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

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-section for the productionof two jetsinassociation with aleptonically decaying 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 a fiducial region chosen to enhance the

electroweak contribution relative to the dominant Drell–Yan Z jj process, which is constrained using

adata-drivenapproach. 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 radiation from the incoming partons in the Drell–Yan process (QCD- Z jj), asshownin Fig. 1(a).Incontrast,the produc-tionofZ jj eventsviat-channelelectroweakgaugebosonexchange (EW- Z jj events),including the vector-bosonfusion (VBF) process shownin

Fig. 1

(b),isamuchrarerprocess.SuchVBFprocessesfor vector-bosonproductionareofgreatinterestasa‘standardcandle’ forother VBF processesatthe LHC: e.g.,the productionof Higgs bosons or thesearch forweakly interacting 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-handed coordinatesystemwith itsoriginat thenominal}

interaction point inthe centre ofthe detector and the z-axisalong the beam

pipe.Inthetransverseplane,the x-axispointsfromtheinteractionpointtothe centreofthe LHCring,the y-axispoints upward,andφ isthe azimuthalangle aroundthe z-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.Aconsequenceoftheexchangeofavectorbosonin

Fig. 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 of EW- Z jj productionwere performed [1]in pp collisionsatacentre-of-massenergy(

√

s)of7 TeVbytheCMS Collaboration,wherethebackground-onlyhypothesiswasrejected atthe2

.

6σ level.ThefirstobservationoftheEW- Z jj processwas performedbytheATLAS Collaborationatacentre-of-massenergy (

√

s)of8 TeV

[2]

.Thecross-sectionmeasurementisinagreement withpredictionsfromthe Powheg-box eventgenerator

[3–5]

and allowed limitsto beplaced onanomalous triplegauge couplings. The CMS Collaboration has also observed and measured [6] the cross-sectionforEW- Z jj productionat8 TeV.ThisLetterpresents measurements ofthecross-section forEW- Z jj productionand in-clusive Z jj productionathighdijetinvariantmassinpp collisions at

√

s

=

13 TeV usingdatacorrespondingtoanintegrated luminos-ityof3.2 fb−1 collectedby theATLAS detectorattheLHC.These measurements allow the dependence of 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-anismsforaleptonicallydecayingZ 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

TheATLASdetectorisdescribed indetailinRefs.

[7,8]

.It con-sistsofaninnerdetectorfortracking,surroundedbyathin super-conducting solenoid, electromagnetic and hadronic calorimeters, andamuonspectrometerincorporatingthreelarge superconduct-ingtoroidalmagnetsystems.Theinnerdetectorisimmersedina 2 Taxial magneticfield and providescharged-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-precisiontrackingchambers. The trackingchamberscoverthe re-gion

|

η

|

<

2

.

7 withthreelayersofmonitoreddrifttubes, comple-mentedby cathodestrip chambersinpartof theforwardregion, wherethehitrateishighest. Themuontriggersystemcoversthe range

|

η

|

<

2

.

4 withresistiveplatechambersinthebarrelregion, andthingapchambersintheend-capregions.

A two-level trigger system is used to select events of inter-est

[9]

. TheLevel-1 triggeris implementedinhardware anduses a subset of the detectorinformation to reduce the event rateto 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 2.2.0eventgenerator

[11]

.Formodellingofthe partonshower,fragmentation,hadronisation andunderlyingevent (UEPS), Powheg-box wasinterfacedto Pythia 8

[12]

witha ded-icated set of parton-shower-generator parameters (tune) denoted AZNLO_{[13] andthe CT10 NLO partondistributionfunction (PDF)} set

[14]

.The renormalisation andfactorisationscales were set to

the Z boson mass. Sherpa predictions used the Comix [15] and OpenLoops [16] matrix elementevent generators,and theCKKW method was used to combine the various final-state topologies from the matrixelement andmatch them to the partonshower

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

[21]

.

The production of QCD- Z jj events was simulated usingthree event generators, Sherpa _2.2.1, Alpgen _{2.14 [22] and} MadGraph5_aMC@NLO 2.2.2 [23]. Sherpa provides Z

+

n-parton predictionscalculatedforup totwopartonsatNLO accuracyand up to four partons at LO accuracy in perturbative QCD. Sherpa predictionsusedthe NNPDF30NLO PDFsettogether withthe tun-ingoftheUEPSparametersdevelopedbythe Sherpa authorsusing the ME+PS@NLO prescription

[19,20]

. Alpgen isanLOevent gener-atorwhichusesexplicitmatrixelementsforuptofivepartonsand was interfacedto Pythia 6.426

[24]

usingthe Perugia2011Ctune

[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 using the NNPDF23LO PDF set [28]. Forreconstruction-level studies,total Z bosonproductionratespredictedbyallthreeevent generators used to produce QCD- Z jj predictions are normalised usingthenext-to-next-to-leading-order (NNLO)predictions calcu-lated withthe FEWZ 3.1 program [29–31] using the CT10 NNLO PDF set [14]. However, when comparing particle-leveltheoretical predictionstodetector-correctedmeasurements,thenormalisation ofquotedpredictions isprovidedbytheeventgeneratorin ques-tionratherthananexternalNNLOprediction.

TheproductionofapairofEWvectorbosons(diboson),where onedecaysleptonicallyandtheotherhadronically, 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 Sherpa2.1.1andthe CT10 NLO PDFset.

Thelargestbackgroundtotheselected Z jj samplesarisesfrom tt and

¯

single-top (W t) production. These were generated using Powheg-box v2and Pythia 6.428withthePerugia2012tune

[25]

, andnormalisedusingthecross-section calculatedatNNLO

+

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]

simulationoftheATLAS detector

[34]

.The effectof additional pp interactions (pile-up)in thesame ornearbybunch crossingswasalsosimulated,using Pythia v8.186withthe A2tune

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

4. Eventpreselection

The Z bosonsaremeasuredintheirdielectronanddimuon de-cay modes.Candidate eventsare selectedusingtriggers requiring atleastone identifiedelectronormuon withtransverse 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 the electromagneticcalorimeter matched toinner-detector tracks

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

>

25 GeV and

|

η

| <

2.47,

exclud-ingthetransitionregionbetweenthebarrelandend-cap calorime-tersat1.37

<

|

η

| <

1.52.Candidatemuonsareidentifiedastracks intheinner detectormatchedandcombinedwithtracksegments inthemuonspectrometer. TheymustsatisfytheMedium identifi-cationrequirementsdescribedinRef.[38],andhavepT

>

25 GeV

and

|

η

| <

2.4. Candidate leptons mustalso satisfya set of isola-tioncriteriabasedonreconstructedtracksandcalorimeteractivity. Events are required to contain exactly two leptons of the same flavour butof opposite charge.The dilepton invariant massmust satisfy81

<

m

<

101 GeV.

Candidate hadronic jets are required to satisfy pT

>

25 GeV

and

|

y

| <

4.4. Theyare reconstructed fromclustersofenergy in the calorimeter[39] usingthe anti-kt algorithm [40,41] with

ra-dius parameter R

=

0

.

4. Jet energies are calibrated by applying pT- and y-dependent correctionsderived from MonteCarlo

sim-ulationwithadditionalinsitucorrectionfactorsdetermined from data [42]. To reduce the impact ofpile-up contributions, all jets with

|

y

|

<

2

.

4 and pT

<

60 GeV are required to be compatible

withhaving originatedfromthe primary vertex (the vertexwith the highest sum of track p2_T), as defined by the jet vertex tag-geralgorithm

[43]

.Selected electronsandmuonsarediscarded if theylie within



R

=

0

.

4 of areconstructedjet. Thisrequirement isimposed to removenon-prompt non-isolated leptons produced in heavy-flavourdecays orfrom thedecay in flight ofa kaon or pion.

5. MeasurementofinclusiveZ j j fiducialcross-sections

5.1. Definitionofparticle-levelcross-sections

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

σ

f

=

N f obs

−

N f bkg L

·

C

f

,

(1)

where N_obsf is the number of events observed in the data pass-ing theselection requirements ofthefiducial regionunder study at detectorlevel, N_bkgf is the corresponding number of expected background(non- Z jj)events,L istheintegratedluminosity corre-spondingtotheanalyseddatasample,and

C

f _{isacorrectionfactor}

appliedtotheobserveddatayields,whichaccountsfor experimen-tal efficiencyanddetector resolutioneffects, andis derived from MCsimulationwithdata-drivenefficiencyandenergy/momentum scalecorrections.Thiscorrectionfactoriscalculatedas:

C

f

₌

N

f det N_particlef

,

where N_detf is the numberof signal events that satisfy the fidu-cial selection criteriaatdetectorlevel in theMC simulation, and

N_particlef isthenumberofsignaleventsthatpasstheequivalent

se-lectionbut atparticle level. These correction factors havevalues between0

.

63 and0

.

77,dependingonthefiducialregion.

Withtheexception ofbackgroundfrommultijetandW

+

jets processes(henceforthreferredtotogether simplyasmultijet pro-cesses),contributionsto N_bkgf areestimatedusingtheMonteCarlo

samples described in Section 3.Background from multijetevents is estimated fromthe data by reversing requirements on lepton identification or isolation to derive a template for the contribu-tion of jets mis-reconstructedas lepton candidates asa function ofdileptonmass.Non-multijet backgroundis subtractedfromthe template usingsimulation.The normalisationisderived byfitting thenominaldileptonmassdistributionineachfiducialregionwith the sumofthe multijettemplate andatemplate comprising sig-nalandbackgroundcontributionsdeterminedfromsimulation.The multijetcontributionisfoundtobelessthan0.3%ineachfiducial region.ThecontributionfromW

+

jets processeswaschecked us-ing MC simulation andfound to be much smaller than the total multijetbackgroundasdeterminedfromdata.

At particlelevel,only final-stateparticles withproperlifetime 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-quiredto satisfy pT

>

25 GeV and

|

η

|

<

2

.

47.Events arerequired

tocontain exactlytwodressedleptons ofthesameflavourbutof oppositecharge,andthedileptoninvariantmassmustsatisfy81

<

m

<

101 GeV. Jetsarereconstructed usingthe anti-kt algorithm

with radius parameter R

=

0

.

4. Prompt leptons andthe photons used to dresstheseleptons are not includedin theparticle-level jet reconstruction. All remaining 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-space regions definedinTable 1. Theseregions arechosen tohavevaryingcontributionsfromEW- Z jj andQCD- Z jj processes. 5.2. Eventselection

Following Ref. [2], events are selected in sixdetector fiducial regions. Asfaraspossible,theseare definedwiththesame kine-matic requirements asthe six phase-space regions in which the cross-sectionismeasured(Table 1).Thisminimisessystematic un-certaintiesinthemodellingoftheacceptance.

Thebaselinefiducialregionrepresentsaninclusiveselectionof eventscontainingaleptonicallydecayingZ bosonandatleasttwo jetswithpT

>

45 GeV,atleastoneofwhichsatisfiespT

>

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.The requirementof 81

<

m

<

101 GeV

sup-presses other sources ofdileptonevents,such astt and

¯

Z

→

τ τ

, aswellasthemultijetbackground.

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

>

1 TeV)isrequired,andinthehigh-pT

fidu-cialregiontheminimumpToftheleadingandsub-leadingjetsis

increasedto85 GeVand75 GeVrespectively.TheEW- Z jj process typicallyproduces harderjettransversemomentaandresultsina 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 requiring mj j

>

250 GeV, a dilepton transverse momentum

of p_T

>

20 GeV,andthatthenormalisedtransverse momentum balance betweenthe twoleptons andthe two highesttransverse

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 is thetransversemomentum vector ofobjecti,



1 and



2labelthetwoleptonsthatdefinethe Z bosoncandidate,and j1

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

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

badlymeasured andit enhancesthe EW- Z jj contribution,where the lower probability of additional radiation 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(N_jetinterval_(p

T>25 GeV)

=

0). A second 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-pressthe EW- Z jj contributionrelativetoQCD- Z jj byrequiringat leastonejetwith pT

>

25 GeV tobereconstructedwithinthe

ra-pidityintervalboundedbythedijetsystem(Ninterval_jet(p

T>25 GeV)

≥

1).

IntheQCD-enrichedregion,thedefinitionofthenormalised trans-versemomentumbalanceismodifiedfromthatgiveninEq.(2)to includeinthe calculationofthe numeratoranddenominator the pT of thehighest pT jet within therapidity interval 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-tronchanneland36786 eventsareselected inthedimuon chan-nel.Thetotalobservedyieldsare inagreementwiththeexpected yieldswithinstatisticaluncertaintiesineachdileptonchannel.The largestdeviationacrossallfiducialregionsisa2σ (statistical) dif-ference between the expected to observed ratio in the electron versusmuonchannelinthehigh-pT region.

Theexpectedcomposition ofthe selecteddata samplesinthe sixZ jj fiducialregionsissummarisedin

Table 2

,averagingacross thedielectronanddimuonchannelsasthesecompositionsinthe

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

whichisdiscussedbelowandaccountedforintheassessmentof systematicuncertainties inthemeasurement.

5.4. SystematicuncertaintiesintheinclusiveZ jj fiducialcross-sections Experimentalsystematicuncertainties affectthe determination of the

C

f _{correction factor and the background estimates. The}

dominantsystematicuncertaintyintheinclusiveZ 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-enriched region (anaverage of0.4additionaljets). Other ex-perimentalsystematicuncertaintiesarisingfromleptonefficiencies relatedto reconstruction, identification, isolation andtrigger, and leptonenergy/momentumscaleandresolutionaswellasfromthe effectofpile-up,amounttoatotalofaround1–2%,depending on thefiducialregion.

The systematic uncertainty arising from the MC modelling of the mj j distributionin the QCD- Z jj and EW- Z jj signal processes

is around 3% in theEW-enriched region, around 1% in the QCD-enriched region, 2% inthe high-mass region, andbelow1% else-where. This is assessed by comparing the correction factors ob-tained by usingthe differentMC event generators listed in Sec-tion3andbyperformingadata-drivenreweightingoftheQCD- Z jj MCsampletodescribethemj jdistributionoftheobserveddatain

agivenfiducialregion.Additionalcontributionsarisefromvarying theQCDrenormalisationandfactorisationscalesupanddownby a factor of two independently, andfrom the propagation of un-certaintiesinthePDFsets.Thenormalisationofthediboson con-tribution isvaried accordingto PDF andscale 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),arisingfromchangesintheextractedZ jj cross-sectionswhen usingmodified top-quarkbackground MC samples withPDFandscalevariations, 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 t¯t 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 production sample 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 calibration ofthe luminosity scaleusing x– 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(takingintoaccounttotaluncertainties)acrossboth channels. Thesecross-sections aredetermined using each 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 betweenQCD- Z jj and EW- Z jj models isassignedasasourceofsystematicuncertainty(1%inallregions excepttheEW-enrichedregion wherethevariationis3% 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.

The largestdifferences betweenpredictions andmeasurement 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 have relative uncertainties with re-spect tothe measurement of 36% and32%. Forthe EW-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 overestimate measured ratesby 33% and57%, withrelative uncertaintiesof16%and15%.Someofthesedifferencesarisefrom a significant mismodellingofthe QCD- Z jj contribution, as inves-tigated and discussed in detail in Section 6.1. Predictions from

Alpgen(QCD- Z jj)+ Powheg (EW- Z jj)are inagreementwiththe 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) andthissignal fraction grows with in-creasing mj j to 26.1% formj j

>

1 TeV. The QCD-enriched region

has an overall expectedEW- Z jj signal fractionof 1.6%increasing to 4.4%formj j

>

1 TeV.The dominantbackgroundto theEW- 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

f EW

,

(3)

with the samenotations asin Eq.(1) andwhere N_{QCD- Zjj}f is the expectednumberofQCD- Z jj eventspassingtheselection require-mentsofthefiducialregionatdetectorlevel,N_bkgf istheexpected number ofbackground(non- Z jj and diboson)events,and

C

_EWf is acorrection factorappliedtotheobservedbackground-subtracted datayields that accountsforexperimental efficiencyanddetector 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) whenusingthe Powheg EW- Z jj pre-diction.

Detector-levelcomparisonsofthemj j distributionbetweendata

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

Measuredand predictedinclusive 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 in the EW-enriched region the EW- Z jj component becomes prominentatlargevaluesofmj j.However,

Fig. 2

(b)demonstrates

that the shape ofthe mj j distribution for QCD- 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_aMCand Sherpa overestimatethedataby 80%and 120%respectively, formj j

=

2 TeV, well outsidethe uncertainties

onthesepredictionsdescribedin

Table 3

.Thesediscrepancieshave beenobserved previously in Z jj [2,48] and Wjj [49–51] produc-tionathighdijetinvariant massandathighjetrapidities.Forthe purposeofextractingthecross-sectionforEW- Z jj production,this mismodellingofQCD- Z jj iscorrected forusinga 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

thedata in the EW-enriched region, aftersubtraction ofnon- Z jj anddibosonbackground,usingalog-likelihoodmaximisation

[52]

. FollowingtheprocedureadoptedinRef.[2],thedatainthe

QCD-enriched region are used toevaluate detector-level shape correc-tion factors for the QCD- Z jj MC predictions bin-by-bin in mj j.

Thesedata-to-simulationratiocorrectionfactorsareappliedtothe simulation-predicted shapeinmj j oftheQCD- Z jj contribution in

the EW-enriched region.This procedure ismotivated by two ob-servations:

(a) theQCD-enrichedregionandEW-enrichedregionaredesigned tobe kinematicallyverysimilar, differingonlywithregardto thepresence/absenceofjetsreconstructedwithintherapidity intervalboundedbythedijetsystem,

(b) the contributionofEW- Z jj totheregion ofhighmj j is

sup-pressedintheQCD-enrichedregion(4.4%formj j

>

1 TeV)

rel-ativetothatintheEW-enrichedregion(26.1%formj j

>

1 TeV)

(also illustratedin Fig. 2);the impactofthe residualEW- Z jj contamination in the QCD-enriched region is assigned as a componentofthesystematicuncertaintyintheQCD- Z jj back-ground.

The shape correction factors in mj j obtained using the three

different QCD- Z jj MC samplesare shown in Fig. 3(a). These are derived asthe ratio ofthe data to simulation in bins 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-enrichedregion.Abinnedfittothecorrection factorsderived indijetinvariant mass isperformedwithalinear fit function(and also witha quadratic fitfunction) toproduce a continuouscorrectionfactor.Thelinearfitisillustratedoverlaidon thebinnedcorrectionfactorsin

Fig. 3

(a).Thenominalvalueofthe EW- Z jj cross-section correspondingtoaparticularQCD- Z jj event generatortemplateisdeterminedusingthecorrectionfactorsfrom the linearfit. The change inresultant EW- Z jj cross-section from using binned correction factors directly is assessed asa system-aticuncertainty.ThechangeintheextractedEW- Z jj cross-section when usinga quadraticfit was found tobe negligible.The vari-ations observed betweenevent generators may be partlydue to differencesinthemodelling ofQCD radiationwithin therapidity interval bounded by the dijet system, which affects the extrap-olationfrom thecentral-jet-enriched QCD-enrichedregion to the central-jet-suppressedEW-enrichedregion.The variationbetween eventgenerators ismuch larger than theeffect ofPDF andscale uncertaintiesinaparticularprediction(indicatedin

Fig. 3

(a)bya shadedband onthepredictionsfrom Sherpa).Estimatingthe un-certainties associated with QCD- Z jj mismodelling from PDF and scale variations around a single generator predictionwould thus resultinan underestimate ofthe truetheoretical uncertainty as-sociated with this mismodelling. In this measurement, the span ofresultantEW- Z jj cross-sectionsextractedfromtheuseofeach ofthethreeQCD- Z jj templates isassessedasasystematic uncer-tainty.ThevariationintheEW- Z jj cross-sectionmeasurementdue toa changeintheEW- Z jj signaltemplate usedinthederivation ofthemj j correctionfactors(from Powheg to Sherpa)isfoundto

benegligible.

To test the dependence of the QCD- Z jj correction factors on the modelling of any additional jet emitted in the dijet rapidity interval, theQCD-enriched control region isdivided into pairs of mutuallyexclusivesubsetsaccordingto the

|

y

|

ofthehighest pT

jet within the rapidity interval bounded by the dijetsystem, the pT ofthatjet,orthevalueofN_jetinterval_{(pT>25 GeV)}.Thecontinuous

cor-rectionfactors are determined fromeach subregion usingboth a linearandaquadraticfittothedata.Correctionfactorsderived in the subregions usingquadratic fits result inthe largest variation in the extractedcross-sections. These fits are shown in Fig. 3(b) forthe Alpgen QCD- Z jj sample, which displays 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.

Themeasured electroweakproductioncross-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 Sherpa_{EW- Z jj templateinthefit.Thecentralvalue oftheresult} quoted isthe averageofthe measured EW- Z jj cross-sections de-terminedwitheachofthesixcombinationsofthethreeQCD- 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-enriched region.Separate uncertaintiesareassignedforthe deter-mination of the QCD- Z jj correction factors in the QCD-enriched region and their propagation into the EW-enriched region. The measurement ofthe EW- Z jj cross-sectionintheEW-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 fittedEW- 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 modelled with 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, anddifferent models for additional QCD radia-tion,theresultsofthecombinedcorrectionandfitproceduregive aconsistentdescriptionofthedata.

6.2. SystematicuncertaintiesintheEW- Z jj fiducialcross-section Thetotalsystematicuncertaintyinthecross-sectionforEW- Z jj production intheEW-enriched fiducialregion is17% (16%inthe EW-enriched mj j

>

1 TeV region). The sources and size of each

systematicuncertaintyaresummarisedin

Table 4

.

Systematic uncertainties associated with the EW- Z jj signal templateusedinthefitandEW- Z jj signalextractionareobtained from thevariation in the measured cross-section whenusing ei-ther ofthe individual EW- Z jj MC samples(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-certaintiesinthe EW- Z jj templates duetovariations 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 of the QCD-enrichedregion is recalculated, which leads to a modification of the QCD- Z jj correction factors. The EW- Z jj cross-section measurement is repeatedwith 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.

Asdiscussed inSection 6.1, theuse ofa QCD-enrichedregion provides away to correctforQCD- Z jj modelling issues andalso constrains theoretical and experimental uncertainties associated withobservablesconstructedfromthetwoleadingjets.

Neverthe-less, the largest contribution to the total uncertaintyarises from modelling uncertainties associated with propagation of the mj j

correctionfactorsforQCD- Z jj intheQCD-enrichedregionintothe EW-enriched region, andthese correction factors depend on the modellingoftheadditionaljetactivityintheQCD- Z jj MCsamples used inthemeasurement. Theuncertainty isassessed 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 inthe QCD-enriched region are also propagated 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 themeasurement procedure,includingrederiving

mj j correction factors in the QCD-enriched region and repeating

fits intheEW-enriched region,using modified 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

impact ofinterferenceon themeasurement is determinedby re-peatingtheEW- Z jj measurementproceduretwice,eitherapplying thiscorrection totheQCD- 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 one ofthetwo fiducial regions andthereforehasa maximal im-pacton the signal extraction. Potential interference betweenthe Z jj anddibosonprocesseswasfoundtobenegligible.

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

Experimentalsystematic uncertainties arisingfrom thejet 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 described inSection6.1hasthe addedbenefitofsignificantly re-ducingthejet-basedexperimentaluncertainties,ascanbeseenin

Table 4fromtheirsmallimpactonthetotalsystematicuncertainty. 6.3. ElectroweakZ jj results

AsintheinclusiveZ jj cross-sectionmeasurements,thequoted EW- Z jj cross-sectionmeasurements are theaverageofthe cross-sectionsdeterminedwitheachofthesixcombinationsofthethree QCD- Z jj MCtemplates andtwo EW- Z jj MC templates.The mea-sured cross-sectionsfor the EW productionof a leptonically de-cayingZ bosonandatleasttwojetssatisfyingthefiducial require-mentsfortheEW-enrichedregionsasgivenin

Table 1

withthe re-quirementsmj j

>

250 GeV andmj j

>

1 TeV areshownin

Table 5

,

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

allowssystematicuncertainties ontheEW- Z jj cross-section mea-surementstobeconstrainedbythebinswiththemostfavourable balanceofEW- Z jj signalpurityandminimal 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 verysimilarrelativesystematicuncertainties intheEW- Z jj cross-sectionmeasurementsatthetwodifferentmj j thresholds,despite

themeasuredrelativeEW- Z jj contributiontothetotal Z 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-ductionratespredictedby Powheg+Pythia to themeasured rates presented here was found to be negligible. Modifications to the mj j distribution shape are already accounted foras a systematic

uncertaintyintheinclusiveZ jj measurements.

Fig. 5. Fiducialcross-sectionsforaleptonicallydecayingZ bosonandatleasttwo jets(soliddatapoints)andEW- Z jj production(opendatapoints)at13 TeV (cir-cles) compared toequivalent results 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 2.2 and3.2 times aslarge as those measuredat8 TeV,asillustratedin

Fig. 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-sectionisextractedinafiducialregion chosen toenhance the EW contributionrelative to thedominant QCD- Z jj process, which is constrained using a data-driven ap-proach.The measuredfiducialEWcross-section is

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)times aslargeasthose measuredat

√

s

=

8 TeV inthe low (high) dijetmassEW-enriched regions. Relativeto measurements at

√

s

=

8 TeV, the increased

√

s allows a region of higher dijet massto beexplored, inwhich theEW- Z jj signalismore promi-nent.The Standard Model predictionsare in agreement withthe EW- Z jj measurements.

The inclusive Z jj cross-section is also measured insix differ-ent fiducialregions withvarying contributions fromEW- Z jj and QCD- Z jj production. At higher dijet invariant masses (

>

1 TeV), particularlycrucialforprecisionmeasurementsofEW- Z jj produc-tionandforsearches fornewphenomena in vector-bosonfusion topologies,predictionsfrom Sherpa (QCD- Z jj)+ Powheg (EW- Z jj) and MG5_aMC (QCD- Z jj) + Powheg (EW- Z jj) are found to sig-nificantlyoverestimatethe observed Z jj productionratesindata. Alpgen(QCD- Z jj)+ Powheg (EW- Z jj)provides a better descrip-tionofthemj j shape.

Acknowledgements

We thankCERN for thevery successful operation ofthe LHC, aswell asthe support stafffromour institutions without whom ATLAScouldnotbeoperatedefficiently.

WeacknowledgethesupportofANPCyT,Argentina;YerPhI, Ar-menia;ARC,Australia;BMWFW andFWF,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 andDNSRC, 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 andNCN, Poland;FCT, Portugal; MNE/IFA, Romania; MES of Russiaand NRC KI, Russian Federation;JINR;MESTD,Serbia; MSSR,Slovakia; ARRSandMIZŠ, Slovenia;DST/NRF,SouthAfrica;MINECO,Spain;SRCandKnutand AliceWallenberg Foundation,Sweden;SERI, SNSF andCantonsof BernandGeneva,Switzerland;MOST,Taiwan;TAEK,Turkey;STFC,

UnitedKingdom;DOEandNSF,UnitedStatesofAmerica. In addi-tion, individualgroups andmembershave receivedsupport from BCKDF,theCanadaCouncil,Canarie,CRC,ComputeCanada,FQRNT, and the Ontario Innovation Trust, Canada; EPLANET, ERC, ERDF, FP7, Horizon2020andMarieSkłodowska-Curie Actions,European Union; Investissementsd’AvenirLabexandIdex, 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 crucial computingsupport from all WLCG partnersis 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].

References

[1]CMSCollaboration,MeasurementofthehadronicactivityineventswithaZ andtwojetsandextractionofthecrosssectionfortheelectroweakproduction ofaZwithtwojetsinppcollisionsat√s=7 TeV,J.HighEnergyPhys.10 (2013)062,arXiv:1305.7389[hep-ex].

[2]ATLASCollaboration,Measurementoftheelectroweakproductionofdijetsin associationwithaZ-bosonanddistributionssensitivetovectorbosonfusion inproton–protoncollisionsat√s=8 TeV usingthe ATLASdetector,J.High EnergyPhys.04(2014)031,arXiv:1401.7610[hep-ex].

[3]P.Nason,AnewmethodforcombiningNLOQCD withshowerMonteCarlo algorithms,J.HighEnergyPhys.11(2004)040,arXiv:hep-ph/0409146. [4]S.Frixione,P.Nason,C.Oleari,MatchingNLOQCDcomputationswithParton

Showersimulations:thePOWHEGmethod,J.HighEnergyPhys.11(2007)070, arXiv:0709.2092[hep-ph].

[5]S. Alioli, et al., Ageneral framework for implementingNLO calculationsin shower MonteCarloprograms: the POWHEGBOX,J. HighEnergy Phys. 06 (2010)043,arXiv:1002.2581[hep-ph].

[6]CMSCollaboration,Measurementofelectroweakproductionoftwojetsin as-sociationwithaZbosoninproton–protoncollisionsat√s=8 TeV,Eur.Phys. J.C75(2015)66,arXiv:1410.3153[hep-ex].

[7]ATLASCollaboration,TheATLASexperimentattheCERNLargeHadronCollider, J.Instrum.3(2008)S08003.

[8] ATLASCollaboration,ATLASInsertableB-Layer,TechnicalDesignReport, ATLAS-TDR-19,2010,https://cds.cern.ch/record/1291633;

ATLAS Insertable B-Layer, Technical Design Report Addendum,

ATLAS-TDR-19-ADD-1,https://cds.cern.ch/record/1451888,2012.

[9]ATLAS Collaboration,Performanceofthe ATLAStriggersystemin2015,Eur. Phys.J.C77(2017)317,arXiv:1611.09661[hep-ex].

[10]F. Schissler, D. Zeppenfeld, Partonshower effectson Wand Z production via vector boson fusion at NLO QCD,J. High Energy Phys. 04 (2013) 057, arXiv:1302.2884[hep-ph].

[11]T.Gleisberg,etal.,EventgenerationwithSHERPA1.1,J.HighEnergyPhys.02 (2009)007,arXiv:0811.4622[hep-ph].

[12]T.Sjöstrand,etal.,AnintroductiontoPYTHIA8.2,Comput.Phys.Commun.191 (2015)159,arXiv:1410.3012[hep-ph].

[13]ATLASCollaboration,Measurementofthe Z/γ∗ bosontransversemomentum distributioninppcollisionsat√s=7 TeV withtheATLASdetector,J.High EnergyPhys.09(2014)145,arXiv:1406.3660[hep-ex].

[14]H.-L.Lai,etal.,Newpartondistributionsforcolliderphysics,Phys.Rev.D82 (2010)074024,arXiv:1007.2241[hep-ph].

[15]T.Gleisberg,S.Höche,Comix,anewmatrixelementgenerator,J.HighEnergy Phys.12(2008)039,arXiv:0808.3674[hep-ph].

[16]F.Cascioli,P.Maierhöfer,S.Pozzorini,Scatteringamplitudeswithopenloops, Phys.Rev.Lett.108(2012)111601,arXiv:1111.5206[hep-ph].

[17]S.Catani,etal.,QCDmatrixelements+partonshowers,J.HighEnergyPhys. 11(2001)063,arXiv:hep-ph/0109231.

[18]S.Schumann,F.Krauss,ApartonshoweralgorithmbasedonCatani–Seymour dipolefactorisation,J.HighEnergyPhys.03(2008)038,arXiv:0709.1027 [hep-ph].

[19]T.Gehrmann,S.Hoche,F.Krauss, M.Schonherr,F.Siegert,NLOQCDmatrix elements+partonshowersine+e−→hadrons,J.HighEnergyPhys.01(2013) 144,arXiv:1207.5031[hep-ph].

[20]S.Höche,etal.,QCDmatrixelements+partonshowers:theNLOcase,J.High EnergyPhys.04(2013)027,arXiv:1207.5030[hep-ph].

[21]NNPDFCollaboration,R.Ball,etal.,PartondistributionsfortheLHCRunII,J. HighEnergyPhys.04(2015)040,arXiv:1410.8849[hep-ph].

[22]M.L.Mangano,F.Piccinini,A.Polosa,M.Moretti,R.Pittau,ALPGEN,agenerator forhardmultipartonprocessesinhadroniccollisions,J.HighEnergyPhys.07 (2003)001,arXiv:hep-ph/0206293.

[23]J.Alwall,etal.,Theautomatedcomputationoftree-levelandnext-to-leading orderdifferentialcrosssections,andtheirmatchingtopartonshower simula-tions,J.HighEnergyPhys.07(2014)079,arXiv:1405.0301[hep-ph]. [24]T.Sjöstrand,S.Mrenna,P.Z.Skands,PYTHIA6.4physicsandmanual,J.High

EnergyPhys.05(2006)026,arXiv:hep-ph/0603175.

[25]P.Z.Skands,TuningMonteCarlogenerators:thePerugiatunes,Phys.Rev.D82 (2010)074018,arXiv:1005.3457[hep-ph].

[26]J.Pumplin, etal.,Newgeneration ofpartondistributionswith uncertainties from global QCD analysis, J. HighEnergy Phys. 07 (2002) 012, arXiv:hep-ph/0201195.

[27] ATLASCollaboration, ATLAS Pythia8Tunes to7TeV Data,

ATL-PHYS-PUB-2014-021,2014–2014,https://cds.cern.ch/record/1966419.

[28]NNPDFCollaboration,R.Ball,etal.,PartondistributionswithLHCdata,Nucl. Phys.B867(2013)244,arXiv:1207.1303[hep-ph].

[29]C.Anastasiou,L.J.Dixon,K.Melnikov,F.Petriello,HighprecisionQCDathadron colliders: electroweak gaugeboson rapiditydistributions at next-to-next-to leadingorder,Phys.Rev.D69(2004)094008,arXiv:hep-ph/0312266. [30]R. Gavin,Y. Li,F.Petriello,S. Quackenbush,FEWZ2.0:acodefor hadronic

Z production at next-to-next-to-leadingorder, Comput.Phys. Commun.182 (2011)2388,arXiv:1011.3540[hep-ph].

[31]Y.Li,F.Petriello,CombiningQCDandelectroweakcorrectionstodilepton pro-ductionintheframeworkoftheFEWZsimulationcode,Phys.Rev.D86(2012) 094034,arXiv:1208.5967[hep-ph].

[32]M. Czakon, A.Mitov, Top++:a program for the calculation ofthe top-pair cross-sectionathadron colliders,Comput. Phys.Commun.185(2014)2930, arXiv:1112.5675[hep-ph].

[33]S.Agostinelli,etal.,GEANT4–asimulationtoolkit,Nucl.Instrum.MethodsA 506(2003)250.

[34]ATLASCollaboration, TheATLASsimulation infrastructure,Eur.Phys. J.C70 (2010)823,arXiv:1005.4568[physics.ins-det].

[35] ATLAS Collaboration, Summary of ATLAS Pythia 8 Tunes,

ATL-PHYS-PUB-2012-003,2012,https://cds.cern.ch/record/1474107.

[36]A.D.Martin,W.J.Stirling,R.S.Thorne,G.Watt,PartondistributionsfortheLHC, Eur.Phys.J.C63(2009)189,arXiv:0901.0002[hep-ph].

[37] ATLASCollaboration,ElectronEfficiency MeasurementswiththeATLAS Detec-torUsingthe2015LHCProton–ProtonCollisionData,ATLAS-CONF-2016-024, 2016,https://cds.cern.ch/record/2157687.

[38]ATLASCollaboration,MuonreconstructionperformanceoftheATLASdetector inproton–protoncollisiondataat√s=13 TeV,Eur.Phys.J.C76(2016)292, arXiv:1603.05598[hep-ex].

[39] W. Lampl,et al.,CalorimeterClusteringAlgorithms: Descriptionand Perfor-mance,ATL-LARG-PUB-2008-002,2008,https://cds.cern.ch/record/1099735. [40]M.Cacciari,G.P.Salam,G.Soyez,Theanti-ktjetclusteringalgorithm,J.High

EnergyPhys.04(2008)063,arXiv:0802.1189[hep-ph].

[41]M.Cacciari,G.P.Salam,G.Soyez,FastJetusermanual,Eur.Phys.J.C72(2012) 1896,arXiv:1111.6097[hep-ph].

[42]ATLASCollaboration,Jetenergyscalemeasurementsandtheirsystematic un-certaintiesinproton–protoncollisionsat√s=13 TeV withtheATLASdetector, arXiv:1703.09665[hep-ex],2017.

[43] ATLASCollaboration,TaggingandSuppressionofPileupJetswiththe ATLAS Detector,ATLAS-CONF-2014-018,2014,https://cds.cern.ch/record/1700870. [44]D.L.Rainwater,R.Szalapski,D.Zeppenfeld,Probingcolorsingletexchangein

Z+twojeteventsattheCERNLHC,Phys.Rev.D54(1996)6680, arXiv:hep-ph/9605444.

[45] ATLASCollaboration,Multi-BosonSimulationfor13 TeVATLASAnalyses, ATL-PHYS-PUB-2016-002,2016,https://cds.cern.ch/record/2119986.

[46]M.Bahr,etal.,Herwig++physicsandmanual,Eur.Phys.J.C58(2008)639, arXiv:0803.0883[hep-ph].

[47]ATLAS Collaboration, Luminosity determination in pp collisions at √s=

8 TeV usingthe ATLASdetector at theLHC, Eur.Phys.J. C76(2016)653, arXiv:1608.03953[hep-ex].

[48]ATLASCollaboration,MeasurementsoftheproductioncrosssectionofaZ bo-soninassociationwithjetsinppcollisionsat√s=13 TeV withtheATLAS detector,Eur.Phys.J.C77(2017)361,arXiv:1702.05725[hep-ex].

[49]DØ Collaboration, V.M. Abazov, et al., Studies of W boson plus jets pro-ductionin p¯p collisionsat √s=1.96 TeV,Phys.Rev.D88(2013)092001, arXiv:1302.6508[hep-ex].

[50]CMSCollaboration,Measurementsofdifferentialcrosssectionsforassociated productionofaWbosonandjetsinproton–protoncollisionsat√s=8 TeV, Phys.Rev.D95(2017)052002,arXiv:1610.04222[hep-ex].

[51]ATLASCollaboration,Measurementsofelectroweak Wjj productionand

con-straintsonanomalousgaugecouplingswiththeATLASdetector,Eur.Phys.J.C 77(2017)474,arXiv:1703.04362[hep-ex].

[52]W. Verkerke, D. Kirkby, The RooFit toolkit for data modeling, arXiv:physics/0306116,2003.

[53] ATLASCollaboration,ATLASComputingAcknowledgements2016–2017,2016,

ATL-GEN-PUB-2016-002,http://cdsweb.cern.ch/record/2202407.

TheATLASCollaboration

M. Aaboud

137d

,

G. Aad

88

,

B. Abbott

115

,

O. Abdinov

12

,

∗

,

B. Abeloos

119

,

S.H. Abidi

161

,

O.S. AbouZeid

139

,

N.L. Abraham

151

,

H. Abramowicz

155

,

H. Abreu

154

,

R. Abreu

118

,

Y. Abulaiti

148a

,

148b

,

B.S. Acharya

167a

,

167b

,

a

,

S. Adachi

157

,

L. Adamczyk

41a

,

J. Adelman

110

,

M. Adersberger

102

,

T. Adye

133

,

A.A. Affolder

139

,

Y. Afik

154

,

T. Agatonovic-Jovin

14

,

C. Agheorghiesei

28c

,

J.A. Aguilar-Saavedra

128a

,

128f

,

S.P. Ahlen

24

,

F. Ahmadov

68

,

b

,

G. Aielli

135a

,

135b

,

S. Akatsuka

71

,

H. Akerstedt

148a

,

148b

,

T.P.A. Åkesson

84

,

E. Akilli

52

,

A.V. Akimov

98

,

G.L. Alberghi

22a

,

22b

,

J. Albert

172

,

P. Albicocco

50

,

M.J. Alconada Verzini

74

,

S.C. Alderweireldt

108

,

M. Aleksa

32

,

I.N. Aleksandrov

68

,

C. Alexa

28b

,

G. Alexander

155

,

T. Alexopoulos

10

,

M. Alhroob

115

,

B. Ali

130

,

M. Aliev

76a

,

76b

,

G. Alimonti

94a

,

J. Alison

33

,

S.P. Alkire

38

,

B.M.M. Allbrooke

151

,

B.W. Allen

118

,

P.P. Allport

19

,

A. Aloisio

106a

,

106b

,

A. Alonso

39

,

F. Alonso

74

,

C. Alpigiani

140

,

A.A. Alshehri

56

,

M.I. Alstaty

88

,

B. Alvarez Gonzalez

32

,

D. Álvarez Piqueras

170

,

M.G. Alviggi

106a

,

106b

,

B.T. Amadio

16

,

Y. Amaral Coutinho

26a

,

C. Amelung

25

,

D. Amidei

92

,

S.P. Amor Dos Santos

128a

,

128c

,

S. Amoroso

32

,

G. Amundsen

25

,

C. Anastopoulos

141

,

L.S. Ancu

52

,

N. Andari

19

,

T. Andeen

11

,

C.F. Anders

60b

,

J.K. Anders

77

,

K.J. Anderson

33

,

A. Andreazza

94a

,

94b

,

V. Andrei

60a

,

S. Angelidakis

37

,

I. Angelozzi

109

,

A. Angerami

38

,

A.V. Anisenkov

111

,

c

,

N. Anjos

13

,

A. Annovi

126a

,

126b

,

C. Antel

60a

,

M. Antonelli

50

,

A. Antonov

100

,

∗

,

D.J. Antrim

166

,

F. Anulli

134a

,

M. Aoki

69

,

L. Aperio Bella

32

,

G. Arabidze

93

,

Y. Arai

69

,

J.P. Araque

128a

,

V. Araujo Ferraz

26a

,

A.T.H. Arce

48

,

R.E. Ardell

80

,

F.A. Arduh

74

,

J-F. Arguin

97

,

S. Argyropoulos

66

,

M. Arik

20a

,

A.J. Armbruster

32

,

L.J. Armitage

79

,

O. Arnaez

161

,