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

TL;DR

This study searches for an invisible $Z'$ boson predicted by a lepton-flavor-violating extension of the standard model using Belle II data, setting upper limits on its production and coupling without observing a signal.

Contribution

First search for an invisible $Z'$ boson in the $e^+ e^- o mbda^+ mbda^- Z'$ process at Belle II, providing new constraints on the model parameters.

Findings

No excess over standard model background observed.

Set upper limits on the cross section and coupling of the $Z'$ boson.

Constraints vary from $3 imes 10^{-3}$ to 1 depending on $Z'$ mass.

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$GeV/c^{2}$.