Assertion: known lepton | /r/HasProperty | muon
Facets:
Source sentences:
text: When Rosenfeld named them, the only known leptons were electrons and muons, whose masses are indeed small compared to nucleons—the mass of an electron (0.511 MeV/c2) and the mass of a muon (with a value of 105.7 MeV/c2) are fractions of the mass of the "heavy" proton (938.3 MeV/c2).
text: The term was chosen to refer to particles of small mass, since the only known leptons in Rosenfeld’s time were muons.