Probing top-Higgs non-standard interactions at the LHC

TL;DR

This paper studies dimension-6 top–Higgs operators within an EFT framework and their impact on Higgs production at the LHC, focusing on both gluon-fusion and $t\bar t h$ channels. It shows that $\mathcal{O}_{HG}$ drives tree-level $gg\to h$ and is tightly constrained by current Higgs-rate data, while loop-induced operators $\mathcal{O}_{hg}$, $\mathcal{O}_{Hy}$, and $\mathcal{O}_{H}$ modify the top–Higgs sector and mix into $c_{HG}$ through renormalization, with constraints partly encoded in the combination $c_y$. The study demonstrates that $t\bar t h$ production provides complementary information and can help disentangle the contributions of these operators, though sensitivity to higher-order $1/\Lambda^4$ terms becomes relevant at large couplings. Overall, the work emphasizes the power of combining Higgs-rate measurements with $t\bar t h$ observables to probe top–Higgs new physics and to distinguish among different operators linked to electroweak symmetry breaking.

Abstract

Effective interactions involving both the top quark and the Higgs field are among the least constrained of all possible (gauge invariant) dimension-six operators in the Standard Model. Such a handful of operators, in particular the top quark chromomagnetic dipole moment, might encapsulate signs of the new physics responsible for electroweak symmetry breaking. In this work, we compute the contributions of these operators to inclusive Higgs and t tbar h production. We argue that: i) rather strong constraints on the overall size of these operators can already be obtained from the current limits/evidence on Higgs production at the LHC; ii) t tbar h production will provide further key information that is complementary to t tbar measurements, and the possibility of discriminating among different contributions by performing accurate measurements of total and differential rates.

Figures (9)

• Figure 1: $gg\to h$ production. The first two diagrams are the contributions to $O_{HG}$ from $O_{hg}$. The third one is induced by $O_{Hy}$ and $O_{H}$. The operators of Eq. (\ref{['htop']}) do not contribute to $\mathcal{O}_{HG}$ (see Sec. \ref{['ggh']}).
• Figure 2: Region allowed at 95% C.L. by the ATLAS upper bound on the Higgs production cross-section ATLAS:2012ae for $\mu_R=\mu_F={m_H}/{2}$ (solid line). The errors are estimated by varying the renormalization and factorization scales from $\mu_R=\mu_F={m_H}/{4}$ (dotted line) to $\mu_R=\mu_F=m_H$ (dashed line). The blue region uses the combination of all channels. The yellow region is obtained using the strongest constraint among the $WW$ and $ZZ$ channels. The red lines show the relative deviation compared to the SM Higgs production rate.
• Figure 3: The dashed blue and solid red lines are the limits from $h\to (WW,ZZ)$ and $h\to \gamma \gamma$ respectively. The $WW/ZZ$ constraints on $c_{HG}$ are stronger only when the branching ratio to $\gamma \gamma$ goes below $10^{-3}$ (SM value), corresponding to $0 \lesssim c_{H\gamma}\lesssim 0.1$. For larger branching ratio, the $\gamma \gamma$ constraints are stronger and do not allow for large values of $c_{H\gamma}$. Note that the allowed region is symmetric along the dotted black lines where $\sigma (gg \to h)=0$ and $\Gamma (h \to \gamma \gamma)=0$. We have checked that a more refined analysis combining all the channels along the lines of Ref. Espinosa:2012ir gives qualitatively similar results, although slightly more constraining of course.
• Figure 4: Diagram leading to the operator $\mathcal{O}_{HG}$ with the particles in the loop labeled by their transformations under $SU(3)\times SU(2)\times U(1)$, i.e., $(c,T,Y)$ if $\bar{c}\otimes c'\ni 8$. If the particles in the loop are bosons, additional diagrams can be obtained by replacing one or two internal lines and their two adjacent vertices by a single vertex.
• Figure 5: Diagrams leading to the operator $\mathcal{O}_{hg}$ if $\bar{c'}\otimes c"\ni 8$, $\bar{c}\otimes c'\ni 3$ and $\bar{c}\otimes c"\ni 3$. The internal fermion and boson lines can be exchanged and the internal bosons do not have to be scalar. Similarly as for Fig. \ref{['fig:OHG']}, additional diagrams can be obtained by removing one internal boson propagator.