First observation of the $Z_b^0(10610)$ in a Dalitz analysis of ${\rm Y}(10860)\to {\rm Y}(nS){\pi }^0{\pi }^0$

We report the first observation of ${\rm Y}(10860)\to {\rm Y}(1,2,3S){\pi }^0{\pi }^0$ decays. The neutral partner of the $Z_b^{\pm }(10610)$, the $Z_b^0(10610)$ decaying to ${\rm Y}(2,3S){\pi }^0$, is observed for the first time with a $6.5\sigma$ significance using a Dalitz analysis of ${\rm Y}(10860)\to {\rm Y}(2,3S){\pi }^0{\pi }^0$ decays. The results are obtained with a $121.4{\mathrm{fb}}^{-1}$ data sample collected with the Belle detector at the ${\rm Y}(10860)$ resonance at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.

Figure 1

The ${\pi }^0{\pi }^0$ missing mass distribution for ${\rm Y}(nS){\pi }^0{\pi }^0$ candidates, using (a) ${\rm Y}(nS)\to {\mu }^{+}{\mu }^{-}$ and (b) ${\rm Y}(nS)\to e^{+}{e}^{-}$ candidates. The $M({\rm Y}(1S){\pi }^{+}{\pi }^{-})$ distribution for ${\rm Y}(2S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$ candidates is shown in (c). Points with error bars represent the data. In each panel, the solid curve shows the fit result, while the dashed curve corresponds to the background contribution.

Figure 2

Dalitz plots for selected (a) ${\rm Y}(1S){\pi }^0{\pi }^0$, (b) ${\rm Y}(2S){\pi }^0{\pi }^0$, and (c) ${\rm Y}(3S){\pi }^0{\pi }^0$ candidates.

Figure 3

Comparison of the (a) $M({\rm Y}(1S){\pi }^0{)}_{\max }$, (b) $M({\pi }^0{\pi }^0)$, and (c) $M({\rm Y}(1S){\pi }^0{)}_{\min }$ distributions for the ${\rm Y}(1S){\pi }^0{\pi }^0$ events in the signal region (points with error bars) and results of the fit (open histograms). Solid red and dashed blue open histograms show the fit with and without $Z_b^0$’s, respectively. Hatched histograms show the background components.

Figure 4

Comparison of the (a) $M({\rm Y}(2S){\pi }^0{)}_{\max }$, (b) $M({\pi }^0{\pi }^0)$, and (c) $M({\rm Y}(2S){\pi }^0{)}_{\min }$ distributions for the ${\rm Y}(2S){\pi }^0{\pi }^0$ events in the signal region (points with error bars) and results of the fit (open histograms). The legends are the same as in Fig. 3. Only solution A is shown. Both solutions give indistinguishable plots.

Figure 5

Comparison of the (a) $M({\rm Y}(3S){\pi }^0{)}_{\max }$, (b) $M({\pi }^0{\pi }^0)$, and (c) $M({\rm Y}(3S){\pi }^0{)}_{\min }$ distributions for the ${\rm Y}(3S){\pi }^0{\pi }^0$ events in the signal region (points with error bars) and results of the fit (open histograms). The legends are the same as in Fig. 3.