This question arises from the fact that the muon has a mass close to the first excited state of the radial vibration of the electron (Prog. Theor. Vol. 47 (1972), No. 3 Cohesive force of electron and Nambu's mass-formula).
At <5.7E-13, it should be the second best upper limit on a branching fraction ever set, at least according to the particle data group (checked kaons, pions and neutrons. The lowest upper limit is for a charge violating neutron decay that would make 71Ga unstable).This has never been observed, despite a LOT of looking.
To make it worse, all the mesons with heavy quarks are heavier than the light baryons, so yes, the original idea that lead to their names is not right any more.The names hadron/meson/lepton are really abused in terms of their meanings.E.g. the tau lepton can by no-means be considered lighter than some mesons, and its mass is comparable to hadrons (protons/neutrons).
That's why they have changed their meanings, leptons are "those" , mesons are the quark-antiquarks, hadrons are the triquarks etc...
There are hundreds of experiments consistent with the muon as a fundamental particle, with incredible precision. There is no theory of a muon as something else that would be consistent with all (or even just half) of those measurement results.I think that my original question "Why the muon is considered a fundamental particle?" was not answered yet.
You don't need recent research for that, this is standard quantum mechanics and known for over 80 years now. Quantum electrodynamics is over 50 years old, too.Considering that recent research using tunnelling microscopy shows clear evidence of the distributed nature of particles
As mfb pointed out, the uncertainty principle has been around a long time. But you imply that recent results show clear evidence of greater "distributed nature of particles". Do you have a reference for that?... recent research using tunnelling microscopy shows clear evidence of the distributed nature of particles, far bypassing the uncertainty principle