„Cosmic rays“ is the term used to indicate protons (or other heavier nuclei) that „shoot“ from the Universe to us. We know that many of these are coming from the Sun, but there are some that have very high energies that come from unknown sources. The Earth is constantly hit by these particles.

When a proton from this radiation hits the Earth atmosphere (primary cosmic ray), it collides with other nuclei and produces, in the nuclear reactions, several particles such as pions and neutrons. Pions quickly decay into muons (secondary cosmic rays). Muons are also very instabile particles, they decay in 2,2 microseconds , but, since they travel very fast and according to the theory of relativity, „their“ time goes slower and they can reach the Earth's ground.

The muon's lifetime (how much time it takes for the  muon to decay) is a complete random process. Generally, there is always the same percentage of probabilitny that a muon decays in a unit of time (ca. 63,2% in 2,2 microseconds.

An "air shower" is the term used to indicate these nuclear reactions, as the resulting particles expand (and eventually collide and divide) in a much wider area, and the imaginary track left by these particles looks roughly like a shower.

Cosmic rays were discovered after radiation; when in 1903 an experiment showed that an isolated radiation relevator shows always some signal, the theory of the existence of a highly penetrating radiation was confirmed. Further experiments and measurements were conduct by the austrian physics Victor Franz Hess and Wener Kolhörster (1912-1914). They discovered that the "rays" were coming by the outer space rather than from the Earth using a balloon and a detector. The detector showed higher rates as the balloon went up. These measurement were conducted in 3 years, with a great precicision. Hess won the Nobel prize with Carl David Anderson, the discoverer of the positron.

Today, further research about cosmic rays sources and physics are done by the international experiments of Auger, IceCube and others.

 

The image shows a typical "air shower", the result   of the collison of  a cosmic ray with the atmosphere.
muons are marked with a "μ", pions with a "π". The image shows also neutrinos (v) and electrons (e).
Image by Wikipedia