  Flavor violating processes in the lepton sector have highly suppressed
branching ratios in the standard model mainly due to the tiny neutrino mass.
This means that observing lepton flavor violation (LFV) in the next round of
experiments would constitute a clear indication of physics beyond the standard
model (BSM). We revisit a discussion of one possible way to search for LFV,
muonium-antimuonium oscillations. This process violates muon lepton number by
two units and could be sensitive to the types of BSM physics that are not
probed by other types of LFV processes. Using techniques of effective field
theory, we calculate the mass and width differences of the mass eigenstates of
muonium. We argue that its invisible decays give the parametrically leading
contribution to the lifetime difference and put constraints on the scales of
new physics probed by effective operators in muonium oscillations.
