Higgs Decays at NLO in the SMEFT

TL;DR

This work delivers a comprehensive NLO electroweak and QCD analysis of Higgs decays within the dimension-6 SMEFT, providing all two- and three-body decays and accessing four-body final states via the narrow width approximation. The calculations are implemented in the public NEWiSH Monte Carlo framework, enabling full SM+SMEFT width predictions and differential distributions with explicit SMEFT-coefficient dependences. By exploring HL-LHC and future FCC-ee (Tera-Z) scenarios, the study reveals how NLO SMEFT corrections induce operator correlations and enhance sensitivities, especially for operators that do not contribute at tree level to Z-pole observables. A scalar singlet case study demonstrates the impact of NLO matching on projected mass reach, highlighting the crucial role of theoretical uncertainties in shaping future collider constraints. The results advocate integrating Higgs-decay NLO SMEFT predictions into global fits and underscore the necessity of advancing production- and decay-level calculations for precise SMEFT analyses.

Abstract

The calculation of precise predictions for Higgs decays is a necessary ingredient for determining Higgs properties at the LHC and future colliders. We compute all two- and three-body Higgs decays at next-to-leading order (NLO) in both QCD and electroweak interactions using the dimension-6 Standard Model Effective Field Theory (SMEFT). Results for four-body Higgs decays that are accurate to NLO QCD/electroweak order in the SMEFT are obtained using the narrow width approximation. Our results are contained in a flexible Monte Carlo program, NEWiSH, that is publicly available and we illustrate the impact of the NLO electroweak corrections for HL-LHC, Tera-Z, and Higgstrahlung projections.

Figures (11)

• Figure 1: Representative diagrams for virtual contributions to the $h\to f\bar{f}Z$ decay process in the SMEFT.
• Figure 2: Representative diagrams for the $h\to f\bar{f}'W^\pm$ decay process in the SMEFT. Dashed blobs represent SMEFT operator insertions.
• Figure 3: Representative diagrams for the $h\to f\bar{f}$ decay process in the SMEFT at NLO. The dashed blobs represent SMEFT operator insertions.
• Figure 4: Representative diagrams for the $h\to VV'$ decay processes in the SMEFT at NLO. Dashed blobs represent SMEFT operator insertions.
• Figure 5: A comparison of the narrow width approximation and full $h \to 4\ell$ results for $C_{ll}[1221]$, where the narrow width approximation works similarly for both $e^+e^-\mu^+\mu^-$ (left) and $4\mu$ (right) channels. Here $m_{Z^*}$ is defined to be the invariant mass of the opposite pair of same flavour opposite sign leptons from the pair with invariant mass closest to $M_Z$.