsstrahlung and Møller scattering.

Figure 2

Missing mass spectrum of the $p(e,e^{\prime }{K}^{+})\Lambda /{\Sigma }^0$ reaction. The mass of the $\Lambda$ particle was subtracted. The $\Lambda$, ${\Sigma }^0$ peaks were used to calibrate the absolute missing mass scale. The line shows the accidental background estimated by the mixed events analysis method.

Figure 3

Binding energy spectrum obtained by the ${}^7\mathrm{Li}(e,e^{\prime }{K}^{+}){}_{\Lambda }{}^7\mathrm{He}$ reaction. The line shows the accidental background estimated by the mixed events analysis method. The events in the unbound region ($-B_{\Lambda }>0$) originate from quasifree $\Lambda$ production.

Figure 4

Binding energy spectra of ${}_{\Lambda }{}^7\mathrm{He}$ measured by the emulsion experiment [6] (top) and the JLab E01-011 (HKS) experiment after background subtraction and acceptance corrections (bottom).