Study of radiative bottomonium transitions using converted photons

We use ($111\pm 1$) million ${\rm Y}(3S)$ and ($89\pm 1$) million ${\rm Y}(2S)$ events recorded by the BABAR detector at the PEP-II B-factory at SLAC to perform a study of radiative transitions between bottomonium states using photons that have been converted to $e^{+}{e}^{-}$ pairs by the detector material. We observe ${\rm Y}(3S)\to \gamma {\chi }_{b0,2}(1P)$ decay, make precise measurements of the branching fractions for ${\chi }_{b1,2}(1P,2P)\to \gamma {\rm Y}(1S)$ and ${\chi }_{b1,2}(2P)\to \gamma {\rm Y}(2S)$ decays, and search for radiative decay to the ${\eta }_b(1S)$ and ${\eta }_b(2S)$ states.

Figure 1

End view of the BABAR inner detector along the beam axis as seen by converted photons. Points indicate the number of converted photon vertices per cross-sectional area, as measured in a subset of the test sample data. From the center outward, features of note include the beam pipe, the SVT (the hexagonal inner layers) and its support structure rods, the support tube, and the inner wall of the DCH.

Figure 2

Efficiency for the conversion and reconstruction of a photon versus photon energy as derived from a sample of generic ${\rm Y}(3S)$ MC events before the optimal selection criteria have been applied.

Figure 3

Raw inclusive converted photon energy spectrum from (a) ${\rm Y}(3S)$ and (b) ${\rm Y}(2S)$ decays. The shaded areas indicate different regions of interest considered in detail in this analysis. Each Roman numeral indicates the corresponding section of this article in which each energy region is discussed.

Figure 4

Pictorial representation of energy levels in the bottomonium system and the radiative transitions studied here. Each Roman numeral indicates the corresponding section of this article in which the labeled transition is discussed.

Figure 5

Fit to the $180\le E_{\gamma }^{*}\le 300\mathrm{MeV}$ region of the ${\rm Y}(3S)$ data (a) for all of the data, and (b) after subtraction of the fitted background contribution. The ${\chi }^2/\mathrm{ndof}$ (where ndof stands for number of degrees of freedom) for the fit is $119.1/110$. The thin gray lines indicate the individual signal components in the fit, as labeled.

Figure 6

Fit to the $300\le E_{\gamma }^{*}\le 600\mathrm{MeV}$ region of the ${\rm Y}(3S)$ data (a) for all of the data, and (b) after subtraction of the fitted background contribution. The ${\chi }^2/\mathrm{ndof}$ for the fit is $316/290$. The thick solid lines indicate the total fit; the thin solid lines indicate the ${\rm Y}(3S)\to \gamma {\chi }_{bJ}(1P)$ components; and the dashed lines indicate the contributions from ${\chi }_{bJ}(1P)\to \gamma {\rm Y}(1S)$ and $h_b(1P)\to \gamma {\eta }_b(1S)$, as labeled above the data points for the ${\rm Y}(3S)\to \gamma {\chi }_{bJ}(1P)$ components, and below the points for all others.

Figure 7

Fit to the $300\le E_{\gamma }^{*}\le 800\mathrm{MeV}$ region of the ${\rm Y}(2S)$ data (a) for all of the data, and (b) after subtraction of the fitted background contribution, where the inset focuses on the ${\rm Y}(2S)\to \gamma {\eta }_b(1S)$ region of the fit. The thin lines indicate the individual fit components. For this fit, ${\chi }^2/\mathrm{ndof}=511.0/487$.

Figure 8

Fit to the $600\le E_{\gamma }^{*}\le 1100\mathrm{MeV}$ region of the ${\rm Y}(3S)$ data (a) for all of the data, and (b) after subtraction of the fitted background contribution, where the inset focuses on the ${\rm Y}(3S)\to \gamma {\eta }_b(1S)$ region of the fit. The thin lines indicate the individual fit components. For this fit, ${\chi }^2/\mathrm{ndof}=442.9/487$.