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Muons entering the Earth are caused by cosmic ray interactions with matter. These interactions usually produce pi mesons initially, which most often decay to muons.

Muons have a mass of 105.7 MeV/c2, which is about 207 times that of the electron. Now at some measurenments of muon decay of outer space coming muons there speed is about 0.994c. That is close to the speed of light, but how can such a heavy particle reach that high speed?

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  • $\begingroup$ Cosmic ray protons can be very energetic. Muons have about 10% the mass of a proton. $\endgroup$ – dukwon Apr 28 '17 at 20:23
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    $\begingroup$ Such a heavy object? It is literally the lightest particle that decays. Only electrons and neutrinos have lower non-zero masses. $\endgroup$ – dmckee Apr 28 '17 at 21:46
  • $\begingroup$ Converting to familiar units, the mass of a muon is still less than a trillionth of a trillionth of a gram, and even at 0.995 c its kinetic energy is still under a nanojoule. That's a pretty good amount for a single particle, but it's not all that much energy, absolutely. $\endgroup$ – hobbs Apr 29 '17 at 5:17
  • $\begingroup$ How fast do you think cosmic rays move? Or should I say "How slow do you think cosmic rays move?" $\endgroup$ – Kyle Kanos Apr 29 '17 at 16:16
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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.

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