  Electromagnetic interactions of the spin 3/2 particle are investigated while
allowing the propagation of the transverse spin 1/2 component present in the
reducible Rarita-Schwinger vector-spinor. This is done by allowing a more
general form for the mass term, while leaving the kinetic terms untouched. We
find that the interaction is consistent and does not lead to superluminal
propagation for a range of the mass parameter where the spin 1/2 component is
lighter than the spin 3/2 component, in contrast to the traditional value
whereby the spin 1/2 component is removed by making it infinitely massive. The
hyperbolicity condition is found to be independent of the magnitude of the
electromagnetic field, and the canonically quantized field is constructed to be
causal. We then provide appropriate projection operators which reproduce the
spin-sum expressions and enable the construction of a physically acceptable
propagator. Finally, we suggest a scheme for extension of the Standard Model
with Rarita-Schwinger multiplets and identify the muon as the spin 1/2
component of the new multiplet which in addition contains a new, heavy partner
of the muon of spin 3/2. The scattering cross-section e+, e- \to {\mu}+, {\mu}-
is calculated within this theory, compared to QED, and finally a bound on the
new particle's mass is obtained from precision electroweak measurements.
