We report on a search for a resonance X decaying to a pair of muons in e+e−→μ+μ−X events in the 0.212–9.000GeV/c2 mass range, using 178fb−1 of data collected by the Belle II 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 6GeV/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.

• Received 7 March 2024
• Accepted 29 April 2024

DOI:https://doi.org/10.1103/PhysRevD.109.112015

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society