Dielectron Widths of the ${\rm Y}(1S,2S,3S)$ Resonances

We determine the dielectron widths of the ${\rm Y}(1S)$, ${\rm Y}(2S)$, and ${\rm Y}(3S)$ resonances with better than 2% precision by integrating the cross section of $e^{+}{e}^{-}\to {\rm Y}$ over the $e^{+}{e}^{-}$ center-of-mass energy. Using $e^{+}{e}^{-}$ energy scans of the ${\rm Y}$ resonances at the Cornell Electron Storage Ring and measuring ${\rm Y}$ production with the CLEO detector, we find dielectron widths of $1.252\pm 0.004({\sigma }_{\mathrm{stat}})\pm 0.019({\sigma }_{\mathrm{syst}})\mathrm{keV}$, $0.581\pm 0.004\pm 0.009\mathrm{keV}$, and $0.413\pm 0.004\pm 0.006\mathrm{keV}$ for the ${\rm Y}(1S)$, ${\rm Y}(2S)$, and ${\rm Y}(3S)$, respectively.

Figure 1

Two of the distributions used to identify hadronic ${\rm Y}$ decays. Solid histograms are data, dashed are simulated ${\rm Y}(1S)$ decays, both with all hadronic selection criteria applied except the one shown. Dotted lines indicate selection thresholds.

Figure 2

The event yield as a function of center-of-mass energy in the region of the ${\rm Y}(3S)$. The top points are data, with the fit superimposed, and the dashed curve represents the sum of all backgrounds. The lower points and lines show the individual non-$1/s$ background contributions.

Figure 3

Mass of the system recoiling against the ${\pi }^{+}{\pi }^{-}$ in ${\rm Y}(2S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ candidates satisfying the two-track trigger, for (a) all events, and (b) events that satisfy the hadronic trigger. The dashed curve represents backgrounds and the solid curve represents the sum of backgrounds and the recoiling ${\rm Y}(1S)$ signal.

Figure 4

The hadronic yield vs center-of-mass energy in the vicinity of the three ${\rm Y}$ resonances. Points represent the data, corrected for fitted beam energy shifts between scans, the solid line is the fit, the dashed line is the sum of all backgrounds, and the insets show high-energy measurements. The pull of each point is shown above.