Measurement of the $t\bar{t}$ production cross-section as a function of jet multiplicity and jet transverse momentum in 7 TeV proton-proton collisions with the ATLAS detector

TL;DR

<3-5 sentence high-level summary>This ATLAS study measures differential $t\bar{t}$ production cross-sections as a function of jet multiplicity up to eight jets and jet $p_T$ up to the fifth jet in 7 TeV $pp$ collisions, using the full 2011 data sample in the single-lepton channel. It employs fiducial, detector-level to particle-level unfolding with iterative Bayesian methods and combines electron and muon channels via BLUE, comparing multiple MC models (ALPGEN, POWHEG, MC@NLO) to probe ISR/FSR modelling. The results show MC@NLO+HERWIG underpredicts high jet multiplicities, while POWHEG+PYTHIA with tuned radiation provides the best overall description; ALPGEN with lowered $\alpha_s$ improves several observables. These measurements constrain higher-order QCD effects in $t\bar{t}$ production and aid MC tuning for precision top-quark studies and related searches.

Abstract

The $t\bar{t}$ production cross-section dependence on jet multiplicity and jet transverse momentum is reported for proton--proton collisions at a centre-of-mass energy of 7 TeV in the single-lepton channel. The data were collected with the ATLAS detector at the CERN Large Hadron Collider and comprise the full 2011 data sample corresponding to an integrated luminosity of 4.6 fb$^{-1}$. Differential cross-sections are presented as a function of the jet multiplicity for up to eight jets using jet transverse momentum thresholds of 25, 40, 60, and 80 GeV, and as a function of jet transverse momentum up to the fifth jet. The results are shown after background subtraction and corrections for all detector effects, within a kinematic range closely matched to the experimental acceptance. Several QCD-based Monte Carlo models are compared with the results. Sensitivity to the parton shower modelling is found at the higher jet multiplicities, at high transverse momentum of the leading jet and in the transverse momentum spectrum of the fifth leading jet. The MC@NLO+HERWIG MC is found to predict too few events at higher jet multiplicities.

Figures (17)

• Figure 1: The relationship between the $p_{\mathrm{T}}$ of the $t\bar{t}$ system in the single-lepton channel and the $p_{\mathrm{T}}$ of the highest $p_{\mathrm{T}}$ jet in $t\bar{t}$ events generated with ALPGEN+ HERWIG. The $p_{\mathrm{T}}$ of the $t\bar{t}$ system is taken at parton level and the leading jet is constructed at particle level.
• Figure 2: The reconstructed jet multiplicities for the jet $p_{\mathrm{T}}$ threshold of 25 Ge V, in the (a) electron ($e+\textrm{jets}$) and (b) muon ($\mu+\textrm{jets}$) channel. The data are compared to the sum of the $t\bar{t}$POWHEG+ PYTHIA MC signal prediction and the background models. The shaded bands show the total systematic and statistical uncertainties on the combined signal and background estimate. The errors bar on the black points and the hatched area in the ratio, show the statistical uncertainty on the data measurements.
• Figure 3: The reconstructed jet $p_{\mathrm{T}}$ for the electron ($e+\textrm{jets}$) channel (a) leading and (b) fifth jet and muon channel ($\mu+\textrm{jets}$) (c) leading and (d) fifth jet. The data are compared to the sum of the $t\bar{t}$POWHEG+ PYTHIA MC signal prediction and the background models. The shaded bands show the total systematic and statistical uncertainties on the combined signal and background estimate. The error bars on the black points and the hatched area in the ratio, show the statistical uncertainty on the data measurements.
• Figure 4: Global correction factors for the acceptance ($f_\mathrm{accpt}$) and particle-level and reconstruction-level inefficiencies ($f_\mathrm{part!reco},f_\mathrm{reco!part}$) to correct the jet multiplicity distribution with $p_{\mathrm{T}}>25$Ge V to particle level (a) in the electron and (b) in the muon channel as described in the text and in eq. (\ref{['eqn:corrections']}). The symbol $n_\mathrm{jet}$ refers to the number of particle-level jets for $f_\mathrm{accpt}$ and $f_\mathrm{part!reco}$ and to the number of reconstructed jets in case of $f_\mathrm{reco!part}$. The distributions are shown with statistical uncertainties only, which are too small to be visible.
• Figure 5: Global correction factors for the acceptance ($f_\mathrm{accpt}$), particle-level and reconstruction-level inefficiencies ($f_\mathrm{part!reco},f_\mathrm{reco!part}$) and misassignment in the $p_{\mathrm{T}}$ ordering of the jets ($f_\mathrm{misassign}$), used to correct the jet $p_{\mathrm{T}}$ distributions to the particle level as described in the text and in eq. (\ref{['eqn:corrections-jetpt']}). The muon-channel correction factors are shown as an example. However, the corresponding distributions of the electron channel (not shown) are similar. The distributions are shown with statistical uncertainties only, which are too small to be visible.