Article
Reference
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
ATLAS Collaboration AKILLI, Ece (Collab.), et al .
Abstract
The cross-section for the production of two jets in association with a leptonically decaying Z boson ( Zjj ) is measured in proton–proton collisions at a centre-of-mass energy of 13 TeV, using data recorded with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 3.2 fb −1 . The electroweak Zjj cross-section is extracted in a fiducial region chosen to enhance the electroweak contribution relative to the dominant Drell–Yan Zjj process, which is constrained using a data-driven approach. The measured fiducial electroweak cross-section is σEWZjj = 119±16(stat.)±20(syst.)±2(lumi.) fb for dijet invariant mass greater than 250 GeV, and 34.2±5.8(stat.)±5.5(syst.)±0.7(lumi.) fb for dijet invariant mass greater than 1 TeV. Standard Model predictions are in agreement with the measurements. The inclusive Zjj cross-section is also measured in six different fiducial regions with varying contributions from electroweak and Drell–Yan Zjj production.
ATLAS Collaboration, AKILLI, Ece (Collab.), et al . 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 , 2017, vol. 775, p. 206-228
DOI : 10.1016/j.physletb.2017.10.040
Available at:
http://archive-ouverte.unige.ch/unige:98341
Disclaimer: layout of this document may differ from the published version.
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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
.The ATLAS Collaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received2October2017
Receivedinrevisedform19October2017 Accepted19October2017
Availableonlinexxxx Editor:W.-D.Schlatter
The cross-section for the productionoftwo jets inassociation with aleptonicallydecaying Z boson (Z jj)ismeasuredinproton–protoncollisionsatacentre-of-massenergyof13 TeV,usingdatarecorded with the ATLASdetector atthe Large Hadron Collider, corresponding to an integrated luminosity of 3.2 fb−1. The electroweak Z jj cross-section is extracted in a fiducial regionchosen to enhancethe electroweak contribution relative to the dominant Drell–Yan Z jj process, whichis constrained using adata-drivenapproach. The measuredfiducialelectroweak cross-sectionis σEWZjj = 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 jjcross-sectionisalsomeasuredinsixdifferentfiducialregions withvaryingcontributionsfromelectroweakandDrell–YanZ jjproduction.
©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
AttheLargeHadronCollider(LHC)eventscontaininga Z boson andatleasttwojets(Z jj)areproducedpredominantlyviainitial- state QCD radiation fromthe incoming partons in the Drell–Yan process(QCD-Z jj),asshowninFig. 1(a).Incontrast,theproduc- tionofZ jj eventsviat-channelelectroweakgaugebosonexchange (EW-Z jj events), including the vector-bosonfusion (VBF) process showninFig. 1(b),isamuchrarerprocess.SuchVBFprocessesfor vector-bosonproductionareofgreatinterestasa‘standardcandle’
forother VBF processesatthe LHC: e.g.,the productionofHiggs bosons orthe search for weakly interacting particles beyondthe StandardModel.
Thekinematicpropertiesof Z jj eventsallowsome discrimina- tionbetweentheQCDandEWproductionmechanisms.Theemis- sionofavirtual W bosonfromthequarkinEW-Z jjeventsresults inthepresenceoftwohigh-energyjets,withmoderatetransverse momentum(pT),separatedbyalargeintervalinrapidity(y)1 and
E-mailaddress:atlas.publications@cern.ch.
1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthe nominal interactionpoint inthe centre ofthe detector and the z-axis along the beam pipe.Inthetransverseplane,thex-axispointsfromthe interactionpointtothe centreoftheLHCring,the y-axispoints upward,andφ istheazimuthalangle aroundthez-axis.Thepseudorapidityisdefinedintermsofthepolarangleθas η= −ln tan(θ/2).Therapidityisdefinedasy=0.5ln[(E+pz)/(E−pz)],whereE andpzaretheenergyandlongitudinalmomentumrespectively.Anangularsepara-
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-pTjetsthan correspondingQCD-Z jjevents.
The first studies ofEW-Z jj productionwere performed [1]in ppcollisionsatacentre-of-massenergy(√
s)of7 TeVbytheCMS Collaboration,wherethebackground-onlyhypothesiswasrejected atthe2.6
σ
level.ThefirstobservationoftheEW-Z jj processwas performedbytheATLAS Collaborationata centre-of-massenergy (√s)of8 TeV[2].Thecross-sectionmeasurementisinagreement withpredictionsfromthePowheg-boxeventgenerator[3–5]and allowed limitstobe placed onanomalous triplegauge couplings.
The CMS Collaboration has also observed and measured [6] the cross-sectionforEW-Z jj productionat8 TeV.ThisLetterpresents measurementsofthecross-sectionforEW-Z jj productionandin- clusive Z jj productionathighdijetinvariantmassinppcollisions at√
s=13 TeV usingdatacorrespondingtoanintegratedluminos- ityof3.2 fb−1 collected by theATLAS detectoratthe LHC.These measurements allow the dependence ofthe 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.
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Fig. 1.Examplesofleading-orderFeynmandiagramsforthetwoproductionmech- anismsforaleptonicallydecaying Zbosonandatleasttwojets(Z jj)inproton–
proton collisions: (a) QCD radiation from the incoming partons (QCD-Z jj)and (b)t-channelexchangeofanEWgaugeboson(EW-Z jj).
tobestudied.Theincreased√
sallowsexploration ofhigherdijet masses, where the EW-Z jj contribution to the total Z jj ratebe- comesmorepronounced.
2. ATLASdetector
TheATLAS detectorisdescribedindetailin Refs.[7,8].It con- sistsofaninnerdetectorfortracking,surroundedbyathinsuper- conducting solenoid, electromagnetic and hadronic calorimeters, andamuonspectrometerincorporatingthreelargesuperconduct- ingtoroidalmagnetsystems.Theinner detectorisimmersedina 2 Taxial magnetic field andprovides 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 calorimeteris 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.The trackingchamberscoverthere- gion|
η
|<2.7 withthreelayersofmonitoreddrifttubes,comple- mented bycathode stripchambersin partoftheforwardregion, wherethehit rateishighest.Themuontriggersystemcoversthe range|η
|<2.4 withresistiveplatechambersinthebarrelregion, andthingapchambersintheend-capregions.A two-level trigger system is used to select events of inter- est[9].The Level-1triggeris implementedinhardware anduses a subset of the detectorinformation to reduce theevent 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 usingtheSherpa2.2.0eventgenerator[11].Formodellingofthe partonshower,fragmentation, hadronisationandunderlyingevent (UEPS),Powheg-boxwasinterfacedto Pythia8 [12]witha ded- icated set of parton-shower-generator parameters (tune) denoted AZNLO[13] andtheCT10NLO partondistribution function(PDF) set[14].The renormalisation andfactorisationscales were setto
the Z boson mass. Sherpa predictions used the Comix [15] and OpenLoops [16] matrix element eventgenerators, andthe CKKW method was used to combine the various final-state topologies from the matrix element andmatch them to the partonshower [17]. The matrix elements were merged with the Sherpaparton shower [18] usingthe ME+PS@LO prescription[19,20], andusing Sherpa’snativedynamicalscale-settingalgorithmtosettherenor- malisation and factorisation scales. Sherpa predictions used the NNPDF30NNLOPDFset[21].
The productionof 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 predictions calculatedforup totwo partonsatNLO accuracyand up to four partons at LO accuracy in perturbative QCD. Sherpa predictionsusedtheNNPDF30NLOPDFsettogetherwiththetun- ingoftheUEPSparametersdevelopedbytheSherpaauthorsusing theME+PS@NLOprescription[19,20].AlpgenisanLOeventgener- atorwhichusesexplicitmatrixelementsforuptofivepartonsand was interfacedto Pythia 6.426[24] using thePerugia2011C tune [25]andtheCTEQ6L1PDFset[26].Onlymatrixelementsforlight- flavourproductioninAlpgenareincluded,withheavy-flavourcon- tributionsmodelledbythepartonshower.MadGraph5_aMC@NLO 2.2.2(MG5_aMC)usesexplicitmatrixelementsforuptofourpar- tonsatLO,andwasinterfacedtoPythia8withtheA14tune[27]
and using the NNPDF23LO PDF set [28]. For reconstruction-level studies,total Z bosonproductionratespredictedbyallthreeevent generators used to produce QCD-Z jj predictions are normalised usingthe next-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-level theoretical predictionstodetector-correctedmeasurements,thenormalisation ofquoted predictionsisprovidedby theeventgenerator inques- tionratherthananexternalNNLOprediction.
TheproductionofapairofEWvectorbosons(diboson),where one decaysleptonically andtheother 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 Sherpa2.1.1andtheCT10NLOPDFset.
Thelargestbackgroundtotheselected Z jj samplesarisesfrom t¯t and single-top (W t) production. These were generated using Powheg-boxv2andPythia6.428withthePerugia2012tune[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] simulationofthe ATLASdetector[34].Theeffectof additional ppinteractions (pile-up) inthe sameornearby bunch crossingswasalsosimulated,usingPythiav8.186withtheA2tune [35] and the MSTW2008LOPDF set [36]. The MC samples were reweightedsothatthedistributionoftheaveragenumberofpile- upinteractions perbunchcrossingmatchesthatobservedindata.
ForthedataconsideredinthisLetter,theaveragenumberofinter- actionsis 13.7.
4. Eventpreselection
The Z bosonsaremeasuredintheirdielectronanddimuonde- cay modes. Candidateeventsare selectedusingtriggers requiring atleastone identifiedelectron ormuonwithtransversemomen- 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
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muontriggerswithoutisolationrequirementsandwiththresholds ofpT=60 GeV and50 GeVrespectively.
Candidateelectronsarereconstructedfromclustersofenergyin theelectromagnetic calorimetermatched to inner-detector tracks [37].TheymustsatisfytheMediumidentificationrequirementsde- scribedinRef.[37]andhavepT > 25 GeV and|
η
|<2.47,exclud- ingthetransitionregionbetweenthebarrelandend-capcalorime- tersat1.37<|η
|<1.52.Candidatemuonsareidentifiedastracks intheinnerdetectormatchedandcombinedwithtracksegments inthemuonspectrometer. TheymustsatisfytheMediumidentifi- cationrequirementsdescribedinRef.[38],andhavepT > 25 GeV and|η
| < 2.4. Candidate leptons mustalsosatisfy a setof isola- tioncriteriabasedonreconstructedtracksandcalorimeteractivity.Events are required to contain exactly two leptons of the same flavour butofopposite charge. The dilepton invariant massmust satisfy81<m<101 GeV.
Candidate hadronic jets are required to satisfy pT > 25 GeV and|y|< 4.4. Theyare reconstructedfrom clustersofenergyin thecalorimeter [39] using the anti-kt algorithm [40,41] withra- dius parameter R=0.4. Jet energies are calibrated by applying pT- and y-dependent correctionsderived fromMonte Carlosim- ulationwithadditionalinsitucorrectionfactorsdeterminedfrom 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 withhavingoriginated fromthe primary vertex(the vertex with the highest sum of track p2T), as defined by the jet vertex tag- geralgorithm [43].Selectedelectrons andmuonsare discardedif theylie within R=0.4 ofa reconstructedjet.Thisrequirement isimposed toremove non-prompt non-isolated leptons produced inheavy-flavour decaysor fromthe decay inflight of a kaon or pion.
5. MeasurementofinclusiveZ j jfiducialcross-sections 5.1.Definitionofparticle-levelcross-sections
Cross-sections are measured forinclusive 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=
Nf
obs
−
NbkgfL
·
Cf,
(1)where Nobsf is the number of events observed in the data pass- ingthe selection requirementsof thefiducialregion understudy atdetector level, Nbkgf is the corresponding number ofexpected background(non-Z jj)events,Listheintegratedluminositycorre- spondingtotheanalyseddatasample,andCf isacorrectionfactor appliedtotheobserveddatayields,whichaccountsforexperimen- talefficiency anddetectorresolution effects,and isderived from MCsimulationwithdata-drivenefficiencyandenergy/momentum scalecorrections.Thiscorrectionfactoriscalculatedas:
Cf
=
Nf det
Nparticlef
,
where Ndetf is the number of signal events that satisfy the fidu- cialselection criteria atdetectorlevel inthe MC simulation, and Nparticlef isthenumberofsignaleventsthatpasstheequivalentse- lectionbut atparticle level. Thesecorrection factors havevalues between0.63 and0.77,dependingonthefiducialregion.
Withtheexception ofbackgroundfrommultijetandW+jets processes(henceforthreferred totogether simplyasmultijetpro- cesses),contributionstoNbkgf areestimatedusingtheMonteCarlo
samples described inSection 3.Background frommultijet events 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 asa function ofdileptonmass.Non-multijetbackgroundissubtracted fromthe templateusingsimulation.The normalisationisderivedby fitting thenominaldileptonmassdistributionineachfiducialregionwith the sumofthemultijet templateanda template comprisingsig- nalandbackgroundcontributionsdeterminedfromsimulation.The multijetcontributionisfoundtobelessthan0.3%ineachfiducial region.ThecontributionfromW+jets processeswascheckedus- ing MC simulation and found to be much smaller than thetotal multijetbackgroundasdeterminedfromdata.
At particlelevel,only final-stateparticleswithproper lifetime c
τ
>10 mm areconsidered.Promptleptonsaredressedusingthe four-momentumcombinationofanelectronormuonandallpho- tons (not originating fromhadron 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.Eventsare required tocontainexactly twodressedleptons ofthesameflavourbutof oppositecharge,andthedileptoninvariantmassmustsatisfy81<m<101 GeV.Jetsarereconstructedusingtheanti-kt algorithm with radius parameter R=0.4. Prompt leptons and the photons used todress theseleptons are notincluded inthe particle-level jet reconstruction. Allremaining final-state particles are included inthe particle-leveljet clustering. Promptleptons witha separa- tionRj,<0.4 fromanyjetarerejected.
The cross-section measurements are performed in the six phase-spaceregions definedin Table 1.These regionsare chosen tohavevaryingcontributionsfromEW-Z jjandQCD-Z jjprocesses.
5.2. Eventselection
Following Ref. [2], events are selected in six detector fiducial regions.As faraspossible,thesearedefinedwiththesamekine- matic requirements as the six phase-space regions in which the cross-sectionismeasured(Table 1).Thisminimisessystematicun- 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.Therequirementof81<m<101 GeV sup- presses other sourcesof dileptonevents,suchast¯t and Z→
τ τ
, aswellasthemultijetbackground.Becausetheenergyscaleofthedijetsystemistypicallyhigher ineventsproducedbytheEW-Z jj processthaninthoseproduced bytheQCD-Z jjprocess,twosubsetsofthebaselineregionarede- fined which probe the EW-Z jj contribution indifferent ways: in thehigh-massfiducialregionahighvalueoftheinvariantmassof thedijetsystem(mj j>1 TeV)isrequired,andinthehigh-pTfidu- cialregiontheminimumpT oftheleadingandsub-leadingjetsis increasedto85 GeV and75 GeVrespectively.TheEW-Z jj process typicallyproducesharder jettransversemomentaandresultsina harderdijetinvariantmassspectrumthantheQCD-Z jjprocess.
Three additional fiducial regions allow the separate contribu- tionsfromtheEW-Z jjandQCD-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 pT > 20 GeV, andthat thenormalisedtransversemomentum balance betweenthetwo leptons andthe twohighest transverse
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Table 1
Summaryoftheparticle-levelselectioncriteriadefiningthesixfiducialregions(seetextfordetails).
Object Fiducial region
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
— pT >20 GeV
Jets |y|<4.4
pTj1>55 GeV pTj1>85 GeV pTj1>55 GeV pTj2>45 GeV pTj2>75 GeV pTj2>45 GeV
Dijet system – mj j>1 TeV – mj j>250 GeV mj j>1 TeV mj j>250 GeV
Interval jets – Njetinterval(p
T>25 GeV)=0 Nintervaljet(pT>25 GeV)≥1
Z jjsystem – pbalanceT <0.15 pbalanceT ,3<0.15
momentumjetssatisfy pbalanceT <0.15.Thelatterquantityisgiven by
pbalanceT
=
pT1
+
pT2+
pTj1+
pTj2pT1
+
pT2+
pTj1+
pTj2,
(2) where pTi isthetransverse momentumvector ofobject i,1 and 2 labelthetwoleptonsthatdefinethe 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 ormultiple parton interactions. The requirement on pbalanceT also helps suppress events in which the pT of one or more jets is badly measured andit enhancesthe EW-Z jj contribution, where the lower probability ofadditional 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(Njetinterval(pT>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-enrichedfiducialregionisdesignedtosup- presstheEW-Z jj contributionrelativeto QCD-Z jj byrequiringat leastone jetwithpT>25 GeV to bereconstructedwithinthera- pidityinterval boundedby thedijetsystem(Nintervaljet(p
T>25 GeV) ≥1).
IntheQCD-enrichedregion,thedefinitionofthenormalisedtrans- versemomentumbalanceismodifiedfromthatgiveninEq.(2)to includeinthe calculationof thenumeratoranddenominator the pT ofthe highest pT jet within the rapidity interval boundedby thedijetsystem(pbalanceT ,3).Inallotherrespects,thekinematicre- quirements in the EW-enriched region and QCD-enriched region areidentical.
5.3. Detector-levelresults
Inthebaselineregion,30686 eventsareselectedinthedielec- tronchannel and36786 eventsare selectedinthedimuonchan- nel.Thetotalobservedyields areinagreementwiththeexpected yieldswithinstatisticaluncertaintiesineachdileptonchannel.The largestdeviationacrossallfiducialregionsisa2
σ
(statistical)dif- ference between the expected to observed ratio in the electron versusmuonchannelinthehigh-pTregion.Theexpected compositionof theselected datasamplesinthe six Z jj fiducialregions issummarisedinTable 2,averagingacross thedielectronanddimuonchannelsasthesecompositionsinthe
two dilepton channels are in agreement within statisticaluncer- tainties.The numbersofselectedeventsindataandexpectations fromtotalsignalplusbackgroundestimatesarealsogivenforeach region. The largest discrepancy between observed and expected yieldsisseeninthehigh-massregion,andresultsfromamismod- elling ofthe mj j spectrumin the QCD-Z jj MC simulations used, whichisdiscussed belowandaccountedforintheassessmentof systematicuncertainties inthemeasurement.
5.4. SystematicuncertaintiesintheinclusiveZ jj fiducialcross-sections Experimental systematicuncertainties affectthedetermination of the Cf correction factor and the background estimates. The dominantsystematicuncertaintyintheinclusiveZ jjfiducialcross- 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 of0.4additional jets).Other ex- perimentalsystematicuncertaintiesarisingfromleptonefficiencies related toreconstruction, identification, isolation andtrigger, and leptonenergy/momentumscaleandresolutionaswellasfromthe effectofpile-up,amounttoatotalofaround 1–2%,depending on thefiducialregion.
The systematicuncertainty arising from the MC modelling of the mj j distribution inthe QCD-Z jj andEW-Z jj signal processes is around 3% in the EW-enrichedregion, around 1% in the QCD- enriched region, 2% in thehigh-mass region, andbelow1% 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 MC sample to describe themj j distributionof theobserved data inagivenfiducialregion.Additionalcontributionsarisefromvary- ingtheQCDrenormalisationandfactorisationscalesupanddown by a factor of two independently, and from the propagation of uncertainties in the PDF sets. The normalisation of the diboson contributionisvariedaccordingtocorrespondingtoPDFandscale variations in these predictions [45], and results in up to a 0.1%
effecton themeasured Z jj cross-sectionsdepending onthefidu- cial region. The uncertainty from varying the normalisation and shape in mj j of the estimated background from top-quark pro- duction is at most 1% (in the high-mass region), arising from changes in the extracted Z jj cross-sectionswhen usingmodified top-quarkbackgroundMC sampleswithPDFandscale variations,
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Table 2
Estimatedcomposition(inpercent)ofthedatasamplesselectedinthesix Z jjfiducialregionsforthedielectronanddimuonchan- nelscombined,usingtheEW-Z jjsamplefromPowheg,andtheQCD-Z jjsamplefromSherpa(normalisedusingNNLOpredictions fortheinclusiveZ cross-sectioncalculatedwithFEWZ).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
suppressedor enhanced additional radiation(generated withthe Perugia2012radHi/Lo tunes [25]), or using an alternative top- quark production sample from MadGraph5_aMC@NLO interfaced toHerwig++v2.7.1[23,46].
Thesystematicuncertaintyintheintegratedluminosityis2.1%.
This is derived following a methodology similar to that detailed inRef. [47], froma calibrationof theluminosity scale using 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(taking intoaccounttotaluncertainties)acrossbothchannels.
Thesecross-sectionsare determined 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)thecross- 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 excepttheEW-enrichedregionwherethe variationis3% 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)are foundtobeinagreementwith themeasurementsinmostcases.Theuncertaintiesinthetheoreti- calpredictionsaresignificantlylargerthantheuncertaintiesinthe correspondingmeasurements.
Thelargest differencesbetween predictionsand 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 have relative uncertainties withre- spectto the measurement of36% 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
asignificantmismodellingoftheQCD-Z jj contribution,asinvesti- gatedanddiscussedindetailinSection6.1.PredictionsfromAlp- gen(QCD-Z jj)+Powheg(EW-Z jj)areinagreementwiththedata forthehigh-mass andEW-enriched(mj j>250 GeV and>1 TeV) regions.
6. MeasurementofEW-Z j jfiducialcross-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 thissignal fractiongrows with in- creasing mj j to 26.1% formj j>1 TeV. The QCD-enriched region hasan overallexpected EW-Z jj signal fractionof 1.6%increasing to4.4%formj j>1 TeV.ThedominantbackgroundtotheEW-Z jj cross-sectionmeasurementisQCD-Z jj production.Itissubtracted inthesamewayasnon-Z jj backgroundsintheinclusivemeasure- mentdescribedinSection5.Althoughdibosonproductionincludes contributionsfrompurelyEWprocesses,inthismeasurementitis consideredaspartofthebackgroundandisestimatedfromsimu- lation.
A particle-level production cross-section measurement of EW-Z jj productioninagivenfiducialregion f isthusgivenby
σ
EWf=
Nf
obs
−
NQCD-Zjjf−
NbkgfL
·
CEWf,
(3)withthe same notations asin Eq.(1) andwhere NQCD-Zjjf is the expectednumberofQCD-Z jjeventspassingtheselectionrequire- mentsofthefiducialregionatdetectorlevel,Nbkgf istheexpected numberofbackground (non-Z jj anddiboson)events,andCEWf is acorrectionfactorappliedtotheobservedbackground-subtracted datayields thataccountsforexperimentalefficiencyanddetector 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 usingthe SherpaEW-Z jj prediction,and0.67 (0.68) when usingthe PowhegEW-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)
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Table 3
MeasuredandpredictedinclusiveZ jjproductioncross-sectionsinthesixfiducialregionsdefinedinTable 1.Forthemeasuredcross-sections, thefirstuncertaintygivenisstatistical,thesecondissystematicandthethirdisduetotheluminositydetermination.Forthepredictions, thestatisticaluncertaintyisaddedinquadraturetothesystematicuncertaintiesarisingfromthePDFsandfactorisationandrenormalisation scalevariations.
Fiducial region InclusiveZ jjcross-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 0.118±0.008 ±0.006 ±0.002 0.156±0.019 0.185±0.023 0.120±0.015 (mj j>1 TeV)
Fig. 2.Detector-levelcomparisonsofthedijetinvariantmassdistributionbetweendataandsimulationin(a) theEW-enrichedregionand(b) theQCD-enrichedregion,forthe dielectronanddimuonchannelcombined.Uncertaintiesshownonthedataarestatisticalonly.TheEW-Z jjsimulationsamplecomesfromthePowhegeventgeneratorand theQCD-Z jjMCsamplecomesfromtheSherpaeventgenerator.ThelowerpanelsshowtheratioofsimulationtodataforthreeQCD-Z jjmodels,fromAlpgen,MG5_aMC, andSherpa.Thehatchedbandcentredatunityrepresentsthesizeofstatisticalandexperimentalsystematicuncertaintiesaddedinquadrature.
that in the EW-enriched region the EW-Z jj component becomes prominentatlargevaluesofmj j.However,Fig. 2(b)demonstrates that the shape of themj j distribution forQCD-Z jj production is poorlymodelledinsimulation.Thesametrendisseenforallthree QCD-Z jj eventgeneratorslistedinSection3.Alpgenprovidesthe best description of the data over the whole mj j range. In com- parison,MG5_aMCandSherpaoverestimatethe databy80% and 120%respectively, formj j=2 TeV, well outsidethe uncertainties onthesepredictionsdescribedinTable 3.Thesediscrepancieshave been observedpreviously in Z jj [2,48] and Wjj [49–51] produc- tionathighdijetinvariantmassandathighjetrapidities.Forthe purposeofextractingthecross-sectionforEW-Z jjproduction,this mismodellingofQCD-Z jj is correctedforusinga data-drivenap- 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 datain the EW-enriched region, after subtractionof non-Z jj anddibosonbackground,usingalog-likelihoodmaximisation[52].
FollowingtheprocedureadoptedinRef.[2],thedataintheQCD-
enriched region are used to evaluatedetector-level shape correc- tion factors for the QCD-Z jj MC predictions bin-by-bin in mj j. Thesedata-to-simulationratiocorrectionfactorsareappliedtothe simulation-predicted shape inmj j ofthe QCD-Z jj contributionin the EW-enriched region. Thisprocedure is motivatedby two ob- servations:
(a) theQCD-enrichedregionandEW-enrichedregionaredesigned tobe kinematicallyvery similar,differingonly withregard to thepresence/absenceofjetsreconstructedwithintherapidity intervalboundedbythedijetsystem,
(b) thecontributionof EW-Z jj tothe regionof highmj j issup- pressedintheQCD-enrichedregion(4.4%formj j>1 TeV)rel- ativetothatintheEW-enrichedregion(26.1%formj j>1 TeV) (also illustrated inFig. 2);the impactof theresidualEW-Z jj contamination in the QCD-enriched region is assigned as a componentofthesystematicuncertaintyintheQCD-Z jjback- ground.
The shape correction factors in mj j obtained using the three different QCD-Z jj MC samples are shown in Fig. 3(a). These are derived asthe ratio of the data to simulation inbins of mj j af- ternormalisationofthetotalyield insimulationtothat observed
