Non-resonant New Physics in Top Pair Production at Hadron Colliders

TL;DR

The paper develops a model-independent, low-energy EFT framework to study non-resonant, top-philic new physics in top quark pair production. It shows that tt̄ observables at hadron colliders can be described by one two-fermion chromomagnetic operator and seven four-fermion operators, with observables sensitive to specific linear combinations (c_hg, c_Vv, c_Vv′, c_Aa, etc.). By combining Tevatron and LHC data, it delineates complementary constraints on these parameters, finds that the LHC cross section mostly constrains the chromomagnetic operator, and demonstrates how invariant-mass, forward-backward, and spin-correlation observables help disentangle the operator structure. The work also discusses implications for Higgs/top compositeness models and highlights the potential of four-top and tt̄bb̄ channels as probes of the strong dynamics hypothesized in such theories. Overall, it provides a cohesive strategy to identify or constrain top-philic new physics beyond the Standard Model using current and future collider measurements.

Abstract

We use top quark pair production as a probe of top-philic non-resonant new physics. Following a low energy effective field theory approach, we calculate several key observables in top quark pair production at hadron colliders (e.g., total cross section, ttbar invariant mass distribution, forward-backward asymmetry, spin correlations) including the interference of the Standard Model with dimension-six operators. We determine the LHC reach in probing new physics after having taken into account the Tevatron constraints. In particular, we show that the gluon fusion process gg -> ttbar which remains largely unconstrained at the Tevatron is affected by only one top-philic dimension-six operator, the chromo-magnetic moment of the top quark. This operator can be further constrained by the LHC data as soon as a precision of about 20% is reached for the total ttbar cross-section. While our approach is general and model-independent, it is particularly relevant to models of Higgs and top compositeness, which we consider in detail, also in connection with ttbar ttbar and ttbar bbbar production.

Figures (15)

• Figure 1: A Feynman representation of the relevant operators for $t\bar{t}$ production at hadron colliders.
• Figure 2: Typical one loop contributions of (a) the dimension-six operators (\ref{['odr']})--(\ref{['odb']}) leading to $\delta c_{Rv}$ and $\delta c_{Lv}$ respectively once the equation of motion \ref{['motion']} is used, and (b) the dimension-eight operator $\left(H\bar{Q}t\right)\left(H\bar{Q}t\right)$ leading to $\delta c_{hg}$ if one chirality-flip is considered in the loop.
• Figure 3: One particle exchange contributions to $\mathcal{L}_{t\bar{t}}$ in Eq. \ref{['eq:ttlag']}: (a) the five four-fermion operators can be directly associated with the exchange of a spin-1 resonance once Fierz transformations are used, (b) the single two-fermion operator $\mathcal{O}_{hg}$ can be indirectly associated with the exchange of a spin-0 or spin-2 resonance coupled to two gluons via a fermion loop.
• Figure 4: Region allowed by the Tevatron constraints (at 2$\sigma$) for $c'_{Vv}=0$. The green region is allowed by the total cross section measurement. The blue region is consistent with the $t \bar{t}$ invariant mass shape. The red lines show the limits that can be set by the LHC at 7 TeV (thin line) and at 14 TeV (thick line) as soon as a precision on the top pair cross section of 10% and 20% respectively is reached. The "$0 \%$"line delimits the region where the new physics contributions are smaller than the theoretical error on the SM cross section. The dashed ($\mu_F=\mu_R=\frac{m_t}{2}$), dotted ($\mu_F=\mu_R=2 m_t$) and solid lines ($\mu_F=\mu_R=m_t=174.3$ GeV) show the estimated theoretical uncertainties.
• Figure 5: Summary plot (taking $\mu_F=\mu_R=m_t$ and defining the exclusion region at $2\sigma$). The yellow region is excluded by the Tevatron. The green (blue) region is excluded by LHC at 7 TeV (14 TeV) after a precision of 10% is reached on the $t\bar{t}$ cross section. In the first plot, it is assumed that the measured cross section is the SM value. If the measured value deviates from the SM one, the center of the unconstrained white region will be translated along the thin black line as indicated. For instance, the second and third plots show the displacement of the white region if the measured $\sigma_{t\bar{t}}$ is respectively $+10\%$ and $-20\%$ of the SM value.