Improved Measurement of ttZ Couplings at the LHC

TL;DR

The paper analyzes measuring ttZ couplings at the LHC by focusing on ttZ production with Z→νν and fully hadronic tt decays, which increases statistics relative to leptonic Z decays. It defines a general ttZ vertex with four form factors, computes signal and backgrounds with full spin correlations, and uses tailored cuts and a χ^2 reconstruction to suppress backgrounds. A binned log-likelihood analysis of the missing transverse momentum distribution, incorporating nominal signal and background uncertainties, yields projected 68% CL limits on the ttZ couplings and shows that including the pTmiss bb+4j final state improves sensitivity by 10–60%, particularly for dimension-five couplings. The results translate into constraints on Littlest Higgs parameter space, illustrating how ttZ measurements at the LHC (and SLHC) can probe heavy top partners and complement indirect EW constraints, with notable precision for the axial vector and magnetic-dipole-type couplings.

Abstract

We consider QCD tt~Z production at the LHC with Z->\barννand all-hadronic tt~ decays, i.e. pp -> p_T(miss)bb~+4 jets, as a tool to measure ttZ couplings. This channel has a significantly larger cross section than those where the Z boson decays leptonically. However, tt~, bb~+4 jet, tt~j and tt~jj production give rise to potentially large backgrounds. We show that these processes can be suppressed to an acceptable level with suitable cuts, and find that adding the p_T(miss)bb~+4 jet channel to the final states used in previous ttZ couplings analyses will improve the sensitivity by 10-60%. We also discuss how the measurement of the ttZ couplings may constrain Little Higgs models.

Figures (4)

• Figure 1: The differential cross sections as a function of missing transverse momentum for $p\hbox{/}_Tb\bar{b}+4j$ production at the LHC. Shown are the SM predictions for $t\bar{t}Z$ production and various backgrounds. We impose the cuts of Eqs. (\ref{['eq:cuts1']}--\ref{['eq:cuts5']}), but do not include the double $b$-tag efficiency common to all curves.
• Figure 2: The differential cross sections as a function of missing transverse momentum for $p\hbox{/}_Tb\bar{b}$+$4j$ production at the LHC. Shown are the SM predictions for $t\bar{t}Z$ production (solid line), the combined $t\bar{t}$, $b\bar{b}+4j$, $t\bar{t}j$ and $t\bar{t}jj$ background, and the predictions for several non-standard $ttZ$ couplings. Only one coupling at a time is allowed to deviate from its SM value. We impose the cuts of Eqs. (\ref{['eq:cuts1']}--\ref{['eq:cuts5']}), but do not include the double $b$-tag efficiency common to all curves.
• Figure 3: Projected $68.3\%$ CL bounds on anomalous $ttZ$ couplings from the LHC with an integrated luminosity of 300 fb$^{-1}$, for: (a) $\Delta F^Z_{1A}$ versus $\Delta F^Z_{1V}$, (b) $\Delta F^Z_{2V}$ versus $\Delta F^Z_{1A}$, (c) $\Delta F^Z_{2V}$ versus $\Delta F^Z_{1V}$, and (d) $\Delta F^Z_{2A}$ versus $\Delta F^Z_{2V}$. Shown are the limits obtained from the $p\hbox{/}_T b\bar{b}$+$4j$ (dashed) and the dilepton and trilepton final states (dotted), and the combined limits (solid). To derive limits for the dilepton and trilepton final states, we use the results of Ref. Baur:2004uw. In each graph, only those couplings which are plotted against each other are assumed to be different from their SM values.
• Figure 4: Projected $68.3\%$ CL bounds on anomalous $ttZ$ couplings from the SLHC with an integrated luminosity of 3000 fb$^{-1}$, for: (a) $\Delta F^Z_{1A}$ versus $\Delta F^Z_{1V}$, (b) $\Delta F^Z_{2V}$ versus $\Delta F^Z_{1A}$, (c) $\Delta F^Z_{2V}$ versus $\Delta F^Z_{1V}$, and (d) $\Delta F^Z_{2A}$ versus $\Delta F^Z_{2V}$. Shown are the limits obtained from the $p\hbox{/}_T b\bar{b}$+$4j$ (dashed) and the dilepton and trilepton final states (dotted), and the combined limits (solid). To derive limits for the dilepton and trilepton final states, we use the results of Ref. Baur:2004uw. In each graph, only those couplings which are plotted against each other are assumed to be different from their SM values.