  The fermion lifetime was usually calculated in terms of spin states. However,
after comparing spin states with helicity states and chirality states, it is
pointed out that a spin state is helicity degenerate, and so it can not be used
for discussion of the dependence of lifetime on the polarization of an
initial-state fermion. Using helicity states, we calculate the lifetime of
polarized muons. The result shows that the lifetime of right-handed polarized
muons is always greater than that of left-handed polarized muons with the same
speed in flight.
