Muon mass

Discovered in cosmic rays in 1936 and mistaken for Yukawa's predicted meson, the muon prompted I. I. Rabi's famous complaint: "Who ordered that?" It behaves exactly like a heavy electron — same charge, same spin, no strong interaction — but decays with a mean life of 2.197 μs into an electron and two neutrinos.

That short life makes muons a standing demonstration of time dilation. Created 15 km up when cosmic rays hit the atmosphere, a muon travelling at 0.998c should manage only about 660 m before decaying, yet they reach sea level in quantity because γ ≈ 16 stretches their clock. The mass matters elsewhere too: because muonic hydrogen's Bohr radius shrinks by the mass ratio of 207, the muon orbits deep inside the proton's charge distribution — which is how the 2010 muonic-hydrogen experiment exposed the "proton radius puzzle".