  The leptonic decay of charged pions is investigated in the presence of
background magnetic fields. In this situation Lorentz symmetry is broken and
new fundamental decay constants need to be introduced, associated with the
decay via the vector part of the electroweak current. We calculate the magnetic
field-dependence of both the usual and a new decay constant non-perturbatively
on the lattice. We employ both Wilson and staggered quarks and extrapolate the
results to the continuum limit. With this non-perturbative input we calculate
the tree-level electroweak amplitude for the full decay rate in strong magnetic
fields. We find that the muonic decay of the charged pion is enhanced
drastically by the magnetic field. We comment on possible astrophysical
implications.
