Form factor (quantum field theory)

In effective field theory, a form factor is a function which gives the properties of a certain particle interaction without including all of the underlying physics. It is measured experimentally when a theoretical calculation is unavailable or too difficult.

For example, at low energies the interaction of a photon with a nucleon is a very complicated calculation involving interactions between the photon and a sea of quarks and gluons, and often the calculation cannot be done.

However the general form of the interaction is known,

$\epsilon _{\mu }N(\alpha (q^{2})\gamma ^{\mu }+\beta (q^{2})q^{\mu }+\kappa (q^{2})\sigma ^{\mu \nu }\gamma _{\nu })N$

where $q^{\mu }$ represents the photon momentum. The three functions, $\alpha ,\beta ,\kappa$, can be measured experimentally, and then the three effective vertices can be used to perform calculations that would otherwise be too difficult.