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

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' -> mu+mu- 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 x 10-8-200 x 10-8 on the effective coupling epsilon 2 x alpha D for the A' mass in the range of 4.0 GeV/c2 < MA' < 9.7 GeV/c2 and for the h' mass Mh' < MA', where epsilon is the mixing strength between the standard model and the dark photon and alpha D is the coupling of the dark photon to the dark Higgs boson. Our limits are the first in this mass range.