Two-proton decay from the isobaric analog state in ${}^8\mathrm{B}$

The isobaric analog state (IAS) in ${}^8\mathrm{B}$ is confirmed to decay by two-proton emission to the isobaric analog state in ${}^6\mathrm{Li}$, which emits a 3.563 MeV $\gamma$ ray. Upper limits on the yield to isospin breaking decay channels are also reported. The three-body correlations from the two-proton decay from ${}^8\mathrm{B}{}_{\text{IAS}}$ to ${}^6\mathrm{Li}{}_{\text{IAS}}$ are statistically different than the two-proton decay of its isospin partner, ${}^8\mathrm{C}{}_{\text{g.s.}}$.

Figure 1

Set of levels relevant for the decay of the IAS in ${}^8\mathrm{B}$. The levels are labeled by their spin-parity $\left(J^{\pi }\right)$ and isospin $\left(T\right)$ quantum numbers. Colors indicate isospin-allowed transitions. The width of the $J^{\pi }=7/2^{-}\mathrm{state}$ in ${}^7\mathrm{Be}$ is not known but is assumed to be wide as in the mirror.

Figure 2

The excitation-energy spectrum from charged-particle reconstruction determined for ${}^8\mathrm{B}$ from detected (a) $2p\text{−}{}^6\text{Li}$ and (b) $2p\text{−}\alpha \text{−}d$ events. These energies assume no missing decay energy due to $\gamma$-ray emission.

Figure 3

$\gamma$-ray energies measured in coincidence with $p\text{−}p\text{−}{}^6\text{Li}$ events. This spectrum includes add back from nearest neighbors and has been event-wise Doppler corrected.

Figure 4

Projected three-body correlations from the decay of ${}^8\mathrm{B}{}_{\text{IAS}}$ to the $2p$ + ${}^6\mathrm{Li}{}_{\text{IAS}}$ exit channel in the [(a), (c)] T and [(b), (d)] Y Jacobi systems. Energy correlations are shown in panels (a) and (b); angular correlations in panels (c) and (d). The right panel shows the same as the left, but now the correlations are associated with the first step of ${}^8\mathrm{C}{}_{\text{g.s.}}$ decay to $2p+{}^6\text{Be}$.