Complementarity of di-top and four-top searches in interpreting possible signals of new physics

TL;DR

The paper addresses the challenge of interpreting potential signals from top-philic BSM scalars at the LHC, where interference between SM di-top production and mixed scalar resonances can distort signatures. It develops a loop-level mixing framework using the Z-factor propagator formalism and compares it to an effective mixing angle approach within a two-scalar (c2HDM-like) model, applying it to reinterpret CMS/ATLAS di-top and four-top searches with MC simulations and K-factors. The study shows that ignoring loop-level mixing can substantially overestimate sensitivity and misidentify resonance structures, while proper treatment yields smooth, robust predictions and reveals complementary information between di-top and four-top channels. Collectively, the results demonstrate that combining di-top and four-top analyses, guided by the Z-factor formalism, enhances the discovery potential and improves the ability to pinpoint the underlying extended Higgs sector in top-rich BSM scenarios.

Abstract

Final states comprising two or more top quarks are important search channels at the Large Hadron Collider for scalar particles predicted in models of physics beyond the Standard Model. While the di-top final state profits from a higher signal cross section, it can be subject to intricate interference patterns. Besides the interference with the large QCD background, in case of the presence of more than one high-mass scalar also large signal--signal interference contributions can occur. We show that in such scenarios it is crucial to account for loop-level mixing for obtaining accurate exclusion bounds. We demonstrate how the interference patterns can obscure the interpretation of possible deviations from the Standard Model expectations. We show that the four-top final state, while giving rise to a smaller signal cross section, provides important complementary information due to its much smaller signal--background interference contributions. Thus, the results obtained from the four-top final state can be instrumental for pinpointing the underlying new physics scenario.

Figures (15)

• Figure 1: Left: Total width of $h_{1,2}$ as a function of $m_{h_{1,\text{tree}}}$ comparing the results of the full pole determination to the effective mixing angle approach. Right: Integral over the squared sum of the Breit-Wigner propagators as a function of $m_{h_{1,\text{tree}}}$ using either the tree-level mixing angle, the effective mixing angle, or the $Z$-factor formalism for the couplings of the mass eigenstates.
• Figure 2: Difference between the simulated BSM contributions and the SM background prediction for the invariant mass distribution of the $t\bar{t}$ system, shown for different bins of the spin–correlation variable ${c_{hel}}$. Results are displayed for a $\mathcal{CP}$-odd scalar (red), a $\mathcal{CP}$-even scalar (blue), and two $\mathcal{CP}$-mixed scalars (green). The gray bands indicate the systematic uncertainties reported by CMS CMS:2019pzc. An integrated luminosity of $36~\mathrm{fb}^{-1}$ at $\sqrt{s} = 13~\mathrm{TeV}$ and an 8% smearing of $m_{t\bar{t}}$ are applied.
• Figure 3: The calculated recast significance is shown in blue, with $8\%$ smearing and the efficiency set to $14.5\%$, in comparison with the CMS expected significance which is shown in red.
• Figure 4: Distributions of jet multiplicity $N_j$ (left) and $b$-jet multiplicity $N_b$ (right) in the analysis region of the CMS four-top analysis implemented in MadAnalysis for a signal with a single scalar which has a mass of $M_{h_1} = 500GeV$ and a width of $\Gamma_{h_1} = 10GeV$. The distributions are scaled to the theory cross-section and a luminosity of $137/\text{fb}$.
• Figure 5: Parton-level invariant mass spectra (including the folding with the gluon parton distribution functions) for di-top production (left) and four-top production (right) for the benchmark point specified in \ref{['eq:benchmark_masscontributions']}. The purple line labelled "No interference" shows the incoherent sum of the two resonant signal contributions. The green line shows the contribution from signal–signal interference, the red line shows the contribution from signal–background interference, and the blue dashed line labelled "Signal" shows the full signal prediction including both interference terms.