Search for an Invisible Z^{'} in a Final State with Two Muons and Missing Energy at Belle II

Area of Science:

• Particle Physics
• High-Energy Physics
• Beyond Standard Model Physics

Background:

• The Standard Model of particle physics has limitations and requires extensions.
• The Lμ–Lτ extension predicts a new Z' boson interacting with heavier leptons.
• Lepton flavor universality violation is a key prediction of this extension.

Purpose of the Study:

• To search for the Z' boson predicted by the Lμ–Lτ extension.
• To investigate the invisible decay of the Z' boson in electron-positron collisions.
• To set limits on the Z' boson's properties and coupling strength.

Main Methods:

• Utilized electron-positron collision data from the Belle II experiment (79.7 fb⁻¹ at 10.58 GeV).
• Searched for the process e⁺e⁻ → μ⁺μ⁻Z' with subsequent invisible Z' decay.
• Analyzed data for an excess beyond the Standard Model background predictions.

Main Results:

• No significant excess of events attributable to the Z' boson was observed.
• Established 90%-confidence-level upper limits on the cross section for the e⁺e⁻ → μ⁺μ⁻Z' process.
• Constrained the coupling of the Z' boson, ranging from 3×10⁻³ to 1 for Z' masses up to 8 GeV/c².

Conclusions:

• The Belle II experiment found no evidence for the predicted Z' boson in this specific search channel.
• The results place constraints on the parameter space of the Lμ–Lτ extension of the Standard Model.
• Further data collection and analysis at Belle II will improve sensitivity to such exotic particles.