A sample of $pp$ collision data, corresponding to an integrated luminosity of 5.5 fb$^{ -1}$ and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the $\Xi^-_b$ baryon to that of the $\Lambda^0_b$ baryon, $r_{ \tau}\equiv\tau_{ \Xi^-_b}/\tau_{ \Lambda^0_b}$. The value ${ r_{ \tau}=1.076\pm0.013\pm0.006}$ is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain ${ r_{ \tau}^{ \rm Run\,1,2} = 1.078\pm0.012\pm0.007}$. Multiplying by the world-average value of the $\Lambda^0_b$ lifetime yields $\tau_{ \Xi^-_b}^{ \rm Run~1,2} = 1.578\pm0.018\pm0.010\pm0.011$ ps, where the uncertainties are statistical, systematic, and due to the limited knowledge of the $\Lambda^0_b$ lifetime. This measurement improves the precision of the current world average of the $\Xi^-_b$ lifetime by about a factor of two, and is in good agreement with the most recent theoretical predictions.
Categories: hep-ex