Why are muons considered to be "elementary particles" in the Standard Model?

According to this article, a muon decays into one electron and two neutrinos.

According to this article, elementary particles or fundamental particles are particles "whose substructure is unknown, thus it is unknown whether it is composed of other particles." I have also seen somewhere that it is a particle that cannot be reduced into other constituent particles.

While perhaps not a sure thing, seems like the decay indicates that the muon may be just a composite particle, perhaps consisting of one electron and two neutrinos?

Based on this, why does the muon fit with the above definition of an elementary or fundamental particle?

I realize there are much more complicated, historical reasons as to why it was included in the Standard Model, but this question is just related so how it fits (or doesn't fit) the stated definition above.

It seems to me that we really can only get solid evidence of elementary vs. composite when we smash the particles together and see what comes out and compare that to all the masses, energies and momentum before and after? Until we do that with muons, how can we know with much certainty?

And perhaps we'll have a better answer with a Muon collider: http://en.wikipedia.org/wiki/Muon_collider/ http://map.fnal.gov/

To that point, seems that electrons may not be fundamental after all: https://www.sciencedaily.com/releases/2016/04/160404111559.htm

Answer

Indeed, we can't know for sure if muons are elementary are not. In this sense, the situation is like the 1950's, where we had a zoo of mesons and hadrons, but didn't yet know they were made of quarks. We wouldn't have direct confirmation of quarks for decades to come, like how we have no indications of muon compositeness now.

Despite this, the quark model was accepted, and all attempts to make muons composite have not been. There are many scientific reasons to reject muon substructure:

• Predictive power. Your model basically says that all the charged leptons are 'really' electrons or positrons with bound neutrinos. This makes no direct predictions of new particles or processes. In contrast, the quark model predicted whole families of new particles.


• Theoretical simplicity. You need to postulate a new force to keep the neutrinos bound to the electrons, since the weak force is certainly not enough to do it. This was okay for the quark model, because we knew there was a force we didn't understand at the time (the strong nuclear force), but we have no indications for such a force. This 5th force should significantly affect the motion of neutrinos, but we haven't seen any such thing.


• Direct observation. Since muons are fairly light, whatever force is binding the electrons and neutrinos together must be fairly weak (since the binding energy $E = mc^2$ is low). This indicates that we should have been able to tear apart a muon into its constituent parts by now, or at least excite its energy levels. This hasn't been observed.


• Occam's razor. There's no need to postulate composite particles to explain decays. For example, excited atoms can decay to ground-state atoms and photons. This process can be described simply and exactly by coupling the atom to the electromagnetic field; it doesn't require a photon to be 'inside' the excited atom. Worse, there are alternative decay channels which emit two photons instead! The 'photon inside' picture isn't even self-consistent.

In short, there's no guarantee that muons aren't composite. But there are many compelling experimental and theoretical reasons suggesting so.