Observation of ${\rm Y}(4S)$ Decays to ${\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ and ${\pi }^{+}{\pi }^{-}{\rm Y}(2S)$

We present the first measurement of ${\rm Y}(4S)$ decays to ${\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ and ${\pi }^{+}{\pi }^{-}{\rm Y}(2S)$ based on a sample of $230\times {10}^6{\rm Y}(4S)$ mesons collected with the BABAR detector. We measure the product branching fractions $\mathcal{B}\mathbf{(}{\rm Y}(4S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)\mathbf{)}\times \mathcal{B}\mathbf{(}{\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}\mathbf{)}=(2.23\pm {0.25}_{\mathrm{stat}}\pm {0.27}_{\mathrm{syst}})\times {10}^{-6}$ and $\mathcal{B}\mathbf{(}{\rm Y}(4S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(2S)\mathbf{)}\times \mathcal{B}\mathbf{(}{\rm Y}(2S)\to {\mu }^{+}{\mu }^{-}\mathbf{)}=(1.69\pm {0.26}_{\mathrm{stat}}\pm {0.20}_{\mathrm{syst}})\times {10}^{-6}$, from which we derive the partial widths $\Gamma \mathbf{(}{\rm Y}(4S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)\mathbf{)}=(1.8\pm 0.4)\mathrm{keV}$ and $\Gamma \mathbf{(}{\rm Y}(4S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(2S)\mathbf{)}=(2.7\pm 0.8)\mathrm{keV}$.

Figure 1

The $M_{\mu \mu }$ vs $\Delta M$ distribution. Dashed lines delimit the regions where $\Delta M$ and $M_{\mu \mu }$ are within $\pm 60\mathrm{MeV}/c^2$ and $\pm 300\mathrm{MeV}/c^2$, respectively, of the values expected for an $mS\to nS$ transition. The remaining region is used to model background. The text discusses the features seen in the data.

Figure 2

The $\Delta M$ distribution for events with $|M_{{\ell }^{+}{\ell }^{-}}-M(1S)|<200\mathrm{MeV}/c^2$ (left column) and $|M_{{\ell }^{+}{\ell }^{-}}-M(2S)|<150\mathrm{MeV}/c^2$ (right column). The solid lines show the best fit to the data. Dashed lines show the background contribution. The two upper plots are for ${\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ candidates and the two lower plots for ${\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ candidates.

Figure 3

The efficiency-corrected $M_{{\pi }^{+}{\pi }^{-}}$ distribution for $4S\to 1S$ transition (left panel) and $4S\to 2S$ transition (right panel). The solid line shows the distribution predicted in Ref. [2]. The dotted histogram shows the selection efficiency in each bin. The experimental resolution in $M_{{\pi }^{+}{\pi }^{-}}$ is less than $5\mathrm{MeV}/c^2$, much smaller than the bin size.