Abstract
This paper presents single lepton and dilepton kinematic distributions measured in dileptonic tbar{t} events produced in 20.2hbox {fb}^{-1} of sqrt{s}=8 TeV pp collisions recorded by the ATLAS experiment at the LHC. Both absolute and normalised differential cross-sections are measured, using events with an opposite-charge emu pair and one or two b-tagged jets. The cross-sections are measured in a fiducial region corresponding to the detector acceptance for leptons, and are compared to the predictions from a variety of Monte Carlo event generators, as well as fixed-order QCD calculations, exploring the sensitivity of the cross-sections to the gluon parton distribution function. Some of the distributions are also sensitive to the top quark pole mass; a combined fit of NLO fixed-order predictions to all the measured distributions yields a top quark mass value of {m_t^{mathrm {pole}}}=173.2pm 0.9pm 0.8pm 1.2 GeV, where the three uncertainties arise from data statistics, experimental systematics, and theoretical sources.
Highlights
The top quark is the heaviest known fundamental particle, with a mass that is much larger than any of the other quarks, and close to the scale of electroweak symmetry breaking
W t background were assessed by comparing the predictions from the baseline Powheg + Pythia6 sample with those from MC@NLO + Herwig, and from two samples generated with AcerMC + Pythia6 utilising different tunes to vary the amount of additional radiation, in all cases normalising the total production cross-section to the approximate NNLO prediction based on Ref. [67]
Lepton and dilepton differential cross-section distributions have 20.2 bfbe−en1omf epaspurceodlliinsitotns→ateμ√νsνb=bevents selected from 8 TeV recorded by the ATLAS detector at the Large Hadron Collider (LHC)
Summary
The top quark is the heaviest known fundamental particle, with a mass (mt ) that is much larger than any of the other quarks, and close to the scale of electroweak symmetry breaking. Going beyond the inclusive production cross-section, measurements of ttproduction as a function of the top quark and ttsystem kinematics properties allow the predictions of QCD calculations and Monte Carlo event-generator programs to be probed in more detail. [32], and mass measurements are made by comparing the measured distributions to predictions from both NLO plus parton shower event generators and fixed-order QCD calculations The former are similar to traditional measurements where the top quark mass is reconstructed from its decay products [33,34,35,36], but rely only on the leptonic decay products of the ttsystem and are less sensitive to experimental uncertainties related to the hadronic part of the final state.
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