Search for a dark leptophilic scalar produced in association with ${\tau }^{+}{\tau }^{-}$ pair in $e^{+}{e}^{-}$ annihilation at center-of-mass energies near 10.58 GeV

A dark leptophilic scalar $({\phi }_L)$ is a hypothetical particle that couples only to leptons rather than quarks. We report on a search for ${\phi }_L$ in the $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}{\phi }_L,{\phi }_L\to {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}(\mathrm{\ell }=e,\mu )$ process using 626~\invfb of data collected by the Belle experiment near the \Y4S resonance. We validate the backgrounds with multiple control regions in data, using a novel multiclass multivariate event classifier. In absence of a signal, we quote upper limits at the 90% confidence level on the coupling between ${\phi }_L$ and leptons. Our bounds, obtained in a blinded approach, are 19% more constraining than the previous limits, averaged over the mass range $0.04\le m_{{\phi }_L}\le 6.5\text{gev}$. We exclude the parameter space below 4~\gev favored by measurement of the anomalous magnetic moment of the muon.