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

TL;DR

This paper remeasures the $B(E2; 2^+ ightarrow 1^+)$ transition strength in $^8$Li using improved methods, finding a value lower than previous reports but still unusually high compared to theoretical predictions.

Contribution

The study provides a revised measurement of the $B(E2)$ strength in $^8$Li, clarifying discrepancies with prior data and theoretical models.

Findings

Revised $B(E2)$ value of 19(^{+7}_{-6})(2) e^2fm^4 in $^8$Li

The new measurement is about three times smaller than previous reports

The $B(E2)$ strength remains anomalously high compared to models.

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.