Search for a μ⁺μ⁻ resonance in four-muon final states at Belle II

We report on a search for a resonance X decaying to a pair of muons in e⁺e⁻→ μ⁺ μ⁻ X events in the 0.212-9.000 GeV/c² mass range, using 178 fb⁻¹ of data collected by the BelleII experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of X beyond the standard model: a Z^ vector boson in the Lμ-Lτ model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90$%$ confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the Lμ-Lτ model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below 6 GeV/c², the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the Lμ-Lτ model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.