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Muons result from cosmic rays, but cosmic rays are protons that decay?

  1. Aug 27, 2014 #1
    I am wondering how Muons are manufactured.

    Wikipedia says they're the decay products of cosmic rays, and that cosmic rays are usually high energy protons.

    So a high energy proton (cosmic ray) decays into a muon after colliding with matter on earth.
    How is consistent with the fact that protons are so-called 'stable particles'?

    Doesn't 'stable' mean 'won't decay'?
  2. jcsd
  3. Aug 27, 2014 #2
    Not quite. A proton will collide with a nucleus in the upper atmosphere, creating many particles. Some of the created particles, for example charged pions, decay to muons. The proton itself does not decay.
  4. Aug 27, 2014 #3


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    decay means that if you have an isolated proton, then it will remain a proton. Decays are of the form [itex] A \rightarrow B+C + ... [/itex]. For example the protons in a nucleus can go through a beta+ decay.

    What happens in the atmosphere is something similar to what happens in a pp collider... you have a proton colliding with the matter [other nuclei] in the atmosphere, and then several particles can come out of this, as for example the pions which give you the muons.
  5. Aug 27, 2014 #4
    OK. So stable means won't decay if it doesn't collide with another particle?

    Do protons that collide with other matter transmogrify into pions, and then the pions decay into muons?
    Does the proton ever disappear or turn into something else?
  6. Aug 27, 2014 #5
    So protons can be decomposed by collisions. Right?

    I mean a proton (UUD) can collide and break into a pion (U/D) and other stuff too. Is this true?
  7. Aug 27, 2014 #6
    'Decaying' is spontaneous decomposition?
  8. Aug 27, 2014 #7


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    stable means that it doesn't decay if it's isolated... The collision is not a decay (you have at least two particles participating). As I wrote above, proton would decay if a process like the
    [itex] p \rightarrow [stuff] [/itex]
    was to happen. There is no such process. What is happening is the hard scattering of protons to neutrons or protons in the atmosphere. The result can be a lot of things, but most of these end up in pions, neutrons and protons. The charged pions then can decay with weak interactions and give muons. And this ends up creating a shower of particles in the upper atmosphere.

    The proton can remain a proton or it can become something else.
  9. Aug 27, 2014 #8


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    It does not decay at all.
    A collision process is not called "decay".

    No, those pions are newly created particles. They get produced together with various other particles. The charged particles decay to muons (and a neutrino).

    Often the proton completely breaks up, it is not meaningful to ask "what does the proton gets transformed to".
  10. Oct 16, 2014 #9
    Baryon number is conserved. So when the collision produces one or more antibaryons, it is not meaningful to ask which of the two or more baryons left is the original proton.

    However, if the collision produces just leptons, mesons and various bosons then there will be just one baryon afterwards, and if not a proton then it is what the proton has been transformed to.
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