Artificial Hydrogen Tests Quantum Theory: Scientific American: "To make the ultra-light isotope, they swapped the proton with a positively charged muon, which has just 11% of the mass of a proton. And to make ultra-heavy hydrogen, they replaced one of the electrons in a helium atom with a negative muon.
Helium has two electrons, two protons and two neutrons. But because it is more massive than an electron, the negative muon orbits the nucleus much more closely, masking the positive charge of one of the protons. In effect, the atom becomes a hydrogen-like composite: a 'nucleus' made of the existing two-proton, two-neutron nucleus and the muon, orbited by the remaining electron. It has a mass of a little over four times that of hydrogen."