Spectroscopy of the neutron-rich hypernucleus ${}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ from electron scattering

The missing mass spectroscopy of the ${}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ hypernucleus was performed using the ${}^7\mathrm{Li}(e,e^{\prime }{K}^{+}){}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ reaction at the Thomas Jefferson National Accelerator Facility Hall C. The $\mathrm{\Lambda }$-binding energy of the ground-state ($1/2^{+}$) was determined with a smaller error than that of the previous measurement, being $B_{\mathrm{\Lambda }}=5.55\pm 0.{10}^{\mathrm{stat}.}\pm 0.{11}^{\mathrm{sys}.}\mathrm{MeV}$. The experiment also provided new insight into charge symmetry breaking in $p$-shell hypernuclear systems. Finally, a peak at $B_{\mathrm{\Lambda }}=3.65\pm 0.{20}^{\mathrm{stat}.}\pm 0.{11}^{\mathrm{sys}.}\mathrm{MeV}$ was observed and assigned as a mixture of $3/2^{+}$ and $5/2^{+}$ states, confirming the “gluelike” behavior of $\mathrm{\Lambda }$, which makes an unstable state in ${}^6\mathrm{He}$ stable against neutron emission.

Figure 1

The binding energy ($-B_{\mathrm{\Lambda }}$) spectrum of ${}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ with an ordinate axis of $\overline{\left(\frac{d\sigma }{d{\mathrm{\Omega }}_K}\right)}$ defined in Ref. [21].

Figure 2

The spectrum of the binding energy ($-B_{\mathrm{\Lambda }}$) and the excitation energy $\{E_{\mathrm{\Lambda }}\equiv -[B_{\mathrm{\Lambda }}-B_{\mathrm{\Lambda }}(\#1)]\}$ for the ${}^7\mathrm{Li}(e,e^{\prime }{K}^{+}){}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ reaction with an ordinate axis of $\overline{\left(\frac{d\sigma }{d{\mathrm{\Omega }}_K}\right)}$ after the accidental $e^{\prime }{K}^{+}$-coincidence distribution was subtracted. The curve is a fit with two Voigt functions.

Figure 3

Measured $\mathrm{\Lambda }$-binding energies of ${}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ (present result and Ref. [18]), ${}_{\mathrm{\Lambda }}^7{\mathrm{Li}}^{*}$ [36, 37], and ${}_{\mathrm{\Lambda }}{}^7\mathrm{Be}$ [36] for the $1/2^{+}$ state with statistical error bars. Colored boxes on the experimental results of ${}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ indicate systematic errors on $B_{\mathrm{\Lambda }}$. The solid and dashed lines represent theoretical calculations without and with a phenomenological even-state $\mathrm{\Lambda }N$ CSB potential, which reproduces $\mathrm{\Lambda }$-binding energies of ${}_{\mathrm{\Lambda }}{}^4\mathrm{He}$ and ${}_{\mathrm{\Lambda }}{}^4\mathrm{H}$ by a four-body cluster model [39].

Figure 4

Obtained energy levels of ${}_{\mathrm{\Lambda }}{}^7\mathrm{He}$ with theoretical predictions [40, 41]. Reported energy levels for ${}^6\mathrm{He}$ are also shown [38].