Search for an invisibly decaying $Z^{\prime}$ boson at Belle II in $e^+ e^{\mathstrut -} \to \mu^+ \mu^{\mathstrut -} (e^{\pm} \mu^{\mp})$ + 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 $5 \times 10^{-2}$ -- $1$ $M_{Z^\prime}\leq 6$ GeV/$c^2$.
Tags
Working group Dark Sector and Low Multiplicity
Principal authors @  Michel Bertemes, Marcello Campajola, Luigi Corona, Giacomo De Pietro, Enrico Graziani, Gianluca Inguglia, Ilya Komarov, Alberto Martini, Laura Zani
Reference Phys.Rev.Lett. 124, 141801 (2020), DOI: 10.1103/PhysRevLett.124.141801
Document BELLE2-PUB-PH-2019-001
Links arXiv:1912.11276, Inspire, PRL
Bibtex 
@article{Belle-II:2019qfb,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{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}",
    eprint = "1912.11276",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2019-002, KEK Preprint 2019-55,",
    doi = "10.1103/PhysRevLett.124.141801",
    journal = "Phys. Rev. Lett.",
    volume = "124",
    number = "14",
    pages = "141801",
    year = "2020"
}