Search for an invisibly decaying Z boson at Belle II in e+eμ+μ(e±μ) plus missing energy final states

Abstract: 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 Z′ in the process e+eμ+μ Z' and of a lepton-flavor-violating Z′ in e+ee±μZ'. 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/c2.

A simulated event in which a Z' boson is produced in association with two muons (blue tracks with green hits) and decays into invisible particles that detection and are shown in yellow.

90%CL upper limits on coupling constant g. Dark blue filled areas show the exclusion regions for gat 90% CL, assuming the LμLτ predicted BF for Zinvisible; light blue areas are for BF(Zinvisible) = 1. The solid and dashed lines are the expected sensitivities in the two hypotheses. The red band shows the region that could explain the muon anomalous magnetic moment(g2)μ±2σ

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