The Speed and Mean Life of Cosmic-Ray Muons

Experiments in relativistic kinematics: the universal speed limit and time dilation. The existence of an absolute limit, c, on the speed of particles is demonstrated by a measurement of the times of flight of cosmic-ray muons between scintillation counters in the laboratory. The mean life of muons at rest is then determined from a measurement of the distribution of radioactive decay times of muons that stop in a large plastic scintillator.

Given their measured short mean life, the measured speed limit, and the large distance they travel from their places of production high in the atmosphere, it is apparent that the muons in flight live much longer relative to an observer in the laboratory than muons at rest, in conformance with the predictions of relativistic kinematics.

Lab Guide (PDF)

A large plastic scintillator. (Image courtesy of Junior Lab staff.)

References

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Griffiths, David. Introduction to Elementary Particles. New York, NY: Wiley, 1987, chapter 1, pp. 1-51. ISBN: 0471603864.

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Perkins, Donald H. Introduction to High Energy Physics. Cambridge, UK: Cambridge University Press, 2000, p. 213. ISBN: 0521621968.
A nice comparison positon spectra from muon (three body) and pion (two body decays).