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$$ \mathcal{B}(B_s^0 \to K_S^0 K_S^0) = [8.3 \pm 1.6 (\rm{stat}) \pm 0.9 (\rm{syst}) \pm 0.8 (\rm{norm}) \pm 0.3 (f_s/f_d)] \times 10^{-6}, $$ 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 $$\frac{\mathcal{B}(B^0 \to K_S^0 K_S^0)}{\mathcal{B}(B_s^0 \to K_S^0 K_S^0)} = [7.5 \pm 3.1 (\rm{stat}) \pm 0.5 (\rm{syst}) \pm 0.3 (f_s/f_d)] \times 10^{-2}.$$