Search for an Invisible <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>Z</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math> in a Final State with Two Muons and Missing Energy at Belle II

Abstract

The ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ extension of the standard model predicts the existence of a lepton-flavor-universality-violating ${Z}^{\ensuremath{'}}$ boson that couples only to the heavier lepton families. We search for such a ${Z}^{\ensuremath{'}}$ through its invisible decay in the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{Z}^{\ensuremath{'}}$. 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 $79.7\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. 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 $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at low ${Z}^{\ensuremath{'}}$ masses to 1 at ${Z}^{\ensuremath{'}}$ masses of $8\text{ }\text{ }\mathrm{GeV}/{c}^{2}$.