Anomalous Higgs Couplings at the LHC

TL;DR

This paper evaluates how physics beyond the Standard Model could modify the Higgs sector using a model‑independent effective field theory with dimension‑6 operators constructed from the Higgs and SM gauge fields. It focuses on operators that can arise at tree level, notably $oldsymbol{\mathcal{O}_{\partial\phi}}$ and $oldsymbol{\mathcal{O}_{\phi}^{(1)}}$, which affect Higgs-fermion and Higgs-gauge couplings, and on $oldsymbol{\mathcal{O}_{\phi}^{(3)}}$ (strongly constrained by EWPT) and $oldsymbol{\mathcal{O}_{\phi}}$ (Higgs self‑couplings). A comprehensive tree‑level mediator decomposition is performed to translate EFT coefficients $\alpha_i$ into heavy states with specific quantum numbers, revealing correlations among observables and signaling which UV completions could be responsible for observed deviations. The analysis shows that early LHC signals of deviations are plausible, but discriminating among mediators relies on patterns across production and decay channels; if a deviation is attributed to a new scalar doublet, then $oldsymbol{\mathcal{O}_{\phi}}$ would be implicated and would likely require next‑generation colliders to probe the Higgs self‑couplings. Overall, the work provides a robust framework for interpreting potential Higgs sector deviations in terms of high‑scale physics.

Abstract

We discuss the impact and potential discovery of physics beyond the Standard Model, coupling to the Higgs sector, at the LHC. Using a model-independent effective Lagrangian approach, pure Higgs and Higgs-gauge operators are analyzed, and their origin in terms of tree-level exchange of unknown heavy messengers is systematically derived. It is demonstrated that early signals at the LHC may result from a simultaneous modification of Higgs-fermion and Higgs-gauge boson couplings induced by those operators, pointing towards singlet scalar or a triplet vector -- barring fine-tuned options. Of course, the Higgs discovery itself will also be affected by such new couplings. With increasing statistics, the remaining options can be discriminated from each other. On the other hand, the discovery of a new scalar doublet may require technology beyond the LHC, since the Higgs self-couplings have to be measured. Our conclusions are based on the complete set of tree-level decompositions of the effective operators unbiased by a specific model.

Figures (6)

• Figure 1: Total decay width of the Higgs boson as a function of $M_H$ for $\alpha_{\partial\phi}$ (left) and $\alpha_{\phi}^{(1)}$ (right), for the values given in the plot legends.
• Figure 2: $\mathcal{O}_{\phi}^{(1)}$ impact on Higgs branching ratios, as a function of $M_H$. The thick (middle) curves represent the SM reference, and the shaded regions mark the range $-0.4 \le \alpha_{\phi}^{(1)}v^2 \le 0.4$ (thin curves for the case $\alpha_{\phi}^{(1)}v^2=-0.4$ and medium thick curves for $\alpha_{\phi}^{(1)}v^2=0.4$).
• Figure 3: Combined LEP and Tevatron experimental 95%C.L. exclusion regions in the ($M_H$,$\alpha_{\partial\phi}v^2$) (left) and ($M_H$,$\alpha_{\phi}^{(1)}v^2$) planes (right), obtained with the program HiggsBounds Bechtle:2008jhBechtle:2011sb. The purple region (right) indicates our prediction for the exclusion region; see main text for explanations.
• Figure 4: Production cross sections of the Higgs boson at the LHC as a function of the Higgs mass $M_H$ in the presence of $\mathcal{O}_{\phi}^{(1)}$, where $\alpha_{\phi}^{(1)}v^2=-0.4$ (left panel) and $\alpha_{\phi}^{(1)}v^2=0.4$ (right panel). The dashed curves represent the corresponding SM predictions (only visible if there are deviations from the SM).
• Figure 5: The significances of the different Higgs searches channels at CMS as a function of the Higgs boson mass in the cases: SM (top), $\alpha_{\partial\phi}v^2=-0.4$ (middle left), $\alpha_{\partial\phi}v^2=0.4$ (middle right), $\alpha_{\phi}^{(1)}v^2=-0.4$ (lower left) and $\alpha_{\phi}^{(1)}v^2=0.4$ (lower right)