Measurements of four-lepton production in pp collisions at root s=8 TeV with the ATLAS detector

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Physics

Letters

B

www.elsevier.com/locate/physletb

Measurements

of

four-lepton

production

in

pp collisions

at

√

s

=

8 TeV with

the

ATLAS

detector

.ATLASCollaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received28September2015

Receivedinrevisedform4December2015 Accepted16December2015

Availableonline18December2015 Editor:W.-D.Schlatter

The four-lepton (4, =e,μ) production cross section is measured in the mass range from 80 to 1000 GeV using 20.3 fb−1 _of_data _in _{pp collisions} _at√_s₌_{8 TeV collected}_with _the _ATLAS _detector atthe LHC. The4events are producedinthe decaysofresonant Z andHiggsbosonsand the non-resonant Z Z continuumoriginating fromqq,¯ gg, andqg initial states.Atotalof476signal candidate eventsare observedwithabackgroundexpectationof26.2±3.6 events,enablingthemeasurementof the integratedcross sectionand the differentialcrosssectionas afunctionofthe invariantmass and transversemomentumofthefour-leptonsystem.

In the mass range above 180 GeV, assuming the theoretical constraint on the qq production¯ cross section calculated withperturbative NNLOQCD and NLO electroweakcorrections, the signal strength of the gluon-fusion component relative to its leading-order prediction is determined to be μgg =

2.4_±1.0(stat.)±0.5(syst.)±0.8(theory).

1. Introduction

This paper presents measurements of the production of four isolatedcharged-leptonsinproton–protoncollisionsata centre-of-massenergyof√s=8 TeV using 20.3 fb−1ofdatacollectedwith theATLAS detectoratthe LHC.Forthe four-lepton (4, =e, μ) production,both the integratedcross section andthe differential cross sections as functions of invariant mass (m4) and trans-versemomentum(p4_T) ofthe4systemare measuredinamass range 80 <m4<1000 GeV. In addition, the 4 signal strength ofgluonfusion (ggF)productionrelativeto itsleading-order (LO) QCD estimate is measured. Thesemeasurements test the validity of the Standard Model (SM) through the interplay of QCD and electroweakeffectsfordifferent4productionmechanismsas de-scribedbytheLOFeynmandiagramsshownin Fig. 1.

The4 signaleventscomefromthedecaysofresonant Z and

Higgsbosonsandthe non-resonantZZ continuumproduced from

qq,¯ gg,andqg initialstates,whicharebrieﬂydiscussedbelow.

•qqqqq¯q-initiated¯¯ 444production

The tree-level diagrams for qq¯ →4 production are shown in Fig. 1(a) andFig. 1(b). The cross section as a function of m4 is shownin Fig. 2 (the dashed blackhistogram). The 4 event pro-ductionatthe Z resonanceoccurspredominantlyviathes-channel

diagram as shown in Fig. 1(a), andwas measured previously by

_E-mail_address:_{atlas.publications@cern.ch}_.

the ATLASandCMScollaborations[1,2].Inthe 4invariant mass regionabovethe Z resonancethe4eventproductionmainly pro-ceeds through the t-channel process as shown in Fig. 1(b). The crosssection signiﬁcantlyincreaseswhenboth Z bosonsare pro-duced on-shell, resulting in a rise in the m4 spectrum around 180 GeV. In addition,a smallportion of the 4 eventswith the

qq initial¯ state canbe produced fromthevector-bosonscattering

(VBS)process.

•gggggg-initiated444production

TheLOdiagramsoftheHiggs-bosonproductionandnon-resonant 4productionviaggF areshownin Fig. 1(c)and Fig. 1(d), respec-tively.Thecrosssectionsasafunction ofm4 areshownin Fig. 2 (the coloured histograms).The featuresofthe4eventsfromthe decaysofHiggs-bosonandcontinuum Z Z productionvia gg F are

describedbelow.

(1) The dominant Higgs-boson production mechanism is ggF.

Other Higgs-boson production mechanisms, vector-boson fu-sion(VBF), vector-bosonassociatedproduction(VH),and top-pair associatedproduction (t¯t H ), contribute lessthan 15% to the on-shell Higgs-boson decay to Z Z∗ event rate. The on-shell Higgs-boson production and decay leads to a narrow resonancearound125GeV,whichhasbeenakeysignaturein theHiggs-bosondiscovery bytheATLAS[3]andCMS[4] col-laborations. The off-shellHiggs-boson productionhas a large destructive interference with continuum ZZ production from

the ggF processes [5–7]. This effect can be observed in the

http://dx.doi.org/10.1016/j.physletb.2015.12.048

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Fig. 1. TheLOFeynmandiagramsfortheq¯q- and gg-initiatedproductionof4:(a)s-channel productionofqq¯→Z(∗)_→+− _with_associated_radiative_decays_to_an additionalleptonpair;(b)t-channel productionofqq¯→Z(∗)_Z(∗)_→₄_;_(c)_Higgs-boson_production_through_gluon_fusion_gg_→_H(∗)_→_{Z Z}(∗)_→₄_;_(d)_non-resonant₄ productionthroughthequark-boxdiagramgg→Z(∗)_Z(∗)_→₄_._The_Z(∗)_notation_stands_for_production_of_{on- and}_off-shell_{Z bosons}_{( Z and}_Z∗₎_and_production_of_off-shell photons(γ∗).

Fig. 2. Thedifferentialcrosssections,dσ/dm4versustheinvariantmassofthefour

leptonsm4,calculatedby MCFM fromtheq¯q andgg initialstatesat√s=8 TeV

forthe2e2μfinalstateintheexperimentalfiducialphasespace(seeTable 2for definition).The inclusivegg→4distributionisthe sumofthe gg→H→4

andthe gg→Z Z→4,andinterference terms.Thecalculationoftheq¯q→4

differentialproductioncrosssectionincludesperturbativeQCDcorrectionsatNLO, whilethe distributionsfromthe gg initialstatearecalculatedat LO.TheNNLO K -factorsareappliedtoon-shellHiggs-bosonproduction.

high-mass tail of the distributions shown in Fig. 2, andhas been usedasa toolto constrainthe totalHiggs-bosonwidth bytheATLASandCMScollaborations[8,9].

(2) The non-resonant Z Z→4 production via ggF, includes the production of off-shell Higgs bosons and continuum ZZ

pro-duction aswellastheir interference.Thisprocessproduces a sizeablenumberof4eventsinthem4>2×mZ massregion anddominatesthetotal gg-initiated4production.

Contributionsfromdifferentprocesseshavedifferentstrengths as a function of m4 (Fig. 2) and p4T. Therefore, differential 4 production cross sections are measured separately as a function of m4 and pT4. The measurement of the integrated cross sec-tion is first performed in the experimental fiducial phase space, andthen extendedto a commonphase spaceforthree 4 chan-nels:4e, 4μ,and 2e2μ.This commonphase spaceis definedby 80 <m4<1000 GeV,m+−>4 GeV, p

Z1,2

T >2 GeV,andthe pres-enceoffourleptonseachwithpT>5 GeV and|η| <2.8.

Currently, the gluon-fusion production is estimated theoreti-cally with only a LO QCD approximation for the gg continuum

production[6,10].Inthisanalysisthemass rangeabove180 GeV isusedtodeterminethesignal strengthofthegluon-fusion

com-ponentwithrespecttoitsLOprediction.Thisisdonebyﬁttingthe observed m4 spectrum using the next-to-next-to-leading-order (NNLO) QCD theoretical prediction,corrected for next-to-leading-order (NLO) electroweak effects, for the production originating

fromtheqq initial¯ state.

2. TheATLASdetector

The ATLAS detector[11] hasa cylindricalgeometry1 and con-sistsofaninnertrackingdetector(ID)surroundedbya2 T super-conducting solenoid, electromagnetic and hadronic calorimeters, andamuonspectrometer(MS)withatoroidalmagneticfield.The IDprovides trackingforchargedparticlesfor|η| <2.5.Itconsists of silicon pixel and strip detectors surrounded by a straw tube tracker that also provides transition radiation measurements for electronidentification.Theelectromagneticandhadronic calorime-tersystemcoversthepseudorapidityrange|η| <4.9.For|η| <2.5, the liquid-argon electromagnetic calorimeter is finely segmented and plays an important role in the electron identification. The MS includes fasttrigger chambers(|η| <2.4) and high-precision trackingchamberscovering|η| <2.7.Athree-leveltriggersystem selectseventstoberecordedforofflinephysicsanalysis.

3. Signalandbackgroundsimulation

Thesignalmodellingforqq¯→4productionusesthe POWHEG-BOX_Monte_Carlo_(MC)_program_[12–14]_,_which_includes perturba-tiveQCDcorrectionsatNLO.Theproductionthroughtheqg initial

state isan NLOcontributiontotheqq process.¯ TheCT10NLO[15] set of parton distribution functions (PDFs), with QCD renormal-isation and factorisation scales (μR, μF) set to m4 are used to calculate the cross section and generate the kinematic distribu-tions. The NNLO QCD [16] and the NLO electroweak (EW) [17] corrections are applied to the NLO cross section calculated by POWHEG-BOXasafunctionofthe 4mass forthe kinematic re-gion where both Z bosons are produced on-shell. Following the sameapproachasdescribed inRef.[8],the4 eventdistributions are re-weighted tomatchthose expectedwhenusingQCD scales ofm4/2.Thisisdone tounifytheQCDscales usedinsimulation

oftheqq and¯ thegg processes.

1 _ATLAS_uses_a_right-handed_coordinate_system_with_its_origin_at_the_nominal in-teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe. Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdeﬁnedintermsof thepolarangleθasη= −ln tan(θ/2).

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Thesignalmodellingoftheon-shellHiggs-bosonproductionvia

theggF and VBF mechanismsuses POWHEG-BOX whichprovides

calculations atNLO QCD, with the CT10NLO PDFs and μR, μF= m4.TheHiggs-bosonproductionviatheVH andt¯tH mechanisms is simulated with PYTHIA8 [18]. The NNLO QCD and NLO EW effects on the cross-section calculationsfor on-shellHiggs-boson productionaresummarisedinRef.[19].Theexpectedeventyields of on-shell Higgs boson are normalised to the higher-order cor-rectedcrosssections.

Thenon-resonant4signalproductionincludesoff-shell Higgs-boson production, continuum Z Z production, and their interfer-ence. The LO MCFM generator [20] is used to simulatethe non-resonant ggF production, with the CT10NNLO [21] set of PDFs with QCD scales of μR, μF set to m4/2; while the LO MAD-GRAPH generator [22] is used to simulatenon-resonant VBF and

VBS productionandtheir interference.TheNNLOQCD corrections

areavailableforoff-shellHiggs-bosonproduction[23] andforthe interference between off-shell Higgs bosons and Z Z pairs from

the gg initialstate[24].However, nohigher-ordercorrectionsare

available for the continuum gg→Z Z process, which dominates the 4 eventsfromthe gg initial state in theregion outsidethe Higgs-bosonresonance.Therefore,theLOcrosssectionisusedfor thenormalisationofthe4 eventsproducedingluon-fusion pro-cesses.

AllthesignalMCgeneratorsareinterfacedto PYTHIA8 for par-tonshower simulation,except MADGRAPH,whichisinterfacedto PYTHIA6[25].

Backgrounds in this analysis include reconstructed 4 events from Z+jets, tt,¯ diboson(ZW, Zγ and doubleDrell–Yan),

tribo-son VVV (V =Z, W ), and VH (H→ VV),and Z+top (t¯t and t)

processes,whicharealsosimulated.

Thereduciblebackgroundfrom Z+jets production,which in-cludes light- and heavy-ﬂavour contributions, is modelled using both SHERPA[26]and ALPGEN [27].The Zγ process issimulated with SHERPA.Thett background¯ ismodelledusing POWHEG-BOX. Background events from ZH production, where Z → and

H→V V (VV = WW or ZZ with two leptons and two

neutri-nos or two leptons and two jets in the ﬁnal state), are simu-latedwith PYTHIA8. The ZW and the tZ processes are simulated with SHERPA and MADGRAPH, respectively.The irreducible back-ground fromVVV and t¯tZ is modelled with MADGRAPH. Finally, thedouble-Drell–YanZZ productionismodelledwith PYTHIA8.

Forbackground modellingthe POWHEG-BOX and MADGRAPH generatorsareinterfacedto PYTHIA6 forthepartonshower, hadro-nisation and underlying-event simulation. The ALPGEN genera-tor is interfaced to HERWIG [28] for the parton shower and to JIMMY [29] for the underlying event simulation. SHERPA uses built-in modelsforboth thepartonshower andunderlying-event description.

Both the signal andbackground MC events are simulated us-ingtheATLASdetectorsimulation[30]basedontheGEANT4[31] framework. Additional pp interactions in the same and nearby bunchcrossings(pile-up)areincludedinthesimulations.TheMC samplesarere-weightedtoreproducethedistributionofthemean numberofinteractionsperbunchcrossingobservedinthedata.

4. Eventreconstructionandselection

Thefollowingeventselectioncriteriaareappliedtotheevents collected with a single-lepton or dilepton trigger. The transverse momentum andtransverseenergy thresholds ofthe single-muon andsingle-electrontriggersare 24 GeV.Twodimuon triggersare used,onewithsymmetricthresholdsat13 GeVandtheotherwith asymmetric thresholds at 18 GeV and 8 GeV. Forthe dielectron triggerthesymmetricthresholdsare12 GeV.Furthermore,thereis

an electron–muon trigger of thresholdsat 12 GeV (electron)and 8 GeV(muon).

Aprimaryvertexreconstructedfromatleastthreetracks,each withpT>0.4 GeV,isrequired.Foreventswithmorethanone pri-mary vertex, the vertex with the largest p2_T of the associated tracksisselected.

Electroncandidatesare reconstructedfromacombinationofa cluster ofenergydeposits intheelectromagneticcalorimeterand atrackintheID.TheyarerequiredtohavepT>7 GeV and|η| < 2.47.Candidateelectronsmustsatisfya loosesetofidentiﬁcation criteriabasedonalikelihoodbuiltfromparameterscharacterising theshowershapeandtrackassociationasdescribedinRef.[32].

Muon identiﬁcationis performed according to severalcriteria basedontheinformationfromtheID,theMS,andthecalorimeter. Thedifferenttypesofreconstructedmuonsare:a)Combined(CB), whichisthecombinationoftracksreconstructedindependentlyin the ID andMS; b) Stand-Alone(SA), where the muon trajectory isreconstructed onlyintheMS;c)Segment-tagged(ST), wherea trackintheIDisassociatedwithatleastonelocaltracksegment in theMS;andd)Calorimeter-tagged (CaloTag), wherea trackin theID isidentiﬁedasamuonifitisassociatedwithaminimum ionising particle’senergydepositinthecalorimeter.

TheacceptanceforboththeCBandSTmuonsis|η| <2.5,while theSAmuonsareusedtoextendthe|η|acceptancetoincludethe region from 2.5 to 2.7, which is not covered by the ID. CaloTag muonsareusedintherapidityrange|η| <0.1 wherethereis in-complete MS coverage. Allmuon candidatesare requiredto have

pT>6 GeV.

In order to reject electrons and muons from hadron decays, onlyisolated leptons areselected.Twoisolationrequirementsare used,onefortheIDandoneforthecalorimeter.FortheID,the re-quirementisthatthescalarsumofthetransversemomenta,pT, ofalltracksinsideaconeof R≡( η)2_{+ ( φ)}2₌₀_._{2 around} thelepton,excludingtheleptonitself,belessthan15%ofthe lep-ton pT. Forthe calorimeter,the ET deposited inside acone of R=0.2 around the lepton, excluding thelepton itself and cor-rectedforcontributionsfrompile-upand,inthecaseofelectrons, shower leakage, isrequired to be lessthan 30% of the muon pT (15%forSAmuons)and20%oftheelectron ET.

Attheclosestapproachofatracktotheprimaryvertex,the ra-tioofthetransverseimpactparameterd0 toitsuncertainty,thed0 signiﬁcance,mustbe smallerthan3.5(6.5) formuons(electrons) to further reject leptons from heavy-ﬂavour decays.The longitu-dinal impact parameter, |z0|,must be lessthan 10 mm for elec-tronsaswellasmuons(no vertexrequirementsareappliedtoSA muons).

Selectionofleptonquadrupletsisdoneseparatelyineachofthe channels4μ, 2e2μ, 4e,keepingonlyasinglequadrupletper chan-nel.Candidate quadrupletsare formedby selectingtwo opposite-sign,same-flavourleptonpairs(+−).Thetwoleading-pTleptons of the quadruplet must have pT>20 and 15 GeV, respectively, whilethethirdleptonmusthave pT>10(8)GeV ifitisan elec-tron (muon).The fourleptons ofa quadrupletare requiredto be separatedfromeachother by R >0.1(0.2) forsame(different) flavour. At most one SA or a CaloTag muon is allowed in each quadruplet.Theinclusionoffinal-stateradiationtochargedleptons followsthesameapproachasdescribedinRef.[33].Eacheventis required to have the triggering lepton(s) matched to one or two oftheselected leptons.Alltheselected 4eventsmustlie inthe 80 <m4<1000 GeV range.

Foreach channel,theleptonpairwiththemassclosest tothe

Z -bosonmassis selectedasthe leadingdilepton pairandits

in-variant mass, m12, is required to be between 50 and 120 GeV. The sub-leading +− pairwith the largest invariant mass,m34, among the remaining possible pairs, is selected in the invariant

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massrange 12 <m34<120 GeV. Inthe 4e and 4μ channels all possible +− pairsarerequiredtohavem+−>5 GeV toreject eventscontaining J/ψ→ +− decays. The transverse momenta oftheleptonpairsmustbeabove2 GeV.

5. Backgroundestimation

Thedominantreduciblebackgroundsforthisanalysisarefrom

Z+jets andt¯t processesandare estimatedfromdata. Contribu-tionsfromZW,Zγ,tZ aswellasfromtheirreduciblebackgrounds

from t¯tZ, VVV, ZH and double-DY processes are estimated from

simulation.

TheZ+jets andt¯t backgroundsareestimatedusingtwo differ-entﬁnal statesindata: +μμand +ee,where ( =e, μ) is the leading-lepton pair. The +μμ background arises from

Z+jets andtt processes¯ wherethe Z+jets contributioninvolves theassociatedproductionofaZbosonandheavy-ﬂavourhadrons, whichdecaysemileptonically,anda componentarising from Z +

light-flavourjetswithsubsequent π/K in-flightdecays.The back-groundfor +ee finalstatesarisesfromassociatedproductionof

a Z boson withother objects namely jets misidentiﬁed as

elec-trons, which can be light-flavour hadrons misidentified as elec-trons,photon conversions reconstructedaselectrons, orelectrons fromsemileptonicdecaysofheavy-flavourhadrons.

Forboththe +μμand +ee cases,thenumbersof

back-groundeventsare estimatedfromaﬁtperformedsimultaneously tothreemutuallyexclusive control regions,each ofthem provid-ing informationon one ormore backgroundcomponents.The ﬁt is based on the mass of the leading dilepton,m12, which peaks

atthe Z massforthe Z+jets componentandhasa broad

distri-butionforthett component.¯ Thethree controlregions are fit si-multaneouslytoextractthedifferentcomponentsofthereducible background,using a profile likelihood approach where the input template shapes for Z +jets and t¯t are obtained from simula-tion. The fitted yields in the control regions are extrapolated to thesignalregion usingefficiencies,referred toastransferfactors, obtainedfromsimulation.Independentvalidationregionsareused tochecktheextrapolations.

Thethreecontrol regionsfor +μμbackgroundare deﬁned basedontheimpactparametersigniﬁcanceandisolationvariables ofthesub-leadingmuonpairandareconstructedasfollows:

•A heavy-ﬂavour-enriched control region where at least one of the muonsin the second pair fails the impact parameter signiﬁcancerequirementwhiletheisolationrequirementis re-laxed;

•A light-ﬂavour-enriched control region where at leastone of the muonsin the second pairfails theisolation requirement butpassestheimpactparametersigniﬁcancecut;

•At¯t-enrichedregionwheretheleadingleptonpairismadeof

opposite-sign anddifferent-ﬂavour leptons.For themuonsof the second pairthere isnocharge requirement, the isolation cut isrelaxedandthemuonsmustnot satisfytheimpact pa-rameterrequirement.

Avalidationregiontocheckthe +μμbackgroundextrapolation is populated by both Z+jets and t¯t. The leading lepton pair is requiredto fulﬁlthe full selectioncriteria, while there isneither isolation nor impact parameter requirements on the sub-leading muonpair. Thisregion isused tocheck the ﬁtresultsandverify thatthedataandMCsimulationagree.

The three control regions for +ee background are defined basedontheimpactparametersignificance,isolationandelectron identificationrequirementsonthesecondelectronpair.Inall con-trolregions atleastone oftheelectrons inthesecond pairmust

Table 1

Numbersofexpectedbackgroundeventsfordifferentprocessesandchannels.

Process 4e 4μ 2e2μ t¯t 0.45±0.24 0.68±0.19 1.3±0.5 Z+jets 0.6±0.29 5.3±1.5 6.3±1.4 Diboson 1.25±0.18 0.83±0.18 2.84±0.34 Triboson 0.67±0.12 0.97±0.14 1.46±0.19 Z+top 0.62±0.15 1.19±0.32 1.7±0.5

notsatisfytheidentiﬁcationcriteria.Theseregionsareconstructed asfollows:

• A Z+jets-enriched control region whereat leastone of the electrons of the second pair fails the track isolation andno calorimeterisolationisrequired;

• AnadditionalZ+jets-enrichedcontrolregionwherenocharge requirementismadeontheelectronsofthesecondpair,while atleastoneof theseelectrons failstheimpact parameter se-lectionandnocalorimeterortrackisolationisrequired;

• At¯t-enrichedregion,wheretheleadingleptonpairisselected

from opposite-sign anddifferent-ﬂavour leptons. There is no charge requirementforthe sub-leadingelectronpair.At least one of the electrons of the second pairfails the calorimeter isolation requirement andneither trackisolation nor impact parameterrequirementsareapplied.

Avalidationregiontocheck the +ee backgroundextrapolation isdefinedbyremovingthecalorimeterisolationandrequiringthat atleastoneelectroninthesub-leadingpairfailstheelectron iden-tification.Eachcandidateinthepairisrequiredtopasstheimpact parameterandthetrackisolationselections.thisregionisusedto checkthefitoutcomeandverifythatthedataandMC simulation agree.

The residual contributions from ZZ and ZW production in all control regions are estimated fromsimulation. The purity ofthe

Z+jets andtt backgrounds¯ inthecontrolregionsisabove95%. Inthevalidationregions,thepost-ﬁtMCpredictionsagreewith thedatawithinthestatisticaluncertainty.

The major uncertainties for the ﬁtted reducible background comefromthenumberofeventsinthecontrolregionsfollowedby thesystematicuncertaintyinthetransferfactors.Thelatteris eval-uatedfromthedifferenceintheselectioneﬃciencydeterminedin dataandsimulationindedicatedcontrolregionsusingleptons ac-companying Z→ +− candidates,where theleptons composing

the Z -boson candidate are required to satisfy isolation and

im-pactparametercriteria.Eventswithfourleptonsareexcluded.For theMCestimatedbackgroundthesystematicuncertaintiesmainly comefromtheoreticalcross-sectionuncertaintiesfordifferent pro-cesses and from luminosity uncertainties in normalisations. The differential distributions for all background processes are taken fromsimulation.

The total number of background events estimated from data andMC simulationis 26.2±3.6. Numbers ofbackground events expectedperchannel estimatedfordifferentprocessesareshown in Table 1.Thebackgroundestimationwascross-checkedwithan alternativemethod,describedinRefs.[1,34],calledthefake-factor method.Theresultsfromthiscross-checkarefoundtobe consis-tentwithinuncertaintieswiththosedescribedabove.

6. Cross-sectionextractionmethod

Twocrosssectionsareextractedfromthenumberofobserved events.Oneistheﬁducialcrosssection, σﬁd

4,intheexperimental phase spacedeﬁnedbythe eventselection criteriaandtheother is the cross section, σext

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Table 2

Listofselection cutswhichdefine the fiducialregion ofthecross-section mea-surement.Same-flavouropposite-signleptonpairsaredenotedasSFOS,theleading leptonpairmassasm12,andthesub-leadingleptonpairmassasm34.The four-momentaofallfinal-statephotonswithin R=0.1 ofaleptonareaddedtothe four-momentumofthatlepton.

Lepton selection

Muons: pT>6 GeV,|η|<2.7

Electrons: pT>7 GeV,|η|<2.5

Lepton pairing

Leading pair: SFOSleptonpairwithsmallest

|mZ−m|

Sub-leading pair: TheremainingSFOSwiththelargest

m

For both pairs: p+−

T >2 GeV Event selection

Lepton p1,2,3

T : >20,15,10(8 ifμ)GeV

Mass requirements: 50<m12<120 GeV

12<m34<120 GeV Lepton separation: R(i,j)>0.1(0.2)

forsame- (different-)ﬂavourleptons

J/ψveto: m(+i,−j)>5 GeV

4mass range: 80<m4<1000 GeV

where electrons andmuons have the same geometric and kine-matic acceptance.The fiducial phase spaceis definedin Table 2. Theextendedphasespaceforthe4cross-sectionextractionis de-finedby80 <m4<1000 GeV,m+−>4 GeV,p

Z1,2

T >2 GeV,and thepresenceoffourleptonseachwithpT>5 GeV and|η| <2.8.

Thecrosssectionmeasurementisperformedusingalikelihood ﬁtdescribedbelow.Foragivenchanneli,theobservednumberof events, Ni

obs, follows aPoisson distribution, Pois(Niobs, Nipred),the meanofwhich,Ni

pred=Nis+Nbi,isthesumoftheexpectationsfor signalandbackgroundyields.Theseyieldsdependontheﬁducial crosssectionandthenuisanceparameters, x,whichrepresentthe experimentalandtheoreticaluncertaintiesas:

Ni_s(σﬁd 4l ,x)=Nsi(σ4lﬁd,0)(1+ k xkSki), (1) N_bi(x)=N_bi(0)(1+ k xkBki), (2)

where S_ki andBi_k are therelativesystematiceffectson thesignal and background, respectively, due to the k-th source of system-aticuncertainty.Thecentralexpectationofthesignalyield, corre-spondingtothesystematicsourcesatthenominalvalue(referred toasthenuisance-freeexpectation),isgivenby:

Ni_s(σ_4lfid,0)=L·C4·Kτ·σ4lfid, (3) where Lis theintegratedluminosity, andC4 isthe ratioofthe number of accepted signal events to the number of generated eventsinthe fiducialphase space.Corrections are appliedto C4 toaccount formeasured differencesintriggerandreconstruction efficiencies betweensimulated anddata samples. The C4 values are53.3%,82.2%and67.7%forthe4e,4μ,and2e2μchannels, re-spectively.Thecontributionfrom τ-leptondecaysisaccountedfor byacorrectionterm Kτ=1+N_τMC/N_sigMC,whereN_τMCisthe num-berof acceptedsimulated4 eventsinwhichatleastone ofthe

Z bosons decays into τ-lepton pairs, and NMC_sig is the number of

acceptedsimulatedZZ eventswithdecaysintoelectronsormuons. Cross-sectionmeasurementsare extractedforasingle channel or any combination of channels, using a likelihood method. The likelihoodfunctionis:

L(σ_4lﬁd,x)=

i

Pois(N_obsi ,Ni_pred(σ_4lﬁd,x))·e−x22_, ₍₄₎

Table 3

ThecombinedrelativeuncertaintiesontheeﬃciencycorrectionfactorC4,

evalu-atedasthesuminquadratureoftheuncertaintiesfromdifferentsources,including electronand muonidentiﬁcationand theoreticaluncertaintiesduetoPDFs,QCD scales,andpartonshowermodelling.Extrauncertaintiesduetohigher-order cor-rectionsforthegg process(NNLOK -factorsforHiggs-bosonproductionappliedto theinclusivegg process)arealsogiven.

Sources C4/C4 4e 4μ 2e2μ Experimental (e) 4.8% – 2.3% Experimental (μ) – 1.8% 0.9% Theoretical 0.1% 0.1% 0.2% Extra gg corrections 0.6% 0.2% 0.3% Combined uncertainty 4.9% 1.9% 2.5% Table 4

TheoreticalrelativeuncertaintiesontheﬁducialacceptanceA4andA4×C4due

toPDFs,QCDscalesandpartonshowermodelling.Extrauncertaintiesdueto higher-ordercorrectionsforthe gg process(NNLO K -factorsforHiggs-bosonproduction appliedtotheinclusivegg process)arealsogiven.

Sources A4/A4 4e 4μ 2e2μ Theoretical 1.2% 1.0% 1.6% Extra gg corrections 4.0% 3.0% 3.9% (A4×C4)/(A4×C4) Theoretical 1.4% 1.1% 1.7% Extra gg corrections 4.6% 3.2% 4.2%

wheretheproductrunsoverthechannelstobeconsidered. Fortheextendedphasespacethelikelihoodfunctionis param-eterisedasafunctionoftheextendedcrosssection similartothe one shown in Eq. (3) and multiplied by the ﬁducial acceptance

A4,whichistheratioofthenumberofeventswithintheﬁducial phase-spaceregiontothetotalnumberofgeneratedeventsinthe extendedphasespace.Theﬁducialacceptance A4isevaluated us-ing simulationto be 41.6%,50.3%,and42.2%,forthe 4e,4μ,and 2e2μ channels, respectively. The differencesare dueto the elec-tronandmuongeometricdetectioncoverage.

Toﬁndthecentral valueofthecrosssection σ,thelikelihood functionismaximisedsimultaneouslywithrespecttothenuisance parametersand σ.Correlationsbetweenthesignalandbackground systematic uncertainties are taken intoaccount in the likelihood ﬁttingprocedure.

7. Systematicuncertainties

Systematicuncertaintiesonthemeasurementarisefrom uncer-taintiesontheintegratedluminosity,theexperimentalcalibrations oftheleptonenergyandmomentum,andtheleptondetection ef-ﬁciencies,aswellasthetheoreticalmodellingofsignalacceptance, andthebackgroundestimation.Theoveralluncertaintyonthe in-tegratedluminosityis±2.8%,whichisderivedfollowingthesame methodology asthat detailedinRef. [35].A summary ofthe rel-ativeuncertainties ofC4, A4,and A4×C4 isgivenin Tables 3

and 4.

The effect on the expected signal eventyields dueto experi-mentalsystematicuncertaintiesisdeterminedfromthe uncertain-ties on lepton energy andmomentum scales and resolutions, as well astheuncertainties on efficienciesof thelepton reconstruc-tion and identification. The major contributions come from the uncertainties onlepton reconstruction andidentification efficien-cies[36–38].

TheuncertaintiesonthesignalacceptanceforbothC4andA4 includetheoreticaluncertaintiesfromthechoiceofQCDscalesand PDFset.Thescalesarevariedindependentlyfrom0.5to2.0times

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Table 5

Summaryoftheobservedandpredictednumbersof4eventsindifferent4channels.NData _denotes_the_selected_number_of_data_candidates._NTotal

expecteddenotesthetotal predictednumberofevents(includingτcontributions)forsignalplusbackground.Nsignalnon-ggandtheN

signal

gg denotethepredictednon-gg signalandthegg signal(noNNLO K -factorhasbeenapplied),respectively.NMC

τ denotestheτ contributions.Nbkgdenotesthetotalestimatednumberofbackgroundevents(fromdataandMCsimulation). Thelisteduncertaintiesoftheexpectednumberofsignaleventsincludestatisticalandexperimentalsystematicuncertainties.

Channel NData _NTotal

expected N signal non-gg Nsignalgg NMCτ Nbkg 4e 85 80±4 68.4±3.4 6.24±0.31 1.28±0.06 3.6±0.5 4μ 156 150.2±2.9 128.2±2.5 11.00±0.21 2.18±0.09 9.0±1.5 2e2μ 235 205±5 172±5 16.0±0.4 3.08±0.13 13.6±2.1 Total 476 435±9 369±9 33.3±0.8 6.54±0.14 26.2±3.6

thenominalvaluesof μR and μF.ThePDFsuncertainties are es-timatedby usingthe envelope[39] ofvariations ofdifferent PDF sets,CT10,MSTW2008[40]andNNPDF2.3[41].

TheC4uncertaintyismostlyexperimentalandoftheorderof 2–5%,whilethe A4 uncertaintyisentirelytheoreticalandofthe orderof 3–5%. A rangeof valuesof the relative uncertainties on theC4aregivenby4.9%,1.9%,and2.5%forthe4e,4μ,and2e2μ, respectively.Theuncertainties on C4 duetohigher-order correc-tions to the gg production processes are lessthan 0.6%. This is estimatedbyapplyinganapproximateNNLO K -factordetermined fortheHiggs-bosonproduction[23],assumingthatitisapplicable tothenormalisationofthecontinuum gg→Z Z productioncross section.

Uncertainties on C4, as a function of m4 and p4T, are also computedforthe differential crosssection measurements. In the mass region (m4<150 GeV), the relative uncertainties on C4 varyintherangeof4–9%,1.7–2.7%,and2–5%forthe4e,4μ,and 2e2μ channel, respectively. In the mass region m4>150 GeV, they are almostconstant as afunction ofm4 andare about4%, 1.8%,and3%forthe4e,4μ,and2e2μchannel,respectively.

The relative uncertainties on A4 are 1.2%, 1.0%, and1.6% for the4e,4μ,and2e2μchannel,respectively, evaluatedby compar-ing POWHEG-BOX and MCFM MCsampleswiththesameapproach fortheQCD scalesandthePDF uncertaintiesasdescribedearlier. TheQCDscaleuncertainties donot changewhengoingfromNLO toNNLOforthesignalnormalisationfortheqq¯→4events[16]. An additional uncertainty (3–4%) is included in the A4 uncer-taintyestimate toaccount fortheuncertaintyoftheHiggs-boson

NNLO K -factor normalisation correction of the non-resonant 4

signalfromgluonfusion(labelled“extraggcorrections”in Tables 3 and 4).

Theoveralluncertaintyonthebackgroundestimationis±14%. Thecontributionsfromdifferentsourcesandchannelsaregivenin Table 1.

8. Results

8.1.Cross-sectionmeasurements

The numbers of expected andobserved events after applying allselectioncriteriaareshownin Table 5.Atotalof476candidate events is observed with a background expectation of 26.2±3.6 events.The observedandpredictedm4 andp4T distributions for theselectedeventsareshownin Fig. 3.

Themeasuredcrosssectionsintheﬁducialandextendedphase space for different 4 channels are summarised in Table 6 and compared to the SM predicted cross sections. The combined 4 cross section in the extended phase space is found to be 73± 4 (stat.)±4 (syst.)±2 (lumi.) fb,comparedtoaSMpredictionof 65±4 fb.Oneshouldnotethatthecrosssectionfornon-resonant

ZZ productionfromthe gg-inducedsignalisonlycalculatedatLO

approximation,whichcouldbesigniﬁcantlyunderestimated.

Fig. 3. DataandMCpredictioncomparisonforselectedeventsasafunctionofthe invariantmassm4(top)andthetransversemomentump4T(bottom)ofthe four-leptonsystem.Thesolidcoloursshowtheexpectedcontributionsfromsignaland backgroundandtheblackpointsrepresentdatawithstatisticalerrorbars.(For in-terpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferred tothewebversionofthisarticle.)

8.2. Differentialcross-sectionmeasurement

Themeasurementofthedifferentialcross-sectionisperformed intheﬁducialphasespacedeﬁnedin Table 2.Theeventsfromall three 4 channels are combined into a common sample forthe unfolding procedure. The unfolding is done as a function of the twokinematicvariablesm4and pT4.Them4spectrumis essen-tial for the studyof the different production mechanisms, while the p4

T spectrumissensitivetohigher-orderQCDcorrectionsand toQCDresummationeffectsatsmallp4

T [10].Thehigh-p 4

T region issensitivetotop-loopeffectsin gg→H productionaswellasto anomaloustriple-bosoncouplings.

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Table 6

Measuredcrosssectionsintheﬁducialphasespace(σﬁd₎_and_extended_phase_space₍_σext_),_compared_to_their_SM_predictions_{(calculations}_described_in_Section₃_)._One shouldnotethatthenon-resonantgg-inducedsignalcrosssectionisonlycalculatedatLOapproximation.

4 Measuredσﬁd_[fb] _SM_σﬁd_[fb] _Measured_σext_[fb] _SM_σext_[fb]

4e 7.4+0.9 −0.8(stat)+ 0.4 −0.3(syst)+ 0.2 −0.2(lumi) 6.9±0.4 17.8+ 2.1 −2.0(stat)+ 1.5 −1.1(syst)+ 0.5 −0.5(lumi) 16.4±1.0 4μ 8.7+₋00..87(stat)+ 0.2 −0.2(syst)+ 0.3 −0.2(lumi) 8.3±0.5 17.3+ 1.5 −1.4(stat)+ 0.9 −0.7(syst)+ 0.5 −0.5(lumi) 16.4±1.0 2e2μ 15.9+1.1 −1.1(stat)+ 0.5 −0.4(syst)+ 0.5 −0.4(lumi) 13.7±0.9 37.7+ 2.7 −2.6(stat)+ 2.5 −2.0(syst)+ 1.1 −1.1(lumi) 32.1±2.0 Total 73+₋44(stat)+ 4 −4(syst)+ 2 −2(lumi) 65±4

Fig. 4. Themeasureddifferentialcross-sectiondistributions(theblackpoints)ofm4(left)andp4T(right),unfoldedintotheﬁducialphasespace,andcomparedtotheory predictions(redhistogram).Thecombinedstatisticalandsystematicuncertaintiesofthemeasurementsareshownastheerrorbarsoftheunfoldedspectra.Thetheoretical predictionsarethesumofthedifferentialcrosssectionsoftheq¯q→4and gg→4processes,wheretheLOcrosssectionsareusedforthenon-resonantgg-induced signals,andthecrosssectionsoftheon-shellHiggsbosonandtheq¯q productionprocessesarecorrectedwiththeNNLOK -factorsforthem4spectrum;exceptforthep4T whereonlytheNLOandLOpredictionsareusedfortheq¯q andthegg processes,respectively.Thetotaltheoreticaluncertaintiesareshownaserrorbandsevaluatedbythe suminquadratureofthecontributionsfrompartonshowers,QCDscales,PDFsets,andelectroweakcorrections.(Forinterpretationofthereferencestocolourinthisﬁgure legend,thereaderisreferredtothewebversionofthisarticle.)

TheiterativeBayesianunfolding[42]isappliedhere.Inthe un-foldingofbinned data,theeffectsoftheexperimentalacceptance andresolutionareexpressedintermsofaresponsematrix,where each element corresponds to the probability of an event in the

i-thgeneratorlevel binbeingreconstructedin the j-th measure-ment bin. The response matrix is combined with the measured spectrumtoformalikelihood,whichisthenmultipliedbyaprior distributiontoproducetheposteriorprobability ofthetrue spec-trum.The SMpredictionisusedastheinitialprior, andoncethe posteriorprobabilityisobtained,itisusedasthepriorforthenext iteration.Thespectrumbecomes insensitivetotheinitialprior af-terafewiterations. Thedifferencesbetweensuccessiveiterations areusedtoestimatethestabilityoftheunfoldingmethod.Inthis analysisfouriterationsareperformed.

Theunfolded distributions areshownin Fig. 4,where the dif-ferential cross section is presentedasa function of m4 and p4T and compared to theory predictions. The data points shown in the ﬁgures are the measurements with combined statistical and systematicuncertainties. The theoretical predictions are the sum of the differential cross sections of the qq¯→4 and gg →4 processes. The LO cross sections are used for the non-resonant

gg-inducedsignals.Thecrosssectionsoftheon-shellHiggsboson

are normalisedto includetheNNLO QCD andNLO EWeffects as summarisedinRef.[19].Theqq production¯ processesarecorrected

withNNLO QCDandtheNLO EW K -factorsforthem4spectrum

form4>2×mZ.Forthep4Tspectrum,theqq signal¯ predictionis calculatedby POWHEG-BOX atNLO.

The uncertainties on the differential cross-section measure-ments are dominatedby the statisticaluncertainties ofthe data. Forexample,in them4 regions betweenthe Z and Higgsboson peaksandbetweentheHiggs-bosonmassmH andm4=180 GeV, the statistical uncertainties are of the order of45% and 20%, re-spectively. In the high-mass region (m4>180 GeV) they are of the order of 10%. Furthermore, one should note that the NNLO QCD correctionsarenot availablefortheqq¯→4 production cal-culationforthemassregionm4<2×mZ.

In the m4 bin of 120–130 GeV, which is dominated by the resonant Higgs-boson contribution, the ratio of data to the MC predictioniscompatiblewiththeATLASmeasurement[33]ofthe Higgs-bosonsignalstrengthof μH=1.44₋+₀0._.40₃₃.Thedatapointsin the m4 spectrum between 140 and 180 GeV are slightly more than 1σ above thetheoretical predictions, wherethe NNLO QCD correction isnot yetavailable. Somediscrepancyis alsoobserved inthelowestbinandintheregionbetween30 and50 GeVofthe

p4

T spectrum.

8.3. Extractionofthegg signalcontributioninthem4>180 GeV

region

The extraction of the signal strength of the non-resonant

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Fig. 5. Comparisonofthem4 spectrabetweenthedata(blackpointswitherror

bars)andtheprediction(redhistogram)afterthelikelihoodﬁtofμgg.Thenon-gg signalfromthetheoreticalprediction(bluehistogram)andthebackground(brown histogram)arealsoshown.Thegg contributionisthedifferencebetweendataand thesumofthenon-gg signalandthebackground.(Forinterpretationofthe refer-encestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionof thisarticle.)

180 GeV,wherethisproductionmodeisdominatedbythe

contin-uum gg→Z Z processthroughaquark-boxdiagramintermediate

state (see Fig. 1(d)). Additional contributions come fromthe off-shellHiggs-bosonproductionandtheinterferencebetweenHiggs bosonandcontinuum Z Z production.

Them4 spectrumischosen asthe discriminanttoextractthe

gg signal strength withrespect to the LO gg prediction: μgg=

σ(data)/σ(LO).

Thecontributionof theqq¯→Z Z production isconstrainedto the best theory knowledge (which accounts for QCD NNLO and EWNLO m4l-dependentcorrections) and μgg isextractedfroma likelihoodﬁtusingthereconstructedm4 distributions.The exper-imental uncertainties are treated as fully correlated between qq¯

andgg processes. Thetheoreticaluncertainties, includingthe

un-certaintiesonthenormalisationoftheqq¯→Z Z→4,theshapes of4 spectrafromboth theqq and¯ gg initial states,andthe ac-ceptance,aretakenintoaccount.Them4 distributionofthedata, theﬁt,theexpectationfromnon-gg signalprocessesandthe back-groundareshownin Fig. 5.Theﬁtresultis μgg=2.4±1.0(stat.) ± 0.5(syst.) ±0.8(theory). Thisresult corresponds to a gg-initiated

crosssection of3.1 fb, which hasthesame relative uncertainties as μgg itselfintheinclusiveﬁducialvolumeasdeﬁnedin Table 2 withtheadditionalrequirementofm4>180 GeV.Thelargest un-certaintyisstatistical.Thetheoreticaluncertaintyismainlydueto thenormalisationuncertaintyoftheqq¯→Z Z process.

Thetheoreticalestimateofm4-dependentK -factorforoff-shell HiggsbosonproductiongiveninRef. [23] isinarangeof2.7–3.1 (withCT10NNLOPDF)andthatgiveninRef.[24] forthe interfer-encetermis2.05–2.45.Thesetheoreticalstudies conﬁrmthatthe gluon soft-collinear approximation predicts similar K -factors for off-shell Higgs-boson and interference, hence supporting the as-sumptionofasimilar K -factorforthe continuum Z Z production.

Thesetheoreticalcalculated K -factorsarecompatiblewiththe re-sultobtainedbythisanalysis,wherethegg-initiated4eventsare producedpredominantlyfromthecontinuum Z Z production.

Applyingthehigher-ordercorrectionstoboththecrosssection oftheoff-shellHiggs-bosonproductionandthecontributionofthe interferenceterm,whilekeepingtheLOcrosssectionforthe con-tinuum gg→Z Z production, thechange ofthe μgg ﬁt resultis negligible(approximately, μgg=0.01).

9. Conclusion

The measurement of four-lepton production in proton–proton collisions at√s=8 TeV ispresentedusingdatacorresponding to anintegratedluminosityof20.3 fb−1collectedwiththeATLAS de-tector attheLHC.Intotal,476 4candidateeventsare observed, witha background expectationof 26.2±3.6 events, in the four-lepton invariant mass range between 80 and 1000 GeV. The 4 productioncrosssectionsaredetermined inbothﬁducialand ex-tended phasespaces.Themeasuredcrosssectionintheextended phase space, deﬁned by 80 <m4<1000 GeV, m+− >4 GeV,

pZ1,2

T >2 GeV, four leptons each with pT>5 GeV and|η| <2.8, is found to be 73±4 (stat.) ± 4 (syst.) ± 2 (lumi.) fb, and is compared to a SM prediction of 65±4 fb. The measurements of the 4 differential cross sections are performed by unfolding the m4 andthe pT4 spectra. In the mass rangeabove 180 GeV, assuming the theoretical constraint on the qq production¯ cross section calculated with perturbative NNLO QCD and NLO elec-troweakcorrections, the signalstrength ofthegluon-fusion com-ponent with respect to the LO prediction is determined to be

μgg=2.4±1.0(stat.) ±0.5(syst.) ±0.8(theory).

Acknowledgements

We thank CERN forthe very successfuloperation of the LHC, aswell as thesupport staff fromour institutionswithout whom ATLAScouldnotbeoperatedeﬃciently.

We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia;BMWFW andFWF,Austria; ANAS, Azer-baijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, CzechRepublic;DNRF,DNSRCandLundbeckFoundation,Denmark; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia; BMBF, HGF, and MPG, Germany; GSRT, Greece; RGC, Hong Kong SAR, China; ISF, I-CORE and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; RCN, Norway;MNiSWandNCN,Poland;FCT,Portugal;MNE/IFA, Roma-nia; MESofRussiaandNRCKI, RussianFederation;JINR;MESTD, Serbia; MSSR,Slovakia; ARRSandMIZŠ,Slovenia; DST/NRF,South Africa; MINECO, Spain;SRCandWallenberg Foundation, Sweden; SERI, SNSF andCantons ofBern andGeneva, Switzerland; MOST, Taiwan;TAEK,Turkey;STFC,UnitedKingdom;DOEandNSF,United States of America. In addition, individual groups and members havereceived supportfromBCKDF,theCanadaCouncil, CANARIE, CRC, Compute Canada, FQRNT, andthe Ontario Innovation Trust, Canada;EPLANET,ERC,FP7, Horizon2020andMarie Skłodowska-Curie Actions, European Union; Investissements d’Avenir Labex andIdex,ANR,RegionAuvergneandFondationPartagerleSavoir, France; DFG and AvH Foundation, Germany; Herakleitos, Thales and Aristeia programmes co-ﬁnanced by EU-ESF and the Greek NSRF;BSF,GIFandMinerva,Israel;BRF,Norway;theRoyalSociety andLeverhulmeTrust,UnitedKingdom.

The crucial computingsupport from all WLCG partnersis ac-knowledged gratefully, in particular from CERN and the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy),NL-T1(Netherlands),PIC(Spain), ASGC(Taiwan),RAL(UK) andBNL(USA)andintheTier-2facilitiesworldwide.

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ATLASCollaboration

G. Aad85, B. Abbott113,J. Abdallah151,O. Abdinov11, R. Aben107, M. Abolins90, O.S. AbouZeid158, H. Abramowicz153,H. Abreu152, R. Abreu116, Y. Abulaiti146a,146b, B.S. Acharya164a,164b,a,

L. Adamczyk38a,D.L. Adams25,J. Adelman108, S. Adomeit100, T. Adye131, A.A. Affolder74, T. Agatonovic-Jovin13,J. Agricola54, J.A. Aguilar-Saavedra126a,126f, S.P. Ahlen22, F. Ahmadov65,b, G. Aielli133a,133b, H. Akerstedt146a,146b,T.P.A. Åkesson81,A.V. Akimov96,G.L. Alberghi20a,20b, J. Albert169, S. Albrand55,M.J. Alconada Verzini71, M. Aleksa30,I.N. Aleksandrov65,C. Alexa26b, G. Alexander153, T. Alexopoulos10,M. Alhroob113, G. Alimonti91a, L. Alio85, J. Alison31,S.P. Alkire35, B.M.M. Allbrooke149,P.P. Allport18,A. Aloisio104a,104b, A. Alonso36,F. Alonso71,C. Alpigiani138, A. Altheimer35,B. Alvarez Gonzalez30,D. Álvarez Piqueras167, M.G. Alviggi104a,104b, B.T. Amadio15, K. Amako66, Y. Amaral Coutinho24a,C. Amelung23,D. Amidei89, S.P. Amor Dos Santos126a,126c, A. Amorim126a,126b, S. Amoroso48,N. Amram153, G. Amundsen23, C. Anastopoulos139, L.S. Ancu49, N. Andari108, T. Andeen35,C.F. Anders58b,G. Anders30,J.K. Anders74,K.J. Anderson31,

A. Andreazza91a,91b_,_{V. Andrei}58a_,_{S. Angelidakis}9_, _{I. Angelozzi}107_,_{P. Anger}44_,_{A. Angerami}35_,

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J. Antos144b, F. Anulli132a,M. Aoki66, L. Aperio Bella18, G. Arabidze90,Y. Arai66, J.P. Araque126a,

A.T.H. Arce45, F.A. Arduh71,J-F. Arguin95, S. Argyropoulos63, M. Arik19a, A.J. Armbruster30,O. Arnaez30, H. Arnold48, M. Arratia28,O. Arslan21, A. Artamonov97, G. Artoni23, S. Asai155, N. Asbah42,

A. Ashkenazi153,B. Åsman146a,146b,L. Asquith149, K. Assamagan25, R. Astalos144a,M. Atkinson165, N.B. Atlay141, K. Augsten128,M. Aurousseau145b, G. Avolio30,B. Axen15, M.K. Ayoub117,G. Azuelos95,d, M.A. Baak30,A.E. Baas58a,M.J. Baca18,C. Bacci134a,134b,H. Bachacou136, K. Bachas154, M. Backes30, M. Backhaus30,P. Bagiacchi132a,132b,P. Bagnaia132a,132b, Y. Bai33a, T. Bain35,J.T. Baines131,

O.K. Baker176, E.M. Baldin109,c, P. Balek129, T. Balestri148, F. Balli84,W.K. Balunas122,E. Banas39, Sw. Banerjee173,A.A.E. Bannoura175,L. Barak30, E.L. Barberio88,D. Barberis50a,50b, M. Barbero85, T. Barillari101, M. Barisonzi164a,164b,T. Barklow143,N. Barlow28, S.L. Barnes84,B.M. Barnett131, R.M. Barnett15,Z. Barnovska5, A. Baroncelli134a,G. Barone23, A.J. Barr120, F. Barreiro82,

J. Barreiro Guimarães da Costa57, R. Bartoldus143,A.E. Barton72, P. Bartos144a, A. Basalaev123,

A. Bassalat117, A. Basye165, R.L. Bates53,S.J. Batista158,J.R. Batley28, M. Battaglia137,M. Bauce132a,132b, F. Bauer136,H.S. Bawa143,e,J.B. Beacham111,M.D. Beattie72, T. Beau80, P.H. Beauchemin161,

R. Beccherle124a,124b, P. Bechtle21, H.P. Beck17,f, K. Becker120,M. Becker83, M. Beckingham170, C. Becot117, A.J. Beddall19b,A. Beddall19b,V.A. Bednyakov65, C.P. Bee148,L.J. Beemster107, T.A. Beermann30,M. Begel25,J.K. Behr120,C. Belanger-Champagne87, W.H. Bell49, G. Bella153, L. Bellagamba20a, A. Bellerive29,M. Bellomo86, K. Belotskiy98, O. Beltramello30,O. Benary153, D. Benchekroun135a, M. Bender100, K. Bendtz146a,146b, N. Benekos10, Y. Benhammou153,

E. Benhar Noccioli49,J.A. Benitez Garcia159b, D.P. Benjamin45,J.R. Bensinger23,S. Bentvelsen107, L. Beresford120,M. Beretta47,D. Berge107,E. Bergeaas Kuutmann166, N. Berger5,F. Berghaus169, J. Beringer15,C. Bernard22,N.R. Bernard86, C. Bernius110,F.U. Bernlochner21,T. Berry77,P. Berta129, C. Bertella83, G. Bertoli146a,146b, F. Bertolucci124a,124b,C. Bertsche113, D. Bertsche113,M.I. Besana91a, G.J. Besjes36,O. Bessidskaia Bylund146a,146b, M. Bessner42, N. Besson136, C. Betancourt48, S. Bethke101, A.J. Bevan76,W. Bhimji15, R.M. Bianchi125,L. Bianchini23,M. Bianco30,O. Biebel100,D. Biedermann16, S.P. Bieniek78, N.V. Biesuz124a,124b, M. Biglietti134a, J. Bilbao De Mendizabal49, H. Bilokon47, M. Bindi54, S. Binet117,A. Bingul19b, C. Bini132a,132b, S. Biondi20a,20b,D.M. Bjergaard45,C.W. Black150,J.E. Black143, K.M. Black22, D. Blackburn138, R.E. Blair6, J.-B. Blanchard136,J.E. Blanco77, T. Blazek144a,I. Bloch42, C. Blocker23, W. Blum83,∗, U. Blumenschein54,S. Blunier32a,G.J. Bobbink107, V.S. Bobrovnikov109,c, S.S. Bocchetta81, A. Bocci45, C. Bock100,M. Boehler48, J.A. Bogaerts30, D. Bogavac13,

A.G. Bogdanchikov109, C. Bohm146a,V. Boisvert77, T. Bold38a, V. Boldea26b,A.S. Boldyrev99,

M. Bomben80,M. Bona76,M. Boonekamp136,A. Borisov130,G. Borissov72,S. Borroni42,J. Bortfeldt100, V. Bortolotto60a,60b,60c,K. Bos107,D. Boscherini20a,M. Bosman12, J. Boudreau125, J. Bouffard2,

E.V. Bouhova-Thacker72,D. Boumediene34, C. Bourdarios117,N. Bousson114,S.K. Boutle53,A. Boveia30, J. Boyd30,I.R. Boyko65,I. Bozic13,J. Bracinik18,A. Brandt8, G. Brandt54,O. Brandt58a, U. Bratzler156, B. Brau86,J.E. Brau116, H.M. Braun175,∗, W.D. Breaden Madden53,K. Brendlinger122, A.J. Brennan88, L. Brenner107,R. Brenner166, S. Bressler172,K. Bristow145c, T.M. Bristow46,D. Britton53,D. Britzger42, F.M. Brochu28,I. Brock21, R. Brock90, J. Bronner101,G. Brooijmans35,T. Brooks77, W.K. Brooks32b, J. Brosamer15, E. Brost116, P.A. Bruckman de Renstrom39,D. Bruncko144b,R. Bruneliere48,A. Bruni20a, G. Bruni20a,M. Bruschi20a,N. Bruscino21,L. Bryngemark81, T. Buanes14, Q. Buat142, P. Buchholz141, A.G. Buckley53, S.I. Buda26b, I.A. Budagov65,F. Buehrer48, L. Bugge119, M.K. Bugge119,O. Bulekov98, D. Bullock8,H. Burckhart30,S. Burdin74,C.D. Burgard48,B. Burghgrave108, S. Burke131, I. Burmeister43, E. Busato34,D. Büscher48,V. Büscher83,P. Bussey53, J.M. Butler22,A.I. Butt3,C.M. Buttar53,

J.M. Butterworth78, P. Butti107,W. Buttinger25, A. Buzatu53, A.R. Buzykaev109,c,S. Cabrera Urbán167, D. Caforio128, V.M. Cairo37a,37b, O. Cakir4a, N. Calace49,P. Calaﬁura15, A. Calandri136,G. Calderini80, P. Calfayan100,L.P. Caloba24a, D. Calvet34, S. Calvet34,R. Camacho Toro31,S. Camarda42,

P. Camarri133a,133b, D. Cameron119,R. Caminal Armadans165,S. Campana30, M. Campanelli78, A. Campoverde148,V. Canale104a,104b,A. Canepa159a,M. Cano Bret33e, J. Cantero82,R. Cantrill126a, T. Cao40, M.D.M. Capeans Garrido30,I. Caprini26b, M. Caprini26b, M. Capua37a,37b, R. Caputo83, R.M. Carbone35, R. Cardarelli133a, F. Cardillo48,T. Carli30,G. Carlino104a,L. Carminati91a,91b, S. Caron106,E. Carquin32a, G.D. Carrillo-Montoya30,J.R. Carter28,J. Carvalho126a,126c, D. Casadei78, M.P. Casado12, M. Casolino12, E. Castaneda-Miranda145a, A. Castelli107, V. Castillo Gimenez167,

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N.F. Castro126a,g, P. Catastini57, A. Catinaccio30,J.R. Catmore119, A. Cattai30, J. Caudron83,

V. Cavaliere165,D. Cavalli91a,M. Cavalli-Sforza12,V. Cavasinni124a,124b,F. Ceradini134a,134b,B.C. Cerio45, K. Cerny129, A.S. Cerqueira24b,A. Cerri149,L. Cerrito76,F. Cerutti15,M. Cerv30, A. Cervelli17,

S.A. Cetin19c,A. Chafaq135a, D. Chakraborty108,I. Chalupkova129, P. Chang165,J.D. Chapman28,

D.G. Charlton18,C.C. Chau158,C.A. Chavez Barajas149,S. Cheatham152, A. Chegwidden90,S. Chekanov6, S.V. Chekulaev159a, G.A. Chelkov65,h,M.A. Chelstowska89,C. Chen64,H. Chen25, K. Chen148,

L. Chen33d,i,S. Chen33c,S. Chen155,X. Chen33f,Y. Chen67, H.C. Cheng89, Y. Cheng31, A. Cheplakov65, E. Cheremushkina130, R. Cherkaoui El Moursli135e, V. Chernyatin25,∗,E. Cheu7, L. Chevalier136, V. Chiarella47,G. Chiarelli124a,124b,G. Chiodini73a, A.S. Chisholm18,R.T. Chislett78, A. Chitan26b,

M.V. Chizhov65,K. Choi61, S. Chouridou9, B.K.B. Chow100,V. Christodoulou78, D. Chromek-Burckhart30, J. Chudoba127, A.J. Chuinard87, J.J. Chwastowski39, L. Chytka115,G. Ciapetti132a,132b,A.K. Ciftci4a,

D. Cinca53,V. Cindro75,I.A. Cioara21,A. Ciocio15, F. Cirotto104a,104b, Z.H. Citron172,M. Ciubancan26b, A. Clark49, B.L. Clark57,P.J. Clark46,R.N. Clarke15, C. Clement146a,146b, Y. Coadou85,M. Cobal164a,164c, A. Coccaro49,J. Cochran64,L. Coffey23, J.G. Cogan143,L. Colasurdo106,B. Cole35, S. Cole108,

A.P. Colijn107,J. Collot55,T. Colombo58c, G. Compostella101, P. Conde Muiño126a,126b,E. Coniavitis48, S.H. Connell145b, I.A. Connelly77, V. Consorti48,S. Constantinescu26b,C. Conta121a,121b,G. Conti30, F. Conventi104a,j,M. Cooke15,B.D. Cooper78,A.M. Cooper-Sarkar120, T. Cornelissen175, M. Corradi20a, F. Corriveau87,k,A. Corso-Radu163,A. Cortes-Gonzalez12,G. Cortiana101, G. Costa91a,M.J. Costa167, D. Costanzo139, D. Côté8,G. Cottin28, G. Cowan77, B.E. Cox84,K. Cranmer110,G. Cree29,

S. Crépé-Renaudin55,F. Crescioli80, W.A. Cribbs146a,146b,M. Crispin Ortuzar120, M. Cristinziani21, V. Croft106,G. Crosetti37a,37b, T. Cuhadar Donszelmann139,J. Cummings176,M. Curatolo47,J. Cúth83, C. Cuthbert150, H. Czirr141,P. Czodrowski3,S. D’Auria53,M. D’Onofrio74,

M.J. Da Cunha Sargedas De Sousa126a,126b, C. Da Via84, W. Dabrowski38a, A. Daﬁnca120, T. Dai89, O. Dale14,F. Dallaire95, C. Dallapiccola86,M. Dam36, J.R. Dandoy31, N.P. Dang48,A.C. Daniells18, M. Danninger168,M. Dano Hoffmann136,V. Dao48, G. Darbo50a,S. Darmora8,J. Dassoulas3,

A. Dattagupta61,W. Davey21, C. David169, T. Davidek129,E. Davies120,l,M. Davies153,P. Davison78, Y. Davygora58a,E. Dawe88,I. Dawson139, R.K. Daya-Ishmukhametova86, K. De8,R. de Asmundis104a, A. De Benedetti113, S. De Castro20a,20b,S. De Cecco80, N. De Groot106,P. de Jong107,H. De la Torre82, F. De Lorenzi64, D. De Pedis132a, A. De Salvo132a,U. De Sanctis149,A. De Santo149,

J.B. De Vivie De Regie117,W.J. Dearnaley72, R. Debbe25, C. Debenedetti137,D.V. Dedovich65, I. Deigaard107,J. Del Peso82,T. Del Prete124a,124b, D. Delgove117, F. Deliot136,C.M. Delitzsch49, M. Deliyergiyev75, A. Dell’Acqua30,L. Dell’Asta22,M. Dell’Orso124a,124b, M. Della Pietra104a,j, D. della Volpe49,M. Delmastro5, P.A. Delsart55,C. Deluca107,D.A. DeMarco158,S. Demers176, M. Demichev65,A. Demilly80, S.P. Denisov130,D. Derendarz39, J.E. Derkaoui135d,F. Derue80, P. Dervan74,K. Desch21, C. Deterre42, P.O. Deviveiros30,A. Dewhurst131, S. Dhaliwal23,

A. Di Ciaccio133a,133b,L. Di Ciaccio5, A. Di Domenico132a,132b, C. Di Donato104a,104b,A. Di Girolamo30, B. Di Girolamo30, A. Di Mattia152, B. Di Micco134a,134b, R. Di Nardo47,A. Di Simone48,R. Di Sipio158, D. Di Valentino29,C. Diaconu85,M. Diamond158, F.A. Dias46,M.A. Diaz32a, E.B. Diehl89,J. Dietrich16, S. Diglio85, A. Dimitrievska13, J. Dingfelder21,P. Dita26b,S. Dita26b,F. Dittus30,F. Djama85,

T. Djobava51b, J.I. Djuvsland58a,M.A.B. do Vale24c,D. Dobos30, M. Dobre26b,C. Doglioni81,

T. Dohmae155,J. Dolejsi129,Z. Dolezal129, B.A. Dolgoshein98,∗,M. Donadelli24d,S. Donati124a,124b, P. Dondero121a,121b,J. Donini34, J. Dopke131,A. Doria104a,M.T. Dova71, A.T. Doyle53,E. Drechsler54, M. Dris10, E. Dubreuil34,E. Duchovni172,G. Duckeck100,O.A. Ducu26b,85, D. Duda107,A. Dudarev30, L. Duﬂot117, L. Duguid77,M. Dührssen30, M. Dunford58a, H. Duran Yildiz4a,M. Düren52,

A. Durglishvili51b,D. Duschinger44,B. Dutta42,M. Dyndal38a,C. Eckardt42,K.M. Ecker101,R.C. Edgar89, W. Edson2,N.C. Edwards46,W. Ehrenfeld21, T. Eifert30, G. Eigen14,K. Einsweiler15,T. Ekelof166,

M. El Kacimi135c, M. Ellert166,S. Elles5, F. Ellinghaus175,A.A. Elliot169, N. Ellis30,J. Elmsheuser100, M. Elsing30, D. Emeliyanov131, Y. Enari155, O.C. Endner83,M. Endo118, J. Erdmann43, A. Ereditato17, G. Ernis175,J. Ernst2, M. Ernst25,S. Errede165, E. Ertel83, M. Escalier117,H. Esch43, C. Escobar125, B. Esposito47, A.I. Etienvre136,E. Etzion153, H. Evans61,A. Ezhilov123, L. Fabbri20a,20b,G. Facini31, R.M. Fakhrutdinov130, S. Falciano132a,R.J. Falla78, J. Faltova129, Y. Fang33a,M. Fanti91a,91b, A. Farbin8, A. Farilla134a,T. Farooque12,S. Farrell15,S.M. Farrington170, P. Farthouat30,F. Fassi135e, P. Fassnacht30,