Key Points Introduction $B_s^0 \to \mu^+\mu^-$ is a FCNC decay heavily suppressed in the SM — new physics can enhance it. The branching fraction is extracted from a simultaneous fit across BDT categories. $B^+ \to J/\psi K^+$ serves as the normalization channel to cancel many systematic uncertainties. Signal MC Fit The signal shape is fixed from MC, with a floating mean shift and resolution scale fitted in data. The combinatorial background uses an exponential; the $J/\psi^+X$ tail uses an error function. Normalization Channel The normalization channel $B^+ \to J/\psi K^+$ cancels many systematic uncertainties. fs/fu is measured from data using $B_s \to J/\psi\phi$ and $B^+ \to J/\psi K^+$. Useful links Long-Lived Particles Subgroup Twiki Page EXO-20-015 CADI page