Measurement of the branching fractions for $\psi \mathrm{}\left(2S\right)\mathrm{}\to e^{+}{e}^{-}$ and $\psi \mathrm{}\left(2S\right)\mathrm{}\to {\mu }^{+}{\mu }^{-}$

We measure the branching fractions of the $\psi \mathrm{}\left(2S\right)\mathrm{}$ meson to the leptonic final states $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ relative to that for $\psi \mathrm{}\left(2S\right)\mathrm{}\to J/\psi {\pi }^{+}{\pi }^{-}.$ The method uses $\psi \mathrm{}\left(2S\right)\mathrm{}$ mesons produced in the decay of B mesons at the ${\rm Y}\mathrm{}\left(4S\right)\mathrm{}$ resonance in a data sample collected with the BABAR detector at the Stanford Linear Accelerator Center. Using previous measurements for the $\psi \mathrm{}\left(2S\right)\mathrm{}\to J/\psi {\pi }^{+}{\pi }^{-}$ branching fraction, we determine the $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ branching fractions to be $0.0078\pm 0.0009\pm 0.0008$ and $0.0067\pm 0.0008\pm 0.0007,$ respectively.