ABSTRACT
Renormalisation provides a solution to the problem of divergences occurring in the calculation of Feynman diagrams that contain closed loops, but is a more general concept that will be encountered even in the absence of divergences. The necessity for renormalisation actually arises from the existence of interactions, which means that the masses which appear in the Lagrangian are not those of the particles which the various fields describe, and that the fields themselves are not “well normalised”, as it clearly appears from the presence of Z factors in the contribution of the single particle states to the relevant propagators.
