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

We report the first search for a nonstandard-model resonance decaying into τ pairs in e+e-→μ+μ-τ+τ- events in the 3.6-10 GeV/c2 mass range. We use a 62.8 fb-1 sample of e+e- collisions collected at a center-of-mass energy of 10.58 GeV by the Belle II experiment at the SuperKEKB collider. The analysis probes three different models predicting a spin-1 particle coupling only to the heavier lepton families, a Higgs-like spin-0 particle that couples preferentially to charged leptons (leptophilic scalar), and an axionlike particle, respectively. We observe no evidence for a signal and set exclusion limits at 90% confidence level on the product of cross section and branching fraction into τ pairs, ranging from 0.7 to 24 fb, and on the couplings of these processes. We obtain world-leading constraints on the couplings for the leptophilic scalar model for masses above 6.5 GeV/c2 and for the axionlike particle model over the entire mass range.