  The ideas and formulas presented in the article will help to bring together
the theoretical predictions for the anomalous magnetic moment of muon and the
results of the "Muon g-2" experiment. In doing so, we are discussing the new
effect exclusively within the Standard Model. In quantum physics a state with
spin perpendicular to a magnetic field can be expressed as a superposition of
energy eigenstates with spins parallel and antiparallel to the field: the
resultant spin precession is due to the energy difference between the two
eigenstates. If the state, like the muon, is unstable and can decay, it will
have a natural energy spread. As a result the frequency of the spin precession
can vary. For a constant magnetic field the measured spin precession velocity
will be spread according to the Lorentzian distribution with width
$\left(\gamma\tau\right)^{-1}$, for Lorentz gamma factor $\gamma=E/ m$, and
particle lifetime $\tau$. Although the true mean and variance of a Lorentzian
distribution is undefined, the latter can be estimated by the maximum
likelihood method to be ${2 \over N (\gamma \tau)^2}$, twice that of a normal
distribution. Thus, the statistical error on the anomalous magnetic moment in
reality should turn out to be wider than with $\chi^2$ analysis of the
experiment.
