In this paper we explore CP discrimination in the associated production of top-quark pairs (tt) with a generic scalar boson (φ) at the LHC. We probe the CPsensitivity of several observables for a varying scalar boson mass and CP-number, either CP-even (φ = H) or CP-odd (φ = A), using dileptonic final states of the ttφ system, with φ → bb. We show that CP-searches are virtually impossible for φ boson masses above a few hundred GeV in this channel. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5 aMC@NLO and reconstructed using a kinematic fit. The most sensitive CP-observables are used to compute Confidence Levels (CLs), as a function of luminosity, for the exclusion of different signal hypotheses with scalar and pseudoscalar boson masses that range from m φ = 40 GeV up to 200 GeV. We finalize by analysing the impact of a measurement (or limit) of the CP-violating angle in the parameter space of a complex two-Higgs doublet model known as the C2HDM.

In this paper we propose a new reconstruction method to explore the low mass region in the associated production of top-quark pairs ($$ t\overline{t} $$ t t ¯ ) with a generic scalar boson (ϕ) at the LHC. The new method of mass reconstruction shows an improved resolution of at least a factor of two in the low mass region when compared to previous methods, without the loss of sensitivity of previous analyses. It turns out that it also leads to an improvement of the mass reconstruction of the 125 GeV Higgs for the same production process. We use an effective Lagrangian to describe a scalar with a generic Yukawa coupling to the top quarks. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5_aMC@NLO and reconstructed using a kinematic fit. The use of CP-sensitive variables allows then to maximize the distinction between CP-even and CP-odd components of the Yukawa couplings. Confidence Levels (CLs) for the exclusion of ϕ bosons with mixed CP (both CP-even and CP-odd components) were determined as a function of the top Yukawa couplings to the ϕ boson. The mass range analysed starts slightly above the ϒΥ mass up to 40 GeV, although the analysis can be used for an arbitrary mass. If no new light scalar is found, exclusion limits at 95% CL for the absolute value of the CP-even and CP-odd Yukawa are derived. Finally, we analyse how these limits constrain the parameter space of the complex two-Higgs doublet model (C2HDM).

In this paper, we use the associated production of top-quark pairs ($$ t\overline{t} $$ t t ¯ ) with a generic scalar boson (ϕ) at the LHC (pp → $$ t\overline{t}\phi $$ t t ¯ ϕ ) to explore the sensitivity of a large set of observables to the sign of the CP mixing angle (α), present in the coupling between the scalar boson and the top quarks. The mass of the scalar boson is set to mϕ = 125 GeV (the Standard Model Higgs boson mass) and its coupling to top-quarks is varied such that α = 0°, 22.5°, 45.0°, 67.5°, 90.0°, 135.0° and 180.0°. Dileptonic final states of the $$ t\overline{t}\phi $$ t t ¯ ϕ system are used (pp → bℓ+νℓ$$ \overline{b} $$ b ¯ ℓ−$$ \overline{\nu} $$ ν ¯ ℓb$$ \overline{b} $$ b ¯ ), where the scalar boson is expected to decay according to ϕ → $$ b\overline{b} $$ b b ¯ . A new method to reconstruct the scalar mass, originally designed for the low mass regime is used, improving the resolution of the Higgs mass by roughly a factor of two. A full phenomenological analysis is performed using Standard Model (SM) background and signal events generated with MadGraph5_aMC@NLO, in turn reconstructed using a kinematical fit. The most sensitive CP-observables are selected to compute Confidence Level (CL) limits as a function of the sign of the top quark Yukawa couplings to the ϕ boson. We also explore the sensitivity to interference terms using differential distributions and angular asymmetries. Given the significant difference between the pure scalar (σ0+) and pure pseudo-scalar (σ0−) production cross section values, it is unlikely the $$ t\overline{t}\phi $$ t t ¯ ϕ channel alone will be sensitive to the sign of the CP-mixing angle or interference terms, even at the end of the LHC. Using the $$ {b}_2^{t\overline{t}\phi } $$ b 2 t t ¯ ϕ and $$ {b}_4^{t\overline{t}\phi } $$ b 4 t t ¯ ϕ variables, exclusion limits at 95% CL for the CP-even and CP-odd components of the top quark Yukawa couplings are expected to be set to $$ \overset{\sim }{\kappa } $$ κ ~ ∈ [-0.698,+0.698] and |κ| ∈ [0.878,1.04], respectively, at the end of the High Luminosity phase of the LHC (HL-LHC) by using the dileptonic decay channel alone.

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