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

TL;DR

This paper reports the first search for invisibly decaying or lepton-flavor-violating $Z^{\u2019}$ bosons at Belle II, setting upper limits on their production cross sections and coupling constants in specific theoretical models.

Contribution

It introduces the first experimental search for invisible and lepton-flavor-violating $Z^{}$ bosons at Belle II, providing new constraints on these processes.

Findings

No excess events observed in the analyzed channels.

Set 90% credibility level upper limits on cross sections.

Derived upper limits on $Z^{}$ coupling constants for $M_{Z'} \u2264 6$ GeV/$c^2$.

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 $5 \times 10^{-2}$ -- $1$ $M_{Z^\prime}\leq 6$ GeV/$c^2$.