This pion-to-muon decay happens higher than 10 km above the Earth’s surface. But the muons have a lifetime at rest of 2.2 μs. This means that the number of muons peak at around 10 km and ...
Abe at al. (T2K Collaboration) "First measurement of the muon neutrino charged current single pion production cross section on water with the T2K near detector," Phys.Rev.D 95,1, 012010 (2017) ...