  The LHC copiously produces muons via different processes, and the muon sample
will be large at the high-luminosity LHC (HL-LHC). In this work we propose to
leverage this large muon sample and utilize the HL-LHC as a muon fixed-target
experiment, with the ATLAS calorimeter as the target. We consider a novel
analysis for the ATLAS detector, which takes advantage of the two independent
muon momentum measurements by the inner detector and the muon system. We show
that a comparison of the two measurements, before and after the calorimeters,
can probe new force carriers that are coupled to muons and escape detection.
The proposed analysis, based on muon samples from $W$ and $Z$ decays only, has
a comparable reach to other proposals. In particular, it can explore the part
of parameter-space that could explain the muon $g-2$ anomaly.
