= Solution
At an <electron-positron collider> tuned to the <charmonium> <resonance>, select the mixed leptonic <tau lepton> decays
$$
\tau^-\to e^-\bar\nu_e\nu_\tau,\qquad \tau^+\to\mu^+\nu_\mu\bar\nu_\tau,
$$
and their charge conjugates. Require exactly two well-reconstructed oppositely charged tracks, one identified <electron> and one identified <muon>, little extra visible activity, visible <energy> below the beam total, and significant <missing momentum> pointing into the instrumented region. The multiple <neutrinos> prevent full reconstruction of each <tau lepton>, but the known initial <four-momentum>, lepton spectra and missing <energy> support a statistical signal extraction. Off-resonance data constrain nonresonant <electron-positron annihilation> into a <tau lepton> pair; that continuum is present at both beam <energies>.<First Measurement of the Branching Fraction of the Decay psi(2S) to tau pairs by the BES Collaboration>[Event selection using mixed electron-muon tau decays and continuum subtraction]{ref}
A resonant <muon> pair is one possible detector background: one <muon> can be misidentified as an <electron>, while radiation escaping along the beam or a badly measured track produces apparent <missing energy>. Particle-identification requirements and rejection of almost back-to-back tracks near the beam <momentum> suppress it. This background needs detector mismeasurement; an ideal measurement would distinguish it.
Above the open-charm threshold, $\psi(3770)\to D\bar D$ supplies an additional resonant background. Semileptonic decays of the two <D mesons> can give an <electron>, a <muon> and <neutrinos>; unreconstructed hadrons let some events pass the selection. This on-shell <D meson>-pair background is kinematically forbidden at $\psi(3686)$.
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