First Observation of the Hadronic Transition $\mathrm{\Upsilon }(4S)\to \eta h_b(1P)$ and New Measurement of the $h_b(1P)$ and ${\eta }_b(1S)$ Parameters

Using a sample of $771.6\times {10}^6$ $\mathrm{\Upsilon }\mathrm{\Upsilon }(4S)$ decays collected by the Belle experiment at the KEKB $e^{+}{e}^{-}$ collider, we observe, for the first time, the transition $\mathrm{\Upsilon }(4S)\to \eta h_b(1P)$ with the branching fraction $\mathcal{B}[\mathrm{\Upsilon }(4S)\to \eta h_b(1P)]=(2.18\pm 0.11\pm 0.18)\times {10}^{-3}$ and we measure the $h_b(1P)$ mass $M_{h_b(1P)}=(9899.3\pm 0.4\pm 1.0)\mathrm{MeV}/c^2$, corresponding to the hyperfine (HF) splitting $\mathrm{\Delta }{M}_{\mathrm{HF}}(1P)=(0.6\pm 0.4\pm 1.0)\mathrm{MeV}/c^2$. Using the transition $h_b(1P)\to \gamma {\eta }_b(1S)$, we measure the ${\eta }_b(1S)$ mass $M_{{\eta }_b(1S)}=(9400.7\pm 1.7\pm 1.6)\mathrm{MeV}/c^2$, corresponding to $\mathrm{\Delta }{M}_{\mathrm{HF}}(1S)=(59.6\pm 1.7\pm 1.6)\mathrm{MeV}/c^2$, the ${\eta }_b(1S)$ width ${\mathrm{\Gamma }}_{{\eta }_b(1S)}=(8_{-5}^{+6}\pm 5)\mathrm{MeV}/c^2$ and the branching fraction $\mathcal{B}[h_b(1P)\to \gamma {\eta }_b(1S)]=(56\pm 8\pm 4)\%$.

Figure 1

$M_{\mathrm{miss}}(\eta )$ distribution after the background subtraction. The solid blue curve shows the fit with the signal PDFs, while the dashed red curve represents the background only hypothesis. The inset shows the $M_{\mathrm{miss}}(\eta )$ distribution before the background subtraction.

Figure 2

$\mathrm{\Delta }{M}_{\mathrm{miss}}$ distribution. The blue solid curve shows our best fit, while the dashed red curve represents the background component.