Strong tW Scattering at the LHC

TL;DR

This work investigates how high-energy growth in certain top-quark scattering amplitudes can reveal non-SM top–Z and top–Higgs couplings. By focusing on tW→tW scattering and its LHC manifestion in pp→t t̄ Wj, the authors show that deviations in Ztt couplings induce amplitudes that grow like p^2/v^2, enabling strong sensitivity at high energies. Recasting the 8 TeV CMS t t̄W analysis yields the strongest direct bounds on Ztt couplings to date, while a dedicated 13 TeV Run-2 search with forward-jet signatures further constrains Δ_R and bar c_R. They also discuss complementary processes (e.g., tZ→th, bW→tZ, tt→hh) and interpret results within both EFT (dimension-6) and chiral Lagrangian frameworks, highlighting the LHC’s potential to test the top–Higgs sector and strongly coupled electroweak symmetry breaking scenarios. Together, the results underscore the importance of high-energy top-quark probes for precision tests of EWSB and for guiding future experimental searches.

Abstract

Deviations of the top electroweak couplings from their Standard Model values imply that certain amplitudes for the scattering of third generation fermions and longitudinally polarized vector bosons or Higgses diverge quadratically with momenta. This high-energy growth is a genuine signal of models where the top quark is strongly coupled to the sector responsible for electroweak symmetry breaking. We propose to profit from the high energies accessible at the LHC to enhance the sensitivity to non-standard top-$Z$ couplings, which are currently very weakly constrained. To demonstrate the effectiveness of the approach, we perform a detailed analysis of $tW \to tW$ scattering, which can be probed at the LHC via $pp\to t\bar{t}Wj$. By recasting a CMS analysis at 8 TeV, we derive the strongest direct bounds to date on the $Ztt$ couplings. We also design a dedicated search at 13 TeV that exploits the distinctive features of the $t\bar{t}Wj$ signal. Finally, we present other scattering processes in the same class that could provide further tests of the top-Higgs sector.

Figures (10)

• Figure 1: $t W \rightarrow t W$ scattering at the LHC. For definiteness, in the inset we show the diagrams corresponding to $tW^{-}\to tW^{-}$.
• Figure 2: Partonic cross section for the process $t W^{-}\to t W^{-}$ as a function of the center of mass energy $\sqrt{\hat{s}}$. The values of $\Delta_{L}$ and $\bar{c}_L^{(1)} = -\bar{c}_L^{(3)}$ are chosen to obtain the same $Zt_L t_L$ coupling for the blue and red solid curves ($\Delta_{L} < 0$) and for the blue and red dashed curves ($\Delta_{L} > 0$). For the $Zt_R t_R$ coupling there is a one-to-one correspondence between $\bar{c}_{R}$ and $\Delta_R$, so we show only one set of curves. A pseudorapidity cut $\left|\eta\right|<2$ has been applied to remove the forward singularity, whereas the soft singularity $\hat{s}\to (m_{W}+m_{t})^{2}$ is evident from the plot. Both singularities arise due to the diagram where a photon is exchanged in the $t$-channel. At large energy, the red, blue and green curves diverge like $\hat{s}$, whereas the SM cross section (dotted black) falls off as $1/\hat{s}$.
• Figure 3: In red, the constraints on top-$Z$ coupling deviations (left panel) and HDO coefficients (right panel) derived from the $t\bar{t}W$ analysis at $8$ TeV. For comparison, in blue we show the constraint obtained from the $8$ TeV $t\bar{t}Z$ analysis.
• Figure 4: Normalized distributions for the signal $(t\bar{t}Wj)_{\mathrm{EW}}$ and the two main backgrounds $(t\bar{t}W+\mathrm{jets})_{\mathrm{QCD}}$ and misID$\ell$ at $13$ TeV, after the $4j$ pre-selection.
• Figure 5: In red, the constraints on top-$Z$ coupling deviations (left panel) and HDO coefficients (right panel) derived from our $4j$$t\bar{t}W$ analysis at $13$ TeV. The solid contour assumes no systematic uncertainty on the background, whereas the dotted one includes a $50\%$ systematic on the misID$\ell$ component. For comparison, in dashed blue we show the constraint obtained from $t\bar{t}Z$, as derived in Ref. RS by means of a NLO-QCD signal-only analysis.