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Renormalization Group

Quantifies the running of coupling constants in quantum field theories, crucial for calculations in theories beyond the Standard Model, like eta_i.
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The statement of the theorem

The running of a coupling constant αi\alpha_i with the energy scale μ\mu is governed by the Renormalization Group Equation (RGE): μdαidμ=βi(αi) \mu \frac{d\alpha_i}{d\mu} = \beta_i(\alpha_i) For a general coupling α\alpha, the beta function is defined as β(α)=μdαdμ\beta(\alpha) = \mu \frac{d\alpha}{d\mu}. In perturbation theory, this is expanded as: β(α)=∑n=0∞βnαn+1 \beta(\alpha) = \sum_{n=0}^{\infty} \beta_n \alpha^{n+1} where β0\beta_0 is the one-loop coefficient.
Source: Wikipedia