Observation of the Radiative Decays of $\mathrm{\Upsilon }(1S)$ to ${\chi }_{c1}$

We report the first observation of the radiative decay of the $\mathrm{\Upsilon }(1S)$ into a charmonium state. The significance of the observed signal of $\mathrm{\Upsilon }(1S)\to \gamma {\chi }_{c1}$ is 6.3 standard deviations including systematics. The branching fraction is calculated to be $\mathcal{B}[\mathrm{\Upsilon }(1S)\to \gamma {\chi }_{c1}]=[{4.7}_{-1.8}^{+2.4}(\text{stat}{)}_{-0.5}^{+0.4}(\text{sys})\times {10}^{-5}]$. We also searched for $\mathrm{\Upsilon }(1S)$ radiative decays into ${\chi }_{c0,2}$ and ${\eta }_c(1S,2S)$, and set upper limits on their branching fractions. These results are obtained from a $24.9{\mathrm{fb}}^{-1}$ data sample collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider at a center-of-mass energy equal to the $\mathrm{\Upsilon }(2S)$ mass using $\mathrm{\Upsilon }(1S)$ tagging by the $\mathrm{\Upsilon }(2S)\to \mathrm{\Upsilon }(1S){\pi }^{+}{\pi }^{-}$ transitions.

Figure 1

(a) The $M_{\text{rec}}({\pi }^{+}{\pi }^{-})$ spectrum for the data collected at the $\mathrm{\Upsilon }(2S)$ energy (points with errors) and expected background from $e^{+}{e}^{-}\to \psi (2S){\gamma }_{\mathrm{ISR}}$ (histogram; not in scale). The curve is the result of the fit, with the signal shape fixed to the MC simulation, the dotted line is the background contribution. (b) The distribution of the CM reconstructed momentum of $\mathrm{\Upsilon }(2S)$ candidates for the MC-simulated events after a mass-constrained fit (open histogram); backgrounds from the radiative return to $\psi (2S)$ and FSR $\mathrm{\Upsilon }(1S)\to {\mu }^{+}{\mu }^{-}{\gamma }_{\mathrm{FSR}}$ (shaded and hatched histograms). The imposed requirements are shown with the vertical dashed lines.

Figure 2

The $J/\psi \gamma$ invariant mass spectrum in the $\mathrm{\Upsilon }(1S)$ data (closed circles with error bars): (a) signal window, (b) 20 times wider $J/\psi$ mass sidebands, (c) 20 times wider $M_{\text{rec}}$ sidebands; continuum data is scaled according to the ratio of luminosities and shown with open circles. Histograms are the background expectation from the MC simulation from (b) $\mathrm{\Upsilon }(1S)\to {\mu }^{+}{\mu }^{-}{\gamma }_{\mathrm{FSR}}$, (c) $e^{+}{e}^{-}\to \psi (2S){\gamma }_{\mathrm{ISR}}$. The solid lines show the result of the simultaneous fit to all these distributions. The dotted line shows the ${\chi }_{c2}$ contribution with its yield set to the 90% confidence level UL.

Figure 3

Invariant mass spectrum for (a) $K_S^0{K}^{\pm }{\pi }^{\mp }$ and (b) $K^{+}{K}^{-}{\pi }^{+}{\pi }^{-}$ modes. Histograms represent the data and curves are result of the fits described in the text.