Remeasuring the anomalously enhanced $B(E2;2^{+}\to 1^{+})$ 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 $2^{+}\to 1^{+}$ 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-${\hat{I}}^3$ coincidences, yielding a revised $B(E2;2^{+}\to 1^{+})$ of $19{(}_{-6}^{+7})(2)$~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.

21 pages, 5 figures