Kavli Affiliate: T. Higuchi
| First 5 Authors: Belle II Collaboration, I. Adachi, K. Adamczyk, L. Aggarwal, H. Ahmed
| Summary:
The $L_{mu}-L_{tau}$ extension of the standard model predicts the existence
of a lepton-flavor-universality-violating $Z^{prime}$ boson that couples only
to the heavier lepton families. We search for such a $Z^prime$ through its
invisible decay in the process $e^+ e^- to mu^+ mu^- Z^{prime}$. We use a
sample of electron-positron collisions at a center-of-mass energy of 10.58GeV
collected by the Belle II experiment in 2019-2020, corresponding to an
integrated luminosity of 79.7fb$^{-1}$. We find no excess over the expected
standard-model background. We set 90$%$-confidence-level upper limits on the
cross section for this process as well as on the coupling of the model, which
ranges from $3 times 10^{-3}$ at low $Z^{prime}$ masses to 1 at $Z^{prime}$
masses of 8$GeV/c^{2}$.
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