Measurements of top-quark pair differential cross-sections in the lepton+jets channel in $pp$ collisions at $\sqrt{s}=8$ TeV using the ATLAS detector

TL;DR

This paper presents measurements of top-quark pair differential cross-sections in proton-proton collisions at 8 TeV using the ATLAS detector, comparing data with simulations to test QCD predictions and parton distribution functions.

Contribution

First measurement of top-quark pair differential cross-sections in the lepton+jets channel at 8 TeV with detailed comparison to various Monte Carlo models.

Findings

Most generators predict a harder top-quark pT spectrum than observed.

Higher-order predictions improve high pT distribution agreement.

Rapidity distributions are poorly modeled with current PDFs, but recent PDFs improve this.

Abstract

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, $t\bar{t}$ system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=8$ TeV}. The observables have been chosen to emphasize the $t\bar{t}$ 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.