Measurement of the branching fraction of the decay $B_s^0\to K_S^0 K_S^0$.

A measurement of the branching fraction of the decay $B_s^0 \to K_S^0 K_S^0$ is performed using proton-proton collision data corresponding to an integrated luminosity of 5 $fb^{-1}$ collected by the LHCb experiment between 2011 and 2016. The branching fraction is determined to be BR($B_s^0 \to K_S^0 K_S^0$) = [8.3 $\pm$ 1.6 (stat) $\pm$ 0.9 (syst) $\pm$ 0.8 (norm) $\pm$ 0.3 ($f_s/f_d$)] $\times 10^{-6}$ , where the first uncertainty is statistical, the second is systematic, and the third and fourth are due to uncertainties on the $B^0 \to \phi K_S^0$ branching fraction and the ratio of hadronization fractions $f_s/f_d$. This is the most precise measurement of this branching fraction to date. Furthermore, a measurement of the branching fraction of the decay $B^0 \to K_S^0 K_S^0$ is performed relative to that of the $B_s^0 \to K_S^0 K_S^0$ channel, and is found to be $\frac{BR(B^0 \to K_S^0 K_S^0)}{BR(B_s^0 \to K_S^0 K_S^0)}$ = [7.5 $\pm$ 3.1 (stat) $\pm$ 0.5 (syst) $\pm$ 0.3 ($f_s/f_d$)] $\times 10^{-2}$.