Abstract
The inclusive and fiducial tbar{t} production cross-sections are measured in the lepton+jets channel using 20.2~hbox {fb}^{-1} of proton–proton collision data at a centre-of-mass energy of 8 TeV recorded with the ATLAS detector at the LHC. Major systematic uncertainties due to the modelling of the jet energy scale and b-tagging efficiency are constrained by separating selected events into three disjoint regions. In order to reduce systematic uncertainties in the most important background, the W text {+,jets} process is modelled using Z+ jets events in a data-driven approach. The inclusive tbar{t} cross-section is measured with a precision of 5.7% to be sigma _{text {inc}}(tbar{t}) = 248.3 pm 0.7 , ({mathrm {stat.}}) pm 13.4 , ({mathrm {syst.}}) pm 4.7 , ({mathrm {lumi.}})~text {pb}, assuming a top-quark mass of 172.5 GeV. The result is in agreement with the Standard Model prediction. The cross-section is also measured in a phase space close to that of the selected data. The fiducial cross-section is sigma _{text {fid}}(tbar{t}) = 48.8 pm 0.1 , ({mathrm {stat.}}) pm 2.0 , ({mathrm {syst.}}) pm 0.9 , ({mathrm {lumi.}})~text {pb} with a precision of 4.5%.
Highlights
The top quark is the most massive known elementary particle
The associated production of an on-shell W boson and a top quark (W t), and single top-quark production in the sand t-channel, were simulated by the Powheg-Box (r2819, r2556) event generator [45,46] with the CT10 parton distribution functions (PDFs) set interfaced to Pythia using the Perugia2011C set of tuned parameters
Systematic uncertainties associated with reconstructed jets, electrons and muons, due to residual differences between data and Monte Carlo (MC) simulations after calibration, and uncertainties in corrective scale factors are propagated through the entire analysis
Summary
The top quark is the most massive known elementary particle. Given that its Yukawa coupling to the Higgs boson is close to unity, it may play a special role in electroweak symmetry breaking [1,2]. The predicted√inclusive ttcross-section at a centre-ofmass energy of s = 8 TeV, assuming a top-quark mass mtop = 172.5 GeV, is σ ( pp → tt) = 253+−1135 pb. The most precise measurement for a centre-of-mass energy of 8 TeV, with a total uncertainty of 3.2%, was performed by the ATLAS Collaboration in the dilepton channel, where both. The most√precise cross-section previously measured in this channel at s = 8 TeV was published by the CMS Collaboration and reached an uncertainty of 6.8% [28]. This analysis supersedes the previous measurement from the ATLAS Collaboration, which achieved a total uncertainty of 9.4% using the same dataset [29]. Two additional software-based trigger levels together reduce the event rate to about 400 Hz on average, depending on the data-taking conditions
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