Abstract
A measurement of the parity-violating decay asymmetry parameter, _b, and the helicity amplitudes for the decay _b0_J/_(_+_-) _0(p_-) is reported. The analysis is based on 1400 _b0 and __b0 baryons selected in 4.6fb-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 _b0 and __b0 samples under the assumption of CP conservation, the value of _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. © 2014 CERN, for the ATLAS Collaboration.