Form factor (QFT)
Encyclopedia
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. Often in this context, form factors are also called "structure functions", since they can be used to describe the structure of the nucleon.
However the general form of the interaction is known,
where
represents the photon momentum. The three functions, 
are the form factors for this interaction, they can be measured experimentally, and then the three effective vertices can be used to perform calculations that would otherwise be too difficult.
Effective field theory
In physics, an effective field theory is, as any effective theory, an approximate theory, that includes appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale, while ignoring substructure and degrees of freedom at shorter distances .-The renormalization...
, 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
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
with a nucleon
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
is a very complicated calculation involving interactions between the photon and a sea of quarks and gluons, and often the calculation cannot be done. Often in this context, form factors are also called "structure functions", since they can be used to describe the structure of the nucleon.
However the general form of the interaction is known,
where
represents the photon momentum. The three functions, are the form factors for this interaction, they can be measured experimentally, and then the three effective vertices can be used to perform calculations that would otherwise be too difficult.