Measurement of the charge asymmetry in top quark pair production in pp collisions at sqrt(s) = 7 TeV using the ATLAS detector

TL;DR

This ATLAS study measures the tt̄ charge asymmetry in 7 TeV pp collisions using 1.04 fb^-1 in the lepton+jets channel. A kinematic fit reconstructs tt̄ events and a Bayesian unfolding corrects the measured |y|-based asymmetry for detector effects, with systematic uncertainties thoroughly evaluated. The inclusive unfolded asymmetry is A_C = -0.019 ± 0.028 (stat) ± 0.024 (syst), in agreement with MC@NLO (A_C = 0.006 ± 0.002) and showing no evidence for beyond-SM contributions; mass-binned results similarly align with SM expectations. A direct comparison to Tevatron results and several NP models disfavors flavour-changing Z' or W' explanations for the Tevatron A_FB excess, thereby constraining new physics scenarios in tt̄ production at the LHC.

Abstract

A measurement of the top-antitop production charge asymmetry A_C is presented using data corresponding to an integrated luminosity of 1.04 fb^-1 of pp collisions at sqrt(s) = 7 TeV collected by the ATLAS detector at the LHC. Events are selected with a single lepton (electron or muon), missing transverse momentum and at least four jets of which at least one jet is identified as coming from a b-quark. A kinematic fit is used to reconstruct the ttbar event topology. After background subtraction, a Bayesian unfolding procedure is performed to correct for acceptance and detector effects. The measured value of A_C is A_C = -0.018 +/- 0.028 (stat.) +/- 0.023 (syst.), consistent with the prediction from the MC@NLO Monte Carlo generator of A_C = 0.006 +/- 0.002. Measurements of A_C in two ranges of invariant mass of the top-antitop pair is also shown.

Figures (6)

• Figure 1: Expected and observed distributions for the invariant mass (plots (a) and (b)) and transverse momentum (plots (c) and (d)) of the reconstructed $t\bar{t}$ system. The left hand panels show distributions in the electron channel, while the right hand panels show distributions in the muon channel. The data are compared to the sum of the $t\bar{t}$ signal contribution and backgrounds. The background contributions from $W$+jets and multijet production have been estimated from data, while the other backgrounds are estimated from simulation. The uncertainty on the combined signal and background estimate includes systematic contributions. Overflows are shown in the highest bin of each histogram.
• Figure 2: Correlations between the true and reconstructed values of $\Delta |y|$ encoded in the unfolding response matrix for the electron (left) and muon (right) channels. The value of an entry in the matrix is proportional to the area of the corresponding box.
• Figure 3: The measured $\Delta |y|$ distribution before unfolding for the electron channel (left) and for the muon channel (right) after $b$-tagging is applied. Data (points) and Monte Carlo estimates (solid lines) are represented. The multijet background and the normalisation of the $W$+jets background are obtained as explained in Section \ref{['Backgrounds']}. The uncertainty on the combined signal and background estimate includes both statistical and systematic contributions.
• Figure 4: The unfolded $\Delta |y|$ distribution for the electron channel (left) and the muon channel (right) after $b$-tagging, compared to the prediction from MC@NLO. The uncertainties on the measurement include both statistical and systematic contributions, which are shown separately. The inner part of the error bars corresponds to the statistical component of the uncertainty, while the outer part corresponds to the systematic component. The error bands on the MC@NLO prediction include uncertainties from parton distribution functions and renormalisation and factorisation scales.
• Figure 5: Unfolded asymmetries in two regions of $m_{t\bar{t}}$ compared to the prediction from MC@NLO. The error bands on the MC@NLO prediction include uncertainties from parton distribution functions and renormalisation and factorisation scales.