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We investigate the spin dynamics and the conservation of helicity in thefirst order $S-$matrix of a Dirac particle in any static magnetic field. Weexpress the dynamical quantities using a coordinate system defined by the threemutually orthogonal vectors; the total momentum$\mathbf{k}=\mathbf{p_f}+\mathbf{p_i}$, the momentum transfer$\mathbf{q}=\mathbf{p_f-p_i}$, and $\mathbf{l}=\mathbf{k\times q}$. We showthat this leads to an alternative symmetric description of the conservation ofhelicity in a static magnetic field at first order. In particular, we show thathelicity conservation in the transition can be viewed as the invariance of thecomponent of the spin along $\mathbf{k}$, and the flipping of its componentalong $\mathbf{q}$, just as what happens to the momentum vector of a ballbouncing off a wall. We also derive a "plug and play" formula for thetransition matrix element where the only reference to the specific fieldconfiguration, and the incident and outgoing momenta is through the kinematicalfactors multiplying a general matrix element that is independent of thespecific vector potential present.

SU(2) Symmetry and Conservation of Helicity for a Dirac Particle in a Static Magnetic Field at First Order