Search for a τ^{+}τ^{-} Resonance in e^{+}e^{-}→μ^{+}μ^{-}τ^{+}τ^{-} Events with the Belle II Experiment

Area of Science:

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

Background:

• The Standard Model of particle physics has limitations and does not explain phenomena like dark matter or neutrino masses.
• Searches for new particles and interactions are crucial for extending our understanding of fundamental physics.
• Lepton flavor universality is a key principle tested in high-energy physics experiments.

Purpose of the Study:

• To conduct the first search for a nonstandard-model resonance decaying into tau lepton pairs.
• To probe models predicting new spin-0 or spin-1 particles interacting with leptons.
• To set exclusion limits on the properties and couplings of hypothetical new particles.

Main Methods:

• Analysis of e^{+}e^{-} collision data collected by the Belle II experiment.
• Selection of events with a muon pair and a tau pair (e^{+}e^{-}→μ^{+}μ^{-}τ^{+}τ^{-}).
• Comparison of observed data with Standard Model predictions in the 3.6-10 GeV/c^{2} mass range.

Main Results:

• No significant excess of events consistent with a new resonance was observed.
• Exclusion limits at 90% confidence level were set on the product of cross section and branching fraction into tau pairs, ranging from 0.7 to 24 fb.
• World-leading constraints were obtained for leptophilic scalar and axionlike particle models.

Conclusions:

• The search found no evidence for the hypothesized nonstandard-model resonances.
• The results provide stringent constraints on specific theoretical models beyond the Standard Model.
• This study contributes to the ongoing quest for new physics by limiting parameter spaces of proposed extensions.