Search for a Dark Photon and an Invisible Dark Higgs Boson in μ+μ- and Missing Energy Final States with the Belle II Experiment

Abstract

The dark photon A′ and the dark Higgs boson h′ are hypothetical particles predicted in many dark sector models. We search for the simultaneous production of A′ and h′ in the dark Higgsstrahlung process e+e-→A′h′ with A′→μ+μ- and h′ invisible in electron-positron collisions at a center-of-mass energy of 10.58 GeV in data collected by the Belle II experiment in 2019. With an integrated luminosity of 8.34 fb-1, we observe no evidence for signal. We obtain exclusion limits at 90% Bayesian credibility in the range of 1.7-5.0 fb on the cross section and in the range of 1.7×10-8-200×10-8 on the effective coupling µ2×αD for the A′ mass in the range of 4.0 GeV/c2 <9.7 GeV/c2 and for the h′ mass Mh′