Search for anomalous t t-bar production in the highly-boosted all-hadronic final state

TL;DR

This CMS study searches for heavy resonances decaying to $t\bar t$ in the all-hadronic, highly boosted regime using 7 TeV data corresponding to 5 fb$^{-1}$. It leverages advanced jet-substructure techniques to identify merged top quarks and distinguishes signal from predominant non-top multijet backgrounds, employing both a resonance search in the $m_{t\bar t}$ spectrum and a model-agnostic enhancement test. The analysis sets 95% CL limits on $\sigma\times B$ for $Z'$ models with 1% and 10% widths and for Randall–Sundrum KK gluons, and constrains any generic enhancement to the $t\bar t$ cross section for $m_{t\bar t}>1$ TeV to be less than about 2.6 times the SM expectation. Overall, no evidence for boosted $t\bar t$ resonances is found, and the work demonstrates the utility of jet-substructure methods in exploring high-mass new-physics scenarios.

Abstract

A search is presented for a massive particle, generically referred to as a Z', decaying into a t t-bar pair. The search focuses on Z' resonances that are sufficiently massive to produce highly Lorentz-boosted top quarks, which yield collimated decay products that are partially or fully merged into single jets. The analysis uses new methods to analyze jet substructure, providing suppression of the non-top multijet backgrounds. The analysis is based on a data sample of proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 5 inverse femtobarns. Upper limits in the range of 1 pb are set on the product of the production cross section and branching fraction for a topcolor Z' modeled for several widths, as well as for a Randall--Sundrum Kaluza--Klein gluon. In addition, the results constrain any enhancement in t t-bar production beyond expectations of the standard model for t t-bar invariant masses larger than 1 TeV.

Figures (6)

• Figure 1: (a) The simulated jet mass for NTMJ MC (light yellow histogram) and ${Z}^\prime\xspace$ MC (open histogram). (b) The type-1 per-jet top-tagging efficiency for ${Z}^\prime\xspace$ MC events is shown as red squares with error bars (see Sec. \ref{['sec:signalestimate']}), and the type-1 per-jet mistag probability for top tagging measured in data is shown as black circles with error bars (see Sec. \ref{['sec:backgroundestimate']}), both as a function of jet $p_{\mathrm{T}}\xspace$.
• Figure 2: (a) The mass of the highest-mass jet (${\mathrm{W}}$-jet), and (b) the mass of the Type-2 top candidate (${\mathrm{W}}+{b}$), in the hadronic hemisphere of moderately-boosted events in the muon control sample. The data are shown as points with error bars, the ${t}\overline{{t}}\xspace$ Monte Carlo events in dark red, the ${\mathrm{W}}+$jets Monte Carlo events in lighter green, and non-${\mathrm{W}}$ multijet (non-${\mathrm{W}}$ MJ) backgrounds are shown in light yellow (see Ref. TOP-10-003 for details of non-${\mathrm{W}}$ MJ distribution derivation). The jet mass is fitted to a sum of two Gaussians in both data (solid line) and MC (dashed line), the latter of which lies directly behind the solid line for most of the region.
• Figure 3: (a) $p_{\mathrm{T}}\xspace$ of the Type-2 top-quark candidate in the muon control sample. The color scheme is the same as in Figs. \ref{['figs:wMass_semileptonic']} and \ref{['figs:topMass_semileptonic']}. (b) $p_{\mathrm{T}}\xspace$ of the ${\mathrm{W}}$-candidate from within the Type-2 top-quark candidate, after a selection on the jet mass of the highest-mass jet in the muon control sample. Overlaid on both (a) and (b) are the corresponding distributions from a ${Z}^\prime\xspace$ MC signal with $m=1$${\,\text{Te\spaceV\space/\space}c^\text{2}}$ (with arbitrary normalization for visualization) to compare kinematics in the muon control region to the signal region.
• Figure 4: (a) The mass of the highest-mass jet (${\mathrm{W}}$-jet), and (b) the mass of the Type-2 top candidate (${\mathrm{W}}+{b}$), in the hadronic hemisphere of moderately-boosted events in the muon control sample. The Figure corresponds to Figs. \ref{['figs:wMass_semileptonic']} and \ref{['figs:topMass_semileptonic']}, except there is an additional requirement on the Type-2 top candidate $p_{\mathrm{T}}\xspace$ to be similar to the signal region. Figure \ref{['figs:semiLepMassDrawPt_ptTopCand_200']} shows the distribution of the Type-2 top candidate $p_{\mathrm{T}}\xspace$.
• Figure 5: Results for (a) 1+1 and (b) 1+2 event selections and background estimates. The yellow (light) histograms are the non-top multijet (NTMJ) estimates from data, as described in the text, and the red (dark) histograms are the MC estimates from SM ${t}\overline{{t}}\xspace$ production. The black points are the data. The hatched gray boxes combine the statistical and systematic uncertainties on the total background. For comparison, expectations for some ${Z}^\prime\xspace$ hypotheses are shown for the assumption of 1% resonance width, with cross sections taken from the expected limits discussed in Sec. \ref{['sec:resonant']}. Also shown are the ratio of the fractional difference between the data and the prediction, shown in red circles, with the $y$-axis on the right of the plot, and the number of standard deviations ($N_{\sigma}$) of the observation from the prediction, shown as a black histogram with the $y$-axis on the left of the plot, where the binning is adjusted to have at least 20 events in each bin.