Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-304/2/i/solution
Past exam of the mathematics course of the University of Cambridge 2018 iii Paper 304 2 i Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-05
Take integer and positive integer , so the interaction is genuinely non-Gaussian. For a connected two-point Feynman diagram with interaction vertices and internal lines, counting half-edges and using the loop order givesAt one loop , hence . There are only two possibilities: a two-interaction vertex bubble diagram for , or a one-interaction vertex tadpole diagram for . The latter is independent of external momentum and changes the mass, not the kinetic term.
For cubic scalar field theory, introduce a positive auxiliary mass if needed to separate infrared divergences from ultraviolet divergences. The one-loop self-energy in the quantum effective action is , withThis Feynman parameter representation follows by shifting the loop momentum; it is directly valid for a translation-invariant regulator, or for the convergent subtracted integral. Differentiating at givesThus the coefficient of the kinetic term changes by .
Only the cubic interaction has a momentum-dependent one-loop two-point correction.There is a terminology qualification: if “wave-function renormalization” means a required ultraviolet counterterm, rather than a finite correction to the field normalization, the large- radial integral behaves as , and the answer isIt is logarithmically divergent at and power divergent above with a momentum cutoff. Among interactions with a relevant coupling or marginal coupling, the divergent case is uniquely . The massless low-dimensional amplitude needs an infrared divergence prescription; one cannot take a local expansion at zero momentum without one. The cases, if admitted, are a source and a Gaussian mass term and have no interaction-induced wave-function renormalization.
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