Physical Review Letters 124
, 141801 (2020)
ISSN: 0031-9007, SCI
Search for an invisibly decaying Z′ boson at Belle II in e^+ e^− → μ^+ μ^− ( e ^± μ ^∓ ) plus missing energy final states
I. Adachi, ..., N. Anh Ky et al. (Belle II collaboration)
Theories beyond the standard model often predict the existence of an additional neutral boson, the Z'.
Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the
first searches for the invisible decay of a Z0 in the process e^+ e^− → μ^+ μ^−Z' and of a lepton-flavor-violating
Z' in e^+e^− → e^∓ μ^∓Z'.We do not find any excess of events and set 90% credibility level upper limits on the
cross sections of these processes.We translate the former, in the framework of an Lμ − Lτ theory, into upper
limits on the Z' coupling constant at the level of 5 × 10^−2 − 1 for MZ' ≤ 6 GeV/c^2.