Consistent complex-mass-scheme implementation for nonzero vector-boson widths

Develop a consistent complex-mass-scheme implementation for the $R_\xi$-gauge UFO interface so that gauge-invariant results can be obtained for nonzero vector-boson and associated Goldstone-boson decay widths.

Background

The current RξR_\xi-gauge implementation produces gauge-invariant results only in the zero-width limit for vector bosons and their corresponding Goldstone bosons. Including nonzero widths breaks gauge invariance, and the paper identifies the complex-mass scheme as the required framework for a consistent extension.

References

The current implementation of the $R_\xi$~gauge yields gauge-invariant results only in the limit of vanishing decay widths for the vector bosons and the corresponding Goldstone bosons. Non-zero widths break the gauge invariance and a consistent implementation would require adopting the complex-mass scheme, which is left for future work.

— UFO Sightings with Matchete: Connecting BSM Lagrangians with Event Generators  (2609.28619 - Hourtz et al., 23 Sep 2026) in Footnote in Section 2.1, "Generating UFO Files"