What are cosmic rays, and muons?
Cosmic rays are high-energy particles that reach the Earth in all directions from our galaxy - and even further in the case of those of higher energies.
The particles that reach the Earth from space are called primary cosmic rays, and those that are created in the atmosphere from the primaries are called secondary cosmic rays.
Studying primary cosmic rays is very difficult, because experiments have to be carried out at high altitudes, well above where most secondary cosmic rays are created. This is done by means of balloons and artificial
satellites.
Primary cosmic rays contain all stable charged particles, among which protons dominate with just under 90%, followed by helium nuclei - or alpha particles - with about 9%. The rest are heavier atomic nuclei produced in stars plus a small percentage of other particles, mostly electrons.
Their energies range from about 10-10 J —1 GeV = 109 eV) to values on the order of 10 J (1108 TeV = 1020 eV). This is tens of millions of times greater than that of a proton accelerated at CERN's LHC and comparable to the kinetic energy gained by an apple falling from a tree or an amateur tennis player's serve, but concentrated in a vastly smaller volume - about 1040 times smaller.
When a primary cosmic ray, for example a proton, stumbles upon the Earth, the first thing it encounters is the atmosphere. There, at an average altitude of about 15 km, a collision with an atomic nucleus occurs, from
which a proton or a neutron and dozens of other high-energy subatomic particles (1), especially pions and kaons, are very often ejected, thus initiating an atmospheric shower.
Neutrinos are undetectable without professional means. However, muons can be "seen" with home-made methods such as fog chambers, where they leave trails much like those of aircraft in the sky (which is not surprising, as the physics is very similar in both cases).
Muons are elementary particles - i.e., as far as we know, they are not composed of more basic elements - extremely similar to electrons: they have the same electric charge, are susceptible to the same interactions,
and so on. However, they are about two hundred times more massive, which is a mystery to many people.
Without a doubt, muons are our favourite particles.
(1) More information about elementary particles at https://particleadventure.org/ or, only in Spanish, in the Experimenta booklet "Cosmic rays":
http://www.muncyt.es/stfls/MUNCYT/Publicaciones/rayos_cosmicos_muncyt.pdf