  A dark leptophilic scalar $(\phi_L)$ is a hypothetical particle from the dark
sector, that couples only to leptons rather than quarks. In the \mev to the
\gev range, such particles are weakly constrained by direct detection
experiments. We report on a search for $\phi_L$ in $e^+e^- \to \tau^+ \tau^-
\phi_L, ~\phi_L \to \ell^+ \ell^- ~(\ell = e, \mu)$ process using 626 fb$^{-1}$
of data collected by the Belle experiment near the $\Upsilon(4S)$ resonance. We
validate the backgrounds with multiple control regions in data, using a novel
multiclass multivariate event classifier. In absence of signal in the $\phi_L$
mass range $0.04~Gev \leq m_{\phi_{L}} \leq 6.5~Gev$, we quote frequentist
limits on the coupling between $\phi_L$ and leptons, where nuisance parameters
describing the systematic variations are obtained from data. Our bounds,
obtained in a blinded approach, are comparable to the Bayesian limits obtained
by the BABAR experiment, and exclude the parameter space below $4~GeV$ favored
by measurement of the anomalous magnetic moment of the muon.

High Energy Physics - Experiment