Lepton Universality and Pauli Exclusion Put in a possibly oversimplified way, lepton universality says that electrons, muons, and taus all behave in the same way except for mass effects. The question is “Does this apply to Pauli exclusion?” Due to the Pauli exclusion principle, only two electrons can be in the s orbital, and hence in the ground state of regular beryllium, two electrons are in the s orbital, and two electrons are in the p orbital. My question is what happens in the ground state of dimuonic beryllium, with two muons and two electrons. Are all four particles in s orbitals, or are two in s orbitals and two in p orbitals? A similar question can be asked about muonic lithium, which would be easier to create experimentally. i.e., in muonic lithium, with one electron replaced by a muon, can all three leptons be in the s orbital?