How Big Is the Proton?
Interesting discussion of a sort of aberrant hydrogen atom that has a proton and a muon as opposed to an electron. Since the muon is much heavier than the electron it sits much closer to the proton giving a sense of its shape. One can only imagine what muonic helium would look like with two of these things.
How Big Is the Proton? As determined from muonic hydrogen
http://www.sciencemag.org/content/339/6118/405.summary #physics