Comparing invariant-mass spectroscopy of 8B with ab initio predictions

TL;DR

This work combines invariant-mass spectroscopy of $^8$B produced by proton knockout from fast $^9$C and projectile fragmentation of $^{13}$O with ab initio SA-NCSM calculations that include continuum coupling and the NNLO$_{opt}$ chiral interaction. The three exit channels $2p+^{6}$Li, $p+^{3}$He+$\alpha$, and $p+^{7}$Be reveal new resonances; momentum correlations and $\gamma$-ray coincidences illuminate decay pathways, including a prompt 2$p$ decay. SA-NCSM predictions enable spin assignments and branching analyses, showing that all new levels are consistent with positive-parity states accessible through $p$-wave knockout, while suggesting a degree of spin mixing in some cases. Overall, the agreement between experiment and theory supports the SA-NCSM in describing unbound $A=8$ systems and underscores the role of continuum coupling in ab initio nuclear structure.

Abstract

Levels in 8B have been investigated experimentally using the invariant-mass technique and compared to ab initio calculations. Data sets obtained using E/A=69-MeV 9C and 13O beams on a Be target have been further analyzed to extend the level scheme of 8B for Ex<10 MeV. New levels were observed in the 2p+6Li, p+3He+alpha, and the p+7Be+gamma exit channels. Momentum correlations between the decay fragments were also investigated in order to deduce the decay pathways and whether the decays are prompt or sequential. This nucleus and its mirror were also investigated in the ab initio symmetry-adapted no-core shell model. Correspondence between the newly observed and predicted levels were made based on the level energy and the decay modes. For positive parity levels with J<=3, all predicted levels can be connected to an experimental counter part (as least tentatively) for Ex<8.4 MeV.

Figures (11)

• Figure 1: Excitation energy in $^8$B obtained from the invariant-mass of detected 2$p$+$^6$Li events assuming the $^6$Li fragment was in its ground state. Data in (a) and (b) were obtained with the $^9$C(2$^{nd}$) and $^{13}$O data sets, respectively. The red curves show fits to these data with contributions from resonance peaks given by the dotted green curves and the background shown as the dashed blue curves. The gate $P1$ about the 8.4-MeV state is used for the correlations shown Fig. \ref{['fig:Corr_2pLi6compare']}.
• Figure 2: (a) Energy spectrum of $\gamma$-rays detected in coincidence with the 2$p$+$^6$Li channel in the $^9$C(2$^{nd}$) data set. The gate $G2$ around the 3.55-MeV full-absorption peak and its first escape peak is used to enhance the yield associated with the 0$^+$ state relative to the 1$^+$ ground state of $^6$Li. (b) $E^*_{n\gamma}$ distribution obtained from the invariant mass of 2$p$+$^6$Li events. The blue histogram is for all events while the red histogram has a $\gamma$ ray in the $G2$ gate. A fit to the latter with the same contributions as in Fig. \ref{['fig:Inv_2pLi6']} is shown by the curves.
• Figure 3: Level diagram of the states observed in the 2$p$+$^{6}$Li, $p$+$^{3}$He+$\alpha$, and $p$+$^7$Be+$\gamma$ channels where the indicated sequential decay pathways have been observed. The 0$^+_2$ state in $^8$B decays by 2$p$ emission to the excited 0$^+$ state in $^6$Li which subsequently decays to the ground state via $\gamma$-ray emission. This 0$^+_2$ state corresponds to the narrow peaks in Figs. \ref{['fig:Inv_2pLi6']}(a) and \ref{['fig:Inv_2pLi6']}(b).
• Figure 4: Spectra obtained from $p$+$^{3}$He+$\alpha$ events produced with the $^9$C(2$^{nd}$) data set. (a) The $^7$Be excitation-energy distribution obtained from the $^3$He+$\alpha$ subevents. The observed peaks are labeled by their known $J^\pi$. The gate $P2$ is used to select or veto the 7/2$^-$ intermediate state in $^7$Be. The $^8$B excitation energy spectrum from the $p$+$^3$He+$\alpha$ events obtained in coincidence with this gate is shown in (b), while (c) is obtained by vetoing with this gate. The solid red curves are fits to the data with peak contributions shown by the dotted green curves and backgrounds indicated by the dashed blue curves. The gate $P3$ around the 8.2-MeV peak is used in the constructions of the correlations shown in Fig. \ref{['fig:corr_pahe3']}.
• Figure 5: Data points correspond to the background-subtracted correlations in the decay of the 8.2-MeV state in $^8$B to the $p$+$^3$He+$\alpha$ channel obtained with the $^{9}$C(1$^{st}$) data set. The projections of the correlations on the (a) $p$-$\alpha$, (b) $\alpha$-$^3$He, and (c) $p$+$^3$He relative-energy axes are shown. The solid green and solid blue curves shown predictions of sequential decay simulations through the $^5$Li(g.s.) and $^7$Be(5/2$^{-}_{1}$) intermediate states, respectively. The solid red curve is the best fit allowing contributions from both sequential decay scenarios.