Editing Muon (section)

== Muon sources ==
[[File:Hard-component-muon-868x1024.png|thumb|Cosmic ray muon passing through lead in cloud chamber]]
Muons arriving on the Earth's surface are created indirectly as decay products of collisions of cosmic rays with particles of the Earth's atmosphere.<ref>{{cite book |last1=Demtröder |first1=Wolfgang |title=Experimentalphysik |volume=1 |date=2006 |publisher=Springer |isbn=978-3-540-26034-9 |page=101 |edition=4}}</ref>

{{Blockquote|About 10,000&nbsp;muons reach every square meter of the earth's surface a minute; these charged particles form as by-products of cosmic rays colliding with molecules in the upper atmosphere. Traveling at relativistic speeds, muons can penetrate tens of meters into rocks and other matter before attenuating as a result of absorption or deflection by other atoms.<ref>{{cite journal |first=Mark |last=Wolverton |date=September 2007 |title=Muons for peace: New way to spot hidden nukes gets ready to debut |journal=[[Scientific American]] |volume=297 |issue=3 |pages=26–28 |doi=10.1038/scientificamerican0907-26 |pmid=17784615 |bibcode=2007SciAm.297c..26W}}</ref>}}

When a cosmic ray proton impacts atomic nuclei in the upper atmosphere, [[pion]]s are created. These decay within a relatively short distance (meters) into muons (their preferred decay product), and [[muon neutrino]]s. The muons from these high-energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the [[speed of light]]. Although their lifetime ''without'' relativistic effects would allow a half-survival distance of only about 456&nbsp;m {{nowrap|( 2.197 μs × ln(2) × 0.9997 {{mvar|c}})}} at most (as seen from Earth), the [[time dilation]] effect of [[special relativity]] (from the viewpoint of the Earth) allows cosmic ray secondary muons to survive the flight to the Earth's surface, since in the Earth frame the muons have a longer [[half-life]] due to their velocity. From the viewpoint ([[inertial frame]]) of the muon, on the other hand, it is the [[length contraction]] effect of special relativity that allows this penetration, since in the muon frame its lifetime is unaffected, but the length contraction causes distances through the atmosphere and Earth to be far shorter than these distances in the Earth rest-frame. Both effects are equally valid ways of explaining the fast muon's unusual survival over distances.

Since muons are unusually penetrative of ordinary matter, like neutrinos, they are also detectable deep underground (700&nbsp;m at the [[Soudan 2]] detector) and underwater, where they form a major part of the natural background ionizing radiation. Like cosmic rays, as noted, this secondary muon radiation is also directional.

The same nuclear reaction described above (i.e. hadron–hadron impacts to produce pion beams, which then quickly decay to muon beams over short distances) is used by particle physicists to produce muon beams, such as the beam used for the muon [[Muon g-2|''g''−2 experiment]].<ref>{{cite press release |publisher=[[Brookhaven National Laboratory]] |date=30 July 2002 |title=Physicists announce latest muon g-2 measurement |url=http://www.bnl.gov/bnlweb/pubaf/pr/2002/bnlpr073002.htm |access-date=2009-11-14 |df=dmy-all |archive-date=8 April 2007 |archive-url=https://web.archive.org/web/20070408005439/http://www.bnl.gov/bnlweb/pubaf/pr/2002/bnlpr073002.htm |url-status=dead }}</ref>