Search for Bottomonium States in Exclusive Radiative ${\rm Y}(2S)$ Decays

We search for bottomonium states in ${\rm Y}(2S)\to (b\overline{b})\gamma$ decays with an integrated luminosity of $24.7{\mathrm{fb}}^{-1}$ recorded at the ${\rm Y}(2S)$ resonance with the Belle detector at KEK, containing $(157.8\pm 3.6)\times {10}^6$ ${\rm Y}(2S)$ events. The $(b\overline{b})$ system is reconstructed in 26 exclusive hadronic final states composed of charged pions, kaons, protons, and $K_S^0$ mesons. We find no evidence for the state recently observed around 9975 MeV ($X_{b\overline{b}}$) in an analysis based on a data sample of $9.3\times {10}^6$ ${\rm Y}(2S)$ events collected with the CLEO III detector. We set a 90% confidence level upper limit on the branching fraction $\mathcal{B}[{\rm Y}(2S)\to X_{b\overline{b}}\gamma ]\times \sum _i\mathcal{B}[X_{b\overline{b}}\to h_i]<4.9\times {10}^{-6}$, summed over the exclusive hadronic final states employed in our analysis. This result is an order of magnitude smaller than the measurement reported with CLEO data. We also set an upper limit for the ${\eta }_b(1S)$ state of $\mathcal{B}[{\rm Y}(2S)\to {\eta }_b(1S)\gamma ]\times \sum _i\mathcal{B}[{\eta }_b(1S)\to h_i]<3.7\times {10}^{-6}$.

Figure 1

The $\Delta M$ distributions for (top) ${\rm Y}(4S)$ off-resonance data and (bottom) ${\rm Y}(2S)$ data events that pass the selection criteria applied for the $[0.03,0.30]\mathrm{GeV}/c^2$ region. Points with error bars are the data, (top) the blue solid curve is the result of the fit for the background-only hypothesis, and (bottom) the result of the fit for the signal-plus-background hypothesis, where blue solid and blue dashed curves are total fit and background components, respectively. The three ${\chi }_{bJ}(1P)$ components indicated by the red dotted curves are here considered as part of the signal. The bottom inset shows an expanded view of the $\Delta M$ distribution in the $[0.035,0.065]\mathrm{GeV}/c^2$ region.

Figure 2

The $\Delta M$ distributions for (top) ${\rm Y}(4S)$ off-resonance data and (bottom) ${\rm Y}(2S)$ data events that pass the selection criteria applied for the $[0.45,0.75]\mathrm{GeV}/c^2$ region. Points with error bars are the data, (top) the blue solid curve is the result of the fit for the background-only hypothesis, and (bottom) the result of the fit for the signal-plus-background hypothesis, where blue solid and blue dashed curves are total fit and background components, respectively.