What are muons?

Muons are sub-atomic elementary particles (i.e. they can’t be broken down into smaller components), similar to electrons but around 200 times heavier. Like electrons, muons have a spin of a half. They can also be either negatively or positively charged (the antiparticle). The muons used in our experiments are formed into beams and implanted into materials. After an average lifetime of ~2.2μs, muons decay into three particles, an electron and two neutrinos. This decay releases information about the environment they were embedded in.

Positive and negative muons can interact with their environment and form can thermalise into the following states:


Bare (diamagnetic muon)

The polarised subatomic particle which did not interact with its environment yet. It is either positively or negatively charged

muonium sketch


A complex in which a positive muon is orbited by an electron. It generally occurs as a result of muons implanting into an insulating sample (and thus they are never seen in metallic samples). It is considered as a light isotope of hydrogen. Their structure being similar, muonium and hydrogen have qualitatively similar chemical behaviour. However, muonium has a mass approximatively 9 times smaller than that of hydrogen, which results in quantitative differences. Muonium is thus generally frequently used to learn about hydrogen behaviour in a material.