Finite width induced modification to the electromagnetic form factors of spin-1 particles

TL;DR

This paper investigates how finite width effects influence the electromagnetic form factors of spin-1 particles, revealing significant modifications for mesons but not for the W boson.

Contribution

It provides a general framework to include finite width effects in the electromagnetic vertex of spin-1 particles without breaking gauge invariance.

Findings

Finite width effects are negligible for the W boson.

Mass of particles in loops significantly affects rho and K* mesons.

Corrections can be comparable to other known effects.

Abstract

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. Considering 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 rho and K* mesons the mass of the particles in the loop makes a significant effect, and can be comparable with corrections of different nature .