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

Abstract

The $L_{\hat{I}}¼-L_{\ddot{I}}„$ 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 $e^{+}{e}^{-}\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}{Z}^{\prime }$. 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$GeV/c^2$.