Resumming relativistic corrections to Γ[J/φ → e ⁺e^-]

We calculate the relativistic corrections to the color-singlet contributions to the leptonic decay width of the spin-triplet S-wave heavy quarkonium resummed to all orders in v. The formula is obtained by using the threshold-expansion method and is consistent with that based on the exact spin-projection operator. The color-singlet nonrelativistic quantum chromodynamics (NRQCD) matrix elements for the J/ψ and ψ(2S) mesons are determined by comparing the resummed formula with the measured leptonic decay widths of the two charmonia. A significant enhancement of the numerical values for the NRQCD matrix elements may result in a large increase in the theoretical predictions for the color-singlet contributions to the S-wave charmonium production cross-sections.