Observation of an enhancement in $e^{+}{e}^{-}\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$, ${\rm Y}(2S){\pi }^{+}{\pi }^{-}$, and ${\rm Y}(3S){\pi }^{+}{\pi }^{-}$ production near $\sqrt{s}=10.89\mathrm{GeV}$

We measure the production cross sections for $e^{+}{e}^{-}\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$, ${\rm Y}(2S){\pi }^{+}{\pi }^{-}$, and ${\rm Y}(3S){\pi }^{+}{\pi }^{-}$ as a function of $\sqrt{s}$ between 10.83 GeV and 11.02 GeV. The data consist of $8.1{\mathrm{fb}}^{-1}$ collected with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We observe enhanced production in all three final states that does not agree well with the conventional ${\rm Y}(10860)$ line shape. A fit using a Breit-Wigner resonance shape yields a peak mass of $[10{888.4}_{-2.6}^{+2.7}(\mathrm{stat})\pm 1.2(\mathrm{syst})]\mathrm{MeV}/c^2$ and a width of $[{30.7}_{-7.0}^{+8.3}(\mathrm{stat})\pm 3.1(\mathrm{syst})]\mathrm{MeV}/c^2$.

Figure 1

The distributions of $\Delta M-[\sqrt{s}-M_{{\rm Y}(nS)}]$ ($n=1,2,3$) for (a)–(f) ${\rm Y}(1S){\pi }^{+}{\pi }^{-}$, (g)–(l) ${\rm Y}(2S){\pi }^{+}{\pi }^{-}$, and (m)–(r) ${\rm Y}(3S){\pi }^{+}{\pi }^{-}$ events with the fit results superimposed. The six columns of plots represent the data samples collected at different energies. The dashed curves show the background components in the fits.

Figure 2

The energy-dependent cross sections for $e^{+}{e}^{-}\to {\rm Y}(nS){\pi }^{+}{\pi }^{-}$ ($n=1,2,3$) processes normalized to the leading-order $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ cross sections. The results of the fits are shown as smooth curves. The vertical dashed line indicates the energy at which the hadronic cross section is maximal.

Figure 3

(a) $R_b$ and (b) the ratio between $\sigma [e^{+}{e}^{-}\to {\rm Y}(nS){\pi }^{+}{\pi }^{-}]$ and $\sigma [e^{+}{e}^{-}\to b\overline{b}]$ as a function of $\sqrt{s}$; (c) the energy-dependent cross section ratios for $e^{+}{e}^{-}\to {\rm Y}(nS){\pi }^{+}{\pi }^{-}$ events, the result of fits with resonant parameters from $R_b$ or PDG averages are superimposed. The horizontal dotted line in (a) is the noninterfering $|A_{nr}{|}^2$ contribution in the fit. The vertical dashed line indicates the energy at which the hadronic cross section is maximal.