Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 304 / 3 / c / i

At lowest derivative order, a general invariant effective Lagrangian through fourth order in the fields has the schematic formHere ${\lambda }_{abcd}$ is any invariant symmetric rank-four tensor, the ${\mathcal{I}}_A$ run over invariant contractions such as $(\mathrm{Tr}{\varphi }^2)\bar{\psi }\psi$ and $\bar{\psi }{\varphi }^2\psi$, and the ${\mathcal{J}}_B$ run over gauge- and Lorentz-invariant four-fermion contractions. The two sign symmetries forbid scalar cubic terms and Yukawa terms $\bar{\psi }\varphi \psi$. The covariant kinetic terms automatically contain the allowed cubic and quartic interactions involving $A_{\mu }$.

The canonical dimensions areand henceA coupling is relevant, marginal, or irrelevant when its dimension is positive, zero, or negative at the Gaussian fixed point. Thus gauge and scalar-quartic interactions are marginal in $d=4$, the mixed two-scalar fermion bilinear is marginal in $d=3$, and four-fermion interactions are marginal in $d=2$. Quantum corrections replace this engineering classification near an interacting fixed point by the eigenvalues of its RG stability matrix.