Future Collider Perspectives on Higgs CP Violation

TL;DR

This work assesses the sensitivity of future colliders to CP violation in the gauge-Higgs sector using a dimension-six EFT framework. By combining MC simulations across HL-LHC, FCC-ee, FCC-hh, and a linear collider with CP-odd observables and ML-enhanced discriminants, the authors extract 95% CL limits on Wilson coefficients governing CP-violating Higgs interactions via a binned likelihood approach. They find consistent, order-of-magnitude improvements over HL-LHC across all channels, with FCC-hh delivering the strongest overall reach and electron-positron colliders offering clean, complementary measurements; ML-based observables significantly boost sensitivity. The results inform the future collider roadmap, indicating that precision CP tests of the Higgs sector could play a central role in addressing the matter-antimatter asymmetry of the universe.

Abstract

The search for new sources of CP violation is a cornerstone of the beyond the Standard Model phenomenology programme at the LHC and beyond. We provide a comprehensive analysis of such searches at a range of future facilities with the aim of informing the currently unfolding future collider roadmap. Focussing on new sources of CP violation specifically in the gauge-Higgs sector, we demonstrate the outstanding potential held by future electron-positron and proton-proton colliders to reveal and identify BSM physics with direct relevance for the observed matter-antimatter asymmetry. In particular, the future colliders will provide an order of magnitude improvement in sensitivity to anomalous CP-violating interactions induced by dimension-six effective field theory operators when compared to the high-luminosity LHC programme.

Figures (6)

• Figure 1: Expected event yields for $e^+e^-\rightarrow ZH$ production at FCC-ee as a function of $\Delta\phi_{\ell\ell}$ in (a) the electron decay channel of the $Z$ boson and (b) the muon decay channel of the $Z$ boson. (c) Expected event yield as a function of the ML-based observable ($O_{\rm NN}^{\rm multi}$) in the electron channel. (d) Expected event yields as a function of $m_{\ell\ell}$ and $\Delta\phi_{\ell\ell}$, for the interference contributions induced by the $\widetilde{\mathcal{O}}_{\Phi\widetilde{W}B}$ operators in the electron channel.
• Figure 2: Expected event yields for $e^+e^-\rightarrow ZH$ production in the $H\rightarrow b\bar{b}$ decay channel at FCC-ee. Distributions are shown as a function of $\Delta\phi_{\ell\ell}$ in (a) the electron decay channel of the $Z$ boson (a) and (b) the muon decay channel of the $Z$ boson.
• Figure 3: Expected event yields for $e^+e^-\rightarrow ZH$ production at LCF with inclusive Higgs boson decays and in the electron decay channel of the $Z$ boson. Distributions are shown as a function of $\Delta\phi_{\ell\ell}$ for different initial-state beam polarisations, namely (a) $\{ e^-_\textrm{pol}, e^+_\textrm{pol}\} = \{+80\%,-30\%\}$ (b) $\{ e^-_\textrm{pol}, e^+_\textrm{pol}\} = \{-80\%,+30\%\}$, (c) $\{ e^-_\textrm{pol}, e^+_\textrm{pol}\} = \{+80\%,+30\%\}$, and (d) $\{ e^-_\textrm{pol}, e^+_\textrm{pol}\} = \{-80\%,-30\%\}$.
• Figure 4: (a) Expected event yields for $ZH$ production at FCC-hh in the $H\rightarrow b\bar{b}$ decay channel as a function of $\Delta\phi_{\ell\ell}$. (b) Expected event yield as a function of the $O_{\rm NN}^{\rm binary}$ observable using a network trained on the interference contribution induced by the $\widetilde{\mathcal{O}}_{\Phi\widetilde{W}}$ operator.
• Figure 5: (a) The expected event yield for $H \rightarrow e^+e^- \mu^+\mu^-$ at FCC-hh as a function of $\Phi_{4\ell}$, for both SM events and the interference contributions predicted for the $\widetilde{\mathcal{O}}_{\Phi\widetilde{B}}$ and $\widetilde{\mathcal{O}}_{\Phi\widetilde{W}B}$ operators. (b) The expected yield of the interference contribution predicted by the $\widetilde{\mathcal{O}}_{\Phi\widetilde{W}B}$ operator as a function of $\Phi_{4\ell}$ and $m_{12}$, where $m_{12}$ is the invariant mass of the SFOS pair that is closest to $m_{Z}$.