Elementary particles (quarks and leptons) are spin 1/2 particles. The charged ones, also have magnetic moment. Is there a clear experimental evidence of the mesurement of the relative orientation between spin and magnetic moment in at least, electrons, positrons and muons?
If positive, what is this relative orientation? Are they parallel or antiparallel vectors?

It is usually argued that for electrons spin and magnetic moment are anti parallel
vectors while for positrons both vectors are parallel. However, my guess is that
PCT invariance of Dirac's equation implies that particles and antiparticles must necessarily
have the same relative orientation, either parallel or anti parallel. This depends on the
selection of the sign of the charge of the object which is considered as the 'particle'.
If the electron is the particle, then, spin and magnetic moment must be parallel vectors
for electrons and positrons. Otherwise, if the positron is considered as the particle, then,
electrons and positrons must have the spin and magnetic moment opposite to each other.

The ground state of positronium (a bound state of an electron and a positron)
the so called parapositronium state, is a 0 spin,
0 orbital angular momentum and also 0 magnetic moment. This implies that the electron
and positron must have their spins opposite to each other, but also their magnetic moments.
The conclusion is that electrons and positrons must have the same relative
orientation between spin and magnetic moment. But this is contrary to what is usually
assumed. I think that this relative orientation between spin and magnetic moment,
at least for electrons, positrons and muons, have never been measured.
This is my starting question on this subject.

The same thing happens to the neutral pion. It is a bound state of the (up-anti up)
and (down-anti down) quarks. It is also a system of 0 spin,
0 orbital angular momentum and also 0 magnetic moment. Because the up and down quarks
have different masses and charges, and therefore different magnetic moments, this implies
that each quark and anti-quark of the bound state must have also their spins and magnetic moments
opposite to each other, thus arriving to the same conclusion that quarks and anti quarks
must have the same relative orientation between spin and magnetic moment.