Fig. 3
From: Low-energy spin-physics experiments with polarized beams and targets at the COSY storage ring
In-principle demonstration that a time-reversed situation is prepared by either a proton or a deuteron spin-flip. a The basic system is shown. b The time reversal operation is applied (momenta and spins are reversed and the particles are exchanged). In order to have a direct comparison between situation (a) and (b), two rotations Ry(π) or Rx(π) by 180∘ about the y- or x- axis are applied, leading to the situations (c) and (d), respectively. This is allowed, since the time reversal operation is invariant under rotations