Higgs characterisation at NLO in QCD: CP properties of the top-quark Yukawa interaction

TL;DR

The paper develops NLO QCD predictions for Higgs interactions with the top quark in a CP-violating or CP-mixed framework, using the Higgs Characterisation EFT to describe CP mixing in gluon fusion with jets and in ttH production. It demonstrates that NLO+PS accuracy significantly reduces theoretical uncertainties and identifies CP-sensitive observables, notably di-jet azimuthal correlations $\Delta\phi_{jj}$ and decay-product correlations in ttX0, as robust probes of the CP nature. The work provides a practical, model-independent toolkit for CP characterization of the Higgs–top coupling and supplies a public EFT model (HC_NLO_X0) for automated NLO+PS analyses. These results enhance the ability to extract CP information from LHC data in the main Higgs production channels.

Abstract

At the LHC the CP properties of the top-quark Yukawa interaction can be probed through Higgs production in gluon fusion or in association with top quarks. We consider the possibility for both CP-even and CP-odd couplings to the top quark to be present, and study CP-sensitive observables at next-to-leading order (NLO) in QCD, including parton-shower effects. We show that the inclusion of NLO corrections sizeably reduces the theoretical uncertainties, and confirm that di-jet correlations in $H+2$ jet production through gluon fusion and correlations of the top-quark decay products in $t\bar tH$ production can provide sensitive probes of the CP nature of the Higgs interactions.

Figures (9)

• Figure 1: Distribution of the invariant mass of the two leading jets in $pp\to X_0jj$ through GF (solid lines) and VBF (dashed) at the 13-TeV LHC. The different hypotheses are defined in tables \ref{['tab:GFscenarios']} and \ref{['tab:VBFscenarios']}.
• Figure 2: Normalized distributions (shape comparison) in $p_T$ and $\eta$ of the resonance $X_0$, with the acceptance cuts for jets (left), plus $m(j_1,j_2)>250$ GeV (centre) and $500$ GeV (right). The three spin-0 hypotheses are defined in table \ref{['tab:GFscenarios']}.
• Figure 3: Same as fig. \ref{['fig:gf_x']}, but for the leading jet.
• Figure 4: Same as fig. \ref{['fig:gf_x']}, but for $\Delta\eta$ and $\Delta\phi$ distributions between the two tagging jets. For $\Delta\phi$, the distribution with the additional $\eta$ jet ordering is also shown by a dashed line for the $0^{\pm}$ case.
• Figure 5: Normalized distributions (shape comparison) of the $p_T$ of the di-jet system (left), the rapidity (centre) and azimuthal (right) difference between the two tagging jets for $pp\to X_0jj$ in GF (solid lines) and VBF (dashed) at the 13-TeV LHC, where the acceptance cuts plus the $m_{jj}>500$ GeV cut are applied. For each GF scenario, the middle panels show the scale and PDF$+\alpha_s$ uncertainties, while the bottom ones give the ratio of NLO+PS to LO+PS results with the total uncertainties.