Measurements of top-quark pair differential cross-sections in the [Formula: see text] channel in pp collisions at [Formula: see text] TeV using the ATLAS detector

Abstract

This article presents measurements of [Formula: see text] differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb[Formula: see text] of proton-proton data at a centre-of-mass energy of [Formula: see text] TeV recorded by the ATLAS experiment at the LHC in 2015. Differential cross-sections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the [Formula: see text] system. The [Formula: see text] events are selected by requiring one electron and one muon of opposite electric charge, and at least two jets, one of which must be tagged as containing a b-hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the Powheg-Box [Formula: see text] Herwig++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the [Formula: see text] system.

Fig. 1

Kinematic distributions for the electron $p_{\text{T}}$ (a), muon $p_{\text{T}}$ (b), b-jet $p_{\text{T}}$ (c), and $E_{\text{T}}^{\text{miss}}$ (d) for the $e^{\pm }{\mathit{\mu }}^{\mp }$ signal selection. In all figures, the rightmost bin also contains events that are above the x-axis range. The dark uncertainty bands in the ratio plots represent the statistical uncertainties while the light uncertainty bands represent the statistical, systematic and luminosity uncertainties added in quadrature. The uncertainties quoted include uncertainties from leptons, jets, missing transverse momentum, background modelling and pile-up modelling. They do not include uncertainties from PDF or signal $t\overline{t}$ modelling

Fig. 2

Kinematic distributions for the $p_{\text{T}}(t)$ (a), |y(t)| (b), $p_{\text{T}}(t\overline{t})$ (c), $t\overline{t}$ $|y_{t\overline{t}}|$ (d), and $m(t\overline{t})$ (e) after reconstruction of the $t\overline{t}$ system. In all figures, the rightmost bin also contains events that are above the x-axis range. The uncertainty bands represent the statistical uncertainties (dark) and the statistical, systematic and luminosity uncertainties added in quadrature (light). The uncertainties quoted include uncertainties on leptons, jets, $E_{\text{T}}^{\text{miss}}$, background and pile-up modelling, and luminosity. They do not include uncertainties on PDF or signal $t\overline{t}$ modelling

Fig. 3

The response matrices for the observables obtained from the nominal $t\overline{t}$ MC, normalised by row to unity. Each bin shows the probability for a particle-level event in bin j to be observed in a reconstruction-level bin i. White corresponds to 0 probability and the darkest green to a probability of one, where the other probabilities lie in between those shades

Fig. 4

Summary of the fractional size of the absolute (a) and normalised (b) fiducial differential cross-sections as a function of $p_{\text{T}}(t)$. Systematic uncertainties which are symmetric are represented by solid lines and asymmetric uncertainties are represented by dashed or dot–dashed lines. Systematic uncertainties from common sources, such as modelling of the $t\overline{t}$ production, have been grouped together. Uncertainties due to luminosity or background modelling are not included. The statistical and total uncertainty sizes are indicated by the shaded bands

Fig. 5

The measured normalised fiducial differential cross-sections compared to predictions from Powheg-Box (top ratio panel), MG5_aMC@NLO, and Sherpa (bottom ratio panel) interfaced to various parton shower programs