Probing Electroweak Top Quark Couplings at Hadron Colliders

TL;DR

The paper analyzes direct probes of top-quark electroweak couplings to photons and Z bosons through ttγ and ttZ production at the Tevatron and LHC. It develops a form-factor framework for ttV vertices, imposes unitarity constraints, and evaluates experimental bounds by simulating signal and backgrounds (including full spin correlations) for multiple final states. At the LHC, ttγ can be measured with roughly 4–7% precision on vector/axial couplings and ~20% on dipole form factors with 300 fb^-1, while ttZ couplings achieve ~15–20% for the axial vector and ~50–55% for dipoles; SLHC upgrades yield further improvements. The Tevatron offers a first, albeit imprecise, test of ttγ couplings, whereas LEP and b→sγ provide indirect constraints on ttZ and dipole moments; overall, hadron colliders provide a valuable, complementary route to constrain top EW interactions alongside future e+e- colliders.

Abstract

We consider QCD t\bar{t}γand t\bar{t}Z production at hadron colliders as a tool to measure the ttγand ttZ couplings. At the Tevatron it may be possible to perform a first, albeit not very precise, test of the ttγvector and axial vector couplings in t\bar{t}γproduction, provided that more than 5 fb^{-1} of integrated luminosity are accumulated. The t\bar{t}Z cross section at the Tevatron is too small to be observable. At the CERN Large Hadron Collider (LHC) it will be possible to probe the ttγcouplings at the few percent level, which approaches the precision which one hopes to achieve with a next-generation e^+e^- linear collider. The LHC's capability of associated QCD t\bar{t}V (V=γ, Z) production has the added advantage that the ttγand ttZ couplings are not entangled. For an integrated luminosity of 300 fb^{-1}, the ttZ vector (axial vector) coupling can be determined with an uncertainty of 45-85% (15-20%), whereas the dimension-five dipole form factors can be measured with a precision of 50-55%. The achievable limits improve typically by a factor of 2-3 for the luminosity-upgraded (3 ab^{-1}) LHC.

Figures (9)

• Figure 1: The differential cross sections as a function of the photon transverse momentum for $\gamma\ell\nu_\ell b\bar{b}jj$ production at (a) Tevatron Run II and (b) LHC. Shown are the SM predictions for $t\bar{t}\gamma$ production (including radiative top decays in $t\bar{t}$ events, solid line), the $t\bar{t}j$ background where one jet is misidentified as a photon (dotted line), the background from single-top production processes (dashed line), and the $W\gamma b\bar{b}jj$ background (histogram). The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}--\ref{['eq:cuts3']}). The photon misidentification probabilities used are described in the text. No particle ID efficiencies are included here.
• Figure 2: The differential cross sections as a function of the photon transverse momentum for $\gamma\ell\nu_\ell b\bar{b}jj$ production at (a) Tevatron Run II and (b) LHC. Shown are the SM predictions for $t\bar{t}\gamma$ production (including radiative top decays in $t\bar{t}$ events, solid line), the combined $t\bar{t}j$, $W\gamma b\bar{b}jj$ and $(t\bar{b}\gamma+\bar{t}b\gamma)+X$ background (long-dashed-dotted line), and the predictions for several non-standard $tt\gamma$ couplings. Only one coupling at a time is allowed to deviate from its SM value. The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}--\ref{['eq:cuts3']}). No particle ID efficiencies are included here.
• Figure 3: The differential cross sections at the LHC as a function of $p_T(Z)$ for ${\ell'}^+{\ell'}^-\ell\nu b\bar{b}jj$ final states. Shown are the SM predictions for $t\bar{t}Z$ production (solid), the single-top background (dashed), the $WZb\bar{b}jj$ background (histogram), and the predictions for several non-standard $ttZ$ couplings. Only one coupling at a time is allowed to deviate from its SM value. The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}), (\ref{['eq:cuts5']}) and (\ref{['eq:cuts4']}).
• Figure 4: The normalized differential signal cross sections at LHC as a function of the $Z\to{\ell'}^+{\ell'}^-$ azimuthal opening angle, $\Delta\Phi({\ell'}{\ell'})$. Shown are the SM distribution (solid line) and the predictions for several non-standard $ttZ$ couplings. Only one coupling at a time is allowed to deviate from its SM prediction. The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}), (\ref{['eq:cuts5']}) and (\ref{['eq:cuts4']}).
• Figure 5: The differential cross sections at the LHC as a function of $p_T(Z)$ for ${\ell'}^+{\ell'}^-b\bar{b}+4j$ final states. The SM is the solid curve. Backgrounds are $Zb\bar{b}+4j$ (dashed), single-top production (dotted), and $WZb\bar{b}jj$ (histogram). The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}) and (\ref{['eq:cuts4']}-\ref{['eq:cuts7']}).