  The rate of muons from LHC $pp$ collisions reaching the surface above the
ATLAS interaction point is measured and compared with expected rates from
decays of $W$ and $Z$ bosons and $b$- and $c$-quark jets. In addition, data
collected during periods without beams circulating in the LHC provide a
measurement of the background from cosmic ray inelastic backscattering that is
compared to simulation predictions. Data were recorded during 2018 in a 2.5
$\times$ 2.5 $\times$ 6.5~$\rm{m}^3$ active volume MATHUSLA test stand detector
unit consisting of two scintillator planes, one at the top and one at the
bottom, which defined the trigger, and six layers of RPCs between them, grouped
into three $(x,y)$-measuring layers separated by 1.74 m from each other.
Triggers selecting both upward-going tracks and downward-going tracks were
used.
