  A search for long-lived particles decaying into muon pairs is performed using
proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the
CMS experiment at the LHC in 2017 and 2018, corresponding to an integrated
luminosity of 101 fb$^{-1}$. The data sets used in this search were collected
with a dedicated dimuon trigger stream with low transverse momentum thresholds,
recorded at high rate by retaining a reduced amount of information, in order to
explore otherwise inaccessible phase space at low dimuon mass and nonzero
displacement from the primary interaction vertex. No significant excess of
events beyond the standard model expectation is found. Upper limits on
branching fractions at 95% confidence level are set on a wide range of mass and
lifetime hypotheses in beyond the standard model frameworks with the Higgs
boson decaying into a pair of long-lived dark photons, or with a long-lived
scalar resonance arising from a decay of a b hadron. The limits are the most
stringent to date for substantial regions of the parameter space. These results
can be also used to constrain models of displaced dimuons that are not
explicitly considered in this paper.
