PHYSICAL REVIEW D 130, 231801 (2023)

ISSN: 2470-0029, SCIE

Search for an Invisible Z0 in a Final State with Two Muons and Missing Energy at Belle II

The Belle II collaboration (I. Adachi, ..., N. Anh Ky, ...)

The Lμ − Lτ extension of the standard model predicts the existence of a lepton-flavor-universality- violating Z0 boson that couples only to the heavier lepton families. We search for such a Z0 through its invisible decay in the process eþe− → μþμ−Z0. 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 Z0 masses to 1 at Z0 masses of 8 GeV=c2.

DOI: DOI: 10.1103/PhysRevLett.130.231801