Observation of $Z_b(10610)$ and $Z_b(10650)$ Decaying to $B$ Mesons

We report the analysis of the three-body $e^{+}{e}^{-}\to B\overline{B}{\pi }^{\pm }$, $B{\overline{B}}^{*}{\pi }^{\pm }$, and $B^{*}{\overline{B}}^{*}{\pi }^{\pm }$ processes, including the first observations of the $Z_b^{\pm }(10610)\to [B{\overline{B}}^{*}+\mathrm{c}.\mathrm{c}.{]}^{\pm }$ and $Z_b^{\pm }(10650)\to [B^{*}{\overline{B}}^{*}{]}^{\pm }$ transitions that are found to dominate the corresponding final states. We measure Born cross sections for the three-body production of $\sigma (e^{+}{e}^{-}\to [B{\overline{B}}^{*}+\mathrm{c}.\mathrm{c}.{]}^{\pm }{\pi }^{\mp })=[17.4\pm 1.6(\text{stat})\pm 1.9(\text{syst})]\mathrm{pb}$ and $\sigma (e^{+}{e}^{-}\to [B^{*}{\overline{B}}^{*}{]}^{\pm }{\pi }^{\mp })=[8.75\pm 1.15(\text{stat})\pm 1.04(\text{syst})]\mathrm{pb}$ and set a 90% C.L. upper limit of $\sigma (e^{+}{e}^{-}\to [B\overline{B}{]}^{\pm }{\pi }^{\mp })<2.9\mathrm{pb}$. The results are based on a $121.4{\mathrm{fb}}^{-1}$ data sample collected with the Belle detector at a center-of-mass energy near the $\mathrm{\Upsilon }(10860)$ peak.

Figure 1

(a) Invariant mass and (b) $M_{\text{miss}}^{*}(B\pi )$ distribution for $B$ candidates in the $B$ signal region. Points with error bars represent the data. The open histogram in (a) shows the result of the fit to data. The solid line in (b) shows the result of the fit to the RS $B\pi$ data; the dashed line represents the background level.

Figure 2

The $M_{\text{miss}}(\pi )$ distribution for the (a) $B{B}^{*}\pi$ and (b) $B^{*}{B}^{*}\pi$ candidate events. Normalization factor is applied for the WS distributions.