Direct measurement of theJ/ψ leptonic branching fraction

The Mark III Collaboration has measured the J/ψ leptonic branching fractions using the process ψ(2S)→${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$J/ψ, J/ψ→${\mathit{l}}^{+}$ ${\mathit{l}}^{\mathrm{-}}$. The results are B(J/ψ→${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$)=(5.92±0.15±0.20)% and B(J/ψ→${\mathrm{\mu }}^{+}$ ${\mathrm{\mu }}^{\mathrm{-}}$)=(5.90±0.15±0.19)%, where the first error is statistical and the second is systematic. Assuming lepton universality, the leptonic branching fraction of the J/ψ is (5.91±0.11±0.20)%. This result is used to obtain the strong coupling constant ${\mathrm{\alpha }}_{\mathit{s}}$ and the QCD scale factor ${\mathrm{\Lambda }}_{\mathrm{MS}}$ (MS¯ denotes the modified minimal-subtraction scheme).