Search for an Invisible Z' in a Final State with Two Muons and Missing Energy at Belle II

Abstract

The Lμ − Lτ extension of the standard model predicts the existence of a lepton-flavor-universality- violating Z' boson that couples only to the heavier lepton families. We search for such a Z' through its invisible decay in the process e⁺e⁻ → μ⁺μ⁻Z'. We use a sample of electron-positron collisions at a center-of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019–2020, corresponding to an integrated luminosity of 79.7 fb⁻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 × 10⁻³ at low Z' masses to 1 at Z' masses of 8 GeV/c².