Muons are generated at the top of the atmosphere in cosmic ray showers .
The cosmic radiation incident at the top of the terrestrial atmosphere includes all stable charged particles and nuclei with lifetimes of order
10^6 years or longer. Technically,“primary” cosmic rays are those particles accelerated at astrophysical sources and
“secondaries” are those particles produced in interaction
of the primaries with interstellar
gas. Thus electrons, protons and helium, as well as carbon, oxygen, iron, and other nuclei
synthesized in stars, are primaries. Nuclei such as lithium
, beryllium, and boron (which
are not abundant end-products of stellar nucleosynthesis)
are secondaries. Antiprotons and positrons are also in large part secondary. Whether a small fraction of these particles
may be primary is a question of current interest.
Apart from particles associated with solar flares, the cosmic
radiation comes from
outside the solar system. The incoming charged particles are “modulated” by the solar wind, the expanding magnetized plasma generated by the Sun,
which decelerates and partially excludes the lower energy galactic cosmic rays from the inner solar system.There is a significant anticorrelation between solar activity (which has an alternating
eleven-year cycle) and the intensity of the cosmic rays with
energies below about 10 GeV. In addition, the lower-energy cosmic rays are affected by the geomagnetic field, which they
must penetrate to reach the top of the atmosphere. Thus the intensity of any component of the cosmic radiation in the GeV range depends both on the location and time
In fig 24.1 the primary energies can be very high, the limit of detection is almost 10^6 GeV per nucleon.
Such a high energy primary hitting a nucleus at the top of the atmosphere will generate the plethora of particles and resonances that one studies at the LHC for example. A lot of these will decay to muons, and thus the muons can have quit highe energy.
Section 24.3 describes the muon cosmic spectrum.
Figure 24.4 gives the surface muon energy distribution where quite high energies are seen, up to 1 TeV.