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ATLAS-CONF-2015-049 15September2015

ATLAS NOTE

ATLAS-CONF-2015-049

14th September 2015

Measurements of the t ¯ t production cross-section in the dilepton and lepton-plus-jets channels and of the ratio of the t ¯ t and Z boson

cross-sections in pp collisions at √

s = 13 TeV with the ATLAS detector

The ATLAS Collaboration

Abstract

This note describes measurements of the inclusive top-quark pair production cross-section with a data sample of 85 pb√ −1 of proton-proton collisions at a centre-of-mass energy of s=13 TeV, collected in 2015 by the ATLAS detector. Two measurements are presen- ted, one with an opposite-sign same-flavour lepton pair in the final state and one with a single lepton and at least four jets in the final state. In both measurements at least one jet is required to be identified as originating from ab-quark. In addition, a measurement of the ratio of thet¯tandZ-boson production cross-sections is presented, based on a measurement of thet¯tcross-section in theeµdilepton final state. All results are consistent with theoretical QCD calculations at next-to-next-to-leading order.

c

2015 CERN for the benefit of the ATLAS Collaboration.

Reproduction of this article or parts of it is allowed as specified in the CC-BY-3.0 license.

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1. Introduction

The top quark is the heaviest known fundamental particle, with a mass close to the scale of electroweak symmetry breaking. At the Large Hadron Collider (LHC), top quarks are primarily produced in quark–

antiquark pairs (t¯t), and the prediction of the corresponding inclusive cross-section (σt) is a substantial challenge for QCD calculational techniques. Precise measurements of σtt¯ are sensitive to the gluon parton distribution function (PDF) of the proton, the strong coupling constantαs, the top-quark massmt, and potential enhancements due to physics beyond the Standard Model.

Calculations ofσtat hadron colliders are available at full next-to-next-to-leading order (NNLO) accuracy in QCD, including the resummation of next-to-next-to-leading logarithmic (NNLL) soft gluon terms [1–

5]. At a centre-of-mass energy of √

s= 13 TeV and assumingmt =172.5 GeV, these calculations give a prediction of 832+−4640pb, including PDF,αsand QCD scale uncertainties. This value has been calculated using thetop++ 2.0program [6]. The PDF andαsuncertainties were calculated using the PDF4LHC prescription [7] with the MSTW2008 68 % CL NNLO [8,9], CT10 NNLO [10,11] and NNPDF2.3 5f FFN [12] PDF sets,1and added in quadrature to the scale uncertainties to give a final relative uncertainty of+4.85.5%. The cross-section at √

s = 13 TeV is predicted to be 3.3 times larger than the cross-section at

√s=8 TeV.

Within the Standard Model, the top quark decays almost exclusively to aWboson and abquark, so the final-state topologies int¯tproduction are governed by the decay modes of the twoWbosons. The most precise ATLAS measurements at √

s=7 and 8 TeV were made usingeµevents withb-tagged jets [13] and the same final state was used for the first ATLAS measurement ofσtat √

s=13 TeV [14]. This document describes two measurements ofσt that are complementary to the measurement presented in Ref. [14].

The first measurement is done in the same-flavour dilepton channel,t¯t→W+bWb¯ →`+`ννbb, where¯

`=e, µ, and the second measurement exploits the lepton-plus-jets channel,t¯t→W+bWb¯ →`+νqq¯0bb¯2. The dilepton measurement selects events with an opposite-sign (OS) same-flavour lepton pair (eeorµµ), and one or two hadronic jets from theb-quarks. The lepton-plus-jets measurement selects events with exactly one lepton and at least four hadronic jets, one of which is identified as originating from a b- quark. Jets originating fromb-quarks are identified (‘tagged’) using ab-tagging algorithm exploiting the long lifetime, high decay multiplicity, hard fragmentation and large mass ofbhadrons [15]. Events with electrons or muons produced via leptonically decaying tau leptons,t→W+b→τ+νb→e++νννb, are¯ included as part of thet¯tsignal in all channels.

In the dilepton analysis, the rates of events with a pair of leptons and one or twob-tagged jets are used to measure simultaneously thet¯t production cross-section and the combined probability to reconstruct andb-tag a jet from a top-quark decay. The main sources of background are Z+jets production and the associated production of aW boson and a single top quark (Wt). Other background contributions arise from diboson production and events where one reconstructed lepton is due to misidentification of a photon or jet (referred to as ‘fake’) or is from a non-prompt (NP) decay. Simulation is used to model all background processes, but the normalisation ofZ+jets production is obtained using data in a control region.

1The values ofαsused are 0.1171±0.0014 for MSTW2008 and 0.1180±0.0012 for the CT10 and NNPDF2.3 PDF sets.

2Charge-conjugate modes are implied throughout.

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In the lepton-plus-jets analysis, the main backgrounds are from the production of aW boson with ad- ditional jets and from non-prompt and fake leptons. The backgrounds are subtracted from the observed number of data events and the t¯tcross-section is extracted using the selection efficiency derived from simulated events.

A measurement of the ratio between thet¯tandZ-boson production cross-sections is also presented, based on the previous measurement of thet¯tcross-section in theeµfinal state [14] and of theZ-boson cross- section in theeeandµµfinal states [16]. The measurement of the t¯tcross-section in theeµfinal state is used to calculate the ratio since it is the most precise ATLAS measurement of thet¯tcross-section at

√s = 13 TeV. This ratio benefits from significant cancellations in systematic uncertainties between the two cross-section measurements, and is sensitive to ratios of the gluon and sea-quark PDFs [17].

The data and Monte Carlo simulation samples used are described in Section2, followed by the object selection in Section 3 and the event selection in the two channels in Section 4. The extraction of the t¯tcross-section is discussed in Section5for the same-flavour dilepton channel and in Section6for the lepton-plus-jets channel. Systematic uncertainties and the results of both measurements are discussed in Section7. The measurement of thet¯t/Zcross-section ratio is presented in Section8. Finally, conclusions are drawn in Section9.

2. Data and simulation samples

The ATLAS detector [18] at the LHC covers nearly the entire solid angle around the collision point, and consists of an inner tracking detector surrounded by a thin superconducting solenoid magnet producing a 2 T axial magnetic field, electromagnetic and hadronic calorimeters, and an external muon spectrometer incorporating three large toroid magnet assemblies. The inner detector consists of a high-granularity silicon pixel detector, including the newly-installed insertable B-layer (IBL) [19], and a silicon microstrip tracker, together providing precision tracking in the pseudorapidity3range|η|< 2.5, complemented by a transition radiation tracker providing tracking and electron identification information for|η|<2.0. A lead liquid-argon (LAr) electromagnetic calorimeter covers the region|η| < 3.2, and hadronic calorimetry is provided by steel/scintillating tile calorimeters for|η|<1.7 and copper/LAr hadronic endcap calorimeters.

The forward region is covered by additional LAr calorimeters with copper and tungsten absorbers. The muon spectrometer consists of precision tracking chambers covering the region|η| < 2.7, and separate trigger chambers covering|η| < 2.4. A two-level trigger system, using custom hardware followed by a software-based level, is used to reduce the event rate to a maximum of around 1 kHz for offline storage.

The analysis is performed using data collected by the ATLAS detector between 13th June and 16th July during the early 2015 pp collision run at √

s = 13 TeV with 50 ns proton bunch spacing. The data correspond to an integrated luminosity of 85 pb−1after requiring that all detector subsystems were fully operational. Events are required to pass either a single electron or single muon trigger, with thresholds set to be almost fully efficient for leptons with pT >25 GeV passing offline selections. Each triggered event

3ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector, and thezaxis along the beam line. Pseudorapidity is defined in terms of the polar angleθasη=ln tanθ/2, and transverse momentum and energy are defined relative to the beamline aspT=psinθandET=Esinθ. The azimuthal angle around the beam line is denoted byφ, and distances in (η, φ) space byR= p

∆η2+ ∆φ2.

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also includes the signals from on average about 19 additional inelastic ppcollisions in the same bunch crossing (pileup).

Monte Carlo simulated event samples are used in this analysis, to compare to the data and to evaluate signal and background efficiencies and uncertainties. Samples were processed either through the full ATLAS detector simulation [20] based on GEANT4 [21], or through a faster simulation making use of parameterised showers in the calorimeters [22]. Additional simulated pp collisions generated with Pythia8.186 [23] were overlaid to model the effects of both in- and out-of-time pileup, from additional ppcollisions in the same and nearby bunch crossings. All simulated events were processed using the same reconstruction algorithms and analysis chain as the data. Small corrections were applied to lepton trigger and reconstruction efficiencies to better model the response observed in data.

The baselinet¯tfull simulation sample was produced using the next-to-leading-order (NLO) matrix ele- ment generator Powheg-Boxv2 [24–26] using the CT10 set of parton distribution functions [10], and in- terfaced to Pythia6.428 [27] for the parton showering and fragmentation. The Perugia 2012 (P2012) [28]

parameter set (tune) with the CTEQ6L PDF set [29] was used for the underlying event (UE) description.

Allt¯t final states involving at least one lepton were included. The EvtGen [30] package was used to model the decays of heavy flavour hadrons. The Powheg model parameterhdamp, which controls mat- rix element to parton shower matching in Powhegand effectively regulates the high-pT radiation, was set to the top-quark mass, 172.5 GeV, a setting which was found to best describe the t¯t system pT at

√s = 7 TeV [31]. The Standard Model expectation of 0.1082 was assumed for theW → `νbranching ratio [32]. Alternativet¯tsimulation samples were generated using Powheginterfaced to Herwig++[33]

and MadGraph5_aMC@NLO [34] interfaced to Herwig++. The effects of initial- and final-state radi- ation (ISR/FSR) were explored using two alternative Powheg +Pythia6 samples, one withhdampset to 2mt, the renormalisation and factorisation scales set to half the nominal value and using the Perugia 2012 radHi UE tune, giving more radiation, and one with the Perugia 2012 radLo UE tune,hdamp=mt and the renormalisation and factorisation scales set to twice the nominal value, giving less radiation [35]. These alternativet¯tsamples were produced using fast simulation and compared to a fast simulation version of the baseline sample. The top-quark mass was set to 172.5 GeV in all the simulation samples.

Events originating fromW+jets production, in which theWboson decays to a charged lepton and neut- rino, were generated using Sherpa2.1.1 [36]. Matrix elements were calculated for up to two additional partons at NLO and four additional partons at LO using the Comix[37] and OpenLoops [38] matrix ele- ment generators and merged with the Sherpaparton shower [39] using the ME+PS@NLO prescription [40]. The CT10 PDF set was used in conjunction with a dedicated parton shower tuning developed by the Sherpaauthors. For the dilepton analysis, inclusiveZ+jets production, where theZboson decays into leptons, was modelled using Powhegwith CT10 PDFs, interfaced to Pythia8.186 and the AZNLO UE tune [41] with CTEQ6L1 PDFs. For the lepton-plus-jets analysis, theZ+jets background was modelled using Sherpasamples, which were generated in an analogous way to theW+jets samples.

Diboson production in association with jets was modelled using Sherpa2.1.1 and CT10 PDFs, including matrix elements calculated for up to one (``ννand````) or zero (```ν) additional partons at NLO and up to three additional partons at leading order using the Comix and OpenLoops matrix element generators, merged with the Sherpa parton shower using the ME+PS@NLO prescription. Single top Wt and t- channel production were modelled using Powheg-Box v2+Pythia6 using the CT10 PDFs and the P2012

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UE tune. In order to remove the overlap with t¯t production, the Wt sample was produced using the

‘diagram removal’ generation scheme [42].

3. Object selection

The analyses make use of reconstructed electrons, muons, jets and the presence of missing transverse momentum. Object reconstruction and selections largely follow those used in theeµanalysis [14], in particular the same kinematic cuts are used for electrons, jets and muons.

Electron candidatesare reconstructed from an isolated electromagnetic calorimeter energy deposit match- ed to an inner detector track, within the fiducial region of transverse energyET > 25 GeV and pseu- dorapidity|η| <2.47. Candidates within the transition region between the barrel and endcap electromag- netic calorimeters, 1.37 < |η| < 1.52, are removed. In the dilepton analysis a medium likelihood-based requirement [43] is used to reduce the background from misidentified and non-prompt leptons, while keeping high efficiency. In the lepton-plus-jets analysis, a tight likelihood-based requirement is used to further suppress the background from multijet production. Electrons are required to be isolated using requirements on the energy of calorimeter topological clusters in a cone of∆R<0.2 around the electron (excluding the deposit associated to the electron) divided by the electronpT, and on the sum of track pT in a variable-sized cone around the electron direction (again excluding the track associated to the elec- tron). The track isolation cone radius is given by the smaller of∆R= 10 GeV/pT(e), where pT(e) is the pT of the electron, and∆R= 0.2,i.e.a cone which increases in size at low pT up to a maximum of 0.2 radians. Selection criteria, dependent onpTandη, are designed to produce a nominal efficiency of 90 % for electrons fromZ →eedecays with pT of 25 GeV which rises to 99 % at 60 GeV. The efficiencies in t¯tevents are somewhat smaller, due to the increased jet activity. To prevent double-counting of electron energy deposits as jets, the closest jet within∆R< 0.2 of a reconstructed electron is removed. Finally, if the nearest jet surviving the selection described below is within∆R< 0.4 of the electron, the electron is discarded, to ensure it is sufficiently separated from nearby jet activity.

Muon candidatesare reconstructed by combining matching tracks reconstructed in both the inner de- tector and muon spectrometer, and required to satisfy pT > 25 GeV and |η| < 2.5. Muons are also required to be isolated, using the same variables as for electrons, with the selection criteria tuned to give similar efficiencies onZ →µµevents. To reduce the background from muons from heavy flavour decays inside jets, muons are removed if they are separated from the nearest jet by∆R < 0.4. However, if this jet has fewer than three associated tracks, the muon is kept and the jet is removed instead; this avoids an inefficiency for high-energy muons undergoing significant energy loss in the calorimeter.

Jetsare reconstructed using the anti-kt algorithm [44,45] with radius parameterR = 0.4, starting from topological clusters in the calorimeters [46]. The effects of pileup on jet energies are accounted for by a jet-area-based correction [47] and the resolution of the jets is improved by using global sequential corrections [48]. Jets are then calibrated to the hadronic energy scale usingE- andη-dependent calibration factors based on MC simulations, with in-situ corrections based on Run 1 data [49,50] and checked with early Run 2 data [51]. Corrections for semileptonicb-hadron decays are not applied. Jets are accepted within the fiducial region pT > 25 GeV and|η| < 2.5. To reduce the contribution from jets associated with pileup, jets with pT < 50 GeV and |η| < 2.4 are required to pass pileup rejection criteria [52].

Reconstructed jets within∆R<0.2 of a selected electron are removed, as discussed earlier.

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Jets areb-tagged as likely to containbhadrons using the MV2c20 algorithm, a multivariate discriminant making use of the long lifetime, high decay multiplicity, hard fragmentation and high mass of bhad- rons [15, 53,54]. Jets are defined as beingb-tagged if the MV2c20 weight is larger than a cut value corresponding to approximately 70%b-tagging efficiency forb-jets int¯tevents, although the exact effi- ciency varies with pT. In simulation, the tagging algorithm gives a rejection factor of about 440 against light-quark and gluon jets, and about eight against jets originating from charm quarks.

The missing transverse momentum (Emiss

T )is defined as the negative of the global vector sumpTof all selected physics objects (electrons, muons, jets) as well as specific “soft terms” accounting for unclassi- fied soft tracks and calorimeter clusters. In this way, the missing transverse momentum is adjusted to take into account the best calibration of the identified physics objects above [55].

4. Event selection

4.1. Selection of same-flavour dilepton events

Events are required to have either exactly two muons or two electrons, which must be of opposite electric charge. TheZ+jets background is suppressed by requiring that the invariant mass of the two leptons is not in the range 81<m``<101 GeV and the missing transverse momentum is above 30 GeV. In addition, m`` is required to be above 60 GeV. This requirement removes low mass dilepton resonances and avoids the need to simulateZ+jets events with low invariant mass. Events passing these selections define the same-flavour dilepton preselected sample. Events are then further classified into those with exactly one or exactly twob-tagged jets.

4.2. Selection of lepton-plus-jets events

Events are required to have exactly one electron or muon and at least four jets. To suppress theW+jets background, at least one of the jets is required to beb-tagged. The non-prompt and fake-lepton back- ground is suppressed by applying requirements on the missing transverse momentum and the transverse mass,mWT,4withEmissT > 40 GeV ormWT >50 GeV in the electron channel, and EmissT +mWT > 60 GeV in the muon channel.

5. Extraction of the t ¯ t cross-section from the dilepton events

The t¯t production cross-section is determined in a similar way to theeµanalysis [13, 14] by counting the numbers of opposite-signeeandµµevents with exactly one (N1ee, N1µµ) and exactly two (Nee2 , N2µµ) b-tagged jets, ignoring any jets that are not b-tagged which may be present, due e.g.to light-quark or

4The transverse mass is defined asmWT = q

2p`TEmissT (1cos∆φ), where∆φis the azimuthal angular difference between the lepton and the missing transverse momentum.

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gluon jets from QCD radiation orb-jets from top quark decays which are notb-tagged. The four event counts can be expressed as:

N1ee = Lσt preselee 2bee(1−Cbeebee)+N1bkg,ee N2ee = Lσt preselee Cbeebeebee+N2bkg,ee

N1µµ = Lσt preselµµ 2bµµ(1−Cµµb bµµ)+N1bkg,µµ

N2µµ = Lσt preselµµ Cbµµbµµbµµ+N2bkg,µµ, (1) whereLis the integrated luminosity of the sample andpreselee andpreselµµ are the efficiencies for at¯tevent to pass the opposite-signeeandµµpreselection requirements. The combined probability for a jet from the quarkqin at →W+qdecay to fall within the acceptance of the detector, be reconstructed as a jet with transverse momentum above the selection threshold, and be tagged as ab-jet, is denoted byb``. Although this quark is almost always ab-quark,b`` also accounts for the approximately 0.2 % of top quarks that decay toW sorWd rather thanWb, slightly reducing the effective tagging efficiency. Due to the small kinematic differences between theeeandµµsamples introduced by the different selection efficiencies, theb`` values are slightly different between the two channels. If the decays of the two top quarks and the subsequent reconstruction of the twob-tagged jets are completely independent, the probability to tag bothb-jetsbb`` is given bybb`` =(b``)2. In practice, small correlations are present for both kinematic and instrumental reasons, and these are taken into account via the tagging correlation coefficientsC``b, defined asC``b = ``bb/(b``)2, which are also determined separately for theeeandµµchannels. These correlation terms also account for the effect on N1`` andN2`` of the small number of mistagged light quark or gluon jets from radiation in thet¯t events. Background events from sources other than t¯t → `+`ννbb¯ also contribute to the event countsN1``andN2``, and are included by the background termsN1bkg,`` andN2bkg,``. The preselection efficiencies, presel`` , and tagging correlations, C``b, are taken from t¯t event simulation.

Compared to theeµanalysis [14], it is important to note that the efficiency of the additional requirements onETmissandmllare included inpresel`` . The measurement in the same-flavour channels is therefore more sensitive to systematic uncertainties that affect the EmissT , in particular the jet energy scale, as discussed in Section7. In addition, the values ofb``andC``b are sensitive to the additional kinematic requirements and hence are different from the values in theeµanalysis. Int¯tsimulation, the value ofb``is about 0.532 (0.535) in theee(µµ) channel. The background contributionsN1bkg,`` and N2bkg,`` are estimated using a combination of simulation and data-based methods, allowing the four equations (1) to be solved for the three unknown parametersσt,beeandbµµ. The equations are solved using a maximum likelihood fit.

A total of 678 (1021) data events pass theee(µµ) opposite-sign preselection. Table1shows the numbers of events with one or twob-tagged jets, together with the estimates of non-t¯tbackground processes and their systematic uncertainties, which are discussed in detail below. The entry ‘NP and fakes’ refers to events where at least one reconstructed lepton is from a non-prompt decay or is due to misidentification of jets or photons. The sample with one b-tagged jet is expected to be about 80% pure in t¯t events in both channels, estimated from simulation, with the dominant background coming from Z+jets and Wtproduction, and smaller contributions from diboson production and events with non-prompt or fake leptons. The sample with twob-tagged jets is expected to be about 93% pure int¯tevents in theeechannel and 96% pure in theµµchannel, withZ+jets andWtproduction being the dominant backgrounds.

The distribution of the number ofb-tagged jets in opposite-sign same-flavour events is shown in Figure1, and compared to the baselinet¯t and background simulation samples, normalised using the theoretical

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Sample N1ee N2ee N1µµ N2µµ

t¯t 84±12 49±18 100±13 58±21

Z(→``)+jets 9.9±2.3 0.6±0.7 18±6 2.5±2.0

Z(→ττ→``νννν)+jets 0.14±0.11 <0.01 0.11±0.12 0.02±0.05

Diboson 0.5±0.4 0.02±0.06 0.8±0.6 0.07±0.08

NP & fakes 2.4±0.5 1.1±0.4 0.27±0.23 0.08±0.16 Single top 8.7±1.6 1.8±0.9 10.3±1.6 2.0±0.9 Total background 21.6±2.8 3.4±1.8 29.4±3.0 4.6±1.8

Total expected 105±12 52±18 129±14 62±21

Observed 103 59 108 65

Table 1: Observed numbers of opposite-signeeandµµevents with one and twob-tagged jets (N1andN2), together with the estimates oftt¯events and non-tt¯backgrounds and the associated uncertainties due to systematic effects, including the limited size of the simulation samples. Uncertainties due to the PDFs are not included.

b-tagged jet multiplicity

0 1 2 3

Events

1 10 102

103

104

105

106

Data t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary

= 13 TeV, 85 pb-1

s e-

e+

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b-tagged jet multiplicity

0 1 2 3

Events

1 10 102

103

104

105

106

Data t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary

= 13 TeV, 85 pb-1

s µ-

µ+

(b)

Figure 1: The number ofb-tagged jets in preselected opposite-sign(a)eeand(b)µµevents. The data are shown compared to the expectation, broken down into contributions fromtt¯(using the baseline Powheg+Pythia6 sample), Wtsingle top,Z+jets, dibosons, and events with non-prompt (NP) or fake electrons or muons, normalised to the recorded integrated luminosity.

prediction of 832 pb for the t¯t cross-section at √

s = 13 TeV. Distributions of the electron and muon pT andη, the jetpT, the number of jets, the missing transverse momentum and dilepton invariant mass are shown for opposite-sign`` events with at least oneb-tagged jet in Figure2for the eechannel and Figure3for theµµchannel. The agreement between data and simulation is good.

The values ofpresel`` are determined from simulation to be about 0.46% and 0.52% in theeeandµµchan- nels, respectively, where the efficiencies include the branching ratios from inclusivet¯tproduction. The

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[GeV]

Electron pT

0 50 100 150 200 250

Events / 25 GeV

0 20 40 60 80 100 120 140 160 180 200

Data t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-,

+e e

(a)

η Electron

2 1 0 1 2

Events / 0.5

0 20 40 60 80 100

120 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-,

+e e

(b)

[GeV]

Jet pT

0 50 100 150 200 250

Jets / 25 GeV

0 50 100 150 200 250

300 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-,

+e e

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Jet multiplicity

1 2 3 4 5

Events

0 20 40 60 80

100 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-,

+e e

(d)

[GeV]

miss

ET

0 50 100 150 200 250

Events / 10 GeV

0 5 10 15 20 25 30 35

40 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

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-,

+e e

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[GeV]

mee

100 200 300

Events / 20 GeV

0 10 20 30 40 50 60

Data t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-,

+e e

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Figure 2: Distributions of the (a)pTof the electrons, (b)ηof the electrons, (c)pTof the jets, (d) jet multiplicity, (e) missing transverse momentumEmissT and (f) invariant mass of the dilepton system, in events with an opposite-sign eepair and at least oneb-tagged jet. The data are compared to the expectation. The last bin in each histogram includes the overflow.

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[GeV]

Muon pT

0 50 100 150 200 250

Events / 25 GeV

0 50 100 150 200

250 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-, µ µ+

(a)

η Muon

2 1 0 1 2

Events / 0.5

0 20 40 60 80 100

Data t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-, µ µ+

(b)

[GeV]

Jet pT

0 50 100 150 200 250

Jets / 25 GeV

0 50 100 150 200 250

300 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-, µ µ+

(c)

Jet multiplicity

1 2 3 4 5

Events

0 20 40 60 80 100

120 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

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-, µ µ+

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[GeV]

miss

ET

0 50 100 150 200 250

Events / 10 GeV

0 10 20 30 40

50 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

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-, µ µ+

(e)

[GeV]

µ

mµ

100 200 300

Events / 20 GeV

0 10 20 30 40 50 60 70

80 Data

t t Single top Z+jets Diboson NP & fakes

ATLAS Preliminary = 13 TeV, 85 pb-1

s

1 b-tag

-, µ µ+

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Figure 3: Distributions of the (a)pT of the muons, (b)ηof the muons, (c) pT of the jets, (d) jet multiplicity, (e) missing transverse momentumEmissT and (f) invariant mass of the dilepton system, in events with an opposite-sign µµpair and at least oneb-tagged jet. The data are compared to the expectation. The last bin in each histogram includes the overflow.

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uncertainties onpresel`` translate directly into uncertainties onσt. Similarly,σtis directly sensitive to the values ofCb`` which are determined from simulation to be about 1.012 and 1.002 in theeeandµµchan- nels, respectively.

5.1. Background estimation

TheWt single top background is normalised to the approximate NNLO cross-section of 71.7±3.8 pb, determined as in Ref. [56]. The diboson background is normalised to the NLO cross-section provided by the generator. TheZ+jets background, in which the Z boson decays to either an electron-positron pair or a muon-anti–muon pair, is first normalised to the NNLO prediction [57] and is subsequently normalized using aZ+jets control region. This region is selected by using the same requirements as the pre-selection, but requiring that the invariant mass of the dilepton pair lies within 10 GeV of theZboson mass. This sample is dominated byZ+jets events. TheZ+jets estimation in the nominal pre-selection is then corrected by a factor, fi, derived using the observed data in the Z-boson dominated control region, as a function of the number ofb-tagged jets, compared to the expectation from simulation:

fi = Ndatai −Nbkgi

NZi MC , (2)

whereNdatai is the number of data events in the control region withi b-tagged jets,Nbkgi is the number of events from other processes expected in the control region withi b-tagged jets (mainlyt¯t,Wtand diboson events) andNZi MCis the number of events predicted by theZ MC sample in the control region withi b- tagged jets. The resultant scale factors, fi, are shown in Table2. TheZ+jets background, in which theZ decays to a tau-anti–tau pair, is normalised to the NNLO prediction [57] but no correction based on the data control region is applied. The background originating from events where at least one reconstructed lepton originates from a hadronic decay or is due to misidentification in the detector is estimated using simulation and is dominated byt¯tevents in the lepton-plus-jets decay mode.

Sample ee µµ

0-Tag 1-Tag 2-Tag 0-Tag 1-Tag 2-Tag

Z+jets 4323.3±7.9 81.3±1.2 3.5±0.3 6892.3±10.5 117.2±1.5 4.9±0.3 non-Zbackground 124.0±1.2 22.2±0.3 10.8±0.2 55.2±0.6 25.2±0.3 13.2±0.2

Data 3697 116 18 5622 175 28

Scale factor 0.82±0.01 1.15±0.14 2.04±1.22 0.81±0.01 1.28±0.12 3.02±1.09

Table 2: Selected events in theZboson dominated control region as a function of the number ofb-tagged jets. The ratios of observed to expected events in this control region are used as scale factors to correct for the mis-modelling of theEmissT andb-tagged jet multiplicity of theZboson Monte Carlo in the pre-selection region.

6. Extraction of the t ¯ t cross-section from the lepton-plus-jets events

The t¯tproduction cross-section is determined by counting the number of events after the selection re- quirements, subtracting the expected number of background events and dividing by the product of the

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Sample e+jets µ+jets

t¯t 2800±400 2620±340

W+jets 340±100 230±60

Single top 192±34 180±30

Z+jets 71±35 45±22

Dibosons 10±5 10±5

Fakes 200±70 130±60

Total background 820±130 600±100 Total expected 3600±500 3220±350

Observed 3439 3314

Table 3: Observed numbers of events in thee+jets andµ+jets channels. The contributions expected from the background processes and the expected number oftt¯events are also shown along with the associated uncertainties due to systematic effects.

efficiency times acceptance to select events,`j, and the integrated luminosity. The two lepton-plus-jets channels are combined together at reconstruction level before performing the extraction of the cross- section. The selection efficiency is extracted from MC simulation and the background is estimated as discussed in Section6.1.

A total of 3439 (3314) data events are selected in thee+jets (µ+jets) channels, respectively. Table3 shows the expected numbers of signal and background events in the two channels. The two samples are expected to be about 80 % pure int¯tevents. Figure4shows the lepton pT, leptonη, the transverse mass and the jetpTfor the selected events. Good agreement is seen between the data and the sum of signal and expected backgrounds.

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[GeV]

Lepton pT

0 50 100 150 200

Events / 10 GeV

0 200 400 600 800 1000 1200 1400 1600 1800

2000 Data

t t Single top Diboson W+jets Z+jets NP & fakes

ATLAS Preliminary

= 13 TeV, 85 pb-1

s l+jets

(a)

η Lepton

2 1 0 1 2

Events / 0.25

0 200 400 600 800 1000 1200

1400 Data

t t Single top Diboson W+jets Z+jets NP & fakes

ATLAS Preliminary

= 13 TeV, 85 pb-1

s l+jets

(b)

[GeV]

W

mT

0 50 100 150 200

Events / 10 GeV

0 200 400 600 800 1000 1200

1400 Data

t t Single top Diboson W+jets Z+jets NP & fakes

ATLAS Preliminary

= 13 TeV, 85 pb-1

s l+jets

(c)

[GeV]

Jet pT

0 50 100 150 200

Jets / 10 GeV

0 2 4 6 8 10

103

×

Data t t Single top Diboson W+jets Z+jets NP & fakes

ATLAS Preliminary

= 13 TeV, 85 pb-1

s l+jets

(d)

Figure 4: Distributions of(a)lepton pT,(b)lepton pseudorapidity,(c)transverse mass of theW boson and(d)jet pT, for all selected jets in the event for both thee+jets andµ+jets channels combined.

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6.1. Background estimation

The number of background events in the lepton-plus-jets channels are estimated using both simulation and data. The largest background originates from the production of aW boson in association with jets.

The shape of this background is estimated using simulation, while the normalisation of the background is estimated by exploiting the expected asymmetry of the charge of leptons originating fromW boson decays. The number ofWboson events in the signal region,N≥1bW,DD, is calculated as:

NW,DD1b = N0bW,DD N0bW,MC

·NW,MC1b (3)

whereN0bW,MCandN≥1bW,MCare the number ofW+jets events with zero and at least oneb-tagged jet predicted by simulation, and

N0bW,DD =

Ndata+ −Nbkg+

Ndata −Nbkg AW

, (4)

whereNdata± are the number of selected events with positive and negative charge,Nbkg± are the number of events expected from single top processes (which are not charge symmetric) and AW is the asymmetry predicted in theW+jets simulation,

AW = NMC+ −NMC

NMC+ +NMC . (5)

The backgrounds from single-top production in the t- and Wt- channels are estimated using the MC samples described in Section 2. Single-top production in the s-channel is expected to be very small compared to the other backgrounds and is neglected in this analysis.

Non-prompt and fake leptons contribute a significant background to the analysis. Such events can pass the selection requirements when either a jet is misidentified as a lepton or a lepton from a hadronic decay fulfills the lepton requirements. This background is estimated using the matrix method [58], where the lepton requirements are relaxed to create a loose sample. The efficiencies for loose leptons to pass the standard (tight) lepton criteria are measured in control samples and then the number of QCD multijet events in the signal region is extracted. In thee+jets channel the efficiencies are parameterised in terms of the difference in azimuthal angle between the electron andEmissT and the number ofb-tagged jets, while in theµ+jets channel the efficiencies are parameterised in terms ofETmissand the difference in azimuthal angle between the muon andEmissT .

7. Systematic uncertainties and results

For each source of uncertainty, the impact on the relevant input parameters (efficiencies, background es- timates and luminosity) is determined and the cross-section extractions are repeated with these parameters simultaneously varied. Systematic correlations between input parameters are thus taken into account. The total uncertainties onσtt¯are calculated by adding the effects of all the individual systematic components in quadrature, assuming them to be independent. The systematic uncertainties on the extracted cross- sectionσt in the dilepton channels are shown in Table4. The systematic uncertainties on the measured cross-section in the lepton-plus-jets channel are shown in Table5. The sources of systematic uncertainty

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Uncertainty ∆σtt¯tt¯(%)

Data statistics 7.6

t¯tNLO modelling 2.6

t¯thadronisation 7.9

Initial/final state radiation 1.5

PDF 3.7

Single-topWtcross-section 0.6 Single-top interference <0.05

Diboson cross-section 0.4

Z+jets→ee/µµmodelling 1.5 Z+jets→ττmodelling 0.1

Electron energy scale 0.3

Electron energy resolution 0.2 Electron identification 3.6

Electron trigger 0.2

Electron isolation 1.0

Muon momentum scale 0.1

Muon momentum resolution 1.1

Muon identification 0.8

Muon trigger 0.6

Muon isolation 1.0

Jet energy scale 1.2

Jet energy resolution 0.2

b-tagging efficiency 0.8 Missing transverse momentum 0.3

NP & fakes 1.5

Analysis systematics 11

Integrated luminosity 10

Total uncertainty 16

Table 4: Summary of the statistical, systematic and total uncertainties on thetproduction cross-sectionσtin the same flavour dilepton channel.

that affect the measurements are discussed in detail below, starting with those common to the two analyses and then the uncertainties specific to each channel.

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Uncertainty ∆σtt (%)

Data statistics 1.5

t¯tNLO modelling 0.6

t¯thadronisation 4.1

Initial/final state radiation 1.9

PDF 0.7

Single top cross-section 0.3

Diboson cross-sections 0.2

Z+jets cross-section 1.0

W+jets method statistics 1.7

W+jets modelling 1.0

Electron energy scale/resolution 0.1

Electron identification 2.1

Electron isolation 0.4

Electron trigger 2.8

Muon momentum scale/resolution 0.1

Muon identification 0.2

Muon isolation 0.3

Muon trigger 1.2

EmissT scale/resolution 0.4

Jet energy scale +−810

Jet energy resolution 0.6

b-tagging 4.1

NP & fakes 1.8

Analysis systematics +−1113 Integrated luminosity +−911

Total uncertainty +17−14

Table 5: Summary of the statistical, systematic and total uncertainties on thetproduction cross-sectionσtmeas- ured in the lepton-plus-jets channel.

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Systematic uncertainties common to both analyses:

tt¯modelling: The modelling uncertainties due to the choice oft¯tgenerator are assessed by com- paring the predictions of the baseline Powheg +Pythia6 sample with the various alternative samples discussed in Section2, all processed using fast simulation. Three separate uncer- tainties are considered: the NLO generator uncertainty (evaluated by considering the relative difference betweenaMC@NLO+Herwig++and Powheg +Herwig++), the shower and had- ronisation uncertainty (evaluated by considering the relative difference between Powheg +Pythia6 and Powheg +Herwig++), and the radiation uncertainty (evaluated by considering half the re- lative difference between the Powheg +Pythia6 samples with more or less radiation).

Parton distribution functions: The uncertainty due to limited knowledge of the proton PDFs are evaluated by reweighting simulated events produced with MadGraph5_aMC@NLO using the error sets of the CT14 [59], MMHT 2014 68 % CL NLO [60] and NNPDF 3.0 PDF [61]

PDF sets. The final uncertainty is calculated as half the envelope encompassing the predic- tions of all three PDF sets along with their associated uncertainties, following the PDF4LHC recommendations [7].

Lepton-related uncertainties: The modelling of the electron and muon trigger efficiencies, iden- tification efficiencies, energy scales and resolutions are studied usingZ → eeandZ → µµ decays in data and simulation using √

s = 13 TeV data. Small corrections are applied to the simulation to better model the performance seen in data [62,63]. These corrections have as- sociated uncertainties that are propagated to the cross-section measurements. The modelling of the isolation requirements made on electrons and muons is studied in √

s = 13 TeV data usingZ decays, and parameterised as functions of the lepton pT, η and the hadronic activ- ity near the lepton. The isolation efficiencies are found to be generally well modelled, and the measurements are extrapolated to thet¯tenvironment to give uncertainties of 1% for both electrons and muons, limited by the size of the availableZ →``samples in data.

Jet-related uncertainties: The lepton-plus-jets channel is sensitive to uncertainties in the jet en- ergy scale and energy resolution since the efficiency fort¯tevents to pass the four jet require- ment is taken from simulation. In the dilepton channel the efficiency to reconstruct andb-tag jets fromt¯tevents is extracted from the data, however jet uncertainties impact the measure- ment viapresel`` primarily due to theETmissrequirement. The jet uncertainties also have a small impact on the tagging correlationC``b. The jet energy scale is varied in simulation according to the uncertainties derived from the √

s = 8 TeV simulation and in-situ calibration, and the uncertainties are extrapolated to √

s = 13 TeV [51]. The uncertainties are evaluated using a simplified model with three orthogonal components which are then added in quadrature.

The jet energy resolution uncertainty is assessed using √

s =8 TeV data, and extrapolated to

√s=13 TeV.

b-tagging uncertainties: In the lepton-plus-jets channel the selection efficiency depends on the efficiency of theb-tagging algorithm predicted by the simulation. In the dilepton channel, the correlationCb``depends weakly on theb-tagging and mistagging efficiencies predicted by the simulation, as it is evaluated from the numbers of events with one and twob-tagged jets. The uncertainties are determined from √

s = 8 TeV data, with some additional uncertainties to account for the presence of the new IBL detector and the extrapolation to √

s=13 TeV. Since

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the definition ofpresel`` does not involveb-tagged jets, it has nob-tagging or mistagging-related uncertainties.

Integrated luminosity: The uncertainty on the integrated luminosity is 9 %. It is derived, follow- ing a methodology similar to that detailed in Ref. [64], from a preliminary calibration of the luminosity scale using a pair ofx-ybeam-separation scans performed in June 2015. The effect on the cross-section measurement is slightly larger than 9 % because theWtsingle top and diboson backgrounds are evaluated from simulation, so they are also sensitive to the assumed integrated luminosity.

Uncertainties on backgrounds in the dilepton channel:

Background cross-sections: The uncertainties on theWt andt-channel single top cross-sections are taken to be 5.3%. The uncertainty on the diboson cross-sections are taken to be 10 %, based on the corresponding theoretical predictions.

Diboson and modelling: A 50 % uncertainty is assigned to the modelling of the number of b- tagged jets in diboson events to account for possible mismodelling of the heavy-flavour frac- tion.

Single topW t/tt¯interference: The uncertainty on the procedure that is used to remove higher order diagrams interfering witht¯tis estimated by comparing MC samples generated using a diagram subtraction scheme and a diagram removal scheme.

Z+jets modelling: The uncertainty of the modelling of theZ+jets background is estimating by comparing the shape of the number of b-tagged jets distribution in the Z+jets simulation between theZ+jets control region and the signal region. This difference is added in quadrature with the statistical uncertainty of the data in the control region.

Non-prompt and fake leptons: The uncertainty on the non-prompt and fake lepton background is taken to be 100 % of the prediction in the simulation based on studies in the previous dilepton analysis that showed that the simulation models this background reasonably well [14].

Uncertainties on backgrounds in the lepton-plus-jets channel:

W+jets modelling: The estimate of theW+jets background in the lepton-plus-jets channel relies on simulation to estimate the charge asymmetry and the ratio of events with at least oneb- tagged jet to events with zerob-tagged jets. An uncertainty on each of these parameters is estimated by taking the difference between the W+jets prediction using the default Sherpa samples discussed in Section 2 and alternative samples generated using Powheg+Pythia8.

This uncertainty already modifies the modelling of heavy flavour jets, but it was checked that adding an additional dedicated uncertainty for the fraction ofW+jets that contain heavy flavour jets does not significantly change the final uncertainty.

Non-prompt and fake lepton background modelling: The non-prompt and fake lepton background shape and normalisation is estimated using a data-driven method. An uncertainty is assigned by varying the parameterisation of the efficiencies used in the matrix method. The maximum difference between the different varied background estimates and the nominal background is used as the uncertainty.

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Single top modelling: Uncertainties on single top modelling are evaluated using alternative sim- ulation samples. An uncertainty due to radiation in thet-channel process is evaluated by considering half the relative difference between the Powheg +Pythia6 samples with more or less radiation. An uncertainty on the interference betweent¯tandWt single top is evaluated by comparingWt single top MC samples generated using the diagram removal and diagram subtraction schemes.

The total systematic uncertainties onσtt¯in the dilepton channel are shown in Table4. The dominant un- certainties on the cross-section result come from the luminosity determination, the statistical uncertainty andt¯tmodelling uncertainties. The total systematic uncertainties on σt in the lepton-plus-jets channel are shown in Table5. The uncertainty is dominated by the jet energy scale uncertainty and the uncer- tainty on the luminosity determination. Since the lepton-plus-jets channel relies on the MC simulation for the jet selection efficiency, it is sensitive to the modelling of additional ppinteractions. It was checked that varying the average number of interactions per bunch crossing in simulation by 16% has less than a 1% impact on the measured cross-section. The uncertainty on the LHC beam energy and the dependence of the cross-section on the assumed mass of the top quark have not been evaluated for this preliminary result, but are expected to be small compared with the total uncertainty.

The results of the cross-section measurements in the dilepton and lepton-plus-jets channels are shown in Table6. The extracted values ofb`` in the dilepton channel were 0.589±0.042 in theeechannel, and 0.584±0.045 in theµµ channel. Both results are in agreement with the expectation from simulation.

All cross-section measurements are consistent with each other within the uncertainties The combined fit to the dilepton channels yields a cross-section measurement of 749 pb with a total relative uncertainty of 16%. The measurement in the lepton-plus-jets channels gives a cross-section measurement of 817 pb with a total relative uncertainty of 17%.

Channel Cross-section measurement

ee 824±88 (stat) ±91 (syst) ±82 (lumi) pb

µµ 683±74 (stat) ±76 (syst) ±68 (lumi) pb

eeandµµcombined 749±57 (stat) ±79 (syst) ±74 (lumi) pb e+jets 775±17 (stat) ±123 (syst) ±85 (lumi) pb µ+jets 862±18 (stat) ±93 (syst) ±94 (lumi) pb e+jets andµ+jets combined 817±13 (stat) ±103 (syst) ±88 (lumi) pb

Table 6: Summary of the measurements of thet production cross-section. For each measurement the statistical, systematic and luminosity uncertainties are shown.

8. Measurement of the ratio of t ¯ t and Z cross-sections

The precision of thet¯tcross-section measurements presented above, as well as that of the corresponding measurement in theeµchannel [14], are limited by the 9 % uncertainty on the integrated luminosity. This uncertainty largely cancels in the measurement of the ratio of inclusivet¯ttoZ-boson cross-sectionsRt/Z,

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which can also provide significant constraints on the ratio of gluon to sea-quark parton distributions in the proton [17].

The ratio is determined from the measurement ofσtt¯in theeµdecay channel described in Ref. [14] and the measurement of theZ-boson cross-section times branching ratio to leptons,σZ·BR(Z → ``), described in Ref. [16]. TheZ cross-section measurement uses the same 85 pb−1dataset as described in Section2, whilst thet¯tmeasurement uses a slightly smaller dataset of 78 pb−1. Thet¯tcross-section measurement is updated to use the set of corrections for electron and muon trigger and identification efficiencies described in Section7, which are also used in theZ measurement. This results in a small change to the measured value ofσt, and a modest reduction in the systematic uncertainty, due to the improved knowledge of the lepton-related uncertainties. The updated result is:

σt =829±50 (stat) ±56 (syst) ±83 (lumi) pb.

Thet¯ttoZcross-section ratioRtt/Z¯ is defined as:

Rt/Z = σt

0.5

σZ→eeZ→µµ, (6)

whereσZ→eeis the inclusiveZ-boson production cross-section multiplied by theZ →eebranching frac- tion and measured in the dielectron channel, andσZ→µµis theZ-boson production cross-section multiplied by theZ →µµbranching fraction and measured in the dimuon channel. The use of equal weights for the electron and muon channels in the denominator maximises the cancellation of lepton-related systematic uncertainties with respect to the numerator, which involves one electron and one muon.

The t¯tand Z cross-section measurements are performed using the same electron and muon identifica- tion, isolation and trigger requirements, and within the same lepton kinematic phase space (except for the requirement 66 < m`` < 116 GeV on the dilepton invariant mass used only in theZ analysis). The uncertainties on lepton identification and trigger requirements, and on momentum scales and resolutions, are therefore taken to be fully correlated in calculating the ratio. The uncertainties do not cancel com- pletely as they depend on lepton pT andη (particularly for electrons), and the leptons from top-quark decays are harder and more central than those fromZ-boson decays. The uncertainties on the efficiencies of the lepton isolation requirements are taken to be uncorrelated betweent¯tandZ, as the hadronic envir- onment is rather different in the two types of events, however, this has no measurable impact on the final result. The PDF uncertainties on the two cross-section measurements, which arise primarily through the simulation-based acceptance and efficiency calculations, are also assumed to be uncorrelated, based on a study of the eigenvector-by-eigenvector variations in thet¯tandZacceptances using the CT10 PDF set.

The ratio ofσttoσZas defined in Equation (6) is measured to be:

Rt/Z =0.445±0.027 (stat)±0.028 (syst)=0.445±0.039,

corresponding to a relative uncertainty of 8.8 %. A detailed breakdown of the uncertainties on the ratio, together with those on the updatedt¯tcross-section measurement, and theZ cross-section measurements from Ref. [16], is given in Table 7. The uncertainty on the ratio is significantly smaller than that on thet¯tcross-section, mostly because of the almost complete cancellation of the uncertainty on the integ- rated luminosity. The statistical and systematic uncertainties onRt/Z are of similar size, and the latter is dominated byt¯tmodelling uncertainties.

Abbildung

Figure 1: The number of b-tagged jets in preselected opposite-sign (a) ee and (b) µµ events
Figure 2: Distributions of the (a) p T of the electrons, (b) η of the electrons, (c) p T of the jets, (d) jet multiplicity, (e) missing transverse momentum E miss T and (f) invariant mass of the dilepton system, in events with an opposite-sign ee pair and
Figure 3: Distributions of the (a) p T of the muons, (b) η of the muons, (c) p T of the jets, (d) jet multiplicity, (e) missing transverse momentum E miss T and (f) invariant mass of the dilepton system, in events with an opposite-sign µµ pair and at least
Table 2: Selected events in the Z boson dominated control region as a function of the number of b-tagged jets
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