A shower of particles
Primary cosmic particles travel enormous distances before they collide with the Earth. They are stable particles, such as protons, light nuclei, gamma rays or even electrons. If they have enough energy, their initial collision can spark off a chain reaction, resulting in the formation of a shower of particles. Some of these, such as the mesons, are extremely short lived whereas others, such as muons, live long enough to reach the ground. These chain reactions produce also several types of neutrinos.