Magnetic focusing in atomic, nuclear and hadronic processes

Processes with oppositely charged spinor particles in initial and/or final states in homogeneous magnetic field B are subject to focusing effects in their relative motion, which yield the amplifying factors in probabilities growing as $eB$. In addition the increasing energy of some Landau levels influences the phase space. As a result some processes in the proper spin states can be enlarged as $\sim \frac{eB}{\kappa^2}$, where $\kappa^2$ is the characteristic 2d phase space factor available without magnetic field. Several examples, including neutron $\beta$ decay, positronium decay and $e^+e^-$ pair production, are quantitatively considered.