Search for an invisibly decaying $Z^{\prime }$ boson at Belle II in $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}(e^{\pm }{\mu }^{\mp })$ plus missing energy final states

Abstract

Theories beyond the standard model often predict the existence of an additional neutral boson, the $Z^{\prime }$. Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the first searches for the invisible decay of a $Z^{\prime }$ in the process $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}{Z}^{\prime }$ and of a lepton-flavor-violating $Z^{\prime }$ in $e^{+}{e}^{-}\to e^{\pm }{\mu }^{\mp }{Z}^{\prime }$. We do not find any excess of events and set 90\% credibility level upper limits on the cross sections of these processes. We translate the former, in the framework of an $L_{\mu }-L_{\tau }$ theory, into upper limits on the $Z^{\prime }$ coupling constant at the level of $5\times 10^{-2}$ -- $1$ $M_{Z^{\prime }}\le 6$ GeV/$c^2$.

9 pages, 5 figures + supplemental materials. Accepted for publication in Physical Review Letters