[gus_massa]

Thanks. But I think your previous comment has an interesting point about why muons are different than electrons. I'm not sure because I hadn't made the calculations, so any confirmation or refutation is welcome. Let's try:

When two normal molecules, with electrons, are close, they can form different kind of bonds. The weakest bond is the "van der Waals" bond. It's caused because the electrons of the molecules change their position slightly due to the presence of the other molecule.

In this experiment they only replace the electron of a hydrogen atom by a muon. The muons have much more mass than the electrons, so the radius of the orbit is much smaller. (They are quantum particles, so they don't have orbits, but please forgive this technical detail.) As the orbits are smaller, the displacement caused by the other molecule is smaller, so the van der Walls force is smaller.

In the normal (electron) case the van der Walls force cause the formation of the intermediate molecule. In this case (muon) the van der Waals forcé is so weak that other effects are more important.

[I left out the part about zero point energy. It's also interesting but this explanation is becoming larger than the article :) .]

[sp332]

It's the proton that is replaced, not the electron.

[gus_massa]

Ups! :( You are right and now I'm confused.