High p_T Top Quarks at the Large Hadron Collider

TL;DR

The paper addresses the challenge of discovering TeV-scale resonances decaying to tt̄ at the LHC by focusing on boosted top quarks, where decay products merge and standard four-jet topologies lose efficiency. It analyzes the lepton+jets channel with 2–3 jets and one or two b-tags, incorporating NLO QCD corrections via MC@NLO and a detailed background study including W+jets and single-top processes. The authors propose jet invariant-mass cuts on hadronic-top jets and cluster transverse-mass cuts to suppress backgrounds, showing that lν+2–3 jets with limited b-tags substantially improve discovery reach for tt̄ resonances, such as KK gluons in bulk RS models. They provide LO estimates of signal significance and discuss how jet algorithms and b-tag performance at high pT affect the results, concluding that boosted-top strategies can extend the LHC’s sensitivity to multi-TeV tt̄ resonances. Overall, the work outlines practical strategies for leveraging boosted-top signatures to enhance searches for new heavy particles decaying to tt̄ at the LHC.

Abstract

Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, t\bar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such particles. The identification of very energetic top quarks is crucial in such an analysis. We consider in detail the t\bar t\to\ell^\pmνb\bar bq\bar q' (\ell=e, μ) final state for high p_T top quarks. In this phase space region, two or more of the final state quarks can merge into a single jet due to the large Lorentz boost of the parent top quark. As a result, a large fraction of t\bar t\to\ell^\pmνb\bar bq\bar q' events with an invariant mass in the TeV region contains less than four observable jets. Requiring one or two tagged b-quarks, we calculate the W+jets, Wb+jets, Wb\bar b+jets, Wbt, and single top plus jets backgrounds for these final states, and identify cuts which help to suppress them. In particular, we discuss whether a cut on the jet invariant mass may be useful in reducing the background in the \ellν+2 jets channel. We also investigate how next-to-leading order QCD corrections affect high p_T top quark production at the LHC. We find that the \ellν+2 jets and \ellν+3 jets final states with one or two $b$-tags will significantly improve the chances for discovering new heavy particles in the t\bar t channel at the LHC.

Figures (18)

• Figure 1: The LO $pp\to t\bar{t}\to\ell\nu b\bar{b}jj$ differential cross section at the LHC for three choices of $R_{min}$ as a function of a) the reconstructed $t\bar{t}$ invariant mass, and b) the reconstructed $p_T$ of the semileptonically decaying top quark. The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}) -- (\ref{['eq:cuts4']}).
• Figure 2: The LO $pp\to t\bar{t}\to\ell\nu +n$ jets differential cross section at the LHC as a function of a) the reconstructed $t\bar{t}$ invariant mass, and b) the reconstructed $p_T$ of the semileptonically decaying top quark. Shown are the cross sections for two (blue), three (red) and four (black) jets in the final state. Two of the jets are assumed to be $b$-tagged. The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}) -- (\ref{['eq:cuts4']}). In addition an isolation cut (see Eq. (\ref{['eq:cuts5']})) with $R_{min}=0.4$ is imposed and jets with $\Delta R<0.4$ are merged.
• Figure 3: The LO $pp\to t\bar{t}\to\ell\nu +n$ jets, $n=2,\,3,\,4$, differential cross section at the LHC as a function of the reconstructed $t\bar{t}$ invariant mass, and the reconstructed $p_T$ of the semileptonically decaying top quark. Two of the jets are assumed to be $b$-tagged. Shown are the cross sections for two, three and four jets in the final state in the SM (black lines) and for two types of KK gluon excitations (see text). The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}) -- (\ref{['eq:cuts4']}). In addition an isolation cut (see Eq. (\ref{['eq:cuts5']})) with $R_{min}=0.4$ is imposed and jets with $\Delta R<0.4$ are merged.
• Figure 4: The NLO to LO $pp\to t\bar{t}\to\ell\nu +n$ jets cross section ratio (solid histograms) at the LHC as a function of the reconstructed $t\bar{t}$ invariant mass. Here, $n$ is the number of $t\bar{t}$ decay jets. The dashed histograms display the fraction of the NLO $pp\to t\bar{t}\to\ell\nu +n$ jets events for which $p_T(t\bar{t})>100$ GeV. Shown are the cross section ratios for two (blue), three (red) and four (black) $t\bar{t}$ decay jets. Two of the jets are assumed to be $b$-tagged. The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}) -- (\ref{['eq:cuts4']}). In addition an isolation cut (see Eq. (\ref{['eq:cuts5']})) with $R_{min}=0.4$ is imposed. $t\bar{t}$ decay jets with a separation $\Delta R<0.4$ have been merged.
• Figure 5: The NLO to LO cross section ratio (solid histograms) for $pp\to t\bar{t}\to\ell\nu +n$ jets at the LHC as a function of the $t\to b\ell\nu$ (black) and $t\to bjj$ (blue) transverse momentum for a) two ($n=2$) and b) three ($n=3$) $t\bar{t}$ decay jets. The dashed histograms display the fraction of the NLO $pp\to t\bar{t}\to\ell\nu +n$ jet events ($n=2,\,3$) for which $p_T(t\bar{t})>100$ GeV. Two of the jets are assumed to be $b$-tagged. The cuts imposed are listed in Eqs. (\ref{['eq:cuts1']}) -- (\ref{['eq:cuts4']}). In addition an isolation cut (see Eq. (\ref{['eq:cuts5']})) with $R_{min}=0.4$ is imposed. $t\bar{t}$ decay jets with a separation $\Delta R<0.4$ have been merged.