Form factor (QFT)

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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,

ε_μN(α(q²)γ^μ + β(q²)q^μ + κ(q²)σ^μνγ_ν)N

where q^μ represents the photon momentum. The three functions, α,β,κ, can be measured experimentally, and then the three effective vertices can be used to perform calculations that would otherwise be too difficult.