  On general grounds based on quantum field theory the decay amplitude for
$\Sigma^0 \to \Lambda \gamma$ consists of a parity conserving magnetic and a
parity violating electric dipole transition moment. Because of the subsequent
self-analyzing weak decay of the $\Lambda$ hyperon the interference between
magnetic and electric dipole transition moment leads to an asymmetry in the
angular distribution. Comparing the decay distributions for the $\Sigma^0$
hyperon and its antiparticle gives access to possible C and CP violation. Based
on flavor SU(3) symmetry the present upper limit on the neutron electric dipole
moment can be translated to an upper limit for the angular asymmetry. It turns
out to be far below any experimental resolution that one can expect in the
foreseeable future. Thus any true observation of a CP violating angular
asymmetry would constitute physics beyond the standard model, even if extended
by a CP violating QCD theta-vacuum-angle term.
