Helicity Selection Rules and Non-Interference for BSM Amplitudes

TL;DR

The paper analyzes how beyond-Standard-Model effects encoded in dimension-6 EFT operators interfere with Standard Model amplitudes in high-energy collider processes. Using the spinor-helicity formalism and the Warsaw basis, it derives helicity selection rules that forbid SM–BSM interference for 2→2 amplitudes involving at least one transverse vector in the massless, tree-level limit, implying that dimension-8 operators can compete with or dominate over dimension-6 effects within EFT validity. Interference can reappear through finite-mass corrections and loop effects, but is generally suppressed by m_W^2/E^2 or by α_s/4π, and exclusive/polarization-sensitive analyses or higher-point processes (2→3) may be needed to probe D=6 operators effectively. The findings motivate careful consideration of dimension-8 contributions in EFT analyses and suggest polarization- and exclusive-channel strategies to maximize sensitivity to BSM physics at colliders.

Abstract

Precision studies of scattering processes at colliders provide powerful indirect constraints on new physics. We study the helicity structure of scattering amplitudes in the SM and in the context of an effective Lagrangian description of BSM dynamics. Our analysis reveals a novel set of helicity selection rules according to which, in the majority of 2 to 2 scattering processes at high energy, the SM and the leading BSM effects do not interfere. In such situations, the naive expectation that dimension-6 operators represent the leading BSM contribution is compromised, as corrections from dimension-8 operators can become equally (if not more) important well within the validity of the effective field theory approach.

Figures (2)

• Figure 1: When the factorization channel goes on-shell, it propagates a well-defined helicity eigenstate and Eq. (\ref{['sumH']}) holds.
• Figure 2: A schematic representation of the relative size of different contributions to the $VVVV$ scattering cross sections, with polarization LLLL (left panel), LLTT (central panel) and TTTT (right panel). LO/NLO denote the leading/next-to-leading contributions to the cross section. In the white region the SM dominates and the leading BSM correction comes from the BSM$_6$-SM interference (denoted as $BSM_6$). BSM non-interference is responsible for the light-shaded blue and orange regions, where the BSM, although it is only a small perturbation around the SM, is dominated by terms of order $E^4/\Lambda^4$, either from (BSM$_6$)$^2$ or from the BSM$_8$-SM interference (denoted as $BSM_8$).