First direct mass-measurement of the two-neutron halo nucleus 6He and improved mass for the four-neutron halo 8He

The first direct mass-measurement of ${}^6$He has been performed with the TITAN Penning trap mass spectrometer at the ISAC facility. In addition, the mass of ${}^8$He was determined with improved precision over our previous measurement. The obtained masses are $m$(${}^6$He) = 6.018 885 883(57) u and $m$(${}^8$He) = 8.033 934 44(11) u. The ${}^6$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) fm and 1.959(16) fm for ${}^6$He and ${}^8$He respectively. We present a detailed comparison to nuclear theory for ${}^6$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.