Bottomonium spectroscopy and radiative transitions involving the χ_(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^+e^− collider at SLAC to perform a study of radiative transitions involving the χ_(bJ)(1P,2P) states in exclusive decays with μ^+μ^−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_(b0)(2P)→γΥ(2S) and χ_(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^+e^− pair due to interaction with detector material, and we measure absolute product branching fractions. This method is particularly useful for measuring Υ(3S)→γχ_(b1,2)(1P) decays. Additionally, we provide the most up-to-date derived branching fractions, matrix elements and mass splittings for χ_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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