Measurement of the Parity-Violating Asymmetry Parameter α_b and the Helicity Amplitudes for the Decay Λ_b^0 → J/ψΛ⁰ with the ATLAS Detector

A measurement of the parity-violating decay asymmetry parameter, α_b, and the helicity amplitudes for the decay Λ_b^0 → J/ψ(μ⁺μ⁻)Λ⁰(pπ⁻) is reported. The analysis is based on 1400 Λ_b^0 and Λ̅_b^0 baryons selected in 4.6 fb⁻¹ of proton-proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the Λ_b^0 and Λ̅_b^0 samples under the assumption of CP conservation, the value of α_b is measured to be 0.30 ± 0.16(stat) ± 0.06(syst). This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.