Positive Muon Decay
Cartoon of positive muon (µ+) decay
showing the case where the neutrino and antineutrino
go off in the same direction together,
which gives the highest energy positron (e+),
roughly 52 MeV.
Since (in this case) the spins of the neutrino and antineutrino
cancel each other, the positron spin must be in the same direction
as that of the muon in order to balance angular momentum.
In the weak interaction governing this decay,
such an ultrarelativistic positron acts pretty much
like an antineutrino and therefore can only be created
with positive helicity (right-handed, with its spin
pointing along its momentum).
Therefore the reaction has maximum probability
when the positron exits along the direction of
the muon's spin (and zero probability of
being emitted in the opposite direction).
When averaged over all possible emission directions
and energies of the neutrino and antineutrino,
the decay asymmetry or anisotropy has a
value of 1/3 (the positron is 2/3 more likely to
be emitted along the muon spin than opposite to it).
So an ensemble of polarized positive muons
``broadcasts'' its polarization in a shower of
high energy positrons that will penetrate through
the walls of cryostats, scintillators and other
experimental apparatus. This makes µSR possible.
Jess H. Brewer
Last modified: Fri Dec 5 09:50:44 EST