An EFT study of the $pp \to \bar{t} t Z(ll) h(bb)$ process at the FCC-$\boldsymbol{hh}$

Abstract

We carry out an Effective Field Theory (EFT) study of the $pp \to \bar{t} t Zh$ process in the $4b + 3 \ell + \ge 2j + \slashed{E}_T$ final state. This process can uniquely probe the $\bar{t} t Zh$ couplings arising from higher dimensional EFT operators and can also provide bounds on $\bar{t} t Z$ coupling deviations. We highlight the importance of the proposed proton-proton Future Circular Collider (FCC-$hh$) to study this process and then perform a complete collider analysis by examining the relevant background processes. This allows us to determine the FCC-$hh$ sensitivity to probe anomalous $\bar{t} t Zh$ couplings.

Figures (5)

• Figure 1: Representative diagrams for $gg/\bar{q}q \to \bar{t} t Zh$. First: a SM-like topology with $Z$ and $h$ radiated from a top line (the emissions can occur from either of the top or anti-top lines). Second: legend illustrating the symbolic red blobs used in the following panels, corresponding to either a $ZZh$ sub-diagram via a $Z$ mediator, or part of the four-point contact interaction $\bar{t}tZh$. Third: $t\bar{t} Zh$ production with hard via a $t$-channel top exchange. Fourth: $t\bar{t}$ production via $s$-channel $gg$ fusion. Fifth: $t\bar{t}$ production via $s$-channel with $\bar{q} q$ initial states. The mediator is either a $Z$ boson or a photon. One of the coloured blobs in the last three diagrams must be replaced by the the structures shown in the second panel. The diagrams shown are representative and illustrate only a subset of the full set of contributions.
• Figure 2: Figure shows, as stacked histograms, the reconstructed masses of the $Z$-boson, the Higgs boson, the hadronic top, and the leptonic top, respectively. The SM $\bar{t}tZh$ sample is denoted in orange and the remaining top + diboson backgrounds are represented in teal. All rates are normalised to the total integrated luminosity of 30 ab$^{-1}$.
• Figure 3: Stacked histograms of the transverse momentum ($p_T$) distributions for the reconstructed $Z$ boson, the Higgs boson, the hadronic top, and the leptonic top, respectively. The plots are shown on a logarithmic scale. The colour coding for the SM background samples follows the same convention as before. Overlaid on these are the signal distributions (not stacked): the cases with couplings $g_{hZt_R} = +0.03$ ($-0.03$) are shown in black (blue), while those with $g_{hZt_L} = +0.03$ ($-0.03$) are shown in red (green).
• Figure 4: Stacked histograms of $m_{Zh}$ and $\slashed{E}_T$ for the SM samples, with the signal samples overlaid (not stacked). The distributions are shown on a logarithmic scale. The labels and colour coding follow the same convention as before.
• Figure 5: The left panel shows the contours obtained when both the squared and interference contributions are included in the $\chi^{2}$ evaluation, while the right panel displays the result when only the interference terms are retained. The red circle denotes the 95% C.L. allowed region in the $(g_{hZt_R},, g_{hZt_L})$ plane for an integrated luminosity of $30~\text{ab}^{-1}$. A 5% systematic uncertainty on the background estimation is included, in addition to the statistical uncertainties beyond 1 TeV. The contour is evaluated using 11 bins: 800 GeV $< m_{hZ} <$ 1.5 TeV as shown in Fig. \ref{['fig::reco_misc']} (left). The colour axis indicates the $\chi^{2}$ values. In the SMEFT interpretation, these bounds can be translated into constraints on the $Z$-coupling deviations by a simple rescaling of the axes, $\delta g^Z_{t_{L,R}} = g_{hZt_{L,R}}/2$ (see Eq. \ref{['expression2']}).