Search for an invisible $Z^\prime$ in a final state with two muons and missing energy at Belle II

The Belle-II collaboration
Phys.Rev.Lett. 130 (2023) 231801, 2023.

Abstract
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$GeV/c^{2}$.

  • Fig. 2 (bottom right)

    Bottom right panel of Fig. 2

    10.17182/hepdata.138160.v1/t1

    Observed 90% CL upper limits on the cross section $\sigma (e^+ e^- \to \mu^+ \mu^- Z', Z' \to $ invisible)...

  • Fig. 3 (top left)

    Top left panel of Fig. 3

    10.17182/hepdata.138160.v1/t2

    Observed 90% CL upper limits on the coupling $g'$ for the fully invisible $L_\mu − L_\tau$ model as functions of...

  • Fig. 4 (middle left)

    Middle left panel of Fig. 4

    10.17182/hepdata.138160.v1/t3

    Observed 90% CL upper limits on the coupling $g'$ for the vanilla $L_\mu − L_\tau$ model as functions of the...

  • No figure

    No figure

    10.17182/hepdata.138160.v1/t4

    Best fit of the cross section.

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