Callan–Symanzik equation

In physics, the Callan–Symanzik equation is a differential equation describing the evolution of the n-point correlation functions under variation of the energy scale at which the theory is defined and involves the beta-function of the theory and the anomalous dimensions. This equation has the following structure

$\left[M\frac{\partial }{\partial M}+\beta(g)\frac{\partial }{\partial g}+n\gamma\right] G^{(n)}(x_1,x_2,\ldots,x_n;M,g)=0$

being $\beta(g)$ the beta function and $\gamma$ the scaling of the fields.

In quantum electrodynamics this equation takes the form

$\left[M\frac{\partial }{\partial M}+\beta(e)\frac{\partial }{\partial e}+n\gamma_2 +m\gamma_3\right]G^{(n,m)}(x_1,x_2,\ldots,x_n;M,e)=0$

being n and m the number of electrons and photons respectively.

It was discovered independently by Curtis Callan[1] and Kurt Symanzik[2][3] in 1970. Later it was used to understand asymptotic freedom.

This equation arises in the framework of renormalization group. It is possible to treat the equation using perturbation theory.