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

TL;DR

This paper reports a search for a hypothetical dark leptophilic scalar particle in electron-positron collisions, setting new constraints on its coupling to leptons and excluding certain parameter spaces relevant to the muon magnetic moment anomaly.

Contribution

The study introduces a novel multivariate classifier for background validation and provides the most stringent limits to date on the scalar's coupling in the specified mass range.

Findings

No signal observed, upper limits set on lepton coupling at 90% CL.

Constraints are 19% more restrictive than previous limits.

Excluded parameter space below 4 GeV related to muon magnetic moment anomaly.

Abstract

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 $e^+e^- \to \tau^+ \tau^- \phi_L, ~\phi_L \to \ell^+ \ell^- ~(\ell = e, \mu)$ 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 $0.04 \leq m_{\phi_L} \leq 6.5~\gev$. We exclude the parameter space below 4~\gev favored by measurement of the anomalous magnetic moment of the muon.