Fiducial and differential cross sections of Higgs boson production measured in the four-lepton decay channel in $\mathit{pp}$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Fiducial and differential cross sections of Higgs boson production measured in the four-lepton decay channel 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:

Received14August2014

Receivedinrevisedform10September 2014

Accepted23September2014 Availableonline28September2014 Editor:W.-D.Schlatter

MeasurementsoffiducialanddifferentialcrosssectionsofHiggsbosonproductionintheH→^{Z Z∗}→⁴ decay channel are presented. The cross sections are determined within a fiducial phase space and corrected for detectionefficiency and resolution effects. Theyare basedon20.3 fb⁻¹ of pp collision data, producedat√

s=8 TeV centre-of-massenergy atthe LHCand recordedbythe ATLASdetector.

Thedifferentialmeasurementsareperformedinbinsoftransversemomentumandrapidityofthefour- leptonsystem,theinvariantmassofthesubleadingleptonpairandthedecayangleoftheleadinglepton pair withrespect to thebeam lineinthe four-lepton restframe,as well as the number ofjetsand the transverse momentumof the leading jet. The measured cross sections are compared to selected theoretical calculationsof the Standard Model expectations. No significantdeviationfrom anyof the testedpredictionsisfound.

PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

1. Introduction

In2012theATLASandCMSCollaborations announcedthedis- covery of a new particle [1,2] in the search for the Standard Model(SM)Higgsboson[3–8]attheCERNLargeHadronCollider (LHC) [9]. Since thisdiscovery,the particle’s massm_H was mea- sured by the ATLAS and CMS Collaborations [10–12]. The result oftheATLAS measurementbasedon 25fb⁻¹ ofdatacollected at centre-of-massenergiesof7TeVand8TeVis125.36±⁰.41 GeV.

Testsofthecouplingsandspin/CPquantumnumbershavebeenre- portedbybothcollaborations[11,13,14]andshowagreementwith thepredictedscalarnatureoftheSMHiggsboson.

In this Letter, measurements of fiducial and differential pro- duction crosssections forthe H→^{Z Z∗}→⁴ decaychannel are reported and compared to selected theoretical calculations. The event selection and the background determination are the same asinRef.[15],whereadetaileddescriptionisgiven.Forthismea- surement,anintegratedluminosityof20.3 fb⁻¹ ofpp collisionsis analyzed.Thedata werecollected attheLHC atacentre-of-mass energyof√

s=^{8 TeV andrecordedwiththeATLASdetector[16].}

The ATLAS detector covers thepseudorapidity range |η_|<4.9 andthefullazimuthalangleφ.¹Itconsistsofaninnertrackingde-

 E-mailaddress:atlas.publications@cern.ch.

1 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpoint(IP)atthecentreofthedetectorandthez-axisalongthebeam

tectorcoveringthepseudorapidityrange|η_|<2.5 surroundedbya superconductingsolenoid, electromagneticandhadroniccalorime- ters,andanexternalmuonspectrometerwithlargesuperconduct- ingtoroidalmagnets.

Fiducial cross sectionsare quoted to minimize the model de- pendenceoftheacceptancecorrectionsrelatedtotheextrapolation tophase-spaceregionsnotcoveredbythedetector.Themeasured fiducial crosssections are correctedfor detectoreffects tobe di- rectlycomparedtotheoreticalcalculations.

The differential measurements are performed in several ob- servables relatedto theHiggsbosonproductionanddecay.These include the transverse momentum pT,H and rapidity |^yH| ^{of the} Higgsboson,theinvariantmassofthesubleadingleptonpairm34 (the leadingandsubleadinglepton pairsaredefinedinSection 3) andthe magnitudeof thecosine ofthe decayangle ofthe lead- ing lepton pair in thefour-lepton rest frame withrespect to the beam axis |^cosθ^∗|^{. The number of jets n}jets and the transverse momentum of the leading jet pT,jet are also included. The dis- tribution of the pT,H observable is sensitive to the Higgs boson productionmechanismsaswellasspin/CPquantumnumbers,and canbeusedtotestperturbativeQCDpredictions.Thisdistribution

pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis points upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedin termsofthepolarangleθasη= −ln[tan(θ/2)].

http://dx.doi.org/10.1016/j.physletb.2014.09.054

0370-2693/PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

hasbeenstudiedextensivelyandprecisepredictionsexist(seee.g.

Refs.[17–21]),includingtheeffectoffinitequarkmasses.Thedis- tributionofthe |^yH|^{observable can beusedto probetheparton} distributionfunctions(PDFs)oftheproton.Thedistributionsofthe decayvariables m34 and |^cosθ^∗| ^{are sensitive to the Lagrangian} structureofHiggsboson interactions, e.g.spin/CP quantum num- bers and higher-dimensional operators. The jet multiplicity and transversemomentumdistributionsaresensitivetoQCDradiation effectsandtotherelativeratesofHiggsbosonproductionmodes.

The distribution of the transverse momentum of the leading jet probesquarkandgluonradiation.

2. Theoreticalpredictionsandsimulatedsamples

TheHiggsbosonproductioncrosssectionsanddecaybranching fractionsaswellastheiruncertaintiesaretakenfromRefs.[21,22].

The cross sections for the gluon-fusion (ggF) process have been calculated to next-to-leading order (NLO) [23–25], and next-to- next-to-leadingorder(NNLO)[26–28]inQCDwithadditionalnext- to-next-to-leadinglogarithm(NNLL) soft-gluonresummation[29].

ThecrosssectionvalueshavebeenmodifiedtoincludeNLOelec- troweak (EW) radiative corrections, assuming factorization be- tween QCD and EW effects [30–34]. The cross sections for the vector-bosonfusion (VBF) processes are calculated withfull NLO QCDandEWcorrections[35–37],andapproximateNNLOQCDcor- rections are included [38]. The cross sections for the associated W H/Z H production processes (V H ) are calculated at NLO [39]

andatNNLO [40]inQCD, andNLO EWradiative corrections[41]

areapplied.ThecrosssectionsforassociatedHiggsbosonproduc- tionwithatt pair¯ (tt H )¯ arecalculatedatNLOinQCD[42–45].

TheHiggs boson branching fractionsfor decaysto four-lepton finalstatesareprovidedby Prophecy4f[46,47],whichimplements thecompleteNLOQCD+ ^EWcorrectionsandinterferenceeffects betweenidenticalfinal-statefermions.

The H→^{Z Z∗} →⁴ signal is modelled using the Powheg MonteCarlo(MC) eventgenerator[48–52], whichcalculates sep- aratelythe ggFandVBFproductionmechanismswithmatrixele- ments up to NLO. The description of the Higgsboson transverse momentumspectrumintheggFprocessisadjustedtofollowthe calculationinRefs. [19,20],whichincludesQCD correctionsupto NLOandQCDsoft-gluonresummationsup toNNLL,aswellasfi- nite quark masses[53]. Powheg is interfacedto Pythia8 [54] for showeringandhadronization,which inturnisinterfacedto Pho- tos[55,56] to modelphoton radiation inthe final state. Pythia8 is used to simulate V H and t¯t H production. The response of the ATLAS detector is modelled in a simulation [57] based on GEANT4[58].

Themeasuredfiducialcross-sectiondistributionsarecompared tothreeggF theoreticalcalculations: Powheg withouttheadjust- mentstothepT,H spectrumdescribedabove, Powheg interfacedto Minlo(Multi-scaleimprovedNLO)[59] and HRes2(v.2.2)[19,20].

Powhegwith Minlo providespredictions forjet-relatedvariables at NLO for Higgs boson production in association with one jet.

The HRes2 program computes fixed-order cross sections for ggF SM Higgs boson production up to NNLO. All-order resummation ofsoft-gluon effects atsmall transverse momenta is consistently includedup to NNLL, usingdynamic factorization andresumma- tionscales.Theprogramimplementstop- andbottom-quarkmass dependenceuptoNLL+^NLO.AtNNLL+^{NNLOlevelonlythetop-} quarkcontributionisconsidered. HRes2doesnotperformshower- ingandQEDfinal-stateradiationeffectsarenotincluded.

Thecontributionsfromtheother productionmodesareadded totheggFpredictions.Atacentre-of-massenergyof8TeVandfor a Higgsboson mass of 125.4 GeV, their relative contributions to

thetotalcrosssectionare87.3%(ggF),7.1%(VBF),3.1%(W H ),1.9%

( Z H )and0.6%(t¯t H ),respectively.

Alltheoretical predictionsarecomputedforaSM Higgsboson with mass 125.4 GeV. They are normalized to the most precise SMinclusivecross-sectionpredictionscurrentlyavailable[60],cor- rectedforthefiducialacceptancederivedfromthesimulation.

The Z Z , W Z , t¯t and Z +jets background events are mod- elledusingthesimulatedsamplesandcrosssectionsdescribed in Ref.[15].

3. Eventselection

The detector level physics object definitions of muons, elec- trons,andjets,andtheeventselectionappliedinthisanalysisare the same as inRef. [15], with the exception of the jet selection andtheadditionalrequirementonthe four-leptoninvariant mass describedbelow.Abriefoverviewisgiveninthissection.

Eventswithatleastfourleptonsareselectedwithsingle-lepton and dilepton triggers. The transverse momentum and transverse energythresholdsforthesingle-muonandsingle-electrontriggers are 24 GeV. Twodimuon triggers are used, one with symmetric thresholdsat13 GeVandtheotherwithasymmetricthresholdsat 18 GeVand8 GeV.Forthedielectrontriggerthesymmetricthresh- olds are 12 GeV. Furthermore there is an electron–muon trigger withthresholdsat12 GeV(electron)and8 GeV(muon).

Higgs boson candidates are formed by selecting two same- flavouropposite-sign(SFOS)leptonpairs(aleptonquadruplet).The leptons must satisfy identification, impact parameter, and track- based and calorimeter-based isolation criteria. Each muon (elec- tron) mustsatisfy transversemomentum pT>6 GeV (transverse energy ET>7 GeV)andbe inthepseudorapidity range|η_|<2.7 (2.47).The highest-pT lepton inthequadrupletmustsatisfy pT>

20 GeV, and the second (third) lepton in p_T order must satisfy pT>15 (10) GeV.The leptons arerequired tobe separatedfrom eachotherbyR≡

(η)²+ (φ)²>0.1 (0.2)whenhavingthe same(different)leptonflavours.

Multiplequadrupletswithinasingleeventarepossible:forfour muons orfour electrons there are two waysto pairthe masses, andforfiveormoreleptonstherearemultiplecombinations.The quadrupletselectionisdoneseparatelyineachchannel:4μ,2e2μ, 2μ2e,4e, keepingonlya singlequadrupletper channel.Here the first flavour index refers to the leading lepton pair, which is the pairwiththeinvariantmassm12closesttothe Z bosonmass[61].

The invariant mass m12 is required to be between 50 GeV and 106 GeV. The subleading pair of each channel is chosen as the remaining pair with mass m₃₄ closest to the Z boson mass and satisfying the requirement 12<m34<115 GeV. Finally, if more thanonechannelhasaquadrupletpassingtheselection,thechan- nelwiththehighestexpectedsignalrateiskept,intheorder:4μ, 2e2μ,2μ2e,4e.A J/ψ vetoisapplied:m(i,j)>5 GeV forSFOS leptonpairs.Onlyeventswithafour-lepton invariantmassinthe range118–129 GeVare kept.This requirementdefinesthesignal masswindowandwaschosenby minimizingtheexpecteduncer- tainty onthetotalsignal yielddetermination,takingintoaccount theexperimentaluncertaintyontheHiggsbosonmass.

Jets are reconstructed fromtopological clustersof calorimeter cellsusingtheanti-kt algorithm[62] withthedistanceparameter R=⁰.4. Inthis analysis, jets[63] are selected by requiring pT>

30 GeV,|^y|<4.4 and,inorderto avoiddoublecountingofelec- tronsthatarealsoreconstructedasjets,R(jet, electron)>0.2.

The events are divided into bins of the variables of interest, whichare computedwiththereconstructedfour-momenta ofthe selected lepton quadruplets or from the reconstructed jets: the transverse momentum p^reco_T,H andthe rapidity |^yrecoH | ^{of the four-} lepton system, the invariant mass of the subleading lepton pair

Table 1

Listofselectioncutswhichdefinethefiducialregionofthecrosssectionmeasure- ment.Thesame flavour oppositesignleptonpairsaredenotedasSFOS,theleading leptonpairmassasm12,andthesubleadingleptonpairmassasm34.

Lepton selection

Muons: pT>6 GeV,|η| <2.7 Electrons: pT>7 GeV,|η| <².47 Lepton pairing

Leading pair: SFOS lepton pair with smallest|^mZ−^m|

Subleading pair: Remaining SFOS lepton pair with smallest|mZ−m| Event selection

Lepton kinematics: pT>20,15,10 GeV

Mass requirements: 50<m12<106 GeV, 12<m34<115 GeV Lepton separation: R(i,j)>0.1(0.2)forsame-

(different-)flavourleptons

J/ψveto: m(i, j) >5 GeV for all SFOS lepton pairs Mass window: 118<m4<129 GeV

m^reco₃₄ ,themagnitudeofthecosineofthedecayangleofthelead- ing lepton pair inthe four-lepton restframe with respect tothe beamaxis|^cosθ^∗reco|^{,thenumberofjetsnreco}_jets^{,andthetransverse} momentumof theleading jet p^reco_T,jet. Inorderto distinguishthem fromthe unfolded variables usedin the crosssection bin defini- tion,theyarelabelledwith“reco”.

4. Definitionofthefiducialregion

Thefiducialselection,outlinedinTable 1,isdesignedtorepli- cateatsimulationlevel,beforeapplyingdetectoreffects,theanal- ysisselection asclosely aspossible inorder to minimize model- dependentacceptanceeffectsonthemeasuredcrosssections.

The fiducialselection is applied to electrons andmuons orig- inating from vector-boson decays before they emit photon radi- ation, referred to as Born-level leptons. An alternative approach would be to correct the lepton momenta by adding final-state radiationphotonswithinaconeofsizeR<0.1 aroundeachlep- ton(dressing).Forthisanalysistheacceptancedifferencebetween Bornanddressed-leptondefinitionsislessthan 0.5%.Particle-level jetsare reconstructedfrom allstable particlesexcept muonsand neutrinosusingtheanti-kt algorithmwiththedistanceparameter R=⁰.4.

Jets are selected by requiring pT >30 GeV, |^y|<4.4 and

R(jet, electron)>0.2. Muons (electrons) must satisfy pT >6 (7) GeVand|η|<2.7 (2.47).Eventsinwhichatleastoneofthe Z bosonsdecaysinto τ leptonsareremoved.Quadrupletsareformed fromtwopairsofSFOS leptons.Theleptons arepaired asinSec- tion3,includingthepossibility ofincorrectlypairing theleptons, whichhappensinabout5%oftheselectedeventsforaSMHiggs boson with mass 125.4 GeV. The leading pair is defined as the SFOSlepton pairwithinvariant massm12 closest tothe Z boson mass and the subleading pair is defined as the remaining SFOS leptonpairwithinvariantmassm34closesttothe Z bosonmass.

Thethreehighest-pT leptonsinthequadrupletare requiredto have pT>20,15,10 GeV,respectively, andthelepton pairsmust have50<m₁₂<106 GeV and12<m₃₄<115 GeV.

The separation between the leptons is required to be

R(i,j)>0.1(0.2)forsame- (different-) flavourleptons.A J/ψ vetois applied: m(i,j)>5 GeV for all SFOS lepton pairs. Fur- thermore,the massof thefour-lepton systemm_4 must be close tomH,i.e.118<m4<129 GeV.

For a SM Higgs boson mass of 125.4 GeV, the acceptance of the fiducial selection (with respect to the full phase space of H→^{Z Z∗}→²2^, where ,^=^e, μ) is 45.7%. The number of eventspassingtheeventselectiondividedbythenumberofevents passingthefiducialselectionis55.3%;about1%oftheeventspass- ingtheeventselectiondonotpassthefiducialselection.

5. Backgroundestimate

The background estimatesused in this analysis are described in detail in Ref. [15]. The irreducible Z Z and the reducible W Z background contributions are estimatedusing simulated samples normalized to NLO predictions. For the jet-related variables, the simulation predictions are compared to data for m4>190 GeV wherethe Z Z backgroundprocessisdominant;shapedifferences betweenthedistributionsindataandsimulationareusedtoesti- matesystematicuncertainties.

Thereducible Z+^{jets andt}t background¯ contributionsarees- timated with data-driven methods. Their normalizations are ob- tained from data control regions and extrapolated to the signal regionusingtransferfactors.The+μμfinalstateisdominated by Z +heavy-flavourjetsandthe+^{ee finalstateby Z} +^light- flavour jets.The misidentificationoflight-flavourjetsaselectrons isdifficult tomodelinthesimulation.Thereforethedistributions for+^{ee aretakenfromdatacontrolregionsand}extrapolatedto thesignal region,whilethebackgrounddistributions for+μμ

aretakenfromsimulatedsamples.

Aftertheanalysisselectionabout9backgroundeventsareex- pected: 6.7events from irreducible Z Z and 2.2events from the reduciblebackground.

The observed distributions compared to the signal and back- ground expectations for the six reconstructed observables p^reco_T,H

|y^reco_H |,m^reco₃₄ ,|cosθ^∗reco|,n^reco_jets,andp^reco_T,jetareshowninFig. 1.The signal prediction includes VBF, Z H , W H , tt H ,¯ and the Powheg ggFcalculationforaHiggsbosonwithmH =125 GeV andisnor- malizedtothemostpreciseSMinclusivecross-sectioncalculation currentlyavailable[60].

6. Observeddifferentialyieldsandunfolding

The extractionof thesignal yield forthe measurement ofthe fiducialcrosssectionisperformedthroughafittothem4 distri- butionusingshapetemplatesforthesignalandbackgroundcontri- butions[15].Inthisfit,theHiggsbosonmassisfixedto125.4 GeV andtheparameterofinterestisthetotalnumberofsignalevents.

Theextractednumberofobservedsignal eventsinthemasswin- dowis23.7⁺₋⁵₅^._.⁹₃(stat.)±⁰.6(syst.).

In the differential cross-section measurements, given the low numberofsignaleventsexpectedineachmeasuredbini,thesig- nalyieldsn^sig_i aredeterminedbysubtractingtheexpectednumber of background events fromthe observed numberof events. This is done within the mass window for each bin of the observable ofinterest.Thetotalnumberofobservedeventsinthemasswin- dowis34andtheextractedsignalyieldis25.1⁺₋⁶₅^._.³₄(stat.)⁺₋⁰₀^._.⁶₄(syst.) events.

The difference betweenthenumberofsignal eventsextracted with the two methods is mainly due to fixing the Higgs boson massto125.4 GeVinthefitmethod.As reportedinRef.[10],the best fitmass inthe H→^{Z Z∗}→⁴ channel alone is124.5 GeV, causingsmallerweightsforsomeeventsinthefit.

After subtracting the background, the measured signal yields are corrected for detector efficiency and resolution effects. This unfoldingisperformedusingcorrectionfactorsderivedfromsim- ulatedSM signal samples.The correctionfactor inthei-th binis calculatedas

c_i= ^Nireco N^fid_i ,

where N^reco_i isthenumberofreconstructedeventsinthei-thbin of theobserved distribution and N^fid_i isthe numberof eventsin