Differential top-antitop cross-section measurements as a function of observables constructed from final-state particles using pp collisions at $\sqrt{s}=7$ TeV in the ATLAS detector

TL;DR

This study presents fiducial, differential tt̄ cross sections constructed from detector-level objects via a hat{t} proxy to minimize model extrapolations. Using lepton+jets tt̄ events at 7 TeV with 4.6 fb⁻¹, ATLAS unfolds and combines hat{t} observables to compare data with various MC models (NLO and LO multi-leg). The results indicate that certain NLO models with alternative PDFs (notably powheg(herapdf)+pythia) provide the best overall agreement, while LO generators show tensions in specific kinematic regions; systematic uncertainties, especially from b-tagging and JES, dominate the precision. The hat{t}-based framework offers a robust, experiment-friendly avenue for testing QCD in tt̄ production and for refining MC tunes without excessive extrapolation to parton level.

Abstract

Various differential cross-sections are measured in top-quark pair ($t\bar{t}$) events produced in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV at the LHC with the ATLAS detector. These differential cross-sections are presented in a data set corresponding to an integrated luminosity of $4.6$ fb$^{-1}$. The differential cross-sections are presented in terms of kinematic variables, such as momentum, rapidity and invariant mass, of a top-quark proxyreferred to as the pseudo-top-quark as well as the pseudo-top-quark pair system. The dependence of the measurement on theoretical models is minimal. The measurements are performed on $t\bar{t}$ events in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of them tagged as originating from a $b$-quark. The hadronic and leptonic pseudo-top-quarks are defined via the leptonic or hadronic decay mode of the $W$ boson produced by the top-quark decay in events with a single charged lepton. Differential cross-section measurements of the pseudo-top-quark variables are compared with several Monte Carlo models that implement next-to-leading order or leading-order multi-leg matrix-element calculations.

Figures (13)

• Figure 1: Simulated event distribution of the parton-level top-quark $p_{\mathrm{T}}$ distribution for all events (green), and the particle-level pseudo-top-quark $p_{\mathrm{T}}$ ($\hbox{$\hat{t}$}_{\rm{h}}$) for events within the fiducial region (blue). In both cases the top-quark that decays hadronically is chosen. The distributions are evaluated for the same event sample based on powheg+ pythia at $\sqrt{s} = 7$ TeV. The upper figure is made for an arbitrary integrated luminosity. The lower figure shows the ratio of the particle-level $\hbox{$\hat{t}$}_{\rm{h}}$ over the parton-level top-quark normalised distributions to emphasise the difference in shape between the two.
• Figure 2: (a) Monte Carlo study using the nominal powheg+ pythia MC sample showing the correlation between the parton-level top-quark $p_{\mathrm{T}}$ and the particle-level hadronic pseudo-top-quark $p_{\mathrm{T}}$ and (b) the correlation between the particle-level hadronic pseudo-top-quark $p_{\mathrm{T}}$ and the hadronic pseudo-top-quark $p_{\mathrm{T}}$ evaluated from reconstructed objects. In each case the correlation is normalised to all the events within a bin on the horizontal axis.
• Figure 3: The reconstructed pseudo-top-quark $p_{\mathrm{T}}\xspace$ in comparison to the MC signal and data-driven background models. The $p_{\mathrm{T}}(\hbox{$\hbox{$\hat{t}$}_{\rm{h}}$})$ distributions: (a) the muon channel and (b) the electron channel. The $p_{\mathrm{T}}(\hbox{$\hbox{$\hat{t}$}_{\rm{l}}$})$ distributions: (c) the muon channel and (d) the electron channel. Signal and background processes are shown in different colours, with "Other" including the small backgrounds from diboson and $Z\!+\!\mathrm{jets}$ production, and non-prompt or fake lepton signatures from multi-jet processes. The data are compared with predictions from background models and expected yields from simulated $t\bar{t}$ events generated using powheg+ pythia with the "C" variant of the Perugia 2011 tunes family. The error bars on the data points show the statistical uncertainty on the data, while the shaded band shows the total systematic and statistical uncertainty on the predicted yields. The systematic uncertainty has a strong bin-to-bin correlation.
• Figure 4: The reconstructed pseudo-top-quark rapidity in comparison to the MC signal and data-driven background models. The |$y(\hbox{$\hbox{$\hat{t}$}_{\rm{h}}$})$| distributions: (a) the muon channel and (b) the electron channel. The |$y(\hbox{$\hbox{$\hat{t}$}_{\rm{l}}$})$| distributions: (c) the muon channel and (d) the electron channel. The powheg+ pythia MC generator with the Perugia 2011C tune is used for the $t\bar{t}$ signal estimate. Signal and background processes are shown in different colours, with "Other" including small backgrounds from diboson and $Z\!+\!\mathrm{jets}$ production, as well as non-prompt and fake leptons from multi-jet processes. The shaded band shows the total systematic and statistical uncertainties on the signal plus background expectation.
• Figure 5: The reconstructed $p_{\mathrm{T}}\xspace$ for the system of pseudo-top-quark pairs $\hbox{$\hbox{$\hat{t}$}_{\rm{l}}$}\hbox{$\hbox{$\hat{t}$}_{\rm{h}}$}$ in comparison to the MC signal and data-driven background models. The $p_{\mathrm{T}}(\hbox{$\hbox{$\hat{t}$}_{\rm{l}}$}\hbox{$\hbox{$\hat{t}$}_{\rm{h}}$})$ distributions: (a) the muon channel and (b) the electron channel. Signal and background processes are shown in different colours, with "Other" including small backgrounds from diboson and $Z\!+\!\mathrm{jets}$ production, as well as non-prompt and fake leptons from multi-jet processes. The powheg+ pythia MC generator with the Perugia 2011C tune is used for the $t\bar{t}$ signal estimate. The shaded band shows the total systematic and statistical uncertainties on the signal plus background expectation.