Bottomonium spectroscopy and radiative transitions involving the χbJ(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 χbJ(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 χb0(2P) → γΥ (2S) and χb0(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) → γχb1,2(1P) decays. Additionally, we provide the most up-to-date derived branching fractions, matrix elements and mass splittings for χb transitions in the bottomonium system. Using a new technique, we also measure the two lowest-order spin-dependent coefficients in the nonrelativistic QCD Hamiltonian.