CP violation in top quark production at the LHC and Two-Higgs-Doublet Models

TL;DR

The paper analyzes CP violation in top-quark pair production at the LHC arising from gluon fusion with neutral-Higgs exchange, building on and refining the Bernreuther–Brandenburg framework. It provides a model-independent description of CP-odd observables in $gg\to t\bar t$, and examines dileptonic observables such as $A_1=E_+-E_-$ and $A_2={\bf p}_{\bar t}\cdot {\bf l}^+-{\bf p}_{t}\cdot {\bf l}^-$, including proposed modifications to improve sensitivity. In a specific minimal CP-violating 2HDM (Model II), CP-violating effects at the LHC are generally at the per-mille level, enhanced for small $\tan\beta$ and with a single light Higgs, but strongly suppressed by cancellations when two light Higgs states lie below the $t\bar t$ threshold. The authors study three mass-spectra scenarios (two light/one heavy, one light/two heavy, and the $t\bar t$ transition region) and show how interference and mixing govern the size of CP violation, often requiring large event samples and precise reconstruction of $M_{t\bar t}$ to observe effects. They conclude that while the general framework is sound, observable CP violation in this channel remains challenging, and future work should consider alternative observables, binning strategies, and higher-order QCD corrections.

Abstract

We discuss CP violation in top-antitop production at the LHC, induced by gluon fusion and final-state Higgs exchange. Results by Bernreuther and Brandenburg are confirmed and further reduced. The lepton energy asymmetry is studied in detail in explicit Two-Higgs-Doublet Models with near-maximal mixing in the neutral Higgs sector. Unless there is only one light Higgs particle, and unless (in Model II) tanbeta \lsim 1, the CP-violating effects are very small, possibly too small to be seen at the LHC.

Figures (15)

• Figure 1: Kinematics of the underlying $g(k_1)+g(k_2)\to t(p_1)+\bar{t}(p_2)$ reaction.
• Figure 2: Lowest-order QCD Feynman diagrams of the underlying $gg\to t\bar{t}$ reaction, the crossed diagram of ($a$) is not shown.
• Figure 3: Feynman diagrams of the underlying $gg\to t\bar{t}$ reaction with neutral non-Standard-Model Higgs exchanges (dashed). The crossed diagrams are not shown (diagram $(g)$ has no crossed partner).
• Figure 4: Parton-level spin-spin correlations (\ref{['Eq:z*bg1+bg2']}) and (\ref{['Eq:z*cg1+cg2']}) in $gg\to t\bar{t}$vs.$\sqrt{\hat{s}}$, for $\gamma_{CP}=1$ and different Higgs masses. Dashed: $m_H=100$, 150, …, 300 GeV; solid: 350, …, 500 GeV.
• Figure 5: Left panel: Lepton energy correlations $\langle A_1\rangle$ in $pp\to t\bar{t}X$vs. Higgs mass for $\gamma_{CP}=1$. Dashed curves labeled "a", "b" and "c" refer to modified observables of eq. (\ref{['Eq:A1-modified']}). Grey (yellow) band: $\langle A_1^{(b)}\rangle$, with 10% uncertainty in $M_{t\bar{t}}$. Right panel: Corresponding signal-to-noise ratio.