Remeasuring the anomalously enhanced $B(E2; 2^+ \rightarrow 1^+)$ in $^8\mathrm{Li}$

Abstract

The large reported $E2$ strength between the $2^+$ ground state and $1^+$ first excited state of $\isotope[8]{Li}$, $B(E2; 2^+ \rightarrow 1^+)= 55(15)\,e^2\fm^4$, presents a puzzle. Unlike in neighboring $A=7\text{--}9$ isotopes, where enhanced $E2$ strengths may be understood to arise from deformation as rotational in-band transitions, the $2^+\rightarrow1^+$ transition in $^8$Li cannot be understood in any simple way as a rotational in-band transition. Moreover, the reported strength exceeds \textit{ab initio} predictions by an order of magnitude. In light of this discrepancy, we revisited the Coulomb excitation measurement of this strength, now using particle-$\gamma$ coincidences, yielding a revised $B(E2; 2^+ \rightarrow 1^+)$ of $19(^{+7}_{-6})(2)$~e$^2$fm$^4$. We explore how this value compares to what might be expected in the limits of rotational models. While the present value is about a factor of three smaller than previously reported, it remains anomalously enhanced.