Which direction does "the mirror" in the Wu experiment flip? I have seen two different setups for the Wu experiment: One where the "imaginary" mirror flips the experiment along a plane parallel to the magnetic field and one where the mirror is aligned perpendicular to the magnetic field. The former case is depicted in the image on the Wikipedia page for the experiment.
Does this make a difference in the reasoning? Which direction is the correct one?
 A: It doesn't matter. The point is that the universe is not symmetric about some mirror flip. If that's true, it's true for any mirror flip.
For example, suppose we had a mirror in the $xy$ plane, which transforms
$$(x, y, z) \to (x, y, -z).$$
If the results differ before and after the flip, the universe is not symmetric about flips in the $xy$ plane. Now, as far as we can tell, physics is rotationally invariant, so the results should not change if we perform a $180^\circ$ rotation about the $z$-axis, which maps
$$(x, y, -z) \to (-x, -y, -z).$$
Now suppose we perform a $180^\circ$ rotation about the $x$-axis. This maps
$$(-x, -y, -z) \to (-x, y, z).$$
So $(x, y, -z)$ and $(-x, y, z)$ behave the same by rotational symmetry. But $(-x, y, z)$ is just $(x, y, z)$ up to a mirror flip in the $yz$ plane. So the universe is not symmetric about flips in the $xy$ plane if and only if it is not symmetric about flips in the $yz$ plane. By mildly adjusting this reasoning you can show that all mirror flips are equivalent. The algebraic steps may look slightly different for different mirror orientations, but it's the same physics.
Incidentally, to avoid having to pick a mirror orientation, physicists usually talk about parity transformations, which reflect about a point,
$$(x, y, z) \to (-x, -y, -z).$$
Of course, in 3D parity asymmetry is totally equivalent to mirror flip asymmetry. That's why all popsci presentations use the mirror, which is easier to visualize. This sometimes leads to confusion.
