Effective field theory versus UV-complete model: vector boson scattering as a case study

Abstract

Effective field theories (EFT) are commonly used to parameterize effects of BSM physics in vector boson scattering (VBS). For Wilson coefficients which are large enough to produce presently observable effects, the validity range of the EFT represents only a fraction of the energy range covered by the LHC, however. In order to shed light on possible extrapolations into the high energy region, a class of UV-complete toy models, with extra SU(2) multiplets of scalars or of fermions with vector-like weak couplings, is considered. By calculating the Wilson coefficients up to energy-dimension eight, and full one-loop contributions to VBS due to the heavy multiplets, the EFT approach, with and without unitarization at high energy, is compared to the perturbative prediction. For high multiplicities, e.g. nonets of fermions, the toy models predict sizable effects in transversely polarized VBS, but only outside the validity range of the EFT. At lower energies, dimension-eight operators are needed for an adequate description of the models, providing another example that dimension-eight can be more important than dimension-six operators. A simplified VBFNLO implementation is used to estimate sensitivity of VBS to such BSM effects at the LHC. Unitarization captures qualitative features of the toy models at high energy but significantly underestimates signal cross sections in the threshold region of the new particles.

Unitarity Bounds on Effective Field Theories at the LHC

“…11, which shows typical curves of the relative error without taking the absolute value to highlight how ∆ crit is approached. 12,13 We emphasize that this behavior of the EFT expansion only happens for the parameter space where M < √ s. If instead the new physics scale M is above the collider energy √ s, the EFT expansion will yield a convergent series as expected. The contrast between these two regimes can be seen in Fig.…”

“…When computing the unitarity bounds on a scenario of interest, it is paramount that these low energy cuts are implemented, since the relevant signal regions often lie in the tails of kinematic distributions (see e.g. [12,51]).…”

“…When confronted with such a result, one should worry that the EFT approach is inconsistent (see e.g. [9][10][11][12]). In this work, we will investigate this question by assessing the impact of the proton's structure on one of the necessary conditions for EFT validity, namely that its scattering amplitudes satisfy partial-wave unitarity.…”

Machine-enhanced CP-asymmetries in the Higgs sector

“…Specifically, we demonstrated this using dimension-six effective field theory predictions for the interference contributions. Improving the sensitivity to CP-violating effects in h → 4 and VBF Higgs production is particularly important for the self-consistency of the dimension-six approach [68,69].…”

From models to SMEFT and back?

“…As a consequence, the effect in m W W is only visible close to m V , where the SMEFT expansion immediately breaks down. The situation improves when we include higher-dimensional operators [22,98]. At dimension six, the matching to our specific model suppresses all energy-enhanced SMEFT contributions to W W production, so the signal is under-estimated across the em W W distribution.…”