  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 $58\pm13$ 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 $0.99^{+0.42}_{-0.07} \, (\text{stat.})\pm 0.17
\text{ (syst.)}\,\mathrm{ps}$ and it is found to be consistent with the
Standard Model.
