  A measurement of the parity-violating decay asymmetry parameter, alpha_b, and
the helicity amplitudes for the decay Lambda_b->J/psi(mu mu)+Lambda(p pi) is
reported. The analysis is based on 1400 Lambda_b and anti-Lambda_b baryons
selected in 4.6/fb 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 Lambda_b
and anti-Lambda_b samples under the assumption of CP conservation, the value of
alpha_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.
