text: The collision of an accelerated proton beam (typical energy 600 MeV) with the nuclei of a production target produces positive pions () via the possible reactions:
  From the subsequent weak decay of the pions (MEAN lifetime  = 26.03 ns) positive muons () are formed via the two body decay:
  Parity violation in the weak interactions implies that only left-handed neutrinos exist, with their spin antiparallel to their linear momentum (likewise only right-handed anti-neutrino are found in nature).  Since the pion is spinless both the neutrino and the   are ejected with spin antiparallel to their momentum in the pion rest frame. This is the key to provide spin-polarised muon beams. According to the value of the pion momentum different types of -beams are available for µSR measurements.