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Distributions of \protect\subref{plots_inv_mass_ee} dielectron and \protect\subref{plots_inv_mass_mm} dimuon reconstructed invariant mass (\mll) after selection, for data and the SM background estimates as well as their ratio. Two selected signals are overlaid; resonant \zpchi\ with a pole mass of 3~\TeV\ and non-resonant contact interactions with left-left (LL) constructive interference and $\Lambda$ = 20~\TeV. The bin width of the distributions is constant in log(\mll), and the shaded band in the lower panel illustrates the total systematic uncertainty, as explained in~\refS{sec:systematics}. The data points are shown together with their statistical uncertainty.

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(a) Upper 95\% CL limits for \zp\ production cross-section times branching ratio to two leptons as a function of \zp\ pole mass (M$_{\zp}$). The signal theory lines are calculated with \pythia~8 using the NNPDF23LO PDF set~\cite{Ball:2012cx}, and corrected to next-to-next-to-leading order in QCD using \vrap~\cite{Anastasiou:2003ds} and the CT14NNLO PDF set~\cite{Dulat:2015mca}. The signal theoretical uncertainties are shown as a band on the \zpssm\ theory line for illustration purposes, but are not included in the \xbr\ limit calculation. (b) Lower 95\% CL limits on the contact interaction (CI) scale $\Lambda$ for different chiral couplings and both constructive and destructive interference scenarios using a uniform positive prior in 1/$\Lambda^2$. For the left-left (LL) and right-right (RR) cases, the ATLAS $\rts = 8$ \TeV{} results~\cite{Aad:2014wca} are shown for comparison. In that publication, the left-right (LR) case was obtained by setting $\eta_{\rm LR} = \eta_{\rm RL} = \pm 1$ and therefore is not directly comparable to the results presented here.Caption not extracted