Evidence for the $h_b(1P)$ meson in the decay ${\rm Y}(3S)\to {\pi }^0{h}_b(1P)$

Using a sample of $122\times {10}^6$ ${\rm Y}(3S)$ events recorded with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider at SLAC, we search for the $h_b(1P)$ spin-singlet partner of the $P$-wave ${\chi }_{bJ}(1P)$ states in the sequential decay ${\rm Y}(3S)\to {\pi }^0{h}_b(1P)$, $h_b(1P)\to \gamma {\eta }_b(1S)$. We observe an excess of events above background in the distribution of the recoil mass against the ${\pi }^0$ at mass $9902\pm 4(\mathrm{stat})\pm 2(\mathrm{syst})\mathrm{MeV}/c^2$. The width of the observed signal is consistent with experimental resolution, and its significance is $3.1\sigma$, including systematic uncertainties. We obtain the value $(4.3\pm 1.1(\mathrm{stat})\pm 0.9(\mathrm{syst}))\times {10}^{-4}$ for the product branching fraction $\mathcal{B}({\rm Y}(3S)\to {\pi }^0{h}_b)\times \mathcal{B}(h_b\to \gamma {\eta }_b)$.

Figure 1

The reconstructed CM energy distribution of the candidate photon in simulated ${\rm Y}(3S)\to {\pi }^0{h}_b(1P)$, $h_b(1P)\to \gamma {\eta }_b(1S)$ events. The shaded region indicates the selected $E^{*}(\gamma )$ signal region.

Figure 2

The result of the fit to the $m(\gamma \gamma )$ distribution in data (data points) for the full range of $m_{\mathrm{recoil}}({\pi }^0)$. The solid histogram shows the fit result, and is essentially indistinguishable from the data; the shaded histogram corresponds to the background distribution.

Figure 3

(a) The $m_{\mathrm{recoil}}({\pi }^0)$ distribution in the region $9.73<m_{\mathrm{recoil}}({\pi }^0)<10\mathrm{GeV}/c^2$ for data (points); the solid histogram represents the fit function described in the text. The data in the $h_b$ signal region have been excluded from the fit and the plot. (b) The $m_{\mathrm{recoil}}({\pi }^0)$ spectrum after subtracting background; in the $h_b$ signal region the data points are shown as squares, and the area with diagonal shading represents the uncertainties from the background fit; the shaded histogram represents the signal function resulting from the fit to the data.