  Elementary particle scatterings and decays in presence of a background
magnetic field are very common in physics, specially after the observation that
the core of the neutron stars can sustain a magnetic field of the order of
$10^{13} {\rm G}$. The important point about these calculations is that they
are done in a background of a gauge field and as a result the calculations are
prone to gauge arbitrariness. In this work we will investigate how this gauge
arbitrariness is eradicated in processes where the initial and final particles
taking part in the interactions are electrically neutral. Some comments on
those processes where the initial or final state consists of electrically
charged particles is presented at the end of the article.
