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1. Unitarity

   Linked via "unitary evolution operator"

   The relationship between gauge choice and unitarity is often complex. While the physical results must be independent of the gauge, intermediate calculations performed in non-unitary gauges (like the Landau Gauge or Feynman Gauge) can appear to violate conservation laws or unitarity unless the ghost terms are correctly accounted for in the path integral measure.
   The Unitary Gauge is a specific choice of [gauge fixing condition](/e…