  High-energy cosmic rays are observed indirectly by detecting the extensive
air showers initiated in Earth's atmosphere. The interpretation of these
observations relies on accurate models of air shower physics, which is a
challenge and an opportunity to test QCD under extreme conditions. Air showers
are hadronic cascades, which eventually decay into muons. The muon number is a
key observable to infer the mass composition of cosmic rays. Air shower
simulations with state-of-the-art QCD models show a significant muon deficit
with respect to measurements; this is called the Muon Puzzle. The origin of
this discrepancy has been traced to the composition of secondary particles in
hadronic interactions. The muon discrepancy starts at the TeV scale, which
suggests that this change in hadron composition is observable at the Large
Hadron Collider. An effect that can potentially explain the puzzle has been
discovered at the LHC, but needs to be confirmed for forward produced hadrons
with LHCb, and with future data on oxygen beams.
