Modifications to the electromagnetic structure of vector bosons due to finite width effects

The inclusion of the unstable features of a spin-1 particle, without breaking the electromagnetic gauge invariance, can be properly accomplished by including higher order contributions as done in the so-called fermion loop scheme (for the W gauge boson), and the boson loop scheme (for vector mesons). This induces a non trivial modification to the electromagnetic vertex of the particle. Considering the modified electromagnetic vertex, we obtain general expressions for the corresponding multipoles as a function of the mass of the particles in the loop. For the W gauge boson no substantial deviations from the stable tree level case is observed. For the ρ meson the mass of the particles in the loop makes a significant effect.