Body face
- second-generation charged lepton: same charge and spin as the electron, ~207× heavier
- mean lifetime 2.2 μs, decays into an electron and two neutrinos
- made in the upper atmosphere by cosmic rays (via pions), ~1 per cm² per minute at the ground
- reaching the ground at all shows time dilation (special relativity)
- cross hundreds of metres of rock → muography (e.g. the hidden void in the Great Pyramid of Giza, 2017)