Precision measurement of the $\Lambda_b^0$ baryon lifetime

The ratio of the $\Lambda_b^0$ baryon lifetime to that of the $\overline{B}^0$ meson is measured using 1.0 fb$^{-1}$ of integrated luminosity in 7 TeV center-of-mass energy $pp$ collisions at the LHC. The $\Lambda_b^0$ baryon is observed for the first time in the decay mode $\Lambda_b^0 \to J/\psi p K^-$, while the $\overline{B}^0$ meson decay used is the well known $\overline{B}^0 \to J/\psi \pi^+ K^-$ mode, where the $\pi^+K^-$ mass is consistent with that of the $\bar{K}^{*0}(892)$ meson. The ratio of lifetimes is measured to be $0.976\pm0.012\pm0.006$, in agreement with theoretical expectations based on the heavy quark expansion. Using previous determinations of the $\overline{B}^0$ meson lifetime, the $\Lambda_b^0$ lifetime is found to be $1.482 \pm 0.018 \pm 0.012$ ps. In both cases the first uncertainty is statistical and the second systematic.