Measurement of the parity violating asymmetry parameter $\alpha_b$ and the helicity amplitudes for the decay $\Lambda_b^0\to J/\psi\Lambda^0$ in the ATLAS experiment

A measurement of the parity violating decay asymmetry parameter, $\alpha_b$, and the helicity amplitudes for the $\Lambda^0_b$ hyperon decay $\Lambda^0_b\to J/\psi (\mu\mu)\Lambda^0(p\pi^-)$ is reported. The analysis is based on about 1400 $\Lambda^0_b$ and$\bar{\Lambda}^0_b$ baryons selected in 4.6/fb of proton-proton collision data recorded by the ATLAS experiment at the LHC with a center-of-mass energy of 7 TeV. By combining the $\Lambda^0_b$ and $\bar{\Lambda}^0_b$ samples under the assumption of CP conservation, the value of $\alpha_b$ was measured to be $0.28\pm0.16 ({\rm stat})\pm 0.06 ({\rm syst})$. This measurement provides a test of several theoretical models that predict $\alpha_b$ values ranging from -0.2 to +0.8.