Finite width induced modification to the electromagnetic form factors of spin-1 particles. D. Garc´ıa Gudi˜ no Instituto de F´ısica, Universidad Nacional Aut´ onoma de M´ exico, AP 20-364, M´ exico D.F. 01000, M´ exico G. Toledo S´ anchez Instituto de F´ısica, Universidad Nacional Aut´ onoma de M´ exico, AP 20-364, M´ exico D.F. 01000, M´ exico and Department of Physics, Florida State University, Tallahasse, FL 32306 (Dated: October 29, 2018) 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, which must be considered in addition to any other contribution computed as stable particles. Con- sidering the modified electromagnetic vertex, we obtain general expressions for the corresponding corrections to the multipoles as a function of the mass of the particles in the loop. For the W gauge boson no substantial deviations from the stable case is observed. For the ρ and K ∗ mesons the mass of the particles in the loop makes a significant effect, and can be comparable with corrections of different nature . PACS numbers: 13.40.Gp, 11.10.St, 14.40.-n 1 Abstract arXiv:1001.0998v2 [hep-ph] 24 Mar 2010