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Physics Letters B
www.elsevier.com/locate/physletb
Measurement of the charge asymmetry in highly boosted top-quark pair production in √
s = 8 TeV pp collision data collected by the ATLAS experiment
.ATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received21December2015
Receivedinrevisedform10February2016 Accepted24February2016
Availableonline2March2016 Editor:W.-D.Schlatter
In the pp→tt process¯ the angular distributions oftop and anti-top quarks are expectedto present a subtle difference,which couldbe enhanced byprocessesnot included inthe Standard Model.This Letterpresentsameasurementofthechargeasymmetryineventswherethetop-quarkpairisproduced withalarge invariantmass.Theanalysisisperformedon20.3 fb−1ofpp collisiondataat√
s=8TeV collectedbytheATLASexperimentattheLHC,usingreconstructiontechniquesspecificallydesignedfor thedecaytopologyofhighlyboostedtopquarks.Thechargeasymmetryinafiducialregionwithlarge invariantmassofthetop-quarkpair(mt¯t>0.75 TeV)andanabsoluterapiditydifferenceofthetopand anti-topquarkcandidateswithin−2<|yt|− |yt¯|<2 ismeasuredtobe4.2±3.2%,inagreementwith theStandardModelpredictionatnext-to-leadingorder.Adifferentialmeasurementinthreet¯t massbins isalsopresented.
©2016CERNforthebenefitoftheATLASCollaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
The charge asymmetry [1,2] in top-quark pair production at hadron collidersconstitutesone ofthe moreinteresting develop- ments in the last decade of top-quark physics. In the Standard Model(SM),aforward–backwardasymmetry( AFB),oforder αs,is expectedataproton–antiproton(pp)¯ collidersuchastheTevatron, witha muchenhancedasymmetry incertainkinematicalregions.
Early measurements [3,4] found a larger AFB than predicted by the SM. Later determinations confirmed this deviationand mea- surements in intervals of the invariant mass, mt¯t, of the system formedby thetop-quarkpair [5–9]found astronger dependence onmtt¯ thananticipated.Recentcalculationsofelectroweakeffects [10]andthefullnext-to-next-to-leading-order(NNLO)corrections [11] to the asymmetry have broughtthe difference betweenthe observedasymmetryattheTevatronandtheSM predictiondown to the 1.5 σ level and reducedthe tensionwith the differential measurementsinmt¯t [12,13].
At the Large Hadron Collider (LHC), the forward–backward asymmetry is not presentdueto thesymmetric initial state, but arelatedcharge asymmetry, AC,isexpectedinthedistributionof thedifferenceofabsoluterapiditiesofthetopandanti-topquarks, AC=N(|y| >0)−N(|y| <0)
N(|y| >0)+N(|y| <0), (1)
E-mailaddress:atlas.publications@cern.ch.
where |y|= |yt|− |y¯t| and y denotes the rapidity of the top and anti-top quarks.1 For quark–antiquark (qq)¯ initial states, the difference in the average momentum carried by valence andsea quarks leads to a positive asymmetry. These quark-initiated pro- cessesarestronglydilutedbythecharge-symmetricgluon-initiated processes,yieldinga SMexpectationforthecharge asymmetryof less than 1%. Many beyond-the-Standard-Model (BSM) scenarios predictanalterationtothisasymmetry.Previousmeasurementsat 7 TeV[14–17]and8 TeV[18–20]byATLASandCMSareconsistent withtheSMprediction.
With a centre-of-mass energy of 8 TeV and a top-quark pair sample ofmillionsofevents,the LHCexperimentscanaccessthe charge asymmetry in a kinematic regime not probed by previ- ous experiments. The development of new techniques involving Lorentz-boostedobjectsandjetsubstructure[21–24]andtheiruse in the analysisof LHC data[25,26] have enabledan efficientse-
1 ATLAS usesaright-handedcoordinatesystemwithitsoriginatthe nominal interactionpoint(IP)inthecentreofthedetectorandthe z-axiscoincidingwith the axisofthebeam pipe.Thex-axispoints fromthe IPtowardsthe centreof theLHCring,andthe y-axispointsupward.Polarcoordinates(r,φ)areusedin thetransverseplane,φbeingtheazimuthalanglearoundthe z-axis.Therapidity y isgivenas y= −12ln[(E+pz)/(E−pz)],whilethepseudorapidityisdefinedin termsofthepolarangleθasη= −ln[tan(θ/2)].Thedistancein(η,φ)coordinates,
R=
(φ)2+ (η)2,isusedtodefineconesizesandthedistancebetweenre- constructedobjects.TransversemomentumandenergyaredefinedaspT=psinθ andET=E sinθ,respectively.
http://dx.doi.org/10.1016/j.physletb.2016.02.055
0370-2693/©2016CERNforthebenefitoftheATLASCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
lectionofhighlyboostedobjectsandanaccuratereconstructionof theirmomentum.
ThisLetterpresents ameasurement ofthe rapidity-dependent charge asymmetry in top-quarkpair production that isbased on techniques specifically designed to deal with the collimated de- caytopologyofboostedtopquarks.Specifically,itisbasedonthe techniquesdescribed inRefs. [27–30].Theanalysisfocusesonthe lepton+jets ( +jets) final state, wherethe hadronictop-quark decay is reconstructed as a single large-radius (large-R) jet and taggedassuch usingjet substructurevariables.The leptonic top- quarkdecayisreconstructed froma singlesmall-radius(small-R) jet,asinglechargedlepton(muonorelectron),andmissingtrans- versemomentum,correspondingtotheneutrinofromtheW bo- sondecay.The eventselection andreconstruction followthepre- scriptionsofRef.[27],whereadetaileddescriptionanddiscussion oftheirperformancecanbefound.
Comparedto previous analyses [18,20] based onthe classical, resolved top-quark selection criteria and reconstruction schemes, thisapproachoffersamoreprecisereconstructionofthett invari-¯ ant massand top-quark directionfor highly boosted top quarks.
Itis therefore possibleto perform accurate measurements ofthe charge asymmetry in eventswitha t¯t invariant mass inthe TeV range.This kinematicregime hasa higher sensitivityfor the SM asymmetryduetoahigherfractionofquark-initiatedprocesses,as wellasforBSMmodelsthatintroducemassivenewstates.
ThisLetter isstructured asfollows.The datasample analysed ispresented inSection 2, along witha description of theMonte Carlo(MC) simulation samples in Section 3. A brief overviewof thereconstructedobjectdefinitionsandoftheeventselectionand reconstruction is given in Sections 4 and5. The observed yields andseveral kinematic distributions are compared to the SM ex- pectations in Section 6. The unfolding technique used to correct thereconstructed |y| spectrum tothe partonlevelis discussed inSection7.Theestimatesofthesystematicuncertaintiesthataf- fect the measurement are described and estimated in Section 8.
TheresultsarepresentedinSection9,andtheirimpactonseveral BSMtheories is discussed in Section 10. Finally, the conclusions arepresentedinSection11.
2. Datasample
ThedataforthisanalysiswerecollectedbytheATLAS[31] ex- perimentinthe 8 TeVproton–proton(pp)collisions atthe CERN LHCin 2012. Collisionevents areselected using isolatedor non- isolated single-leptontriggers, wherethe isolated triggers havea thresholdof24 GeVonthetransversemomentum (pT) ofmuons or on the transverse energy of electrons. The non-isolated trig- gershavehigherthresholds:60 GeV forelectrons and36 GeV for muons.Thecontributionfromeventswithleptonspassingonlythe non-isolatedtriggersbuthaving pT belowthesehigherthresholds isnegligible.Thecollected datasetislimitedtoperiodswithsta- blebeamconditions whenall sub-systems were operational. The samplecorrespondstoanintegratedluminosityof20.3±0.6 fb−1. 3. MonteCarlosimulation
Samplesof MC simulatedevents are used to characterise the detector response and efficiency to reconstruct tt events,¯ esti- matesystematicuncertainties,andpredictthebackgroundcontri- butions from various physics processes. The response of the de- tectorandtriggerissimulated[32] usingadetailedmodelimple- mentedinGEANT4 [33].Simulatedeventsarereconstructedwith thesamesoftware asthedata.Additional pp interactions,simul- taneously present in the detector (pile-up), are generated using Pythia 8.1 [34] with the MSTW2008 leading order PDF set [35]
andthe AUET2set oftune parameters (tune).Thepile-up events arereweightedtothenumberofinteractionsperbunchcrossingin data(on average21in2012).Forsome samplesusedtoevaluate systematicuncertainties,thedetaileddescriptionofthe calorime- terresponseisparameterisedusingtheATLFAST-IIsimulation[32].
Forallsamplesthetop-quarkmassissettomtop=172.5 GeV.
The nominalsignal t¯t sample is produced usingthe Powheg- Box (version 1, r2330) generator [36], which is based on next- to-leading-order (NLO) QCD matrix elements. The CT10 [37] set of parton distribution functions (PDF) is used. The hdamp pa- rameter, which controls the matrix element (ME) to parton shower (PS) matching in Powheg-Box and effectively regulates the high-pT radiation, is set to the top-quark mass. The parton shower, hadronisation, and the underlying event are simulated with Pythia 6.427 [38] using the CTEQ6L1 PDF set and the Pe- rugia 2011[39] tune.Electroweak corrections areapplied to this sample through a reweighting scheme; they are calculated with Hathor2.1-alpha [40] implementing the theoretical calculations of Refs. [41–43]. Alternative samplesare used to evaluate uncer- tainties in modelling the tt signal.¯ These include samples pro- duced with MC@NLO 4.01 [44] interfaced with Herwig 6.520 [45] and Jimmy 4.31 [46], as well as samples generated with Powheg-Box + Herwig/Jimmy and Powheg-Box + Pythia, both with hdamp=infinity. Samples are also produced with differing initial- andfinal-stateradiation(ISR/FSR), usingthe AcerMC gen- erator [47] interfacedwith Pythia. All tt samples¯ arenormalised to cross-section at NNLO+next-to-next-to-leading logarithmic (NNLL)accuracy2[49–54]: σt¯t=253+−1315pb.
Leptonicdecaysofvector bosonsproduced inassociationwith severalhigh-pT jets, referred to asW+jets and Z+jets events, withup tofiveadditionalfinal-statepartons intheleading-order (LO) matrix-elements, are produced with the Alpgen generator [55] interfacedwith Pythia 6.426 forparton fragmentationusing theMLMmatchingscheme[56].Heavy-flavourquarksareincluded intheMEcalculationstomodeltheW bb,¯ W cc,¯ W c,Z bb and¯ Z c¯c processes.TheW+jets samplesarenormalisedtotheinclusiveW bosonNNLOcross-section[57,58].
Singletop-quarkproductionissimulatedusing Powheg-Box in- terfaced with Pythia 6.425 using the CTEQ6L1 PDF set and the Perugia2011tune.Thecross-sectionsmultipliedbythesumofthe branchingratiosfortheleptonicW decayemployedforthesepro- cessesare28pb(t-channel)[59],22 pb(W t production)[60],and 1.8 pb(s-channel)[61],obtainedfromNNLO+NNLLcalculations.
Diboson production is modelled using Sherpa [62] with the CT10 PDF set, and the yields are normalised to the NLO cross- sections: 23 pb (W W → νqq), 0.7 pb ( Z Z → qq), 6.0 pb (W Z→ νqq)and4.6 pb( Z W→ qq).
4. Objectdefinitions
Electron candidates are reconstructed using charged-particle tracks in the inner detector associated with energy deposits in theelectromagneticcalorimeter.Muoncandidatesareidentifiedby matchingtracksegmentsinthemuonspectrometerwithtracksin the inner detector. Leptoncandidates are required to be isolated usingthe“mini-isolation”criteriadescribedinRef.[63].
Jets are reconstructed using the anti-kt algorithm [64] imple- mentedinthe FastJet package[65]withradiusparameterR=0.4 (small-R) or R=1.0 (large-R), usingasinput calibratedtopolog- ical clusters[66] ofenergy depositsin the calorimeters.The jet- trimming algorithm [67] is applied to the large-R jetsto reduce
2 Thetop++2.0[48]calculationincludestheNNLOQCDcorrectionsandresums NNLLsoftgluonterms.Thequotedcross-sectioncorrespondstoatop-quarkmass of172.5 GeV.
theeffectofsoftanddiffuseradiation,suchasthat frompile-up, multiplepartoninteractionsandinitial-stateradiation.Large-R jets aretrimmedbyreclusteringtheconstituentswiththekt algorithm [68,69]witharadius parameter Rsub=0.3 andretainingsub-jets that have a momentum exceeding 5% of that of the large-R jet ( fcut=0.05). For small-R jets,a pile-up correction based onthe jet area, thenumber ofprimary vertices,the bunch spacing, and jet η is applied. Both jet collectionsare calibrated to the stable- particlelevelasafunctionofpT and η (andmassforlarge-R jets) [25]. The stable-particlelevel refers to generator-leveljets recon- structed fromparticles witha lifetimeof atleast10 ps. Small-R jets are b-tagged using an algorithm that exploits the relatively largedecaytimeofb-hadronsandtheirlargemass[70,71].
The missing transverse momentum (with magnitude EmissT ) is computedasthenegativevectorsumoftheenergyofallcalorime- tercells, takinginto account the calibrationof reconstructed ob- jects,andthepresenceofmuons.
5. Eventselectionandreconstruction
Each event must have a reconstructed primary vertex with five or more associated tracks of pT>400 MeV. The events are required to contain exactly one reconstructed lepton candidate, whichmust then be geometricallymatched to thetrigger object.
To reduce the multi-jet background,the magnitude of the miss- ingtransversemomentumandthe W -bosontransversemassmWT mustsatisfy ETmiss>20 GeV and EmissT + mTW > 60 GeV,where mWT =
2pleptonT EmissT (1−cosφ) (2)
andφ istheazimuthalanglebetweentheleptonandthemiss- ing transverse momentum. At least one small-R jet (R=0.4) must be found close to, but not coincident with, the lepton (R( ,jetR=0.4)<1.5).
The leptonic top-quark candidate is reconstructed by adding the highest-pT jet amongthose satisfying the above criteria, the selectedcharged lepton andthereconstructed neutrino. The lon- gitudinal componentofthe neutrinomomentum iscalculated by constraining the lepton-plus-missing-momentum system to have the W boson mass andsolving theresulting quadratic equation.
If two real solutions are found, the one that yields the smallest longitudinalmomentum forthe neutrinoisused.If norealsolu- tionexists,themissingtransversemomentum vectorisvaried by theminimalamountrequiredtoproduceexactlyonerealsolution.
Thehadronically decayingtop quarkis reconstructedasasin- gle trimmed jet with R=1.0. The selected jet must have pT >
300 GeV, must be well separated from both the charged lepton (φ ( ,jetR=1.0)>2.3) and the small-R jet associated with the leptonictop-quarkcandidate(R(jetR=1.0,jetR=0.4)>1.5).Asub- structure analysis of the large-R jet is used to tag the boosted top-quarkcandidate:theinvariantmassofthejetmtrimjet aftercali- brationtotheparticlelevel[26] mustbelargerthan100 GeVand thekt splittingscale3
dtrim12 mustexceed40 GeV.
Finally, atleast one of the highest-pT small-R jets associated with the decay of a top-quark candidates (R( ,jetR=0.4)<1.5 or R(jetR=1.0,jetR=0.4)<1.0) must be b-tagged. Events with a
3 Thektsplittingscale[72]isobtainedbyreclusteringthelarge-R jetcompo- nentswiththe kt algorithmwitharadiusparameterR=0.3.Thefirstsplitting scale
dtrim12 correspondstothescaleat whichthe lasttwosub-jetsaremerged intoone:
dtrim12 =min(pT,1,pT,2)× R1,2,where1and2denotethetwosub-jets mergedinthelaststepofthektalgorithm.
Table 1
Observedand expectednumberofeventsinthesignalregion.Thetwocolumns correspondtothee+jets andμ+jets selecteddatasamples.Thesystematicun- certaintiesoftheSMexpectationincludethosefromdetector-relateduncertainties, uncertaintiesinthenormalisation,theluminosityuncertaintyandtheuncertainty inthecross-sectionpredictionsusedtonormalisetheexpectedyields.
e+jets μ+jets
t¯t 4100±600 3600±500
W+jets 263±32 264±32
Single top 140±20 138±19
Multi-jet 44±8 4±1
Z+jets 40±27 16±11
Dibosons 20±7 18±7
t¯t V 37±19 33±17
Prediction 4600±600 4100±500
Data 4141 3600
reconstructedt¯t massoflessthan750 GeVarerejected,astheper- formance of the reconstruction ofboostedtop quarks is strongly degradedatlowmass.
Theselection andreconstructionschemesyieldgoodefficiency andtt mass¯ determinationforhigh-masspairs.DetailedMCstud- iespresentedinRef.[27]showthatthemassresolutionisapproxi- mately6%foralargerangeoft¯t mass,startingatmtt¯∼1 TeV.The measurement of the top andanti-top-quark rapidities are nearly unambiguous.Thequalityofthetopquarkrapidity reconstruction canbeexpressed intermsofthedilutionfactorD=2p−1,where p istheprobabilityofacorrectassignmentofthe|y|sign.Adi- lution factor D= 1 indicates perfect charge assignment.The MC simulationpredicts avalueofapproximately D=0.75forthese- lectedsample.Theremainingdilutionislargelyduetoeventswith small values of the absolute rapidity difference; if events with
||y||< 0.5 are excluded, the MC simulation predicts a dilution factorgreaterthan0.9.
6. ComparisonofdatatotheSMtemplate
A template for the expected yield of most SM processes is based on Monte Carlo simulation, where the production rate is normalisedusingthepredictionoftheinclusivecross-sectionspec- ified in Section 3. Exceptions are the W +jets background and themulti-jetbackground.The W+jets backgroundnormalisation andheavy-flavourfractionsarecorrectedwithscalefactorsderived from data, asin Ref. [27], usingthe observed asymmetry in the yields ofpositively and negatively charged leptons. The multi-jet backgroundestimateisfullydata-driven,usingthematrixmethod.
Thismethodusestheselectionefficienciesofleptonsfromprompt and non-prompt sources to predict the number of events with non-promptleptons inthesignalregion.Thesemethodsandtheir resultsaredocumentedindetailinRef.[27].
The event yields are compared to the SM expectation in Ta- ble 1.Thedistributionsoftwokeyobservables,theinvariantmass ofthet¯t systemandthedifferenceoftheabsoluterapiditiesofthe candidatetopandanti-topquarksareshowninFig. 1,forthecom- binationofthee+jets and μ +jets channels.Theobservedevent yieldisapproximately10%lessthantheMCprediction,theresult ofthesoftertop-quarkpTspectrumindata,whichisalsoreported inRefs.[73–75].
Since AC is measured asa ratio,it isnot sensitive to the ab- solutecross-section.Theimpactofthedifferencesintheexpected and observed shapes of the distributions in Fig. 1 on the mea- surementisestimatedbyreweightingthesimulated|y|andtop quark pT distributionstomatchthedataandfound tobe negligi- ble.
