Discussion Overview
The discussion revolves around the calculation of cross sections in particle physics, specifically focusing on the treatment of photon spin states in relation to unpolarised cross sections involving fermions and bosons. The scope includes theoretical considerations and conceptual clarifications regarding the nature of photon spin states.
Discussion Character
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- One participant states that when calculating unpolarised cross sections, initial spin states of fermions are averaged and final spin states are summed, questioning why this does not apply to bosons like photons.
- Another participant asserts that the photon only has spin states -1 and +1, challenging the claim of three spin states.
- A different participant mentions averaging over boson spin states as well, noting that the photon is a spin-one particle without a mass state of m=0.
- One participant questions why the photon cannot have a spin state of m=0, referencing the total angular momentum number and the expected range of spin quantum numbers.
- A participant introduces a semiclassical perspective, stating that there is no frame in which the photon is at rest, implying that its spin axis is always aligned with its direction of motion.
- Another participant discusses the four polarization states of the photon, suggesting that two are unphysical and can be eliminated through gauge invariance, leaving two physical spin states corresponding to right and left circular polarizations.
- A participant expresses urgency for a response, indicating a desire for further engagement in the discussion.
Areas of Agreement / Disagreement
Participants express differing views on the nature and number of photon spin states, with some asserting the existence of three states while others limit it to two. The discussion remains unresolved regarding the treatment of photon spin in cross section calculations.
Contextual Notes
There are limitations in the discussion regarding the assumptions made about photon spin states and the implications of gauge invariance. The mathematical steps involved in the calculations are not fully explored.