Search for resonant t t-bar production in lepton+jets events in pp collisions at sqrt(s) = 7 TeV

TL;DR

This study addresses the search for heavy resonances decaying to ttbar without specifying a particular model. It employs two complementary strategies (threshold and boosted) in the semileptonic ttbar channel using CMS data at 7 TeV with 4.4–5.0 fb^-1. A template-based analysis of the reconstructed M_ttbar spectrum is used, with data-driven multijet background estimation and robust treatment of systematic uncertainties. No excess is observed, leading to 95% CL limits that exclude topcolor Z' bosons and KK gluons up to several TeV, depending on width. The results provide the most stringent constraints to date on ttbar resonant production in the 0.5–2 TeV range and constrain relevant beyond-the-Standard-Model scenarios.

Abstract

A model-independent search for the production of heavy resonances decaying into top-antitop quark pairs is presented. The search is based on events containing one lepton (muon or electron) and at least two jets selected from data samples corresponding to an integrated luminosity of 4.4-5.0 inverse femtobarns collected in pp collisions at sqrt(s) = 7 TeV. Results are presented from the combination of two dedicated searches optimized for boosted production and production at threshold. No excess of events is observed over the expected yield from the standard model processes. Topcolor Z' bosons with narrow (wide) width are excluded at 95% confidence level for masses below 1.49 (2.04) TeV and an upper limit of 0.3 (1.3) pb or lower is set on the production cross section times branching fraction for resonance masses above 1 TeV. Kaluza-Klein excitations of a gluon with masses below 1.82 TeV (at 95% confidence level) in the Randall-Sundrum model are also excluded, and an upper limit of 0.7 pb or lower is set on the production cross section times branching fraction for resonance masses above 1 TeV.

Figures (5)

• Figure 1: The distribution of the minimum $\Delta R$ of all three possible pairings between the three quarks $(\mathrm{q}_1, \mathrm{q}_2, {b})$ of the hadronic top-quark decay for SM production and two different ${Z}^\prime$ mass hypotheses. For events with $\Delta R_{\text{min}}$ smaller than the parameter $R=0.5$ in the jet clustering, jets merge and fewer than three jets are reconstructed.
• Figure 2: Comparison of the reconstructed $M_{{t}\overline{{t}}\xspace}$ in data and SM predictions for the threshold analysis with (a) 3 jets of which ${\ge}1$ b tagged, (b) 4 jets, none of which is b tagged, (c) 4 jets of which one is b tagged, (d) 4 jets of which ${\ge}2$ are b tagged. Expected signal contributions for narrow-width topcolor ${Z}^\prime\xspace$ models at different masses are also shown. For clarity, a cross section times branching fraction of 20$\text{\,pb}$ is used for the normalization of the ${Z}^\prime\xspace$ samples.
• Figure 3: Comparison of the reconstructed $M_{{t}\overline{{t}}\xspace}$ in data and SM predictions for the boosted analysis with (a) no b-tagged jets, (b) ${\ge}1$ b-tagged jets. Comparison of the jet multiplicity distribution in data and SM background predictions for the boosted analysis with (c) no b-tagged jets, (d) ${\ge}1$ b-tagged jets. Expected signal contributions for narrow-width topcolor ${Z}^\prime\xspace$ models at different masses are also shown. A cross section times branching fraction of 1.0$\text{\,pb}$ is used for the normalization of the ${Z}^\prime\xspace$ samples.
• Figure 4: The 95% CL upper limits on the product of the production cross section $\sigma_{{Z}^\prime\xspace}$ and the branching fraction $B$ of hypothesized resonances that decay into $\tt$ as a function of the invariant mass of the resonance. The ${Z}^\prime$ production with $\Gamma_{{Z}^\prime}/m_{{Z}^\prime}=$ 1.2% (a) and 10% (b) compared to predictions based on Jain11124928. The $\pm1$ and $\pm2$ s.d. excursions from the expected limits are also shown. The vertical dashed line indicates the transition between the threshold and the boosted analyses, chosen based on the sensitivity of the expected limit.
• Figure 5: The 95% CL upper limits on the product of the production cross section $\sigma_\mathrm{KK}$ and the branching fraction $B$ of Kaluza--Klein excitation of gluon production from Agashe:2006hk, compared to the theoretical prediction of that model. The ${\pm}1$ and ${\pm}2$ s.d. excursions from the expected limits are also shown.