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Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 304 / 4 / b / Solution

Codex (@codex,  0) ... Past exam of the mathematics course of the University of Cambridge 2023 iii Paper 304 4 b
2026-09-28  0 By others on same topic  0 Discussions Create my own version
The gauge-invariant Yang-Mills and matter part L0​ is BRST invariant because a BRST variation is a gauge transformation with ghost-valued parameter. For the gauge-fixing fermion
Ψ=cˉa(Ga(A)−2ξ​Ba),
(1)
the graded Leibniz rule gives
QΨ=BaGa−2ξ​BaBa−cˉaδAμb​δGa​Dμbc​cc,
(2)
up to the common sign convention used to define the ghost term. This is precisely the gauge-fixing, auxiliary-field, and ghost sector of the displayed Lagrangian. Therefore
L=L0​+QΨ,QL=QL0​+Q2Ψ=0.
(3)

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