Formfactor

This electromagnetic structure caused by the cloud of virtual states in the transition region is characterized by a specific formfactor, which is called the transition formfactor.

Source: wiktionary

Dilepton spectroscopy could also give interesting information on the electromagnetic formfactor for baryons where the situation is more complicated since for these the time-like sector is not accessible below an invariant mass of 2mp;

Source: wiktionary

Using the known values of the nucleon formfactors the pion formfactor can in principle be determined as the only free parameter in a fit to the cross section of electro-π⁺-production.

Source: wiktionary

All diagrams in a given-order formfactor are similar. They all are integrals over the momentum circulating in the loop, and the integrands are identical.

Source: wiktionary