Measurement of the cross-section for electroweak production of dijets in association with a Z boson in pp collisions at √s= 13 TeV with the ATLAS detector

Citation for this paper:

Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; … &

Zwalinski, L. (2017). Measurement of the cross-section for electroweak production

of dijets in association with a Z boson in pp collisions at √s= 13 TeV with the ATLAS

detector. Physics Letters B, 775, 206-228. DOI: 10.1016/j.physletb.2017.10.040

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Measurement of the cross-section for electroweak production of dijets in association

with a Z boson in pp collisions at √s= 13 TeV with the ATLAS detector

M. Aaboud et al. (The ATLAS Collaboration)

2017

© 2017 Aaboud et al. This is an open access article distributed under the terms of the

Creative Commons Attribution License.

http://creativecommons.org/licenses/by/4.0

This article was originally published at:

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

the

cross-section

for

electroweak

production

of

dijets

in

association

with

a

Z boson

in

pp collisions

at

√

s

=

13 TeV

with the ATLAS

detector

.

The

ATLAS

Collaboration



a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received2October2017

Receivedinrevisedform19October2017 Accepted19October2017

Availableonline27October2017 Editor:W.-D.Schlatter

The cross-sectionfor the productionoftwo jets inassociation witha leptonicallydecaying Z boson

(Z jj)ismeasuredinproton–protoncollisionsatacentre-of-massenergyof13 TeV,usingdatarecorded with the ATLAS detector atthe LargeHadron Collider, corresponding to an integrated luminosity of 3.2 fb−1. The electroweak Z jj cross-section is extracted in afiducial region chosen to enhance the electroweak contribution relative to the dominant Drell–Yan Z jj process, which is constrained using adata-driven approach.The measuredfiducialelectroweak cross-sectionis

σ

EWZjj =119±16 (stat.)±

20(syst.)±2(lumi.)fbfordijetinvariantmassgreaterthan250 GeV,and34.2±5.8(stat.)±5.5(syst.)±

0.7(lumi.)fbfordijetinvariantmassgreaterthan1 TeV.StandardModelpredictionsareinagreement withthemeasurements.TheinclusiveZ jj cross-sectionisalsomeasuredinsixdifferentfiducialregions withvaryingcontributionsfromelectroweakandDrell–YanZ jj production.

©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

1. Introduction

AttheLargeHadronCollider(LHC)eventscontainingaZ boson andatleasttwojets( Z jj)areproducedpredominantlyvia initial-state QCD radiationfrom the incoming partons in the Drell–Yan process(QCD- Z jj), as showninFig. 1(a).In contrast,the produc-tionofZ jj eventsviat-channelelectroweakgaugebosonexchange (EW- Z jj events), includingthe vector-boson fusion (VBF) process showninFig. 1(b),isamuchrarerprocess.SuchVBFprocessesfor vector-bosonproductionareofgreatinterestasa‘standardcandle’ forother VBFprocesses attheLHC: e.g., theproductionof Higgs bosons orthe search for weaklyinteracting particles beyondthe StandardModel.

Thekinematicpropertiesof Z jj eventsallowsome discrimina-tionbetweentheQCDandEWproductionmechanisms.The emis-sionofavirtualW bosonfromthequarkinEW- Z jj eventsresults inthepresenceoftwohigh-energyjets,withmoderatetransverse momentum(pT),separatedbyalargeintervalinrapidity( y)1 and

 _{E-mailaddress:atlas.publications@cern.ch.}

1 _{ATLASusesaright-handedcoordinatesystemwith itsoriginat thenominal}

interactionpoint in the centreof the detector and the z-axisalong the beam pipe.Inthetransverse plane,thex-axispointsfromtheinteractionpointtothe centreoftheLHCring,the y-axispointsupward,and φisthe azimuthalangle aroundthez-axis.Thepseudorapidityisdefinedintermsofthepolarangleθas η= −ln tan(θ/2).Therapidityisdefinedasy=0.5ln[(E+pz)/(E−pz)],whereE

andpzaretheenergyandlongitudinalmomentumrespectively.Anangular

separa-thereforewithlargedijetmass(mj j)thatcharacterisestheEW- Z jj

signal.AconsequenceoftheexchangeofavectorbosoninFig. 1(b) is that there is no colour connection between the hadronic sys-temsproducedbythebreak-upofthetwoincomingprotons.Asa result,EW- Z jj eventsarelesslikelytocontainadditionalhadronic activityintherapidityintervalbetweenthetwohigh-pT jetsthan

correspondingQCD- Z jj events.

The first studies ofEW- Z jj production were performed [1]in pp collisionsatacentre-of-massenergy(

√

s)of7 TeVbytheCMS Collaboration,wherethebackground-only hypothesiswasrejected atthe2

.

6

σ

level.ThefirstobservationoftheEW- Z jj processwas performed bytheATLASCollaborationatacentre-of-massenergy (

√

s)of8 TeV[2].Thecross-sectionmeasurementisinagreement with predictionsfromthe Powheg-box eventgenerator[3–5] and allowed limits tobe placedon anomaloustriple gaugecouplings. The CMS Collaboration has also observed and measured [6] the cross-sectionforEW- Z jj productionat8 TeV.ThisLetterpresents measurementsofthecross-sectionforEW- Z jj productionand in-clusive Z jj productionathighdijetinvariantmassinpp collisions at

√

s

=

13 TeV usingdatacorrespondingtoanintegrated luminos-ityof3.2 fb−1 collected bythe ATLASdetectorattheLHC.These measurements allow the dependenceof the cross-section on

√

s

tionbetweentwoobjectsisdefinedasR=(φ)2_{+ (η)}2_{,whereφandη}

aretheseparationsinφandηrespectively.Momentuminthetransverseplaneis denotedbypT.

https://doi.org/10.1016/j.physletb.2017.10.040

0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

Fig. 1. Examplesofleading-orderFeynmandiagramsforthetwoproduction mech-anismsforaleptonicallydecaying Z bosonandatleasttwojets( Z jj)inproton– proton collisions: (a) QCD radiation from the incoming partons (QCD- Z jj) and (b) t-channelexchangeofanEWgaugeboson(EW- Z jj).

tobestudied.Theincreased

√

s allowsexplorationofhigherdijet masses, where the EW- Z jj contribution to the total Z jj rate be-comesmorepronounced.

2. ATLASdetector

The ATLASdetectorisdescribedindetailinRefs. [7,8].It con-sistsofaninnerdetectorfortracking,surroundedbyathin super-conducting solenoid, electromagnetic and hadronic calorimeters, and amuonspectrometerincorporatingthreelarge superconduct-ing toroidalmagnetsystems.Theinner detectorisimmersedina 2 T axialmagnetic field and provides charged-particle trackingin therange

|

η

|

<

2

.

5.

The calorimeters cover the pseudorapidity range

|

η

|

<

4

.

9. Electromagnetic calorimetry is provided by barrel and end-cap lead/liquid-argon(LAr)calorimetersintheregion

|

η

|

<

3

.

2.Within

|

η

|

<

2

.

47 the calorimeter is finely segmented in the lateral di-rection of the showers, allowing measurement of the energy and position of electrons, and providing electron identification in conjunction with the inner detector. Hadronic calorimetry is provided by the steel/scintillator-tile calorimeter, segmented into three barrel structures within

|

η

|

<

1

.

7, and two hadronic end-cap calorimeters. A copper/LAr hadronic calorimeter covers the 1

.

5

<

|

η

| <

3

.

2 region, and a forward copper/tungsten/LAr calorimeterwithelectromagnetic-showeridentificationcapabilities coversthe3

.

1

<

|

η

| <

4

.

9 region.

The muon spectrometer comprises separate trigger and high-precision trackingchambers. Thetrackingchamberscoverthe re-gion

|

η

|

<

2

.

7 withthreelayersofmonitoreddrifttubes, comple-mented by cathodestripchambersin partoftheforwardregion, wherethehitrateishighest.Themuontriggersystemcoversthe range

|

η

|

<

2

.

4 withresistiveplate chambersinthebarrelregion, andthingapchambersintheend-capregions.

A two-level trigger system is used to select events of inter-est [9].The Level-1trigger isimplementedinhardware and uses a subset ofthe detector informationto reduce the event rate to around100 kHz.Thisisfollowedbythesoftware-basedhigh-level triggersystemwhichreducestheeventratetoabout1 kHz. 3. MonteCarlosamples

The production of EW- Z jj events was simulated at next-to-leading-order (NLO) accuracy in perturbative QCD using the Powheg-box v1 Monte Carlo (MC) event generator [4,5,10] and, alternatively, at leading-order (LO) accuracy in perturbative QCD usingthe Sherpa 2.2.0 eventgenerator[11].Formodellingofthe partonshower,fragmentation,hadronisationandunderlyingevent (UEPS), Powheg-box was interfacedto Pythia 8[12] with a ded-icated setof parton-shower-generator parameters (tune) denoted AZNLO [13]and the CT10 NLO partondistributionfunction (PDF) set [14]. Therenormalisation and factorisationscales were set to

the Z boson mass. Sherpa predictions used the Comix [15] and OpenLoops_{[16] matrixelement event generators, and theCKKW} method was used to combine the various final-state topologies fromthe matrix element and match them to the partonshower

[17]. The matrix elements were merged with the Sherpa parton shower [18] usingthe ME+PS@LO prescription [19,20], and using Sherpa_{’snativedynamicalscale-settingalgorithmtosetthe} renor-malisation and factorisation scales. Sherpa predictions used the NNPDF30NNLO PDFset[21].

The production of QCD- Z jj events was simulated using three event generators, Sherpa 2.2.1, Alpgen 2.14 [22] and MadGraph5_aMC@NLO 2.2.2 [23]. Sherpa provides Z

+

n-parton predictionscalculatedforuptotwo partonsatNLO accuracyand up to four partons at LO accuracy in perturbative QCD. Sherpa predictionsusedthe NNPDF30NLO PDFsettogetherwiththe tun-ingoftheUEPSparametersdevelopedbythe Sherpa authorsusing the ME+PS@NLO prescription[19,20]. Alpgen isanLOevent gener-atorwhichusesexplicitmatrixelementsforuptofivepartonsand was interfacedto Pythia 6.426 [24] usingthePerugia2011C tune

[25]andtheCTEQ6L1PDFset[26].Onlymatrixelementsfor light-flavourproductionin Alpgen areincluded,withheavy-flavour con-tributionsmodelledbythepartonshower. MadGraph5_aMC@NLO 2.2.2(MG5_aMC)usesexplicitmatrixelementsforuptofour par-tonsatLO,andwasinterfacedto Pythia 8withthe A14tune[27]

and usingthe NNPDF23LO PDF set [28]. For reconstruction-level studies,totalZ bosonproductionratespredictedbyallthreeevent generators used to produce QCD- Z jj predictions are normalised usingthenext-to-next-to-leading-order (NNLO)predictions calcu-latedwith the FEWZ 3.1program [29–31] using the CT10 NNLO PDF set [14]. However, whencomparing particle-level theoretical predictionstodetector-correctedmeasurements,thenormalisation ofquotedpredictionsisprovidedby theeventgeneratorin ques-tionratherthananexternalNNLOprediction.

TheproductionofapairofEWvectorbosons(diboson),where onedecaysleptonicallyandtheother hadronically,orwhereboth decay leptonically and are produced in association with two or more jets, through W Z or Z Z production with at least one Z boson decaying to leptons, was simulated separately using Sherpa_{2.1.1andthe CT10 NLO PDFset.}

ThelargestbackgroundtotheselectedZ jj samplesarisesfrom tt and

¯

single-top (W t) production. These were generated using Powheg-box v2and Pythia 6.428withthePerugia2012tune[25], andnormalisedusingthecross-sectioncalculatedatNNLO

+

NNLL (next-to-next-to-leading log) accuracy using the Top++2.0 pro-gram[32].

All the above MC samples were fully simulated through the Geant4[33] simulationoftheATLASdetector[34].Theeffectof additional pp interactions (pile-up)inthe sameornearbybunch crossingswasalsosimulated,using Pythia v8.186withthe A2tune

[35] and the MSTW2008LO PDF set [36]. The MC samples were reweightedsothatthedistributionoftheaveragenumberof pile-upinteractionsperbunchcrossingmatchesthatobservedindata. ForthedataconsideredinthisLetter,theaveragenumberof inter-actionsis 13.7.

4. Eventpreselection

The Z bosonsaremeasuredintheirdielectronanddimuon de-cay modes.Candidate eventsareselectedusingtriggers requiring atleastoneidentified electronormuonwith transverse momen-tum thresholds of pT

=

24 GeV and 20 GeV respectively, with

additional isolation requirements imposed in these triggers. At higher transverse momenta, the efficiency of selecting candidate events is improved through the use of additional electron and

muontriggerswithoutisolationrequirementsandwiththresholds ofpT

=

60 GeV and50 GeVrespectively.

Candidateelectronsarereconstructedfromclustersofenergyin theelectromagnetic calorimeter matched to inner-detectortracks

[37].TheymustsatisfytheMedium identificationrequirements de-scribedinRef.[37]andhavepT

>

25 GeV and

|

η

| <

2.47,

exclud-ingthetransitionregionbetweenthebarrelandend-cap calorime-tersat1.37

<

|

η

| <

1.52.Candidatemuonsareidentifiedastracks intheinnerdetectormatchedand combinedwithtrack segments inthemuonspectrometer.TheymustsatisfytheMedium identifi-cationrequirementsdescribedinRef.[38],andhavepT

>

25 GeV

and

|

η

| <

2.4. Candidateleptons must alsosatisfy aset of isola-tioncriteriabasedonreconstructedtracksandcalorimeteractivity. Events are required to contain exactly two leptons of the same flavour butofopposite charge. The dileptoninvariant massmust satisfy81

<

m

<

101 GeV.

Candidate hadronic jets are required to satisfy pT

>

25 GeV

and

|

y

| <

4.4. They arereconstructed fromclusters ofenergy in thecalorimeter [39] using theanti-kt algorithm [40,41] with

ra-dius parameter R

=

0

.

4. Jet energies are calibrated by applying pT- and y-dependentcorrections derived fromMonteCarlo

sim-ulationwithadditionalinsitucorrection factorsdeterminedfrom data[42]. To reduce the impact of pile-up contributions,all jets with

|

y

|

<

2

.

4 and pT

<

60 GeV are required to be compatible

with havingoriginated fromtheprimary vertex (the vertex with the highest sum of track p2

T), as defined by the jet vertex

tag-geralgorithm[43].Selectedelectronsand muonsarediscarded if theylie within



R

=

0

.

4 ofareconstructed jet.Thisrequirement isimposed toremove non-prompt non-isolatedleptons produced inheavy-flavour decaysor fromthe decayin flight ofa kaon or pion.

5. MeasurementofinclusiveZ j j fiducialcross-sections 5.1.Definitionofparticle-levelcross-sections

Cross-sectionsare measured for inclusive Z jj production that includes the EW- Z jj and QCD- Z jj processes, as well as diboson events.Theparticle-levelproductioncross-sectionforinclusive Z jj productioninagivenfiducialregion f isgivenby

σ

f

=

N f obs

−

N f bkg L

·

C

f

,

(1)

where N_obsf is the number ofevents observed in the data pass-ingthe selection requirementsofthe fiducialregion understudy atdetector level, N_bkgf is the corresponding number of expected background(non- Z jj)events, L istheintegratedluminosity corre-spondingtotheanalyseddatasample,and

C

f _{isacorrectionfactor}

appliedtotheobserveddatayields,whichaccountsfor experimen-talefficiency and detectorresolutioneffects, and is derived from MCsimulationwithdata-drivenefficiencyandenergy/momentum scalecorrections.Thiscorrectionfactoriscalculatedas:

C

f

₌

N f

det N_particlef

,

where N_detf is the number ofsignal eventsthat satisfy the fidu-cialselection criteria at detectorlevelin theMC simulation, and N_particlef isthenumberofsignaleventsthatpasstheequivalent se-lectionbutat particle level.These correction factors havevalues between0

.

63 and0

.

77,dependingonthefiducialregion.

Withthe exceptionofbackgroundfrommultijetand W

+

jets processes(henceforthreferred totogethersimplyas multijet pro-cesses),contributionstoN_bkgf areestimatedusingtheMonteCarlo

samples described inSection 3. Background frommultijetevents is estimated from the data by reversing requirements on lepton identification or isolation to derive a template for the contribu-tion of jetsmis-reconstructed as lepton candidates as a function ofdileptonmass.Non-multijetbackgroundissubtractedfromthe templateusingsimulation.Thenormalisationisderived byfitting thenominaldileptonmassdistributionineachfiducialregionwith the sumofthemultijet templateand a templatecomprising sig-nalandbackgroundcontributionsdeterminedfromsimulation.The multijetcontributionisfoundtobelessthan0.3%ineachfiducial region.ThecontributionfromW

+

jets processeswaschecked us-ing MC simulationand found tobe muchsmaller than the total multijetbackgroundasdeterminedfromdata.

At particlelevel,onlyfinal-state particleswith properlifetime c

τ

>

10 mm areconsidered.Promptleptonsaredressedusingthe four-momentumcombinationofanelectronormuonandall pho-tons (not originating from hadron decays) within a cone of size



R

=

0

.

1 centred on the lepton. These dressed leptons are re-quiredtosatisfy pT

>

25 GeV and

|

η

|

<

2

.

47.Eventsarerequired

tocontainexactlytwo dressedleptonsofthesameflavour butof oppositecharge,andthedileptoninvariantmassmustsatisfy81

<

m

<

101 GeV.Jetsarereconstructedusingtheanti-kt algorithm

with radius parameter R

=

0

.

4. Promptleptons and the photons used todress theseleptons arenot includedin theparticle-level jet reconstruction. Allremaining final-stateparticles are included in theparticle-leveljet clustering. Promptleptons witha separa-tion



Rj,

<

0

.

4 fromanyjetarerejected.

The cross-section measurements are performed in the six phase-spaceregions definedinTable 1.Theseregions are chosen tohavevaryingcontributionsfromEW- Z jj andQCD- Z jj processes. 5.2. Eventselection

Following Ref. [2], events are selected in six detectorfiducial regions. Asfaraspossible,thesearedefinedwiththesame kine-matic requirements as the six phase-spaceregions in which the cross-sectionismeasured(Table 1).Thisminimisessystematic un-certaintiesinthemodellingoftheacceptance.

Thebaselinefiducialregionrepresentsaninclusiveselectionof eventscontainingaleptonicallydecaying Z bosonandatleasttwo jetswith pT

>

45 GeV,atleastoneofwhichsatisfies pT

>

55 GeV.

The two highest-pT (leading and sub-leading) jets in a given

event define the dijet system. The baseline region is dominated by QCD- Z jj events.Therequirement of81

<

m

<

101 GeV

sup-presses other sourcesofdilepton events,such as tt and

¯

Z

→

τ τ

, aswellasthemultijetbackground.

Becausetheenergyscaleofthedijetsystemistypicallyhigher ineventsproducedbytheEW- Z jj processthaninthoseproduced bytheQCD- Z jj process,twosubsetsofthebaselineregionare de-fined which probe theEW- Z jj contributionin differentways: in thehigh-massfiducialregionahighvalueoftheinvariantmassof thedijetsystem(mj j

>

1 TeV)isrequired,andinthehigh-pT

fidu-cialregiontheminimum pT oftheleadingandsub-leadingjetsis

increased to85 GeVand 75 GeVrespectively.TheEW- Z jj process typicallyproducesharderjet transversemomentaandresultsina harderdijetinvariantmassspectrumthantheQCD- Z jj process.

Three additional fiducial regions allow the separate contribu-tionsfromtheEW- Z jj andQCD- Z jj processestobemeasured.The EW-enriched fiducial region is designed to enhance the EW- Z jj contribution relative to that from QCD- Z jj, particularly at high mj j. The EW-enriched region is derived from the baseline

re-gion requiringmj j

>

250 GeV, a dilepton transverse momentum

of p

T

>

20 GeV, andthat thenormalisedtransversemomentum

Table 1

Summaryoftheparticle-levelselectioncriteriadefiningthesixfiducialregions(seetextfordetails). Fiducial region

Object Baseline High-mass High-pT EW-enriched EW-enriched, QCD-enriched

mj j>1 TeV

Leptons |η| <2.47, pT>25 GeV,Rj,>0.4

Dilepton pair 81<m<101 GeV

– p T >20 GeV Jets |y| <4.4 pj1 T >55 GeV p j1 T >85 GeV p j1 T >55 GeV pj2 T >45 GeV p j2 T >75 GeV p j2 T >45 GeV

Dijet system – mj j>1 TeV – mj j>250 GeV mj j>1 TeV mj j>250 GeV

Interval jets – Ninterval

jet(pT>25 GeV)=0 N interval jet(pT>25 GeV)≥1 Z jj system – pbalance T <0.15 p balance,3 T <0.15

momentumjetssatisfy pbalance

T

<

0

.

15.Thelatterquantityisgiven

by pbalance_T

=





p1 T

+ 

p 2 T

+ 

p j1 T

+ 

p j2 T









p1 T



 +





p 2 T



 +





p j1 T



 +





p j2 T





,

(2)

where p



_Ti isthe transversemomentumvector ofobjecti,



1 and



2labelthetwoleptonsthatdefinethe Z bosoncandidate,and j1

and j2 refer to the leading and sub-leading jets. These

require-ments help remove events in which the jets arise from pile-up or multiple partoninteractions. The requirement on pbalance_T also helps suppress events in which the pT of one or more jets is

badly measured and it enhances the EW- Z jj contribution, where the lower probability of additionalradiation causes the Z boson and the dijet system to be well balanced. The EW-enriched re-gion requires a veto [44] on any jets with pT

>

25 GeV

recon-structed within the rapidity interval bounded by the dijet sys-tem (Ninterval

jet(pT>25 GeV)

=

0). Asecond fiducial region,denoted

EW-enriched(mj j

>

1 TeV),hasidenticalselectioncriteria,exceptfora

raisedmj j thresholdof1 TeVwhichfurtherenhancestheEW- Z jj

contributiontothetotal Z jj signalrate.

Incontrast,theQCD-enrichedfiducialregionisdesignedto sup-presstheEW- Z jj contributionrelativeto QCD- Z jj by requiringat leastonejetwith pT

>

25 GeV to bereconstructedwithinthe

ra-pidity intervalboundedbythedijetsystem(Ninterval_jet(p

T>25 GeV)

≥

1).

IntheQCD-enrichedregion,thedefinitionofthenormalised trans-versemomentumbalanceismodifiedfromthatgiveninEq.(2)to includein thecalculation ofthenumeratorand denominator the pT ofthe highest pT jet within the rapidityinterval bounded by

thedijetsystem(pbalance_T ,3).Inallotherrespects,thekinematic re-quirements in the EW-enriched region and QCD-enriched region areidentical.

5.3. Detector-levelresults

Inthebaselineregion,30686 eventsareselectedinthe dielec-tron channeland 36786 eventsare selectedinthedimuon chan-nel.Thetotalobservedyieldsareinagreementwith theexpected yieldswithinstatisticaluncertaintiesineachdileptonchannel.The largestdeviationacrossallfiducialregionsisa2

σ

(statistical) dif-ference between the expected to observed ratio in the electron versusmuonchannelinthehigh-pTregion.

The expectedcompositionof theselecteddata samplesinthe six Z jj fiducialregionsissummarisedinTable 2,averagingacross thedielectronand dimuonchannelsas thesecompositionsinthe

two dilepton channels are in agreement within statistical uncer-tainties.Thenumbersofselectedeventsindataand expectations fromtotalsignalplusbackgroundestimatesarealsogivenforeach region. The largest discrepancy between observed and expected yieldsisseeninthehigh-massregion,andresultsfroma mismod-ellingof the mj j spectrum inthe QCD- Z jj MC simulations used,

whichisdiscussedbelowandaccountedforintheassessmentof systematicuncertainties inthemeasurement.

5.4. SystematicuncertaintiesintheinclusiveZ jj fiducialcross-sections Experimentalsystematic uncertaintiesaffectthedetermination of the

C

f correction factor and the background estimates. The dominantsystematicuncertaintyintheinclusive Z jj fiducial cross-sections arises from the calibration of the jet energy scale and resolution. This uncertainty varies from around 4% in the EW-enriched region to around 12% in the QCD-enriched region. The largeruncertaintyintheQCD-enrichedregionisduetothehigher average jet multiplicity (an average of 1.7 additional jets in ad-dition to the leading and sub-leading jets) compared with the EW-enrichedregion (an averageof0.4additional jets).Other ex-perimentalsystematicuncertaintiesarisingfromleptonefficiencies relatedto reconstruction,identification, isolation and trigger, and leptonenergy/momentumscaleandresolutionaswellasfromthe effectofpile-up,amounttoatotalofaround1–2%,depending on thefiducialregion.

The systematic uncertaintyarising from the MC modelling of themj j distributionin theQCD- Z jj and EW- Z jj signal processes

is around 3% inthe EW-enriched region,around 1% in the QCD-enriched region,2% in thehigh-mass region, and below1% else-where. This is assessed by comparing the correction factors ob-tained by using the different MC event generators listed in Sec-tion3andbyperformingadata-drivenreweightingoftheQCD- Z jj MCsampletodescribethemj j distributionoftheobserveddatain

agivenfiducialregion.Additionalcontributionsarisefromvarying theQCDrenormalisationandfactorisationscalesup anddownby a factor of two independently, and fromthe propagationof un-certaintiesinthePDFsets.Thenormalisationofthediboson con-tributionis varied according toPDF and scale variations inthese predictions[45],andresultsinuptoa0

.

1% effectonthemeasured Z jj cross-sections depending on the fiducial region. The uncer-taintyfromvaryingthenormalisationandshapeinmj jofthe

esti-matedbackgroundfromtop-quarkproductionisatmost1%(inthe high-massregion),arisingfromchangesintheextracted Z jj cross-sectionswhen usingmodified top-quarkbackgroundMC samples with PDFand scalevariations,suppressedorenhanced additional

Table 2

Estimatedcomposition(inpercent)ofthedatasamplesselectedinthesixZ jj fiducialregionsforthedielectronanddimuon chan-nelscombined,usingtheEW- Z jj samplefrom Powheg,andtheQCD- Z jj samplefrom Sherpa (normalisedusingNNLOpredictions fortheinclusiveZ cross-sectioncalculatedwith FEWZ).Uncertaintiesinthesamplecontributionsarestatisticalonly.Alsoshown arethetotalexpectedyieldsandthetotalobservedyieldsineachfiducialregion.Uncertaintiesinthetotalexpectedyieldsare statistical(first)andsystematic(second),seeSection5.4fordetails.

Process Composition [%]

Baseline High-mass High-pT EW-enriched EW-enriched, QCD-enriched

mj j>1 TeV QCD-Z jj 94.2±0.4 86.8±1.6 92.3±0.4 93.4±0.9 72.9±2.1 95.4±0.8 EW-Z jj 1.5± <0.1 10.6±0.2 2.6± <0.1 4.8± <0.1 26.1±0.5 1.6± <0.1 Diboson 1.6± <0.1 1.5±0.7 2.0±0.5 1.0±0.5 0.8±0.4 1.8±0.4 tt¯ 2.6± <0.1 1.1±0.1 3.1±0.1 0.7± <0.1 0.1±0.1 1.2±0.1 Single-t <0.2 <0.2 <0.2 <0.1 <0.1 <0.1 Multijet <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 Total expected 64800 2220 21900 11100 640 7120 ±130±5220 ±20±200 ±40±1210 ±50±520 ±10±40 ±30±880 Total observed 67472 1471 22461 11630 490 6453

radiation (generated with the Perugia2012radHi/Lo tunes [25]), or using an alternative top-quark productionsample from Mad-Graph5_aMC@NLOinterfacedto Herwig++ v2.7.1[23,46].

Thesystematicuncertaintyintheintegratedluminosityis2.1%. This is derived following a methodology similar to that detailed inRef. [47], froma calibrationofthe luminosity scale usingx– y beam-separationscansperformedinJune2015.

5.5.InclusiveZ jj results

The measured cross-sections in the dielectron and dimuon channels are combined and presented here as a weighted aver-age(takingintoaccount totaluncertainties)across bothchannels. Thesecross-sections are determined usingeach ofthe correction factors derived from the six combinations of the three QCD- Z jj (Alpgen, MG5_aMC, and Sherpa) and two EW- Z jj (Powheg and Sherpa)MCsamples.Foragivenfiducialregion(Table 1)the cross-section averaged over all six variations is presented in Table 3. The envelope of variation between QCD- Z jj and EW- Z jj models isassignedasasourceofsystematicuncertainty(1%inallregions exceptthe EW-enrichedregionwherethevariationis3% andthe high-massregionwherethevariationis2%).

The theoretical predictions from Sherpa (QCD- Z jj)+ Powheg (EW- Z jj), MG5_aMC (QCD- Z jj) + Powheg (EW- Z jj), and Alpgen (QCD- Z jj)+ Powheg (EW- Z jj)arefoundtobeinagreementwith themeasurementsinmostcases.Theuncertaintiesinthe theoreti-calpredictionsaresignificantlylargerthantheuncertaintiesinthe correspondingmeasurements.

Thelargest differences between predictions and measurement are in the high-mass and EW-enriched (mj j

>

250 GeV and

>

1 TeV) regions. Predictions from Sherpa (QCD- Z jj) + Powheg (EW- Z jj) and MG5_aMC (QCD- Z jj) + Powheg (EW- Z jj) exceed measurements in the high-mass region by 54% and 34% respec-tively,where the predictions haverelative uncertainties with re-spectto the measurement of36% and 32%. For theEW-enriched region, Sherpa (QCD- Z jj) + Powheg (EW- Z jj) describes the ob-served rates well, but MG5_aMC (QCD- Z jj) + Powheg (EW- Z jj) overestimatesmeasurementsby28%witharelativeuncertaintyof 11%. In the EW-enriched (mj j

>

1 TeV) region the same

predic-tions overestimatemeasured rates by33% and 57%, with relative uncertaintiesof16%and15%.Someofthesedifferencesarisefrom asignificant mismodelling of theQCD- Z jj contribution, as inves-tigated and discussed in detail in Section 6.1. Predictions from

Alpgen (QCD- Z jj)+ Powheg (EW- Z jj)areinagreementwith the data for the high-mass and EW-enriched (mj j

>

250 GeV and

>

1 TeV)regions.

6. MeasurementofEW- Z j j fiducialcross-sections

The EW-enriched fiducial region (defined in Table 1) is used to measure the production cross-section of the EW- Z jj process. The EW-enriched region has an overall expected EW- Z jj signal fraction of4.8% (Table 2) and this signal fractiongrows with in-creasing mj j to 26.1% formj j

>

1 TeV. The QCD-enriched region

has an overallexpected EW- Z jj signal fractionof1.6% increasing to4.4%formj j

>

1 TeV.ThedominantbackgroundtotheEW- Z jj

cross-sectionmeasurementisQCD- Z jj production.Itissubtracted inthesamewayasnon- Z jj backgroundsintheinclusive measure-mentdescribedinSection5.Althoughdibosonproductionincludes contributionsfrompurelyEWprocesses,inthismeasurementitis consideredaspartofthebackgroundandisestimatedfrom simu-lation.

A particle-level production cross-section measurement of EW- Z jj productioninagivenfiducialregion f isthusgivenby

σ

_EWf

=

N f obs

−

N f QCD-Zjj

−

N f bkg L

·

C

_EWf

,

(3)

with thesame notations as inEq. (1)and where N_{QCD- Zjj}f is the expectednumberofQCD- Z jj eventspassingtheselection require-mentsofthefiducialregionatdetectorlevel,N_bkgf istheexpected numberof background(non- Z jj anddiboson) events, and

C

_EWf is acorrectionfactorappliedtotheobservedbackground-subtracted datayields thataccounts forexperimentalefficiencyanddetector resolutioneffects,andisderivedfromEW- Z jj MCsimulationwith data-drivenefficiencyandenergy/momentumscalecorrections.For the mj j

>

250 GeV (mj j

>

1 TeV) region this correction factor

is determined to be 0

.

66 (0

.

67) when using the Sherpa EW- Z jj prediction, and 0

.

67 (0

.

68) when usingthe Powheg EW- Z jj pre-diction.

Detector-levelcomparisonsofthemj jdistributionbetweendata

and simulation in (a) the EW-enriched region and (b) the QCD-enriched region are shown in Fig. 2. It can be seen in Fig. 2(a)

Table 3

Measuredandpredictedinclusive Z jj productioncross-sectionsinthesixfiducialregionsdefinedinTable 1.Forthemeasuredcross-sections,the firstuncertaintygivenisstatistical,thesecondissystematicandthethirdisduetotheluminositydetermination.Forthepredictions,thestatistical uncertaintyisaddedinquadraturetothesystematicuncertaintiesarisingfromthePDFsandfactorisationandrenormalisationscalevariations.

Fiducial region Inclusive Z jj cross-sections [pb]

Measured Prediction

value ±stat. ±syst. ±lumi. Sherpa(QCD-Z jj) MG5_aMC (QCD-Z jj) Alpgen(QCD-Z jj) +Powheg (EW-Z jj) +Powheg (EW-Z jj) +Powheg (EW-Z jj) Baseline 13.9 ±0.1 ±1.1 ±0.3 13.5±1.9 15.2±2.2 11.7±1.7 High-pT 4.77 ±0.05 ±0.27 ±0.10 4.7±0.8 5.5±0.9 4.2±0.7 EW-enriched 2.77 ±0.04 ±0.13 ±0.06 2.7±0.2 3.6±0.3 2.4±0.2 QCD-enriched 1.34 ±0.02 ±0.17 ±0.03 1.5±0.4 1.4±0.3 1.1±0.3 High-mass 0.30 ±0.01 ±0.03 ±0.01 0.46±0.11 0.40±0.09 0.27±0.06 EW-enriched (mj j>1 TeV) 0.118 ±0.008 ±0.006 ±0.002 0.156±0.019 0.185±0.023 0.120±0.015

Fig. 2. Detector-levelcomparisonsofthedijetinvariantmassdistributionbetweendataandsimulationin(a) theEW-enrichedregionand(b) theQCD-enrichedregion,forthe dielectronanddimuonchannelcombined.Uncertaintiesshownonthedataarestatisticalonly.TheEW- Z jj simulationsamplecomesfromthe Powheg eventgeneratorand theQCD- Z jj MCsamplecomesfromthe Sherpa eventgenerator.ThelowerpanelsshowtheratioofsimulationtodataforthreeQCD- Z jj models,from Alpgen, MG5_aMC, and Sherpa.Thehatchedbandcentredatunityrepresentsthesizeofstatisticalandexperimentalsystematicuncertaintiesaddedinquadrature.

that inthe EW-enriched region the EW- Z jj component becomes prominent atlargevaluesofmj j.However,Fig. 2(b)demonstrates

that the shape of themj j distribution forQCD- Z jj productionis

poorlymodelledinsimulation.Thesametrendisseenforallthree QCD- Z jj eventgeneratorslistedinSection3. Alpgen providesthe best description of the data over the whole mj j range. In

com-parison, MG5_aMC and Sherpa overestimatethe databy80%and 120% respectively,formj j

=

2 TeV, well outsidethe uncertainties

onthesepredictionsdescribedinTable 3.Thesediscrepancieshave been observed previously in Z jj [2,48] and Wjj [49–51] produc-tionathigh dijetinvariantmassand athighjetrapidities.Forthe purposeofextractingthecross-sectionforEW- Z jj production,this mismodelling ofQCD- Z jj is correctedforusinga data-driven ap-proach,asdiscussedinthefollowing.

6.1. CorrectionsformismodellingofQCD- Z jj productionandfitting procedure

ThenormalisationoftheQCD- Z jj backgroundisextractedfrom a fit of the QCD- Z jj and EW- Z jj mj j simulated distributions to

the data inthe EW-enriched region, after subtraction ofnon- Z jj anddibosonbackground,usingalog-likelihoodmaximisation[52]. FollowingtheprocedureadoptedinRef.[2],thedatainthe

QCD-enriched region are usedto evaluate detector-levelshape correc-tion factors for the QCD- Z jj MC predictions bin-by-bin in mj j.

Thesedata-to-simulationratiocorrectionfactorsareappliedtothe simulation-predictedshape inmj j oftheQCD- Z jj contributionin

the EW-enrichedregion. Thisprocedure is motivatedby two ob-servations:

(a) theQCD-enrichedregionandEW-enrichedregionaredesigned tobekinematically verysimilar,differingonlywith regard to thepresence/absenceofjetsreconstructedwithintherapidity intervalboundedbythedijetsystem,

(b) thecontribution ofEW- Z jj tothe regionof high mj j is

sup-pressedintheQCD-enrichedregion(4.4%formj j

>

1 TeV)

rel-ativetothatintheEW-enrichedregion(26.1%formj j

>

1 TeV)

(alsoillustrated inFig. 2);theimpactof theresidualEW- Z jj contamination in the QCD-enriched region is assigned as a componentofthesystematicuncertaintyintheQCD- Z jj back-ground.

The shape correction factors in mj j obtainedusing the three

different QCD- Z jj MC samples are shown in Fig. 3(a). Theseare derived as the ratioof the data to simulation inbins of mj j

Fig. 3. Binneddata-to-simulationnormalisedratioshapecorrectionfactorsasafunctionofdijetinvariantmassintheQCD-enrichedregion.(a) Ratioforthreedifferent QCD- Z jj MCsampleswithuncertaintiescorrespondingtothecombinedstatisticaluncertaintiesinthedataandQCD-ZjjMCsamplesaddedinquadrature.ScaleandPDF uncertaintiesin Sherpa predictionsareindicatedbytheshadedbands.Linesrepresentfitstotheratiosusingalinearfit.(b) RatioforsubregionsoftheQCD-enrichedregion forthe Alpgen MCsample.Curvesrepresenttheresultoffitswithaquadraticfunctionforthevarioussubregions.

indataintheQCD-enriched region.Abinned fittothecorrection factorsderived indijet invariant massisperformed with alinear fitfunction (and alsowith a quadratic fitfunction) toproduce a continuouscorrectionfactor.Thelinearfitisillustratedoverlaidon thebinnedcorrectionfactorsinFig. 3(a).Thenominalvalueofthe EW- Z jj cross-sectioncorrespondingtoaparticularQCD- Z jj event generatortemplateisdeterminedusingthecorrectionfactorsfrom thelinear fit.The change in resultant EW- Z jj cross-section from usingbinned correction factors directly is assessed as a system-aticuncertainty.ThechangeintheextractedEW- Z jj cross-section whenusing a quadratic fitwas found to be negligible.The vari-ations observed between event generators maybe partlydue to differencesin themodellingofQCD radiationwithin therapidity interval bounded by the dijet system, which affects the extrap-olationfromthe central-jet-enriched QCD-enriched regionto the central-jet-suppressedEW-enrichedregion.Thevariationbetween event generatorsis much largerthan theeffect ofPDF and scale uncertaintiesinaparticularprediction(indicatedinFig. 3(a)bya shadedbandonthepredictionsfrom Sherpa).Estimatingthe un-certainties associated with QCD- Z jj mismodelling from PDF and scale variations around a single generator prediction wouldthus result inan underestimateofthe truetheoretical uncertainty as-sociated with this mismodelling. In this measurement, the span ofresultant EW- Z jj cross-sectionsextracted fromtheuseofeach ofthethreeQCD- Z jj templatesisassessedasasystematic uncer-tainty.ThevariationintheEW- Z jj cross-sectionmeasurementdue toachangeintheEW- Z jj signaltemplateusedinthederivation ofthemj j correctionfactors(from Powheg to Sherpa)isfoundto

benegligible.

To test the dependence of the QCD- Z jj correction factors on the modelling ofany additional jet emitted in the dijet rapidity interval,the QCD-enriched control region isdivided into pairs of mutually exclusivesubsetsaccording tothe

|

y

|

ofthehighest pT

jet within the rapidity interval boundedby the dijet system, the pTofthatjet,orthevalueofN_jetinterval_{(pT>25 GeV)}.Thecontinuous

cor-rectionfactors are determinedfrom each subregion usingboth a linearandaquadraticfittothedata.Correctionfactorsderivedin thesubregions using quadraticfits result in the largestvariation inthe extracted cross-sections. These fits are shown in Fig. 3(b) forthe Alpgen QCD- Z jj sample, whichdisplays the largest vari-ation between subregions of the three event generators used to produce QCD- Z jj predictions. Within statistical uncertainties the measuredEW- Z jj cross-sectionsarenotsensitivetothedefinition ofthecontrolregionused.

ThenormalisationsofthecorrectedQCD- Z jj templatesandthe EW- Z jj templatesareallowedtovaryindependentlyinafittothe background-subtractedmj j distributionintheEW-enrichedregion.

Themeasuredelectroweakproductioncross-sectionisdetermined from the data minus the QCD- Z jj contribution determined from thesefits(Eq.(3)).AsthechoiceofEW- Z jj templatecaninfluence the normalisation of the QCD- Z jj template in the EW-enriched regionfit,themeasuredEW- Z jj cross-sectiondeterminationis re-peated for each QCD- Z jj template using either the Powheg or SherpaEW- Z jj templateinthefit.Thecentralvalue oftheresult quoted isthe averageofthemeasured EW- Z jj cross-sections de-terminedwith eachofthesixcombinationsofthethreeQCD- Z jj and two EW- Z jj templates, with the envelope of measured re-sultsfromthesevariationstakenasanuncertaintyassociatedwith the dependence on the modelling of the templates in the EW-enrichedregion.Separateuncertaintiesareassignedforthe deter-mination of the QCD- Z jj correction factors in the QCD-enriched region and their propagation into the EW-enriched region. The measurementoftheEW- Z jj cross-section intheEW-enriched re-gionformj j

>

1 TeV isextractedfromthesamefitprocedure,with

dataandQCD- Z jj yieldsintegratedformj j

>

1 TeV.

Fig. 4(a) shows a comparison in the EW-enriched region of

the fitted EW- Z jj and mj j-reweighted QCD- Z jj templates to the

background-subtracted data, from which the measured EW- Z jj cross-section isextracted.Fig. 4(b)demonstrateshow thedatain the EW-enriched region is modelledwith the fitted EW- Z jj and mj j-reweightedQCD- Z jj templates,forthethreedifferentQCD- Z jj

event generators (and their corresponding correction factors de-rived in the QCD-enriched region shown in Fig. 3(a)). Despite significantly different modelling of the mj j distribution between

event generators, and different models foradditional QCD radia-tion,theresultsofthecombinedcorrectionandfitproceduregive aconsistentdescriptionofthedata.

6.2. SystematicuncertaintiesintheEW- Z jj fiducialcross-section Thetotalsystematicuncertaintyinthecross-sectionforEW- Z jj productioninthe EW-enrichedfiducialregion is17% (16%inthe EW-enriched mj j

>

1 TeV region). The sources and size of each

systematicuncertaintyaresummarisedinTable 4.

Systematic uncertainties associated with the EW- Z jj signal templateusedinthefitandEW- Z jj signalextractionareobtained fromthe variation inthe measured cross-section when using ei-ther oftheindividual EW- Z jj MCsamples (Powheg and Sherpa)

Fig. 4. (a) ComparisonintheEW-enrichedregionofthesumofEW- Z jj andmj j-reweightedQCD- Z jj templatestothedata(minusthenon- Z jj backgrounds).The

normal-isationofthetemplatesisadjustedtotheresultsofthefit(seetextfordetails).TheEW- Z jj MCsamplecomesfromthe Powheg eventgeneratorandtheQCD- Z jj MC

samplecomesfromthe Alpgen eventgenerator.(b) TheratioofthesumoftheEW- Z jj andmj j-reweightedQCD- Z jj templatestothebackground-subtracteddatainthe

EW-enrichedregion,forthreedifferentQCD- Z jj MCpredictions.Thenormalisationofthetemplatesisadjustedtotheresultsofthefit.Errorbarsrepresentthestatistical uncertaintiesinthedataandcombinedQCD- Z jj plusEW- Z jj MCsamplesaddedinquadrature.Thehatchedbandrepresentsexperimentalsystematicuncertaintiesinthe

mj jdistribution.

Table 4

SystematicuncertaintiescontributingtothemeasurementoftheEW- Z jj cross-sectionsformj j >250 GeVand

mj j >1 TeV.UncertaintiesaregroupedintoEW- Z jj signalmodelling,QCD- Z jj backgroundmodelling,QCD-EW

interference,non- Z jj backgrounds,andexperimentalsources.

Source Relative systematic uncertainty [%]

σmj j>250 GeV

EW σ

mj j>1 TeV

EW

EW-Z jj signal modelling (QCD scales, PDF and UEPS) ±7.4 ±1.7 EW-Z jj template statistical uncertainty ±0.5 ±0.04 EW-Z jj contamination in QCD-enriched region −0.1 −0.2 QCD-Z jj modelling (mj jshape constraint / third-jet veto) ±11 ±11

Stat. uncertainty in QCD control region constraint ±6.2 ±6.4 QCD-Z jj signal modelling (QCD scales, PDF and UEPS) ±4.5 ±6.5 QCD-Z jj template statistical uncertainty ±2.5 ±3.5

QCD-EW interference ±1.3 ±1.5

¯

tt and single-top background modelling ±1.0 ±1.2

Diboson background modelling ±0.1 ±0.1

Jet energy resolution ±2.3 ±1.1

Jet energy scale +5.3/−4.1 +3.5/−4.2 Lepton identification, momentum scale, trigger, pile-up +1.3/−2.5 +3.2/−1.5

Luminosity ±2.1 ±2.1

Total ±17 ±16

comparedtotheaverageofthetwo,takenasthecentralvalue. Un-certainties intheEW- Z jj templatesduetovariations ofthe QCD scales,ofthePDFs,andoftheUEPSmodelarealsoincludedasare statisticaluncertaintiesinthetemplatesthemselves.

FollowingtheextractionoftheEW- Z jj cross-sectioninthe EW-enrichedregions,thenormalisationsoftheEW- Z jj MCsamplesare modified to agree with the measurements and the potential EW contamination ofthe QCD-enriched region is recalculated,which leads to a modification of the QCD- Z jj correction factors. The EW- Z jj cross-section measurement is repeated with these mod-ified QCD- Z jj templates and the change in the resultant cross-sections is assigned as a systematic uncertainty associated with theEW- Z jj contaminationoftheQCD-enrichedregion.

As discussed inSection 6.1,the useofa QCD-enriched region provides a way to correctfor QCD- Z jj modelling issues and also constrains theoretical and experimental uncertainties associated withobservablesconstructedfromthetwoleadingjets.

Neverthe-less, the largest contributionto the total uncertainty arisesfrom modelling uncertainties associated with propagation of the mj j

correctionfactorsforQCD- Z jj intheQCD-enrichedregionintothe EW-enriched region, and these correction factors depend on the modellingoftheadditionaljetactivityintheQCD- Z jj MCsamples usedin themeasurement.The uncertaintyisassessed by repeat-ing the EW- Z jj cross-section measurement with mj j-reweighted

QCD- Z jj MCtemplates from Alpgen, MG5_aMC,and Sherpa, and assigning the variation of the measured cross-sections from the central EW- Z jj result as a systematic uncertainty. Statistical un-certaintiesfromdataandsimulationinthemj j correction factors

derived in the QCD-enriched region are alsopropagated through tothemeasuredEW- Z jj cross-sectionasasystematicuncertainty. Uncertainties associated with QCD renormalisation and factori-sation scales, PDF error sets, and UEPS modelling are assessed by studying the change in the extracted EW- Z jj cross-sections whenrepeating themeasurementprocedure,includingrederiving

mj j correction factors inthe QCD-enriched region and repeating

fitsinthe EW-enrichedregion, usingmodified QCD- Z jj MC tem-plates.StatisticaluncertaintiesintheQCD- Z jj templateinthe EW-enrichedregionarealsopropagatedasasystematicuncertaintyin theEW- Z jj cross-sectionmeasurement.

Potential quantum-mechanical interference between the QCD- Z jj andEW- Z jj processesisassessedusing MG5_aMCto de-riveacorrectiontotheQCD- Z jj templateasafunctionofmj j.The

impactofinterference onthe measurementis determinedby re-peatingtheEW- Z jj measurementproceduretwice,eitherapplying thiscorrectiontotheQCD- Z jj templateonlyintheQCD-enriched regionoronlyintheEW-enrichedregionandtakingthemaximum change in the measured EW- Z jj cross-section as a symmetrised uncertainty. This approach assumes the interference affects only oneof thetwo fiducialregions and thereforehasa maximal im-pact on the signal extraction.Potential interference between the Z jj anddibosonprocesseswasfoundtobenegligible.

Normalisation and shape uncertainties in the estimated back-groundfromtop-quarkand dibosonproductionareassessed with varied background templates as described in Section 5.4, albeit withsignificantly largeruncertainties intheEW-enriched fiducial regioncomparedtothebaselineregion.

Experimentalsystematic uncertainties arising fromthejet en-ergy scale and resolution, from lepton efficiencies related to re-construction, identification, isolation and trigger, and lepton en-ergy/momentumscaleandresolution,andfrompile-upmodelling, are independently assessed by repeating the EW- Z jj measure-ment procedure using modified QCD- Z jj and EW- Z jj templates. Here, the QCD-enriched QCD- Z jj template constraint procedure describedinSection 6.1hastheaddedbenefitofsignificantly re-ducingthejet-basedexperimentaluncertainties,ascanbeseenin

Table 4fromtheirsmallimpactonthetotalsystematicuncertainty.

6.3.ElectroweakZ jj results

Asintheinclusive Z jj cross-sectionmeasurements,thequoted EW- Z jj cross-sectionmeasurementsare theaverageofthe cross-sectionsdeterminedwitheachofthesixcombinationsofthethree QCD- Z jj MC templatesand two EW- Z jj MC templates.The mea-suredcross-sections for the EW production ofa leptonically de-caying Z bosonandatleasttwojetssatisfyingthefiducial require-mentsfortheEW-enrichedregionsasgiveninTable 1withthe re-quirementsmj j

>

250 GeV andmj j

>

1 TeV areshowninTable 5,

wheretheyarecomparedtopredictionsfrom Powheg+Pythia.The useofadifferentialtemplatefitinmj j toextracttheEW- Z jj signal

allowssystematic uncertaintiesontheEW- Z jj cross-section mea-surementstobeconstrainedbythebinswiththemostfavourable balanceofEW- Z jj signalpurity andminimal shapeand normali-sationuncertainty.Forthemj j

>

250 GeV region,althoughallmj j

bins contribute to the fit, the individually most-constraining mj j

intervalisthe900–1000 GeVbin.Theuseofthismethodresultsin verysimilarrelativesystematicuncertaintiesintheEW- Z jj cross-sectionmeasurementsatthetwodifferentmj j thresholds,despite

themeasuredrelativeEW- Z jj contributiontothetotalZ jj ratefor mj j

>

1 TeV beingmorethansixtimestherelativecontributionof

EW- Z jj formj j

>

250 GeV.

The EW- Z jj cross-sections at

√

s

=

13 TeV are in agreement withthepredictionsfrom Powheg+Pythia forbothmj j

>

250 GeV

and mj j

>

1 TeV. The effect on the measurement of inclusive

Z jj productionrates(Section5.5)fromcorrectingtheEW- Z jj pro-ductionratespredicted by Powheg+Pythia tothemeasured rates presented here was found to be negligible. Modifications to the mj j distribution shape are alreadyaccounted foras a systematic

uncertaintyintheinclusive Z jj measurements.

Fig. 5. Fiducialcross-sectionsforaleptonicallydecayingZ bosonandatleasttwo jets(soliddatapoints)andEW- Z jj production(opendatapoints)at13 TeV (cir-cles) compared toequivalentresults at 8 TeV[2] (triangles) and to theoretical predictions(shaded/hatchedbands).MeasurementsofZ jj at13 TeVarecompared topredictionsfrom Sherpa (QCD- Z jj)+ Powheg (EW- Z jj), MG5_aMC(QCD- Z jj)+ Powheg(EW- Z jj),and Alpgen (QCD- Z jj)+ Powheg (EW- Z jj),whilemeasurements ofEW- Z jj productionarecomparedto Powheg (EW- Z jj).Resultsat8 TeVare com-paredtopredictionsfrom Powheg+Pythia (QCD- Z jj +EW- Z jj).Thebottompanel showstheratioofthevarioustheorypredictionstodataasshadedbands.Relative uncertaintiesinthemeasureddataarerepresentedbyanerrorbarcentredatunity.

Fig. 6. MeasurementsoftheEW- Z jj processpresentedinthisLetterata centre-of-massenergyof13 TeV,comparedwithpreviousmeasurementsat8 TeV[2],for twodifferentdijetinvariantmassthresholds,mj j>0.25 TeV andmj j>1 TeV.The

errorbarsonthemeasurementsrepresentstatisticalandsystematicuncertainties addedinquadrature.Predictionsfromthe Powheg eventgeneratorwiththeirtotal uncertaintyarealsoshown.

Fig. 5 shows a summary of the fiducial cross-sections for a

leptonically decaying Z boson and at least two jets at 13 TeV comparedtoequivalentresultsat8 TeV[2]andtotheoretical pre-dictionswiththeiruncertainties.Asignificantriseincross-section is observed between

√

s

=

8 TeV and

√

s

=

13 TeV within each fiducial region. In the EW-enriched region, formj j thresholds of

250 GeVand1 TeV,themeasuredEW- Z jj cross-sectionsat13 TeV are found to be respectively 2.2and 3.2times as large as those measuredat8 TeV,asillustratedinFig. 6.

Table 5

MeasuredandpredictedEW- Z jj productioncross-sectionsintheEW-enrichedfiducialregionswithand withoutanadditionalkinematicrequirementofmj j>1 TeV.Forthemeasuredcross-sections,thefirst

uncertaintygivenisstatistical,thesecondissystematicandthethirdisduetotheluminosity determi-nation.Forthepredictions,thequoteduncertaintyrepresentsthestatisticaluncertainty,plussystematic uncertaintiesfromthePDFsandfactorisationandrenormalisationscalevariations,alladdedin quadra-ture.

Fiducial region EW-Z jj cross-sections [fb]

Measured Powheg+Pythia

EW-enriched, mj j>250 GeV 119 ±16 ±20 ±2 125.2±3.4

EW-enriched, mj j>1 TeV 34.2 ±5.8 ±5.5 ±0.7 38.5±1.5

7. Summary

Fiducial cross-sections for the electroweak production of two jetsinassociationwithaleptonicallydecaying Z bosoninproton– proton collisions are measured at a centre-of-mass energy of 13 TeV, using data corresponding to an integrated luminosity of 3.2 fb−1 recorded with the ATLAS detector at the Large Hadron Collider.TheEW- Z jj cross-sectionisextracted inafiducialregion chosen to enhance theEW contributionrelative to thedominant QCD- Z jj process, which is constrained using a data-driven ap-proach. Themeasured fiducialEW cross-sectionis

σ

EWZjj

=

119

±

16

(

stat

.)

±

20

(

syst

.)

±

2

(

lumi

.)

fb fordijetinvariantmassgreater than250 GeV, and34

.

2

±

5

.

8

(

stat

.)

±

5

.

5

(

syst

.)

±

0

.

7

(

lumi

.)

fb fordijetinvariantmassgreaterthan1 TeV.Acomparisonwith pre-viouslypublishedmeasurementsat

√

s

=

8 TeV ispresented,with measured EW- Z jj cross-sections at

√

s

=

13 TeV found tobe 2.2 (3.2) timesas large as thosemeasured at

√

s

=

8 TeV inthe low (high) dijet mass EW-enrichedregions. Relativeto measurements at

√

s

=

8 TeV, the increased

√

s allows a region of higherdijet mass tobe explored,in whichtheEW- Z jj signalis more promi-nent. The StandardModel predictions are inagreement with the EW- Z jj measurements.

The inclusive Z jj cross-section is alsomeasured insix differ-ent fiducial regions with varying contributions fromEW- Z jj and QCD- Z jj production. At higher dijet invariant masses (

>

1 TeV), particularlycrucialforprecisionmeasurementsofEW- Z jj produc-tion and forsearchesfor newphenomena invector-boson fusion topologies,predictionsfrom Sherpa (QCD- Z jj)+ Powheg (EW- Z jj) and MG5_aMC (QCD- Z jj) + Powheg (EW- Z jj) are found to sig-nificantlyoverestimate theobserved Z jj productionratesindata. Alpgen (QCD- Z jj)+ Powheg (EW- Z jj)provides a better descrip-tionofthemj j shape.

Acknowledgements

We thank CERN forthe very successful operation ofthe LHC, as well as thesupport staff fromourinstitutions without whom ATLAScouldnotbeoperatedefficiently.

WeacknowledgethesupportofANPCyT,Argentina;YerPhI, Ar-menia; ARC,Australia;BMWFWandFWF,Austria;ANAS, Azerbai-jan; SSTC,Belarus;CNPqandFAPESP, Brazil;NSERC,NRC andCFI, Canada;CERN;CONICYT,Chile;CAS,MOSTandNSFC,China; COL-CIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Re-public; DNRF and DNSRC,Denmark; IN2P3-CNRS, CEA-DSM/IRFU, France; SRNSF, Georgia; BMBF, HGF, and MPG, Germany; GSRT, Greece;RGC,HongKongSAR,China;ISF,I-COREandBenoziyo Cen-ter, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; NWO, Netherlands; RCN, Norway;MNiSW and NCN, Poland;FCT, Portugal; MNE/IFA, Romania; MES of Russia and NRC KI, Russian Federation;JINR;MESTD,Serbia; MSSR,Slovakia; ARRSandMIZŠ, Slovenia;DST/NRF,SouthAfrica;MINECO,Spain;SRCandKnutand Alice WallenbergFoundation, Sweden; SERI,SNSF and Cantonsof BernandGeneva,Switzerland;MOST,Taiwan;TAEK,Turkey;STFC,

UnitedKingdom;DOEand NSF,UnitedStatesofAmerica.In addi-tion,individual groupsand members havereceived supportfrom BCKDF,theCanadaCouncil,Canarie,CRC,ComputeCanada,FQRNT, and the Ontario Innovation Trust, Canada; EPLANET, ERC, ERDF, FP7,Horizon 2020and MarieSkłodowska-CurieActions,European Union; Investissementsd’AvenirLabexand Idex, ANR,Région Au-vergne and Fondation Partager le Savoir, France; DFG and AvH Foundation,Germany;Herakleitos,ThalesandAristeiaprogrammes co-financedbyEU-ESFandtheGreekNSRF;BSF,GIFandMinerva, Israel; BRF,Norway; CERCA Programme Generalitat de Catalunya, Generalitat Valenciana, Spain; the Royal Society and Leverhulme Trust,UnitedKingdom.

The crucialcomputing support fromall WLCG partners is ac-knowledged gratefully, in particular from CERN, the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Swe-den),CC-IN2P3(France),KIT/GridKA(Germany),INFN-CNAF(Italy), NL-T1(Netherlands),PIC(Spain),ASGC(Taiwan),RAL(UK)andBNL (USA),theTier-2facilitiesworldwideandlargenon-WLCGresource providers.Majorcontributorsofcomputingresourcesare listedin Ref.[53].

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TheATLASCollaboration

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M. Adersberger

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T. Adye

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A.A. Affolder

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Y. Afik

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T. Agatonovic-Jovin

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C. Agheorghiesei

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H. Akerstedt

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J. Albert

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P. Albicocco

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C. Alexa

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G. Alexander

155

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T. Alexopoulos

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M. Alhroob

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B. Ali

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M. Aliev

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B.W. Allen

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A. Alonso

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C. Alpigiani

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A.A. Alshehri

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M.I. Alstaty

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B. Alvarez Gonzalez

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D. Álvarez Piqueras

170

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M.G. Alviggi

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B.T. Amadio

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