This question is closely related to my previous question Bound states in QED.
Muonium is a system of electron and anti-muon. This article in wikipedia claims that muonium is unstable.
QUESTION: Why it is unstable? Is it due to presence of weak interactions, or it can be explained only in the framework of QED (involving both electrons and muons and their anti-particles).
To compare with, the positronium, consisting from electron and positron, in an unstable particle (see e.g. this article in wikipedia). It can annihilate to two photons. This effect can be explained by the usual QED. This explanation does not seem to work for the muonium.