Muons are leptons – with the same charge but greater mass than an electron. They are readily made in the laboratory, but quickly decay, with an average lifetime of
measured in the muons rest frame.
Muons, labelled
in the diagram below, are created by cosmic rays as they strike atoms in the upper part of the atmosphere, at a typical speed of 0.99c . As they travel towards the ground many of them decay, but a significant fraction reach the ground.
In the absence of special relativistic effects, virtually no muons would reach the ground. A particle with a lifetime oftravelling near the speed of light travels about
before decaying. Specially relativity predicts a mean lifetime derived from the time dilation formula:
In the muons rest frame, the 10 km distance to be travelled will be shorter because of the length contraction effect of special relativity:
A significant fraction of muons will reach the ground.