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

Kavli Affiliate: T. Higuchi

| First 5 Authors: Belle II Collaboration, I. Adachi, K. Adamczyk, L. Aggarwal, H. Ahmed

| Summary:

We report on a search for a resonance $X$ decaying to a pair of muons in
$e^{+}e^{-}rightarrow mu^+ mu^- X$ events in the 0.212-9.000 GeV/$c^{2}$
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 $Z^{prime}$
vector boson in the $L_{mu}-L_{tau}$ 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 $L_{mu}-L_{tau}$
model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses
below 6 GeV/$c^{2}$, the corresponding constraints on the couplings of these
processes to the standard model range from 0.0008 to 0.039 for the
$L_{mu}-L_{tau}$ 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.

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