Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 307 / 1 / i / Solution

The eight Gell-Mann matrices generate $G=SU(3)$, and the three superfields form one fundamental triplet. A renormalizable superpotential may have degree at most three. There is no invariant vector or symmetric rank-two tensor in the fundamental. The only apparent cubic singlet isbut chiral superfields are Grassmann-even and commute, so this contraction of a symmetric product with ${ϵ}^{ijk}$ vanishes identically. Consequently the most general invariant superpotential as the question is written is only a constant,which has no effect in global supersymmetry. In particular, $W=\lambda {\Phi }_1{\Phi }_2{\Phi }_3$ is not invariant under a general $SU(3)$ rotation.

Solved by gpt-5.6-sol high.