$CP$ tests of Higgs couplings in $t\overline{t}h$ semileptonic events at the LHC

The $CP$ nature of the Higgs coupling to top quarks is addressed in this paper, in single charged lepton final states of $t\overline{t}h$ events produced in proton-proton collisions at the LHC. Pure scalar ($h=H$) and pseudoscalar ($h=A$) Higgs boson signal events, generated with MadGraph5_aMC@NLO, are fully reconstructed using a kinematic fit. Angular distributions of the decay products, as well as $CP$-sensitive asymmetries, are exploited to separate and gain sensitivity to possible pseudoscalar components of the Higgs boson and reduce the contribution from the dominant irreducible background $t\overline{t}b\overline{b}$. Significant differences are found between the pure $CP$-even and -odd signal hypotheses as well as with respect to the Standard Model background, in particular the $t\overline{t}b\overline{b}$ contribution. Such differences survive the event reconstruction, allowing one to define optimal observables to extract the Higgs couplings parameters from a global fit. A dedicated analysis is applied to efficiently identify signal events and reject as much as possible the expected Standard Model background. The results obtained are compared with a similar analysis in the dilepton channel. We show that the single lepton channel is more promising overall and can be used in combination to study the $CP$ nature of the Higgs coupling to top quarks.

Figure 1

Reconstructed transverse momenta of the Higgs boson (top left) and top quark (bottom left), as a function of the values obtained at the parton level, for semileptonic decays of $t\overline{t}h$ signal events. The corresponding distributions for dileptonic $t\overline{t}h$ signal events are shown on the right. The (rec w/o truth match) label refers to final objects from the KLFitter full kinematic reconstruction, without trying any matching with leptons, quarks, or bosons from the parton level. The color bar (at the right end of each plot) represents the total number of events in each bin with the red code on top corresponding to more events than the blue code on bottom.

Figure 2

The angle between the momentum direction of the top quark, in the $t\overline{t}h$ system, and the $t\overline{t}h$ direction, in the lab frame (${\theta }_t^{t\overline{t}h}$), as a function of the angle between the Higgs momentum, in the $\overline{t}h$ frame, and the momentum of the $b$ quark from the Higgs boson, in the Higgs boson frame (${\theta }_{b_h}^h$). The top (bottom) distributions are for the scalar $h=H$ (pseudoscalar, $h=A$) $t\overline{t}h$ signal. The plots on the left (right) show the semileptonic (dileptonic) channel of $t\overline{t}h$ decays.

Figure 3

The same as Fig. 2, after event selection and full kinematic reconstruction.

Figure 4

Angular distributions: (top left) $\cos ({\theta }_h^{\overline{t}h})\cos ({\theta }_{{\ell }^{-}}^h)$ and (top right) $\sin ({\theta }_h^{t\overline{t}h})\sin ({\theta }_{{\overline{b}}_{\overline{t}}}^{\overline{t}})$; (middle left) $\sin ({\theta }_h^{t\overline{t}h})\cos ({\theta }_{b_h}^{\overline{t}})$ and (middle right) $\sin ({\theta }_{\overline{t}}^{t\overline{t}h})\sin ({\theta }_{b_h}^h)$; (bottom left) $\sin ({\theta }_h^{t\overline{t}h})\sin ({\theta }_{\overline{t}}^{t\overline{t}})$ and (bottom right) $b_4$ [15]. These are shown after event selection and full kinematic reconstruction. The light blue line represents the $t\overline{t}h$ SM model signal ($h=H$ and $CP=+1$), and the dark blue line corresponds to the pure pseudoscalar distribution $t\overline{t}h$ ($h=A$ and $CP=-1$). The filled region corresponds to the $t\overline{t}b\overline{b}$ dominant background.

Figure 5

Distributions of (top) $x_Y=\sin ({\theta }_h^{t\overline{t}h})\cos ({\theta }_{b_h}^{\overline{t}})$ (left) and $x_Y=\sin ({\theta }_h^{t\overline{t}h})\sin ({\theta }_{{\overline{b}}_{\overline{t}}}^{\overline{t}})$ (right) after final event selection and kinematic reconstruction at 13 TeV for $100{\mathrm{fb}}^{-1}$ with the contributions from the full SM background and fake data; (middle) expected limits at 95% C.L. in the background-only hypothesis, for $|\cos (\alpha )|=0$, 1. Limits on $\sigma \times \mathrm{BR}(h\to b\overline{b})$ (left) and $\mu$ (right) obtained with the $\sin ({\theta }_h^{t\overline{t}h})\sin ({\theta }_{{\overline{b}}_{\overline{t}}}^{\overline{t}})$ distribution for integrated luminosities of 100, 300, and $3000{\mathrm{fb}}^{-1}$ are shown. The lines correspond to the median, while the narrower (wider) bands correspond to the $1\sigma (2\sigma )$ intervals. (Bottom) The same limits as presented for the middle plots, but here for the $b_4$ distribution.