Abstract
Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, tbar{t} system and event-level kinematic observables in proton–proton collisions at a centre-of-mass energy of sqrt{s}=8,mathrm{TeV}. The observables have been chosen to emphasize the tbar{t} production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb^{-1}, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.
Highlights
The shape of the distributions of the kinematical variables of this background is evaluated with a Monte Carlo simulation, and the event yields are normalized to the most recent calculations of their cross-sections, as described in Sect
The Alpgen+Pythia W +jets samples, normalized to the inclusive W boson NNLO crosssection, are used as a starting point while the absolute normalization and the heavy-flavour fractions of this process, which are affected by large theoretical uncertainties, are determined from data
Normalized differential cross-sections are shown in order to remove systematic uncertainties on the normalization
Summary
The single-top-quark background is the largest background contribution, amounting to approximately 4 % of the total event yield and 40 % of the total background estimate. Approximately 3–4 % of the total event yield and 35 % of the total background estimate is due to W +jets events. The estimation of this background is performed using a combination of MC simulation and data-driven techniques. The systematic uncertainties due to the overall normalization and the heavy-flavour fraction of W +jets events are obtained by varying the data-driven scale factors within the statistical uncertainty of the W +jets MC sample. The overall impact of this uncertainty is less than 1 %
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