The Belle II Collaboration

23 July 2020

Abstract: This note presents plots associated with the search for an invisible $Z^{\prime}$ in $e^+ e^- \rightarrow \mu^+ \mu^-$ + missing energy final states targeting early phase 3 data. Plots include a distribution of the remaining backgrounds expected from simulation after event selection and a MVA classification. Furthermore we show the expected 90\% CL$_s$ upper limits obtained for different simulated analysis conditions for both the cross section $\sigma ($e$^+ $e$^- \rightarrow\ \mu^+ \mu^-\ $inv.$)$ as well as the associated $Z^{\prime}$ coupling constant in the framework of an $L_{\mu}$ − $L_{\tau}$ theory.