Massive and massless Majorana particles of arbitrary spin: Covariant gauge couplings and production properties

It is shown that massive particles that are their own antiparticles can possess only one specific type of multipolar moments---the so-called toroidal moments. Unlike the familiar charge and magnetic moments, these moments do not directly interact with the (external) static electromagnetic fields but only with the external current, and hence lead to contact interactions. On the other hand, massless Majorana particles, with the exception of those with spin 1/2, do not have any electromagnetic form factor. It is also found that regardless of the spin of the Majorana particle the differential cross section for the production of a Majorana pair by a spin-1 particle at a high-energy e+e- machine has a unique angular distribution