Pseudo-observables in Higgs decays

TL;DR

This work develops a model-independent parameterization of Higgs decay amplitudes in the absence of light new states by introducing pseudo-observables defined from on-shell Higgs decay amplitudes. It decomposes $h\to 4f$, $h\to f\bar f\gamma$, and $h\to \gamma\gamma$ into neutral- and charged-current contributions with a Higgs–gauge–current three-point structure, introducing universal $\kappa_{VV}$ and a set of contact terms $\epsilon_X$ that encode New Physics effects and possible symmetry violations. The authors provide SM references for these pseudo-observables, analyze how flavor universality, CP invariance, custodial symmetry, and linear vs. non-linear EFT reduce the independent parameter count, and illustrate how differential distributions (e.g., $h\to e^+e^-\mu^+\mu^-$) sensitize the extraction of contact terms. The framework supports higher-order QED/QCD corrections, connects to linear EFT operator mappings, and offers a practical tool to test EFT structure and electroweak symmetries with precision Higgs data. Overall, it furnishes a principled, extensible, and experimentally accessible language for constraining or revealing new physics in Higgs decays.

Abstract

We define a set of pseudo-observables characterizing the properties of Higgs decays in generic extensions of the Standard Model with no new particles below the Higgs mass. The pseudo-observables can be determined from experimental data, providing a systematic generalization of the "kappa-framework" so far adopted by the LHC experiments. The pseudo-observables are defined from on-shell decay amplitudes, allow for a systematic inclusion of higher-order QED and QCD corrections, and can be computed in any Effective Field Theory (EFT) approach to Higgs physics. We analyze the reduction of the number of independent pseudo-observables following from the hypotheses of lepton-universality, CP invariance, custodial symmetry, and linearly realized electroweak symmetry breaking. We outline the importance of kinematical studies of $h\to 4\ell$ decays for the extraction of such parameters and present their predictions in the linear EFT framework.

Figures (3)

• Figure 1: Normalized differential $h\to e^+e^- \mu^{+}\mu^{-}$ decay distribution in $m_{12}\equiv\sqrt{q_1^2}$ in the SM. Tree level predictions and full $\mathcal{O}(\alpha)$ electroweak corrections with Prophecy4F Monte Carlo generator Bredenstein:2006rh are shown with blue and red dots, respectively. The solid black line is obtained after integrating the analytic formula (Eq. \ref{['eq-diff-dis']}) over $q_2^2$ for $\kappa_{ZZ}=1$ and $\epsilon_X=0$.
• Figure 2: (a) Total decay rate for $h\to e^+e^- \mu^+\mu^-$ as a function of $\kappa_{ZZ}$ and $\epsilon_{Zf_R}$ in units of the SM-predicted rate. (b) Deviations in the normalized single differential distributions in $m_{12}\equiv \sqrt{q_1^2}$ (solid-blue line) and $m_{34}\equiv \sqrt{q_2^2}$ (dashed-red line) from the SM expectations for the benchmark point $(\kappa_{ZZ},\epsilon_{Zf_R})=(0.88,-0.10)$. (c) Ratio of the double differential distribution with the SM prediction for the same benchmark point. No cuts are applied on the Higgs decay products.
• Figure 3: The same plots as in Fig. \ref{['fig:tot']} for the case $\epsilon_{Zf_R}=\epsilon_{Zf'_R}$. The benchmark point in the plots (b) and (c) is $(\kappa_{ZZ},\epsilon_{Zf_R},\epsilon_{Zf'_R})=(0.88,-0.05,-0.05).$