Muons are fundamental, charged particles with:
spin 1/2
magnetic moment 3.2 x magnetic moment of proton
mass 0.11 x mass of proton
lifetime 2.2 µs: decay into a positron (and a couple of neutrinos)

Muonium: positive muon + electron -
hydrogen atom analogue.

In English: Introduction to Muons - ISIS

In English: About µSR : Muon Properties - PSI

In English: Muons as microscopic probes in condensed matter
National Centre of Competence in Research - MANEP - Materials with Novel Electronic Propierties

In English: µSR , an Introduction by Jess H. Brewer, Canadian Inst. for Advanced Research and Dept. of Physics & Astronomy, Univ. of British Columbia, Vancouver, BC, Canada

In English, Nederlands, Slovensko & Deutsch: Muon - From Wikipedia, the free encyclopedia

In Japanese: What is a Muon?

In Russian: Introduction to µSR , by Victor N.Duginov - JINR Dubna

Latest news on precise measurements of the muon magnetic moment: New g-2 Measurement Deviates Further From Standard Model Brookhaven National Laboratory - Press release