μ Figure 1. Cosmic ray cascade induced by a cosmic ray proton striking an air molecule Not all of the particles produced in the cascade in the upper atmosphere survive down to sea-level due to their interaction with atmospheric nuclei and their own spontaneous decay. The flux of sea-level muons is approximately I per minute per cm"(see http://pdq.lbl.govformoreprecisenumbers)withameankineticenergyofabout 4 Gev Careful study [pdg Ibl. gov] shows that the mean production height in the atmosphere of the muons detected at sea-level is approximately 15 km. Travelling at the speed of light, he transit time from production point to sea-level is then 50 usec. Since the lifetime of at-rest muons is more than a factor of 20 smaller, the appearance of an appreciable sea level muon flux is qualitative evidence for the time dilation effect of special relativity Muon physics4 Muon Physics Figure 1. Cosmic ray cascade induced by a cosmic ray proton striking an air molecule nucleus. Not all of the particles produced in the cascade in the upper atmosphere survive down to sea-level due to their interaction with atmospheric nuclei and their own spontaneous decay. The flux of sea-level muons is approximately 1 per minute per cm2 (see http://pdg.lbl.gov for more precise numbers) with a mean kinetic energy of about 4 GeV. Careful study [pdg.lbl.gov] shows that the mean production height in the atmosphere of the muons detected at sea-level is approximately 15 km. Travelling at the speed of light, the transit time from production point to sea-level is then 50 µsec. Since the lifetime of at-rest muons is more than a factor of 20 smaller, the appearance of an appreciable sea￾level muon flux is qualitative evidence for the time dilation effect of special relativity. π + p µ + ν π 0 γ γ e + e e + − e − π − µ − ν n π +