Article
Reference
Measurements of four-lepton production in pp collisions at √s = 8 TeV with the ATLAS detector
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al.
Abstract
The four-lepton (4 ℓ , ℓ=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 at the LHC. The 4 ℓ events are produced in the decays of resonant Z and Higgs bosons and the non-resonant ZZ continuum originating from qq¯ , gg , and qg initial states. A total of 476 signal candidate events are observed with a background expectation of 26.2±3.6 events, enabling the measurement of the integrated cross section and the differential cross section as a function of the invariant mass and transverse momentum of the four-lepton system.
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Measurements of four-lepton production in pp collisions at √s = 8 TeV with the ATLAS detector. Physics Letters. B , 2016, vol. 753, p. 552-572
DOI : 10.1016/j.physletb.2015.12.048
Available at:
http://archive-ouverte.unige.ch/unige:79078
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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 ofdata in pp collisions at√
s=8 TeV collectedwith the ATLAS detector atthe LHC. The4events are producedinthe decaysofresonant Z and Higgsbosonsand thenon- resonant Z Z continuum originating 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).
©2015CERNforthebenefitoftheATLASCollaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
This paper presents measurements of the production of four isolatedcharged-leptonsinproton–protoncollisionsatacentre-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(p4T) 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 electroweakeffectsfordifferent4productionmechanismsasde- scribedbytheLOFeynmandiagramsshowninFig. 1.
The4 signaleventscomefromthedecaysofresonant Z and Higgsbosonsandthe non-resonantZZ continuumproduced from qq,¯ gg,andqginitialstates,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 eventpro- ductionatthe Zresonanceoccurspredominantlyviathes-channel diagram as shown in Fig. 1(a), andwas measured previously by
E-mailaddress:[email protected].
the ATLASandCMScollaborations[1,2].Inthe 4invariant mass regionabovethe Z resonancethe4eventproductionmainlypro- ceeds through the t-channel process as shown in Fig. 1(b). The crosssection significantlyincreaseswhenboth Z bosons arepro- 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 4productionviaggFareshowninFig. 1(c)andFig. 1(d),respec- tively.Thecrosssectionsasafunction ofm4 areshowninFig. 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),andtop- 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
0370-2693/©2015CERNforthebenefitoftheATLASCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
Fig. 1.TheLOFeynmandiagramsfortheqq- and¯ gg-initiatedproductionof4:(a)s-channel productionofqq¯→Z(∗)→+− withassociatedradiativedecaystoan additionalleptonpair;(b)t-channel productionofqq¯→Z(∗)Z(∗)→4;(c)Higgs-bosonproductionthroughgluonfusiongg→H(∗)→Z Z(∗)→4;(d)non-resonant4 productionthroughthequark-boxdiagramgg→Z(∗)Z(∗)→4.TheZ(∗)notationstandsforproductionofon- andoff-shellZbosons(ZandZ∗)andproductionofoff-shell photons(γ∗).
Fig. 2.Thedifferentialcrosssections,dσ/dm4versustheinvariantmassofthefour leptonsm4,calculatedbyMCFMfromtheqq¯andgginitialstatesat√
s=8 TeV forthe2e2μfinalstateintheexperimentalfiducialphasespace(seeTable 2for definition).The inclusivegg→4distributionisthe sumofthe gg→H→4 andthe gg→Z Z→4,andinterference terms.Thecalculationoftheqq¯→4 differentialproductioncrosssectionincludesperturbativeQCDcorrectionsatNLO, whilethe distributionsfromthe gginitialstatearecalculatedat 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 p4T. 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, pTZ1,2>2 GeV,andthepres- 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-fusioncom-
ponentwithrespecttoitsLOprediction.Thisisdonebyfittingthe observed m4 spectrum using the next-to-next-to-leading-order (NNLO) QCD theoretical prediction,corrected fornext-to-leading- order (NLO) electroweak effects, for the production originating fromtheqq¯ initialstate.
2. TheATLASdetector
The ATLAS detector[11] hasa cylindricalgeometry1 andcon- sistsofaninnertrackingdetector(ID)surroundedbya2 Tsuper- 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.Theelectromagneticandhadroniccalorime- 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¯→4productionusesthePOWHEG- BOXMonteCarlo(MC)program[12–14],whichincludesperturba- tiveQCDcorrectionsatNLO.Theproductionthroughtheqginitial 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 kinematicre- 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¯ andthegg processes.
1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.
Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedintermsof thepolarangleθasη= −ln tan(θ/2).
Thesignalmodellingoftheon-shellHiggs-bosonproductionvia theggF andVBF mechanismsuses POWHEG-BOXwhichprovides calculations atNLO QCD, with the CT10NLO PDFs and μR,μF= m4.TheHiggs-bosonproductionviatheVHandt¯tHmechanisms 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-shellHiggs- boson production, continuum Z Z production, and their interfer- ence. The LO MCFMgenerator [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 VBSproductionandtheir interference.TheNNLOQCD corrections areavailableforoff-shellHiggs-bosonproduction[23] andforthe interference between off-shell Higgs bosons and Z Z pairs from the gg initial state[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.
AllthesignalMCgeneratorsareinterfacedtoPYTHIA8forpar- tonshower simulation,exceptMADGRAPH,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 andt) processes,whicharealsosimulated.
Thereduciblebackgroundfrom Z+jets production,which in- cludes light- and heavy-flavour contributions, is modelled using bothSHERPA[26]andALPGEN[27].The Zγ process issimulated withSHERPA.Thett¯backgroundismodelledusingPOWHEG-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 withSHERPAandMADGRAPH, respectively.The irreducible back- ground fromVVV and t¯tZ is modelled with MADGRAPH. Finally, thedouble-Drell–YanZZproductionismodelledwithPYTHIA8.
Forbackground modellingthe POWHEG-BOXandMADGRAPH generatorsareinterfacedtoPYTHIA6forthepartonshower,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.Foreventswithmorethanonepri- mary vertex, the vertex with the largest
p2T 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 wherethereisin- 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,there- quirementisthatthescalarsumofthetransversemomenta,
pT, ofalltracksinsideaconeof R≡
( η)2+( φ)2=0.2 around thelepton,excludingtheleptonitself,belessthan15%ofthelep- ton pT. Forthe calorimeter,the
ET deposited inside acone of R=0.2 around the lepton, excluding thelepton itself andcor- rectedforcontributionsfrompile-upand,inthecaseofelectrons, shower leakage, isrequired to be lessthan 30% of the muon pT (15%forSAmuons)and20%oftheelectron ET.
Attheclosestapproachofatracktotheprimaryvertex,thera- 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 forelec- tronsaswellasmuons(no vertexrequirementsareappliedtoSA muons).
Selectionofleptonquadrupletsisdoneseparatelyineachofthe channels4μ,2e2μ,4e,keepingonlyasinglequadrupletperchan- nel.Candidate quadrupletsare formedby selectingtwoopposite- sign,same-flavourleptonpairs(+−).Thetwoleading-pTleptons of the quadruplet must have pT>20 and 15 GeV, respectively, whilethethirdleptonmusthave pT>10(8)GeV ifitisanelec- 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-boson massis 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
