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Physics Letters B
www.elsevier.com/locate/physletb
Measurement of the jet mass in high transverse momentum Z ( → bb ) γ production at √ s = 13 TeV using the ATLAS detector
.The ATLASCollaboration
a rt i c l e i n f o a b s t r a c t
Articlehistory:
Received17July2019
Receivedinrevisedform27November2020 Accepted30November2020
Availableonline3December2020 Editor: M.Doser
The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum Z→bb decays are measuredin Zγ events inproton–proton collisions at√
s=13 TeV.
The data analysed were collected between 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb−1. Photons are required to have a transverse momentum pT>175 GeV. The Z→bb decay is reconstructed using a jet with pT>200 GeV, foundwith the anti-kt R=1.0 jet algorithm,and groomed to removesoftand wide- angleradiationand tomitigatecontributionsfromthe underlyingevent andadditional proton–proton collisions.Twodifferentbutrelatedmeasurementsareperformedusingtwojetgroomingdefinitionsfor reconstructingthe Z→bb decay:trimmingandsoftdrop.Thesealgorithmsdifferintheirexperimental and phenomenological implications regarding jet mass reconstruction and theoretical precision. To identifyZ bosons,b-taggedR=0.2 track-jetsmatchedtothegroomedlarge-R calorimeterjetareusedas aproxyfortheb-quarks.Thesignalyieldisdeterminedfromfitsofthedata-drivenbackgroundtemplates tothedifferentjetmassdistributionsforthetwogroomingmethods.Integratedfiducialcross-sections andunfolded jetmassspectraforeachgroomingmethodare comparedwithleading-ordertheoretical predictions.TheresultsarefoundtobeingoodagreementwithStandardModelexpectationswithinthe currentstatisticalandsystematicuncertainties.
©2020TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
Contents
1. Introduction . . . . 2
2. ATLASdetector . . . . 2
3. DataandMonteCarlosimulation . . . . 2
4. Eventreconstructionandselection . . . . 3
5. Signalandbackgroundestimation . . . . 4
6. Definitionoftheobservableandcorrectionfordetectoreffects . . . . 5
7. Systematicuncertainties . . . . 5
8. Results . . . . 7
8.1. Fitresultsandsignificanceestimate . . . . 7
8.2. Integratedfiducialcross-sectionmeasurement . . . . 7
8.3. Differentialfiducialcross-sectionmeasurement . . . . 7
9. Conclusion . . . . 7
Declarationofcompetinginterest . . . . 9
Acknowledgements . . . . 9
References . . . . 9
TheATLASCollaboration . . . . 11
E-mailaddress:atlas.publications@cern.ch.
https://doi.org/10.1016/j.physletb.2020.135991
0370-2693/©2020TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
1. Introduction
ThisLetter presentsa measurement ofthefiducial anddiffer- ential jet mass cross-sectionsofhightransverse momentum (pT) Z bosonsthatdecayintobb pairs¯ andareproducedinassociation witha photon, denotedby Z(→bb¯)γ.The analysisuses proton–
proton(pp)collisiondatacollectedin2015and2016bytheATLAS detector [1] attheLargeHadronCollider(LHC)atacenter-of-mass energyof√
s=13 TeV.Thismeasurementoftheunfoldedjetmass spectrum of hadronically decaying Z bosonsat theLHC explores the experimental features and phenomenological implications of techniquesusedtoreconstructboostedbosons –coloursinglets – decayingintobb.¯ Similarmeasurementsofgluons –colouroctets – decayingintobb pairs¯ havealsobeenmadebytheATLAS Collab- oration [2].The Z(→bb¯)γ processprovidesawell-definedexper- imental signature formeasuring massiveboosted Z bosonsusing high-pT jetscontaining pairsofb-quarks.A detailed study ofthe Z→bb signal¯ isimportant forassessingsystematicuncertainties andidentification techniquesfor themeasurement of H→bb in¯ the high-pT range, as well asfor potential TeV-scale resonances decaying intodibosons,one ofthem beinga Z boson ora Higgs bosondecayingintobb [3,4].¯
The Z(→bb¯)γ channeloffersadvantagesinaccessingthe Z→ bb signal¯ compared to the inclusive channels studied in Run 1 by ATLAS [5] and in Run 2 by CMS [6] since it provides both a useful trigger signature via the photon and an opportunity to directly estimate background processes usingthe data. Initial re- sultsofthemodellingofjetkinematicsinthe Z(→bb¯)γ channel using 13 TeV data collected by ATLAS are presented in Ref. [7].
The measurement described in this Letter selects bb decays¯ of a Z boson contained within a single jet, referred to as a Z -jet, with transversemomentum pZ -jetT >200 GeV anda photon with transverse momentum pγ
T >175 GeV. The high-pT requirement enhancesthe signal overthe dominant γ +jets backgroundpro- duction, which has a softer pT spectrum. The candidate Z -jet is reconstructed using a ‘groomed’ anti-kt [8] jet with radius pa- rameter R=1.0 (large-R jet). A multivariate algorithm is used to determinewhether R=0.2 track-jetsthat are associatedwith the large-R jet are b-tagged, i.e. ifthey contain b-hadron decay products.TheapproachtotaggingpresentedinthisLetterisbuilt uponafoundationofstudiesfromLHCrunsat√
s=7 and8 TeV, includingextensivestudiesofjetreconstructionandgroomingal- gorithms [9–11] and detailed investigations of track-jet-based b- tagginginboostedtopologies [7,12].
Twodifferentjetgroomingalgorithmsareusedtoperformthe measurement: ‘trimming’ [10], and‘soft drop’ [11,13].The exper- imental and phenomenological implications for jet mass recon- structionandtheoreticalprecisionaredifferentforthetwogroom- ingalgorithms. ThetrimmingalgorithmisthedefaultusedinAT- LAS to study boosted bosons, chosen as a result ofoptimisation studiesperformedfromLHCrunsat√
s=8 and13 TeV [14].The soft-dropcalculationsachieveadifferenttheoreticalprecisionand offer advantages such as the formal absence of non-global loga- rithms. The distribution ofthe soft-drop massfor QCD processes hasnowbeencalculatedbothatnext-to-leadingorder(NLO)with next-to-leading-logarithm (NLL) accuracy [15,16] and at leading order (LO) with next-to-next-to-leading-logarithm (NNLL) accu- racy [17,18].Thislevel ofprecision forajet substructureobserv- able at a hadron collider is surpassed only by the calculation of thrust ine+e− interactions [19].Similar calculationsarenot cur- rentlyavailablefortrimmedjets.
Thedoubledifferentialcross-sectionofsoft-dropjetsasafunc- tionofthemassandtransversemomentumwerepreviouslymea- suredbyATLAS [20] andCMS [21] inbalanceddijeteventsat√
s= 13 TeV.The trimmedjetmassdistributionindijetandW/Z +jets eventswas measuredbyCMSat√
s=7 TeV [22].Whileprevious
analyses measured thecross-section of quark andgluon-initiated jetsfordifferentgroomingalgorithms,thisanalyses measuresthe massoflarge-R jetscontaining thehadronicdecayproductsof Z bosonsinZ(→bb¯)γ eventsat√
s=13 TeV.
2. ATLASdetector
The ATLAS detectorat theLHC is a multipurposeparticle de- tector with a forward–backward symmetric cylindrical geometry and a near 4π coverage in solid angle.1 It consists of an inner detector (ID) for tracking surrounded by a thin superconducting solenoidprovidinga2 Taxialmagneticfield,electromagneticand hadroniccalorimeters,andamuonspectrometer.TheIDcoversthe pseudorapidity range|η|<2.5. It consistsofsilicon pixel,silicon microstrip,andtransitionradiationtrackingdetectors.A newinner pixel layer, the insertable B-layer [23,24], was added at a mean radius of 3.3 cm during the period between Run 1 and Run 2 oftheLHC.Lead/liquid-argon(LAr)samplingcalorimetersprovide electromagnetic(EM)energymeasurements withhighgranularity (|η|<3.2). The hadronic calorimeter uses a steel/scintillator-tile samplingdetectorin the centralpseudorapidity range (|η|<1.7) anda copper/LArdetector inthe region 1.5<|η|<3.2. The for- ward regions (3.2<|η|<4.9) are instrumented withcopper/LAr andtungsten/LAr calorimetermodules optimised for electromag- netic and hadronic measurements, respectively. A muon spec- trometer with an air-core toroid magnet system surrounds the calorimeters.Threelayersofhigh-precisiontrackingchamberspro- videcoverageintherange|η|<2.7,whilededicatedfastchambers allow triggering in the region |η|<2.4. The ATLAS trigger sys- temconsistsofahardware-based first-leveltriggerfollowedby a software-basedhigh-leveltrigger [25].
3. DataandMonteCarlosimulation
Thedatawere collectedin pp collisionsattheLHCwith√ s= 13 TeV and a 25 nsproton bunch crossing interval during 2015 and2016. The full data sample corresponds to an integratedlu- minosityof 36.1 fb−1 afterrequiringthat alldetectorsubsystems were operational during data recording. The uncertainty in the combined2015–2016integratedluminosity is2.1% [26], obtained usingtheLUCID-2 detector[27] for theprimary luminosity mea- surements.Collisioneventswererecordedwithatriggerselecting eventswithatleastone photoncandidatewithtransverseenergy ET>140 GeV.
MonteCarlo(MC) eventsamplesthat includeanATLAS detec- torsimulation [28] based on Geant 4 [29] areusedtomodelthe Zγ signal and the small tt¯+γ and Wγ background contribu- tions.Inaddition, γ+jets MCeventsamplesareusedtostudythe triggermodelling. Inadditionto thehard scatter,each eventwas overlaid with additional pp collisions (pile-up) according to the distribution ofthe average number of pp interactions per bunch crossing,μ,observedindata.Theseadditionalpp collisionswere generatedwith Pythia 8.1 [30] using theATLAS A2set of tuned parameters [31] andtheNNPDF23LO [32] partondistributionfunc- tion(PDF)set.Simulatedeventswerethenreconstructedwiththe samealgorithmsasthoserunoncollisiondata.
1 ATLASusesaright-handedcoordinatesystemwithitsoriginat thenominal interactionpointinthecentreofthedetector.Thepositivex-axisisdefinedbythe directionfromtheinteractionpointtothecentreoftheLHCring,withthepositive y-axispointingupwards,whilethebeamdirectiondefinesthez-axis.Cylindrical coordinates(r,φ)areusedinthetransverse plane,φ beingtheazimuthal angle aroundthez-axis.Thepseudorapidityηisdefinedintermsofthepolarangleθ byη= −ln tan(θ/2).Rapidityisdefinedasy=0.5ln[(E+pz)/(E−pz)]whereE denotestheenergyand pzisthecomponentofthemomentumalongthebeam direction.TheangulardistanceR isdefinedas
(y)2+ (φ)2.
The Zγ signal was modelled using the LO Sherpa 2.1.1 [33]
generator, withthe CT10NLO [34] PDFset;thesample isflavour inclusive ( Z(→qq)γ). An alternative Zγ sample was produced with MadGraph 5.2 [35],whichgeneratedLOmatrixelementsthat werethenpartonshoweredwith Pythia 8.1usingtheNNPDF23LO PDF set andtheATLAS A14 setof tuned parameters [36] forthe underlying event.This alternativesignal sample is usedto deter- mine the systematicuncertaintyassociated withthe signal mod- elling.
The γ+jets sampleswerealsogeneratedwith Sherpa 2.1.1and theCT10NLOPDFset.Thematrixelementwasconfiguredtoallow aphotonwithuptothreepartonsinthefinalstate.Thett¯+γ pro- cesses were modelled by MadGraph 5.2 interfacedto Pythia 8.1.
NLO corrections were applied to the tt¯+γ cross-section [37].
The Wγ MCsampleswithhadronicallydecaying W bosonswere generated using Sherpa 2.1.1,witha configurationsimilar tothat usedforthe Zγ sample.PredictionsforWγ productionwerenor- malisedaccordingtothecross-sectionsprovidedbythegenerator.
4. Eventreconstructionandselection
Eventsarerequiredtohaveareconstructedprimaryvertex,de- fined as the vertex with at least two reconstructed tracks with pT>0.4 GeV andwiththehighestsumofsquaredtransversemo- mentaofassociatedtracks [38].
Hadronicallydecayinghigh-pT Z→bb candidates¯ areidentified usinglarge-R jetstocapturebothb-quarks,sincetheywillbevery closeduetothehighLorentzboost.Thetwo differentjetgroom- ingalgorithmsconsideredintheanalysis,trimming andsoftdrop, differintheirpile-upmitigationandmassresolutionperformance.
Trimmed calorimeter jets Trimmed calorimeter jets are recon- structed from noise-suppressedtopological clusters(topoclusters) of calorimeter energy deposits calibrated to the local hadronic scale (LC) [39], using the anti-kt algorithm with radius parame- ter R=1.0 implemented in FastJet [40,41].Trimmedcalorimeter jetsarethosejetstowhichthetrimmingalgorithm [10] isapplied.
The aim of this algorithm is to improve the jet mass resolu- tion and its stability with respect to pile-up by discarding the softer components of jets that originate from initial-state radia- tion,pile-upinteractions,ortheunderlyingevent.Thisisdoneby reclusteringtheconstituentsoftheinitiallarge-R jet,usingthekt algorithm [42,43], into subjets with radius parameter Rsub=0.2 andremoving anysubjetthat hasa pT lessthan5% ( fcut) ofthe parentjet pT.Thejetmassmjet,themainobservableinthisanal- ysis, is definedas themagnitude ofthe four-momentum sumof constituentsinsideajet.Itisreferred toasthecalorimeter-based mass ifit is calculated using the topoclustersasconstituents, or as the track-assisted jet mass [44] if it is estimated by using tracking information.Thejet massfortrimmed jetsisdefinedas theweighted combinationofthecalorimeter-based massandthe track-assistedjetmass [44],whereeachinputmassisweightedby afactorproportionaltotheirinverse-squaredmassresolution.
Soft-drop calorimeter jets Soft-drop calorimeter jets are formed by the applicationof the soft-drop algorithm [11] to the anti-kt R=1.0 jets described above, with additional topological cluster preprocessing that isdescribed below.The soft-drop algorithm is designed to remove soft andwide-angle radiation and also con- tamination from pile-up. In the first step of the grooming algo- rithm,theanti-kt R=1.0 jetsarereclusteredwiththeCambridge–
Aachen (C/A) [45,46] algorithm sothat theconstituents arecom- bined purely accordingto their angularseparation. Thesoft-drop algorithm then reverses the C/A algorithm clustering historyand removes thesofter subjetata specific step of theC/A clustering historyunlessthesoft-dropconditionisfulfilled:
min(pT1,pT2) pT1+pT2
>zcut
R12 R0
β ,
where zcut andβ are algorithm parameters, pT1 and pT2 arethe transversemomentaofthedeclusteredsubjetsateachhistorystep,
R12 isthedistancebetweenthesubjetsinthe(η,φ) spaceand R0 isathresholdcorresponding tothejetradius.The parameters β=0 and zcut=0.1 areusedin theanalysis,based onthestud- iesinRef. [47]. Thefinal measurementis performedforjetmass mjet>30 GeV,whichimpliesthatanycollineardivergenceisreg- ulated andthe measurement remains protected against collinear singularities.Thesoft-dropjetmassexhibitsapile-updependence withthechosenparametersandthereforeaspecialversionofpile- up suppressed topological clustersare used to construct the jets that are then groomed withthe soft-drop algorithm. Specifically, theSoftKiller(SK)algorithm [48] isusedinconjunctionwithCon- stituent Subtraction (CS) [49,50] based on the studies presented inRef. [47]. CS is applied before the SK algorithm.The CS is an extension of the pile-up subtraction based on jet area [51]. The algorithmproceedsasfollows.First,virtualparticleswithinfinites- imally small pT (ghosts) are addedto theevent(eachcovering a fixed area in the η–φ plane) with energy density matching the medianenergydensityoftheevent.Second,theaddedghostsare matchedto the topologicalclustersin η–φ spaceand onlythose withinR=0.25 ofthetopoclusterarefurtherconsideredforthe pile-upremovalprocedure.Thealgorithmproceedstheniteratively througheach topocluster–ghost pairinorder ofascending R.If thepT ofthetopoclusterislargerthanthatofthematchedghost, thepT oftheindividualtopoclusteriscorrectedbysubtractingthe pT of theghost andtheghost are removed. Otherwisethe pT of the topocluster is subtracted from the pT of the ghost and the pT ofthetopoclusterissettozero.TheSKalgorithmexploitsthe characteristicthat particlesoriginatingfrompile-upcollisions are softerthan those from thehard-scattering collision andremoves particlesthatfallbelowacertain pT threshold,determinedonan event-by-event basis. The pile-up suppressed topological clusters afterCSandSKareusedasinputtothesoft-drop jetreconstruc- tion.Thecalorimeter-basedjetmassisusedforsoft-dropjets.
Allgroomedjets A dedicatedMC-based calibration,similar tothe procedure used inRef. [44], is applied to correctthe jet pT and massofboth thetrimmedjetsandthesoft-drop jetsto thepar- ticle level. To account for semileptonic decays of the b-hadrons, thefour-momentumofthe closestreconstructed muoncandidate withinR=0.2 oftheb-taggedtrack-jetistakenintoaccountin thecalorimeter-based componentofthejet massobservable (see belowforthedescriptionofthetrack-jetdefinitionandb-tagging).
MuoncandidatesareidentifiedbymatchingIDtrackstofulltracks ortracksegmentsreconstructedinthemuonspectrometer.Muons are required to have pT>10 GeV and |η|<2.4, and to satisfy theloose identification criteriaofRef. [52], whichimpose quality requirements on the tracks,but no isolation criteria are applied.
A calibration is applied to correct the muon transverse momen- tum,andreconstruction andidentificationefficiencyscalefactors, derivedfrom J/ψ→μ+μ− and Z→μ+μ− events [52], areap- pliedtosimulation.Large-R jetsarerequiredtohavepT>200 GeV and|η|<2.0.A comparisonofthecalibrated Z -jet massdistribu- tion andthe particle-leveljet mass distribution fortrimmedjets andsoft-dropjetsisshowninFig.1.Particle-leveljets,usedinthe unfoldingprocedure described in Section 6,are builtfrom stable final-stateparticles (definedasthose withproper lifetime τ cor- respondingto cτ >10 mm) excluding muonsand neutrinos and using the same jet reconstruction algorithms used for calorime- terjets. Similarly to themuon-in-jet correction at reconstruction leveldescribedinSection4,particle-levelmuonsareaddedtothe particle-leveljet if they are within R=0.2 ofa b-hadron. The
