First Direct Mass Measurement of the Two-Neutron Halo Nucleus ${}^6\mathrm{He}$ and Improved Mass for the Four-Neutron Halo ${}^8\mathrm{He}$

The first direct mass measurement of ${}^6\mathrm{He}$ has been performed with the TITAN Penning trap mass spectrometer at the ISAC facility. In addition, the mass of ${}^8\mathrm{He}$ was determined with improved precision over our previous measurement. The obtained masses are $m({}^6\mathrm{He})=6.018885883(57)\mathrm{u}$ and $m({}^8\mathrm{He})=8.03393444(11)\mathrm{u}$. The ${}^6\mathrm{He}$ value shows a deviation from the literature of $4\sigma$. With these new mass values and the previously measured atomic isotope shifts we obtain charge radii of 2.060(8) and 1.959(16) fm for ${}^6\mathrm{He}$ and ${}^8\mathrm{He}$, respectively. We present a detailed comparison to nuclear theory for ${}^6\mathrm{He}$, including new hyperspherical harmonics results. A correlation plot of the point-proton radius with the two-neutron separation energy demonstrates clearly the importance of three-nucleon forces.

Figure 1

Time-of-flight resonance spectra of ${}^6\mathrm{He}^{+}$ and ${}^8\mathrm{He}^{+}$. The solid line (red) is a fit of the theoretical line shape [22]. The shortest time of flight is achieved when the ions are excited at the cyclotron frequency, i.e., when ${\nu }_{RF}={\nu }_c$.

Figure 2

Correlation plot of the ${}^6\mathrm{He}$ point-proton radius $r_{\mathrm{pp}}$ versus two-neutron separation energy $S_{2n}$. The experimental range (bar) is compared to theory based on different ab initio methods using different NN interactions only (open symbols) and including $3N$ forces fit to light nuclei (filled symbols). The ab initio methods are indicated in the legend and the nuclear forces next to the symbols (for details see text). Theoretical error bars are shown where available.