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Two types of pseudoscalar quarkonium electromagnetic decay processes, i.e.decay to a lepton pair, and to a lepton pair plus a photon (Dalitz decay), areanalyzed at the leading order in NRQCD expansion. The former type of processes,highly suppressed in the Standard Model, have been hoped to act as thesensitive probes of the possible new physics. The latter type of processesgenerally possess much greater decay rates than the former, owing to severalconspiring factors. The recently launched BES-III program, with $10^8$ $\eta_c$samples to be anticipated in the coming years, may be able to observe theDalitz decays $\eta_c\to e^+e^-\gamma$ and $\eta_c\to \mu^+\mu^-\gamma$, whichhave branching ratios of order $10^{-6}$. When the radiated photon becomes verysoft, the Dalitz decay events will be experimentally tagged as the exclusivelepton pair events. It is found that, those quasi-two-body events that arisefrom $\eta_c\to e^+ e^-\gamma$ with photon energy less than the minimumsensitivity of the electromagnetic calorimeter, can vastly outnumber theliteral $\eta_c\to e^+ e^-$ events, however this amplification is still notdramatic enough for the BES-III experiment to establish these events.Consequently, the expectation of looking for new physics signature in the$\eta_c\to l^+l^-$ channel is obscured, unless the contamination from $\eta_c\to l^+l^-\gamma$ has been taken into account carefully.

Observation prospects of leptonic and Dalitz decays of pseudoscalar quarkonia