It's about the Goldhaber experiment, where the exercise I'm trying to solve is:
Photons are filtered according to their helicity thanks to the magnetic field. For a given magnetic field, the electrons contained in the magnet see all spins flipped to a preferred direction. When the magnetic field is inverted, all electrons spins are flipped in the other direction. Considering that photons can have two different helicities and can interact with electrons to change their quantum states, explain why photons of a given helicity for a given magnetic field configuration can loose more energy in the magnet than photons with the other helicity.
The solution states that: If the photons have their spin in the same direction of the electrons, the formers will less likely be able to flip the spins of the latters. Thus, the photon will have a smaller probability to interact with the electrons and loose some energy in the magnet. Then they will have a larger probability to interact with the samarium scatterer. Thus, photons are filtered according to their helicity, depending of the magnetic field applied (as they will loose, in average, enough energy to not exitate a samarium atom of the scatterer).
Can someone explain why the bold text holds?