Discussion Overview
The discussion revolves around the evaluation of Feynman diagrams in quantum field theory (QFT) specifically focusing on photon-photon scattering without external fermion lines. Participants explore the implications of missing elements in the matrix element expression and the resulting mathematical structure.
Discussion Character
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- One participant questions whether the matrix element should be a scalar, noting that the absence of spinor bi-products might necessitate taking a trace.
- Another participant suggests that the issue may stem from missing polarization indices for the external photon lines.
- A participant asserts that contracting the polarization vector with a gamma matrix still results in a matrix, indicating a potential misunderstanding of the mathematical structure involved.
- It is noted that for every closed fermion loop, a trace over the spin indices is required.
Areas of Agreement / Disagreement
The discussion includes multiple competing views regarding the mathematical treatment of the Feynman diagram, and it remains unresolved whether the initial assumptions about the matrix element are correct.
Contextual Notes
Participants express uncertainty about the implications of missing elements in the calculation, such as polarization indices and the need for traces, which may depend on specific definitions and assumptions in QFT.