Measurement of the $\mathit{B}\left(\mathit{E}2\right)$ of ${}_{\Lambda }^7\mathrm{Li}$ and Shrinkage of the Hypernuclear Size

We report on the first measurement of a hypernuclear $\gamma$-transition probability. $\gamma$ rays emitted in the $E2\mathrm{}(5/2^{+}\to 1/2^{+})$ transition of ${}_{\Lambda }^7\mathrm{Li}$ were detected by a large-acceptance germanium detector array (Hyperball), and the lifetime of the parent state ( $5/2^{+}$) was determined by the Doppler shift attenuation method. The obtained result, ${5.8}_{-0.7\mathrm{}}^{+0.9\mathrm{}}\pm 0.7\mathrm{ps}$, was then converted into the reduced transition probability [ $B\left(E2\mathrm{}\right)$] to be $B({E2;5/2\mathrm{}}^{+}\to {1/2}^{+})=3.6\mathrm{}\pm {0.5}_{-0.4\mathrm{}}^{+0.5\mathrm{}}{e}^2{\mathrm{fm}}^4$. Compared with the $B\left(E2\mathrm{}\right)$ of the corresponding $E2\mathrm{}(3^{+}\to 1^{+})$ transition in the ${}^6\mathrm{Li}$ nucleus, our result gives evidence that the size of the ${}^6\mathrm{Li}$ core in ${}_{\Lambda }^7\mathrm{Li}$ is smaller than the ${}^6\mathrm{Li}$ nucleus in the free space.