Using a current-algebra sum rule derived from an equal-time commutator between charge densities, we have set a lower bound on the muon-pair production cross sections. In spite of the fact that our estimate is conservative, the lower bound turns out to be very large at energies larger than ∼100 GeV ($\gtrsim 0.50\times {10}^{-30\mathrm{}}$ ${\mathrm{cm}}^2$ at 500 GeV, the maximum available energy at the National Accelerator Laboratory). Comparing it with recent experimental data at lower energies, we predict that the muon-pair production cross section will keep increasing until it turns over owing to higher-order electromagnetic interactions through unitarity.

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