Measurement of the $B_s^0\to \mu \mu$ Effective Lifetime with the ATLAS Detector

This paper reports the first ATLAS measurement of the $B_s^0\to \mu \mu$ effective lifetime. The measurement is based on the data collected in 2015-2016, amounting to 26.3 fb${}^{-1}$ of 13 TeV LHC proton-proton collisions. The proper decay-time distribution of $58\pm 13$ background-subtracted signal candidates is fit with simulated signal templates parameterised as a function of the $B_s^0$ effective lifetime, with statistical uncertainties extracted through a Neyman construction. The resulting effective measurement of the $B_s^0\to \mu \mu$ lifetime is $0.99_{-0.07}^{+0.42}(\text{stat.})\pm 0.17\text{ (syst.)}\mathrm{p}\mathrm{s}$ and it is found to be consistent with the Standard Model.