Measurement of jet shapes in top pair events at sqrt(s) = 7 TeV using the ATLAS detector

TL;DR

This work presents the first ATLAS measurement of jet shapes in tt̄ events at √s = 7 TeV, comparing $b$-jets from top decays with light-quark jets from hadronic W decays. Using 1.8 fb^{-1} of data in dilepton and single-lepton channels, the analysis computes differential and integrated jet shapes ρ(r) and Ψ(r), and performs a robust unfolding to particle level to compare with NLO MC predictions. The results show that $b$-jets are broader than light jets, particularly at low jet p_T, and are well described by MC@NLO/Herwig and Powheg/Pythia with small colour-reconnection effects. The findings provide valuable constraints on colour radiation modelling in tt̄ events and aid in refining jet structure predictions in top-quark analyses. Overall, the measurement enhances understanding of jet substructure in a top-quark-rich environment and supports the reliability of current MC simulations for top-quark physics at the LHC.

Abstract

A measurement of jet shapes in top-quark pair events using 1.8 fb(-1) of sqrt(s) = 7 TeV pp collision data recorded by the ATLAS detector at the LHC is presented. Samples of top-quark pair events are selected in both the single-lepton and dilepton final states. The differential and integrated shapes of the jets initiated by bottom-quarks from the top-quark decays are compared with those of the jets originated by light-quarks from the hadronic W-boson decays in the single-lepton channel. The light-quark jets are found to have a narrower distribution of the momentum flow inside the jet area than b-quark jets.

Figures (22)

• Figure 1: Example LO Feynman diagrams for $gg\rightarrow t\bar{t}$ in the dilepton (left) and single-lepton (right) decay modes.
• Figure 2: The $p_{\mathrm{T}}$ distributions for $b$-tagged jets in the single-lepton (top) and dilepton (bottom) samples along with the sample composition expectations.
• Figure 3: The distribution of light-jet $p_{\mathrm{T}}$ (top) and of the invariant mass of light-jet pairs (bottom) along with the sample composition expectations. The latter shows a peak at the $W$ mass, whose width is determined by the dijet mass resolution.
• Figure 4: Distribution of $R = 0.4$$b$-jets in the single-lepton channel as a function of the differential jet shapes $\rho(r)$ for different values of $r$.
• Figure 5: Distribution of $R = 0.4$ light jets in the single-lepton channel as a function of the differential jet shapes $\rho(r)$ for different values of $r$.