Double isobaric analog of ${}^{11}$Li in ${}^{11}$B

The ground state of ${}^{11}$Li is the preeminent example of a two-neutron-halo nucleus and is part of an isobaric sextet. Another member of this sextet, the double isobaric analog of ${}^{11}$Li${}_{\mathrm{g}.\mathrm{s}.}$ in ${}^{11}$B, has been identified in the ${}^{12}$Be($p,2n$)${}^{11}$B reaction. The state was discovered through its two-proton decay branch. From detected $2p+{}^9$Li events, its excitation energy was determined to be 33.57(8) MeV using the invariant mass method. With the known masses of ${}^{11}$Li and its isobaric analog state in ${}^{11}$Be, three members of the same sextet have measured masses for the first time, permitting the masses of the remaining members to be extrapolated using the isobaric multiplet mass equation. All members of this multiplet are expected to have a two-nucleon halo structure and this was found consistent with the evolution of the mass across the sextet. The momentum correlations of the detected protons were found to have both “diproton” and “cigar” components.

Figure 1

Distribution of total decay kinetic energy $E_T$ determined from the detected 2$p$+${}^9$Li events. The solid curve shows a fit to the data with a single Breit-Wigner peak including the experimental resolution; the dashed curve indicates the estimated background.

Figure 2

Mass excesses of the three known members of the $A$ $=$ 11 sextet plotted as a function of isospin projection. The solid band shows the prediction for a homogeneous sphere with the same radius as the $T$ $=$ 1/2, $A$ $=$ 11 doublet. The dashed curve shows the quadratic IMME curve which passes through the three data points.

Figure 3

Three-body correlations displayed as energy distributions in the Jacobi (a) Y and (b) T systems. For comparison, the correlations measured for the two-proton decay of ${}^6$Be [13], normalized to the same number of events, are shown by the curves.