"If the muon decays at rest (a good approximation for muons that stop in the detector)..." http://www2.fisica.unlp.edu.ar/~veiga/experiments.html

This is the worst possible approximation, Einsteinians. The muon crashes into the detector at a speed close to the speed of light. To teach that, after the crashing, the muon is "at rest", and that non-crashing muons live longer because they undergo time dilation... Actually, "Einstein's schizophrenic world" is not precise; "Einstein's oligophrenic world" is better.

Pentcho Valev