Study of dipion bottomonium transitions and search for the $h_b(1P)$ state

We study inclusive dipion decays using a sample of $108\times {10}^6{\rm Y}(3S)$ events recorded with the BABAR detector. We search for the decay mode ${\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{h}_b(1P)$ and find no evidence for the bottomonium spin-singlet state $h_b(1P)$ in the invariant mass distribution recoiling against the ${\pi }^{+}{\pi }^{-}$ system. Assuming the $h_b(1P)$ mass to be $9.900\mathrm{GeV}/c^2$, we measure the upper limit on the branching fraction $\mathcal{B}[{\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{h}_b(1P)]<1.2\times {10}^{-4}$, at 90% confidence level. We also investigate the ${\chi }_{bJ}(2P)\to {\pi }^{+}{\pi }^{-}{\chi }_{bJ}(1P)$, ${\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(2S)$, and ${\rm Y}(2S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ dipion transitions and present an improved measurement of the branching fraction of the ${\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(2S)$ decay and of the ${\rm Y}(3S)-{\rm Y}(2S)$ mass difference.

Figure 1

The $m_R$ spectrum: data (points) with the fitted model (solid line) superimposed. The short-dashed line is the contribution from the continuum component. Also shown are the ${{\rm Y}}^{2\to 1}$ (dashed curve around $9.79\mathrm{GeV}/c^2$) and ${{\rm Y}}^{3\to 2}$ (long-dashed curve around $10.02\mathrm{GeV}/c^2$) components. The $h_b$ signal component is excluded from the superimposed model.

Figure 2

The $m_R$ spectrum after subtraction of the continuum background component. The curves represent the fitted model (solid), the ${{\rm Y}}_{\mathrm{ISR}}^{2\to 1}$ (dotted), $K_S^0$ (double-dot-dashed), ${\chi }_b^{1,1}$ (dashed), and ${\chi }_b^{2,2}$ (dash-dotted) components. Inset: expanded view in the $h_b$ region after subtraction of continuum and peaking backgrounds.

Figure 3

The $m_R$ spectrum in the ${\chi }_b^{J^{\prime },J}$ region after subtraction of continuum and $K_S^0$ background components: points represent data, while the curves represent the fitted model (solid), the ${\chi }_b^{1,1}$ (dashed), ${\chi }_b^{2,2}$ (dash-dotted), and ${{\rm Y}}^{3\to 2}$ (long-dashed) components.

Figure 4

Results for the ${\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{h}_b$ branching fraction (points with statistical and systematic uncertainties) as a function of the assumed $h_b$ mass. The triangles indicate the upper limits at 90% CL.