Bottomonium spectroscopy and radiative transitions involving theχbJ(1P,2P)states atBaBar

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Abstract

We use $(121\ifmmode\pm\else\textpm\fi{}1)$ million $\mathrm{\ensuremath{\Upsilon}}(3S)$ and $(98\ifmmode\pm\else\textpm\fi{}1)$ million $\mathrm{\ensuremath{\Upsilon}}(2S)$ mesons recorded by the BABAR detector at the PEP-II ${e}^{+}{e}^{\ensuremath{-}}$ collider at SLAC to perform a study of radiative transitions involving the ${\ensuremath{\chi}}_{b\mathrm{J}}(1P,2P)$ states in exclusive decays with ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\gamma}\ensuremath{\gamma}$ final states. We reconstruct twelve channels in four cascades using two complementary methods. In the first we identify both signal photon candidates in the electromagnetic calorimeter (EMC), employ a calorimeter timing-based technique to reduce backgrounds, and determine branching-ratio products and fine mass splittings. These results include the best observational significance yet for the ${\ensuremath{\chi}}_{b0}(2P)\ensuremath{\rightarrow}\ensuremath{\gamma}\mathrm{\ensuremath{\Upsilon}}(2S)$ and ${\ensuremath{\chi}}_{b0}(1P)\ensuremath{\rightarrow}\ensuremath{\gamma}\mathrm{\ensuremath{\Upsilon}}(1S)$ transitions. In the second method, we identify one photon candidate in the EMC and one which has converted into an ${e}^{+}{e}^{\ensuremath{-}}$ pair due to interaction with detector material, and we measure absolute product branching fractions. This method is particularly useful for measuring $\mathrm{\ensuremath{\Upsilon}}(3S)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{b1,2}(1P)$ decays. Additionally, we provide the most up-to-date derived branching fractions, matrix elements and mass splittings for ${\ensuremath{\chi}}_{b}$ transitions in the bottomonium system. Using a new technique, we also measure the two lowest-order spin-dependent coefficients in the nonrelativistic QCD Hamiltonian.

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