Bottomonium spectroscopy and radiative transitions involving the χ[subscript bJ](1P,2P) states at BaBar

Abstract

We use (121 ± 1) million Υ(3S) and (98 ± 1) million Υ(2S) mesons recorded by the BABAR detector at the PEP-II e[superscript +]e[superscript -] collider at SLAC to perform a study of radiative transitions involving the χ[subscript bJ](1P,2P) states in exclusive decays with μ[superscript +]μ[superscript -]γγ 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 χ[subscript b0](2P) → γΥ(2S) and χ[subscript b0](1P) → γΥ(1S) transitions. In the second method, we identify one photon candidate in the EMC and one which has converted into an e[superscript +]e[superscript -] pair due to interaction with detector material, and we measure absolute product branching fractions. This method is particularly useful for measuring Υ(3S) → γχ[subscript b1,2](1P) decays. Additionally, we provide the most up-to-date derived branching fractions, matrix elements and mass splittings for χ[subscript 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.