In quantum mechanics, chirality is preserved for massless particles.
But in Standard Model, is there any reason for chirality to be preserved for massless particles in annihilation diagrams? (unless it would not be allowed like right-handed neutrinos)
For example, if I have a left-handed neutrino and antineutrino colliding, propagating as z-boson...
Is there any reason for the outgoing massless particles to be only left? Or can they be right also? Outgoing particles could be massless electron and positron.
I don't think Z-boson conserves any chirality, but I'm not 100% sure.