Measurement of the production cross-section of a single top quark in association with a Z boson in proton–proton collisions at 13 TeV with the ATLAS detector

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Article

Reference

Measurement of the production cross-section of a single top quark in association with a Z boson in proton–proton collisions at 13 TeV with

the ATLAS detector

ATLAS Collaboration AKILLI, Ece (Collab.), et al .

Abstract

The production of a top quark in association with a Z boson is investigated. The proton–proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of s=13TeV are used, corresponding to an integrated luminosity of 36.1fb−1 . Events containing three identified leptons (electrons and/or muons) and two jets, one of which is identified as a b -quark jet are selected. The major backgrounds are diboson, tt¯ and Z+jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is 4.2 σ in the data and the expected significance is 5.4 σ . The measured cross-section for tZq production is 600±170(stat.)±140(syst.)fb .

ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Measurement of the production cross-section of a single top quark in association with a Z boson in proton–proton collisions at 13 TeV with the ATLAS detector. Physics Letters. B , 2018

DOI : 10.1016/j.physletb.2018.03.023

Available at:

http://archive-ouverte.unige.ch/unige:103327

Disclaimer: layout of this document may differ from the published version.

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurement of the production cross-section of a single top quark in association with a Z boson in proton–proton collisions at 13 TeV with the ATLAS detector

.TheATLASCollaboration

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

Articlehistory:

Received10October2017

Receivedinrevisedform27February2018 Accepted10March2018

Availableonline14March2018 Editor:M.Doser

TheproductionofatopquarkinassociationwithaZbosonisinvestigated.Theproton–protoncollision data collectedby theATLAS experiment atthe LHCin2015 and 2016atacentre-of-mass energyof

√_s

=¹³^{TeV are}ûsed,correspondingtoan integratedluminosityof36.1fb⁻¹.Eventscontainingthree identifiedleptons(electronsand/ormuons)andtwojets,oneofwhichisidentifiedasab-quarkjetare selected.The majorbackgroundsarediboson,tt¯and Z+jets production.Aneuralnetwork isused to improvethebackgroundrejectionandextract thesignal.Theresultingsignificanceis4.2σ inthedata andtheexpectedsignificanceis5.4σ.Themeasuredcross-sectionfort Zqproductionis600±¹⁷⁰^(stat.)± 140(syst.) fb.

©²⁰¹⁸^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

At hadron colliders, the top quark is typically produced in t¯t pairsthrough thestrong interaction orasa single topor antitop quarkthroughtheelectroweakinteraction.Thetopquarkwasﬁrst observedviat¯tproductionattheTevatron[1,2].Thiswasfollowed bythe observationofsingle top-quarkproduction[3–5] inthet- ands-channels, also atthe Tevatron.The associated t W production was ﬁrst observed in 8 TeV proton–proton collisions at the LargeHadronCollider(LHC)[6,7].Thesesingle-top-quarkchannels allowa directdetermination ofthe dominantt W bvertexandof the magnitude of the CKM matrix element |^Vtb| ^{[8] using} ^their measuredcross-sections.

With increasing energy and integrated luminosity, the ability to study rare Standard Model (SM) phenomena becomes possible.Inthe caseofsingle top-quarkproduction, examplesinclude pp→^{t Zq} ^{[9] and} ^pp→^{t H} ^[10]. ^The ^pp→^{t Zq} ^process ^involves W W Z andt Z couplings andhas not beenobserved so far [11].

Fig.1showstypical lowest-order Feynman diagramsforthepro- cess. This channel probes two SM couplings in a single process, whereas the similar ﬁnal state t¯t Z only probes thet Z coupling.

Thet¯t Z processhasbeenmeasuredbytheATLAS[12,13] andCMS [14] collaborations.In addition, theproduction of pp→^{t Zq} îs â SMbackgroundtothet H finalstate[10].

ThisLetter presentsevidenceoftheproductionofasingle top quarkinassociationwitha Z bosoninthet-channelprocess pp→

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

t Zq,wheretheZ bosondecaysintoelectronsormuonsandtheW bosonfromthetopquarkdecaysleptonically.

2. ATLASdetector

TheATLAS experiment[15] attheLHC isa multi-purposepar- ticledetectorwitha forward–backwardsymmetriccylindricalge- ometry anda near4π coverage in solidangle.¹ It consistsofan innerdetector(ID)surroundedbyathinsuperconductingsolenoid providing a 2 T axial magnetic ﬁeld, electromagnetic andhadron calorimeters, and a muon spectrometer. The inner detector cov- ersthepseudorapidityrange |η_|<2.5. Itconsistsofsiliconpixel, siliconmicro-stripandtransitionradiationtrackingdetectors.The innermost pixellayer, the insertable B-layer, was added between Run 1 and Run 2 of the LHC, at a radius of 33 mm around a new, thinner, beam pipe [16]. Lead/liquid-argon (LAr) sampling calorimeters provide electromagnetic (EM) energy measurements withhighgranularity. Ahadron (steel/scintillator-tile)calorimeter covers the central pseudorapidity range (|η_|<1.7). The end-cap and forward regions are instrumented with LAr calorimeters for both theEMandhadronicenergy measurementsup to |η|=⁴.9.

1 ATLASuses aright-handedcoordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdeﬁnedinterms ofthepolarangleθasη= −ln tan(θ/2).Distancesintheη–φplanearemeasured inunitsofR≡

(η)²+(φ)². https://doi.org/10.1016/j.physletb.2018.03.023

0370-2693/©²⁰¹⁸^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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Fig. 1.ExampleFeynmandiagramsofthelowest-orderamplitudesforthet Zqprocess.Inthefour-ﬂavourscheme,theb-quarkoriginatesfromgluonsplitting.Thelargest contributingamplitudetothecross-sectionwherethe ZbosoniscoupledtotheW bosonisshownin(a)while(b)showsoneofthefourdiagramswithradiationoffa fermion.

The muon spectrometer surrounds the calorimetersand isbased on three large air-core toroidal superconducting magnets with eight coilseach. The ﬁeld integral of the toroidsranges between 2.0and6.0 T m acrossmostofthedetector. Themuon spectrometer includes a system of precision tracking chambers and fast detectorsfortriggering.Atwo-leveltriggersystemisusedtoselect events.Theﬁrst-leveltriggerisimplementedinhardwareanduses asubsetofthedetectorinformationtoreducetheacceptedrateto atmost100 kHz.Thisisfollowedbyasoftware-basedtriggerlevel thatreducestheacceptedeventrateto1 kHzonaverage.

3. Dataandsimulationsamples

The pp collision data sample used in this measurement was collectedwiththeATLASdetectorattheLHCduringthe2015and 2016data-takingperiods,correspondingtointegratedluminosities of3.3 fb⁻¹and32.8 fb⁻¹,respectively,foratotalof36.1 fb⁻¹,after requiringthatthedetectorisfullyoperational.Eventsare consid- erediftheywere acceptedby atleastoneofthe single-muonor single-electrontriggers[17,18].Theelectrontriggersselectaclus- ter in the calorimeter matched to a track. Electrons must then satisfyidentiﬁcationcriteriabasedonamultivariatetechniqueus- ing a likelihood discriminant. In 2015, electrons had to satisfy a

‘medium’identiﬁcationrequirementandhavea transverseenergy of E_T>24 GeV.In2016, electrons hadto satisfya ‘tight’identiﬁ- cationtogether withan isolation criterion andhave ET>26 GeV.

Toavoideﬃciencyloss duetoisolation athigh ET,an additional triggerwas used,selecting ‘medium’electrons with E_T>60 GeV.

Muons are triggered on by matching tracks reconstructed inthe muonspectrometerandintheinnerdetector.In2015,muonshad tosatisfya‘loose’isolationrequirementandhaveatransversemo- mentumofpT>20 GeV.In2016,theisolationcriteriaweretight- ened andthe threshold increased to p_T=^{26 GeV.} ^In^both ^years, anothermuontriggerwithoutanyisolationrequirementwasused, selectingmuonswithp_T>50 GeV.

Inorder toevaluate the effectsof thedetectorresolution and acceptanceonsignalandbackgroundandtoestimatetheSMback- ground, a full Geant4-based detector simulation was used [19, 20]. Event generators were used to estimate the expected signal and background contributions and their uncertainties. The top- quark mass in the eventgenerators described below was set to 172.5 GeV.Multiple inelastic pp collisions(referred toaspile-up) are simulated with Pythia 8.186 [21], and are overlaid on each Monte Carlo (MC) event. Weights are assigned to the simulated eventssuchthatthedistributionofthenumberofpile-upinterac- tionsin thesimulationmatchesthe correspondingdistributionin thedata. Allsimulation samplesare processedthrough thesame reconstructionalgorithmsasthedata.

MonteCarlot Zqsignal sampleswere generatedatleadingor- der(LO)inQCDusingMG5_aMC@NLO 2.2.1[22] inthefour-ﬂavour

scheme, treating the b-quark asmassive, withthe CTEQ6L1 [23]

LO parton distribution functions (PDFs). The Z boson was simulated to be on-shell and off-shell Z/γ^∗ contributions andtheir interference are not taken intoaccount. Following the discussion in Ref. [24], the renormalisation andfactorisation scales (μr and

μ_f) used in MG5_aMC@NLO are set to μr=μ_f₌4

m²_b+^p²_T_,_b^, wheretheb-quarkistheexternaloneproducedfromgluonsplit- ting inthe event. This choiceis motivatedby the total scale de- pendence being dominated by this external b-quark, shown in Fig. 1.The partonshower andthehadronisation ofsignal events were simulatedwith Pythia 6 [25] using thePerugia2012 set of tuned parameters [26]. The t Zq total cross-section, calculated at next-to-leading order(NLO) using MG5_aMC@NLO 2.3.3 withthe NNPDF3.0_nlo_as_0118 [27] PDF,is800 fb, withanuncertaintyof +6.1

−7.4%.Theuncertaintyiscomputedbyvaryingtherenormalisation andfactorisationscalesbyafactoroftwoandbyafactorof0.5.

Acomparisonoftheeventkinematicsbeforepartonshowering between theLO MG5_aMC@NLO 2.2.1 sample anda sample gen- eratedusingNLOMG5_aMC@NLO 2.3.3showedagreementwithin 10%,justifyingtheuseofaLOsampleforthedetectorsimulation.

Monte Carlo simulated events are used to estimate the SM background that can produce three leptons andat leasttwo jets in the final state. In t¯t production,if both W bosons decay into leptons (referred to as‘prompt’)and eithera b- or c-hadron decays into a lepton (referred to as ‘non-prompt’) that is isolated, thefinalstatecanmimicthet Zqfinalstate.Thenominaltt¯simu- latedsamplewasgeneratedatNLOwiththePowheg-Box[28–30]

eventgenerator usingtheCT10 PDFs[31].The cut-off parameter, h_damp, for the ﬁrst emission of gluons was set to the top-quark mass.The eventswerethen processedusingPythia6 toperform thefragmentationandhadronisation,andtogeneratetheunderly- ingevent.

Eventsfromtheassociatedproductionofatt¯pairandaboson (W/Z/H) provideadditional modesforthe productionofleptons inthefinalstate.Fort¯t+^W ^the^MC^simulatedêvents^were^gener- atedusingMG5_aMC@NLO 2.2.2[22],whilethett¯+^H ând^tt¯+^Z MC simulatedeventswere generatedusingMG5_aMC@NLO 2.2.3.

The generated events were then processed with Pythia 8 [21]

to perform thefragmentationandhadronisation, andto generate theunderlyingevent,usingtheNNPDF2.3LOPDF setandtheA14 tune [32].

Processes that include the production of W W, W Z and Z Z events were simulatedusingSherpa2.1.1 atLOwith upto three additional partons and the CT10 PDF set. In the trilepton topol- ogy,thedibosonbackgroundconsistsmainlyof W Z events,while the contribution to the background from W W ﬁnal states, corresponding to the case where a jet is misidentiﬁed asa lepton, is negligible. The Z Z background gives a small contribution of 9% ofall dibosonevents. The gluon-induced dibosonproduction,

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which amounts to about10% of the quark-induced dibosonproduction,isthereforenegligibleinthet Zqsignalregion,andisnot includedinthedibosonsamples.Inordertoestimatethesystem- aticuncertainty,additionaldibosonsampleswere simulatedusing thePowheg-Boxgenerator incombinationwithPythia8andthe CTEQ6L1PDFsets.

Of the aforementioned single-top-quark production channels, onlythet W channel contributestothe trilepton ﬁnalstate. This sample was produced usingthe NLO Powheg-Box eventgenera- torwiththe CT10PDF set.Theevents were thenprocessed with Pythia 6 to perform the fragmentation and hadronisation, and producethe underlyingevent. Asample oft W Z events was produced using the MG5_aMC@NLO 2.2.3 generator and showered withPythia8,usingtheNNPDF3.0_NLOPDFsetandtheA14tune.

4. Objectreconstruction

The reconstruction of the basic physics objects used in this analysisisdescribed inthe following.Theprimary vertexis cho- senasthe proton–protonvertex candidatewith thehighestsum ofthe squared transverse momenta of all associated tracks with pT>400 MeV.

Electroncandidatesare reconstructed fromenergy depositsin theelectromagnetic calorimeterthat matcha reconstructed track [33–36]. Theclustersare requiredtobewithin |η_|<2.47 exclud- ingthetransitionregionbetweenthebarrelandend-capcalorime- tersat 1.37<|η|<1.52. Electroncandidates must also satisfy a transverseenergyrequirementof ET>15 GeV.Alikelihood-based discriminant is constructed from a set of variables that enhance the electron selection, while rejecting photon conversions and hadronsmisidentifiedaselectrons[34].An|η_|- and pT-dependent selection on the likelihood discriminant is applied, such that it has an 80% efficiency when used to identify electrons from the Z-bosondecay.Thisworkingpointcorrespondstoanapproximate rejectionfactoragainstjetsof700ata pTof40 GeV.Electronsare further required to be isolated using criteria based on ID tracks andtopological clustersin the calorimeter, withan isolation efficiency of 90(99)% for pT=²⁵(60)GeV. Correction factors are appliedtosimulatedelectrons totake intoaccountthesmalldif- ferencesinreconstruction, identificationand isolation efficiencies betweendataandMCsimulation.

Muoncandidatesarerequiredtohave|η|<².5 andp_T>15 GeV, andarereconstructedbycombiningareconstructedtrackfromthe innerdetectorwithonefromthemuonspectrometer [37].Tore- jectmisidentifiedmuoncandidates,primarily frompionandkaon decays, several quality requirements are imposed on the muon candidate.An isolationrequirementbasedon IDtracksandtopo- logical clusters in the calorimeter is imposed, and results in an isolationefficiencyof90(99)%for p_T=²⁵(60)GeV.Theoverallef- ficiencyobtained for muons from W-boson decays in simulated pp→^tt¯ events is 96% and the rejection factor for non-prompt muonswith p_T>20 GeVis approximately 600. As for electrons, correction factors are applied to muonsto account forthe small differencesbetweendataandsimulation.

Jetsarereconstructedfromtopologicalclustersusingtheanti-k_t algorithm[38,39] with theradius parametersetto R=⁰.4. They arereconstructedforpT>30 GeVintheregionwith|η_|<4.5.To account forinhomogeneities andthe non-compensatingresponse ofthecalorimeter,thereconstructedjetenergiesarecorrectedus- ingpT- and η-dependentfactorsthatarederivedinMCsimulation andvalidatedindata.Anyremainingdifferencesinthejetenergy scaleare correctedusinginsitu techniques,where awell-deﬁned referenceobject is momentum-balanced with a jet [40]. Tosup- press pile-up,a discriminant calledthe jet-vertex-tagger (JVT) is constructedusing a two-dimensional likelihood method[41]. For

jetswithpT<60 GeVand|η|<2.4 aJVTrequirementcorrespond- ingtoa92%eﬃciency,whilerejecting98%ofjetsfrompile-upand noise,isimposed.

Toidentifyjetscontaininga b-hadron(b-tagging),amultivari- ate algorithm is employed [42]. This algorithm uses the impact parameterandreconstructed secondaryvertexinformationof the tracks containedinthejet asinput fora neuralnetwork. Dueto itsuseoftheinner detectors,thereconstruction ofb-jetsisdone intheregionwith|η|<2.5.Jetsinitiatedbyb-quarksareselected by settingthealgorithm’s outputthreshold suchthat a 77%b-jet selection efficiency is achieved in simulated tt¯ events.With this setting,themisidentificationrateforjetsinitiated bylight-flavour quarksorgluonsis1%,whileitis17%forjetsinitiatedbyc-quarks [43]. Correctionfactorsarederived andappliedto correctforthe smalldifferencesinb-quark selectionefficiencybetweendataand MCsimulation [42].

The missing transverse momentum, with magnitude E^miss_T , is calculatedasthenegativeofthevectorsumofthetransversemo- mentaofallreconstructedobjects, p^miss_T .Inadditiontotheidenti- ﬁedjets,electronsandmuons,atrack-based‘soft’termisincluded inthe p^miss_T calculation,by consideringtracksassociatedwiththe primaryvertexintheeventbutnotwithanidentiﬁedjet,electron, ormuon[44,45].

Toavoidcaseswherethedetectorresponsetoasinglephysical objectisreconstructed astwo separate ﬁnal-stateobjects,several stepsarefollowedtoremovesuchoverlaps,followingRef. [46].

5. Signal,controlandvalidationregions

Thereconstructedt Zq finalstateconsistsofthreechargedlep- tons(electronand/or muon), ab-taggedjet,an additionaljet and E^miss_T .Reconstructing the Z bosonandthetopquark isimportant inordertoidentifyspecificfeaturesthathelptoseparatethesignal fromthebackground.Forexample,theZ-bosonmassdistributions cancontributetothereductionoftop-quarkbackgrounds,asthese do notinclude a Z boson in thefinal state, while theuntagged- jetpseudorapiditydistributiondiffersinshapebetweent Zqsignal eventsanddibosonandt¯t Z events,whichconstitutesome ofthe largestbackgrounds.

Thesignal region(SR)definitionreflectsthet Zq final stateby selectingonlyeventsthathaveexactlythreechargedleptons,one b-tagged jetandoneadditionaljet,referredtoastheuntaggedjet asnob-taggingrequirementisapplied.Inordertobetterseparate the t Zq signal frombackground,additional requirementsare im- posedonthepropertiesoftheselectedobjects.Thethreeleptons aresortedbytheirpT,irrespectiveofflavour,andrequiredtohave transverse momenta of at least 28, 25 and 15 GeV, respectively.

BothjetsarerequiredtohavepT>30 GeV.

Anopposite-sign,same-ﬂavour(OSSF)leptonpairisrequiredin orderto reconstructthe Z boson.Inthe μee andeμμ channels, thepairisuniquelyidentiﬁed.Fortheeeeand μμμ events,both possible combinations are considered and the pair that has the invariantmassclosesttothe Z-bosonmassischosen.The W boson is reconstructed fromthe remaining lepton and the missing transverse momentum,using asconstraintthe W-boson mass to evaluate the z componentof the neutrino momentum.² The top quark is reconstructed from the reconstructed W boson and the b-taggedjet.

Tosuppressbackgroundsourcesthatdonotcontaina Z boson, the invariant mass of the leptons is required to be between 81 and 101 GeV. Because a W boson is expected in the ﬁnal state,

2 Incaseofanimaginarysolution,thep^miss_T valueisvarieduntilonerealsolution isfound.