Measurement of the branching fractions for $J/\psi \to {\ell }^{+}{\ell }^{-}$

We present measurements of the branching fractions for $J/\psi \to e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ using 3M $\psi (2S)$ decays collected with the CLEO detector operating at the CESR $e^{+}{e}^{-}$ collider. We obtain $\mathcal{B}(J/\psi \to e^{+}{e}^{-})=(5.945\pm 0.067\pm 0.042)\%$ and $\mathcal{B}(J/\psi \to {\mu }^{+}{\mu }^{-})=(5.960\pm 0.065\pm 0.050)\%$, leading to an average of $\mathcal{B}(J/\psi \to {\ell }^{+}{\ell }^{-})=(5.953\pm 0.056\pm 0.042)\%$ and a ratio of $\mathcal{B}(J/\psi \to e^{+}{e}^{-})/\mathcal{B}(J/\psi \to {\mu }^{+}{\mu }^{-})=(99.7\pm 1.2\pm 0.6)\%$, all consistent with, but more precise than, previous measurements.

Figure 1

For $\psi (2S)\to {\pi }^{+}{\pi }^{-}{\ell }^{+}{\ell }^{-}$ dimuon (left) and dielectron (right) candidate events in the $\psi (2S)$ data (solid circles), MC simulation of signal (solid histogram), distributions of the dilepton mass (top), the mass recoiling against the ${\pi }^{+}{\pi }^{-}$ pair (middle), and the dipion invariant mass. The arrows shown in each plot indicate nominal cut values, which are applied for the other plots in the figure.

Figure 2

For $\psi (2S)\to {\pi }^{+}{\pi }^{-}{\ell }^{+}{\ell }^{-}$ dimuon (left) and dielectron (right) candidate events in the $\psi (2S)$ data (solid circles) and MC simulation of signal (solid histogram), the polar angles of the positively charged lepton (top) and of the $J/\psi$ (bottom).

Figure 3

Fit result (solid histogram) of the ${\pi }^{+}{\pi }^{-}$ recoil mass spectrum in data (solid circles) as described in the text. The dashed curve represents the background shape.

Figure 4

Track multiplicity distribution for $J/\psi$ decays produced in $\psi (2S)\to {\pi }^{+}{\pi }^{-}J/\psi$. Left: Signal MC for nine decay modes, right: data distribution obtained from the sideband-subtracted inclusive ${\pi }^{+}{\pi }^{-}$ samples (solid circles) and the fit to MC samples of different multiplicities (solid line).