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

Abstract

The tt 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 known 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. Keywords: Hadron-Hadron ScatteringArXiv ePrint: 1407.0891Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. Article funded by SCOAP 3 .doi:10.1007/JHEP01(2015)020 Tables of results with systematic uncertainties 38 The ATLAS collaboration 51 JHEP01(2015)020 IntroductionFinal states of proton-proton (pp) collisions at the Large Hadron Collider (LHC) [1] often include jets arising from QCD bremsstrahlung due to the strongly interacting partons in the initial state and the high centre-of-mass energy of the scattering process that allows for radiation in a large kinematic phase space. In this paper, an inclusive measurement of jets in top-antitop (tt) final states is presented, which is sensitive to the production mechanism -1 - JHEP01(2015)020of additional jets in these events. The events studied have a high partonic-system centreof-mass energy and are complex final states consisting of several coloured partons, with sensitivity to various hard scales. The production of additional jets in tt events is sensitive to higher-order perturbative QCD effects. The uncertainties associated with these processes are a significant source of uncertainty in precision measurements, such as the measurement of the top-quark mass [2] or the inclusive tt production cross-section at the LHC [18]. Several theoretical approaches are available to model tt processes, including NLO QCD calculations, parton-shower models and methods matching fixed-order QCD with the parton shower. The aim of this paper is to test these theoretical approaches by making a direct measurement of jet activity in tt events. Furthermore, tt production with additional jets is a dominant background in certain Higgs boson production processes and decay modes and to many searches for new physics phenomena [3,4].Tests similar to those presented in this paper have been performed at lower energies, using measurements of je...

Measurement of lepton differential distributions and the top quark mass in $$t\bar{t}$$ t t ¯

“…Measurements by ATLAS [18][19][20][21] and CMS [22][23][24] have generally demonstrated good agreement with the predictions of leading-order (LO) multi-leg and next-to-leadingorder (NLO) event generators and calculations, though the top quark p T spectrum is measured to be softer than the predictions by both experiments; this distribution appears to be sensitive to the additional corrections contributing at NNLO [25]. Measurements of jet activity in tt events [26][27][28][29] are also sensitive to gluon radiation and hence the tt production dynamics, without the need to fully reconstruct the kinematics of the tt system. However, all these measurements require sophisticated unfolding procedures to correct for the detector acceptance and resolution.…”

“…The effects of additional radiation in tt events were further studied using two additional Powheg + Pythia6 samples, one using the Perugia 2012 radHi tune [54], with h damp set to 2m t and factorisation and renormalisation scales µ F and µ R reduced from their event generator defaults by a factor of two, giving more parton shower radiation; and one with the Perugia 2012 radLo tune [54], µ F and µ R increased by a factor of two and h damp = m t , giving less parton shower radiation. The parameters of these samples were chosen to span the uncertainties in jet observables measured by ATLAS in tt events at √ s = 7 TeV [26,55,64]. The top quark mass was set to 172.5 GeV in all these samples, consistent with recent measurements by ATLAS [35] and CMS [36].…”

Off-shell top quarks with one jet at the LHC: a comprehensive analysis at NLO QCD

“…+12.06 (+2%) 23) and for the NNPDF3.0 PDF set our integrated cross section are as follows 1.34 Table 6. Integrated cross section for the pp → e + ν e µ −ν µ bbj + X production process at the LHC with √ s = 13 TeV.…”

“…Both the ATLAS and CMS collaborations are carefully examining pp → tt+jets production. The studies performed at the LHC include measurements of jet activity in top-quark events, measurements of tt production with a veto on additional central jet activity and measurements of heavy flavor composition of tt events [21][22][23][24][25][26][27]. For example, the ATLAS experiment has measured using 4.6 fb −1 of data at √ s = 7 TeV the fiducial tt cross section as a function of the light jet multiplicity for up to eight jets with jet p T thresholds of 25, 40, 60, and 80 GeV.…”

Top-Quark Physics at the LHC

“…The ATLAS and CMS collaborations studied the distributions of jet multiplicities and additional jets due to QCD radiation in detail [76][77][78]. The multiplicity distributions of jets for tt events in the single-lepton channel as measured by the CMS collaboration is shown in figure 7.…”