Photon Transitions in ${\rm Y}(2S)$ and ${\rm Y}(3S)$ Decays

We have studied the inclusive photon spectra in ${\rm Y}(2S)$ and ${\rm Y}(3S)$ decays using a large statistics data sample obtained with the CLEO III detector. We present the most precise measurements of electric dipole ($E1$) photon transition rates and photon energies for ${\rm Y}(2S)\to \gamma {\chi }_{bJ}(1P)$ and ${\rm Y}(3S)\to \gamma {\chi }_{bJ}(2P)$ ($J=0$, 1, 2). We measure the rate for a rare $E1$ transition ${\rm Y}(3S)\to \gamma {\chi }_{b0}(1P)$ for the first time. We also set upper limits on the rates for the hindered magnetic dipole ($M1$) transitions to the ${\eta }_b(1S)$ and ${\eta }_b(2S)$ states.

Figure 1

Fit of the off resonance and ${\rm Y}(1S)$ photon spectra to photon backgrounds in the ${\rm Y}(2S)$ (top) and ${\rm Y}(3S)$ (bottom) data. The energy regions used in the fit are shaded. The total fitted background is represented by the solid line. The${\rm Y}(2S)$ and ${\rm Y}(3S)$ data are shown by points. The fitted contributions of the off resonance (dashed line) and ${\rm Y}(1S)$ spectra (dotted line) are also shown. See the text for explanation of various photon lines observed in the data.

Figure 2

Fit to the ${\rm Y}(2S)\to \gamma {\chi }_{bJ}(1P)$ ($J=2$, 1, 0) photon lines in the data. The points represent the data (top plot). Statistical errors on the data are smaller than the point size. The solid line represents the fit. The dashed line represents total fitted background. The background subtracted data (points with error bars) are shown at the bottom. The solid line represents the fitted photon lines together. The dashed lines show individual photon lines.

Figure 3

Fit to the ${\rm Y}(3S)\to \gamma {\chi }_{bJ}(2P)$ ($J=2$, 1, 0) photon lines in the data. See caption of Fig. 2 for the description. Small solid line peaks in the bottom plot show the ${\chi }_{bJ}(2P)\to \gamma {\rm Y}(1D)$ and ${\rm Y}(2S)\to \gamma {\chi }_{bJ}(1P)$ contributions.

Figure 4

Fit to six photon lines due to ${\rm Y}(3S)\to \gamma {\chi }_{bJ}(1P)$ and ${\chi }_{bJ}(1P)\to \gamma {\rm Y}(1S)$ ($J=2$, 1, 0) in the data. See caption of Fig. 2 for the description. The dashed line in the bottom plot shows the ${\rm Y}(3S)\to \gamma {\chi }_{b0}(1P)$ contribution by itself.