Search for ttbar resonances in final states with exactly one or two leptons using 140 fb$^{-1}$ of pp collision data at $\sqrt{s}=13$ TeV with the ATLAS experiment

Abstract

A search for heavy spin-1 and spin-2 resonances decaying into a top-antitop-quark pair has been performed with 140 fb$^{-1}$ of proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Final states with either exactly one electron or muon, or exactly two leptons ($ee$, $μμ$ or $eμ$), large missing transverse momentum, and two jets, at least one of which must be identified as likely containing a b-hadron decay, are considered. The search targets resonances with both narrow and broad widths relative to the detector resolution, and with masses in the range of 0.4-5.0 TeV. No significant deviation from the Standard Model prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for hypothetical $Z'$ bosons, Kaluza-Klein gravitons, and Kaluza-Klein gluons that decay into top-quark pairs.

Figures (10)

• Figure 1: Representative leading-order Feynman diagrams for the s-channel production and subsequent decay to $t\bar{t}$ of the (a) $Z'\xspace_{\mathrm{TC2}}\xspace\,$ , (b) $G_{\textrm{KK}}$, and (c) $g_{\textrm{KK}}$ signals.
• Figure 2: Selection efficiency times acceptance (Eff x Acc) for the $\ell+\mathrm{jets}$ final state as a function of the $t\bar{t}$ invariant mass at the parton level before the emission of FSR, for the \ref{['fig:accXeff_zprime']}$Z'\xspace_{\mathrm{TC2}}\xspace\,$, \ref{['fig:accXeff_kkgrav']}$G_{\textrm{KK}}$, and \ref{['fig:accXeff_kkg']}$g_{\textrm{KK}}$ signals. The selections in the resolved topology only (short-dashed magenta), the merged topology only (dashed blue) and the inclusive topology (solid black) are shown. The error bars correspond to the statistical uncertainty. All $t\bar{t}$ decay modes are considered.
• Figure 3: Distribution of the reconstructed $\frac{\Delta\phi_{\ell\ell}}{\pi}$ in \ref{['fig:dPhi:prefit:incl_SR']} the signal regions and \ref{['fig:dPhi:prefit:incl_VR']} the validation regions of the 2-lepton channel. Each bin corresponds to one of the signal or validation regions. The distributions and the uncertainty band are obtained before the final fit to the data (pre-fit). "Other Bkg." combines the $t\bar{t}\xspace+V$, $t\bar{t}\xspace+H$, diboson and fakes backgrounds together. In \ref{['fig:dPhi:prefit:incl_SR']}, a 3 $\text{Te V}$$Z'\xspace_{\mathrm{TC2}}\xspace\,$ signal (green dashed and dotted line), a 1 $\text{Te V}$$G_{\textrm{KK}}$ signal (blue dotted line) and a 2 $\text{Te V}$$g_{\textrm{KK}}$ signal (red short-dashed and dotted line) are overlaid for illustration purposes, normalised to the total background. The lower panels show the ratio of the data to the total SM background prediction. The error bands include both the statistical and systematic uncertainties.
• Figure 4: Signal distributions in $m_{t\bar{t}}$ after the signal region selection of the 1-lepton channel for the \ref{['fig:reco:1L_Zp']}$Z'\xspace_{\mathrm{TC2}}\xspace\,$, \ref{['fig:reco:1L_KKgrav']}$G_{\textrm{KK}}$, and \ref{['fig:reco:1L_KKglu']}$g_{\textrm{KK}}$ signals for various representative values of the resonance mass. Events from the Resolved $1b$, Resolved $2b$, and Merged signal regions are included.
• Figure 5: Signal distributions in $m_{\ell\ell bb}$ after the signal region selection of the 2-lepton channel for the \ref{['fig:reco:2L_Zp']}$Z'\xspace_{\mathrm{TC2}}\xspace\,$, \ref{['fig:reco:2L_KKgrav']}$G_{\textrm{KK}}$, and \ref{['fig:reco:2L_KKglu']}$g_{\textrm{KK}}$ signals for various representative values of the resonance mass. Events from the five signal regions are combined.