Observation of the decay $\Lambda_b^0\to J/\psi(\mu^+\mu^-) \Lambda^0 (p^+\pi^-)$ with the ATLAS experiment at the LHC

The production of $\Lambda_b^0$ baryons is observed with the ATLAS experiment at the LHC through their decays $\Lambda_b^0\to J/\psi(\mu^+\mu^-) \Lambda^0 (p^+\pi^-)$. The decays are reconstructed in 1.2 fb$^{-1}$ of the 2011 LHC data. The mass fit yields $579 \pm 31(\rm stat.)$ signal events and the fitted $\Lambda_b^0$ mass value, $M = 5620.6 \pm 1.6(\rm stat.)$ MeV, is consistent with the world average value, $M_{\Lambda_b^0} = 5620.2 \pm 1.6$ MeV.