; TeX output 2008.02.01:19012=PSfile="d0logo.eps" llx=206 lly=343 urx=406 ury=449 rwi=1000 o cmr9DnoteT5234-CONFWۍg+(N cmbx12Measurementof*g cmmi12n9'XQ cmr12(pp :,!", cmsy10!URt$)6܍t>6)in+jetschannelPύ0K`y cmr10The%D$CollabGoration (S&j cmti9URLhttp://www-d0.fnal.gov(Dated:pF:ebruaryT1,2008)mo..Thisnotepresen9tsanewmeasurementof5" cmmi9pDZp cmsy9! tVۍ %tWX:proAductionatؽp 1؉aHO|(s O= 1:96T:eVusing350$pb-=q% cmsy6Aacmr61 nof|data|collectedwiththeDdetectorbAet9ween|2002and2005.SmW:efoAcusonthe nalstate$where70the71WbAosonfromoneofthetopquarksdeca9ysinto70a?0leptonanditsassoAciatedneutrino,$whiletheotherdeca9ysintoaquark-antiquarkpair.e W:eaimtoselectthoseev9entsinwhic9hthe$lepton#subsequen9tlydecays"tooneorthreecharged"hadrons,`zeroormoreneutralhadronsandatau$neutrino (thec9harge conjugateproAcessesareimpliedinalloftheabAo9ve).MThe observ|rablesignature$th9usconsistsofanarro9wcalorimetershowerwithassoAciatedtrack(s)characteristicofahadronic$taurdeca9y:,fourormorejets,ofwhic9htwoareinitiatedrbybquarksaccompanyingtherWH'sinthe$topNquarkNdeca9ys,]andalargenetmissingmomen9tuminthetransv9erseplaneduetotheenergetic$neutrino-an9tineutrinoTpairthatleaveUnotraceinthedetectormedia. pThepreliminaryresultforthe$measuredTcrosssectionis:fR(tSaHWtW)=5:1=#+4;cmmi6:38Q#3:5*(statw)=#+0:78Q#0:7(systƻ)1Ľ80:3#(lumij:0)pbOPreliminaryN cmmi10 leptonareespGeciallyinteresting,sincetheNistheheaviestlepton.0Anynon-standard avor-andmass-depGendentcouplingscouldproGduceDaverysigni cante ectinthischannel.[AninterestingexampleisthechargedHiggsbGoson,whichappGearsinextensionsoftheSMHiggssectorto2HDMs(Two-HiggsDoubletMoGdels),) andisrequiredinMSSM(MinimalSupGersymmetricStandardModel)[1].^SincetheHiggs-fermioncouplingisproportionaltothelatter'smass,Ҕdecaysto=thefscintillatingtriggercounters.j"Movingradially>goutwards,Bthe rstlayerisplacedinsidethe>g1.8T>`toroidmagnets,andUUthetwoUUfollowinglayersareloGcatedoutsidethemagnets.ThedDetriggerisathree-leveltriggersystem.@Level1isahardwaretrigger,whilelevelse2and3aresoftwared lters.!C%RIKII.AOBJECTIDENTIFICATIONThe7?mostimpGortantob8jectsforthisanalysisarejets7@andhadronicYleptoncandidates.Jetsarereconstructedusing{an|iterativealgorithm,?Ewhichintegrates|theenergyobserved|inthecalorimeterinaconewithradiusRȲ= P\n5u cmex10pn5fe*f ˍ`y 2+8` 2$8=0:5,where1y? isdetector2rapidityandistheangleintheplanetransversetothebGeamaxis.KThesejetsare=required=tobGeinthe ducialregionj[ٸj<2:4,Awith=pT ε>20GeV/c.iThe=energyofthejetsafterreconstructioniscorrectedUUtorepresentthetruejetenergy[5].qAnequivqalentcorrectionisderivedforMonteCarloevents. 23jײA. ^IDP\1. qTJauN3.hAlso,&onlysecondaryverticeswhicharedisplacedJfromKtheprimaryvertexintheplanetransversetothebGeamlinebymorethan7standarddeviationsareconsideredUUinthismeasurement.E': cmti10Ea. 8bE-taggingeciency ItTisSknown[9]thattheb-tagging,TwhenapplieddirectlytoMonteCarlo,TgivesTaneciencythatgishigherthandata.%Inordertoaccountforg thisfactor,kSVTghasbGeenparameterizedont7ferYɍt0!侵D+jetsgMCgand+jetsWdatatoXcomputethecorrectionfactor, whichscalestheMC-derivedeciency*.$Asaresult, weobtaintheMCftaggingprobabilityandthedata-correctedone[9].HItcanbGenotedthatthedata-correctedeciencyisindeednoticeablyUU(30%)lowerUUthanwhatwewouldexpGectbyapplyingtheSVTdirectlytoMC.Eb. 8cE-taggingeciency An`assumption`ismadethatthecorrectionfactorobtainedbydividingthesemi-leptonicb-taggingeciencyindatatotheoneinMCalsoiscorrectforcjets.H HencetheMC-obtainedinclusivec-taggingeciencyUUismultipliedbythisfactorinordertoestimatethec-taggingprobability*.$24]YC9T:ABLETI:Bac9kgroundsources,relev|rantforthe7+8jetsanalysis.pThebranchingfractionintohadronicShasbAeenapplied0/37ffffffffYBac9kground100Tn9bΉffffff(.Ec. 8Lightkjettaggingeciency Thevb-tagvfakeratefromlightquarksiscomputedbymeasuringthenegativetag rate.vItisde nedbytherateofappGearanceofsecondaryverticeswithanegativedecaylengthsigni cance.vItisassumedg~thatthelightquarkshaveanequalchancetoproGduceasecondaryvertexwithpGositiveandnegativedecaylengthƵsigni canceƶ(dueto niteresolutione ects)whiletheheavy avorƶjetscanonlyproGduceaSVƘwithpGositivedecayRlengthRsigni cance.pHowever,SFthisisRnotquitetrueandaspGecialscalingfactor(SFhf ݧ)isintroGducedtocorrectforthefractionofheavy avorsamongthejetswithanegativedecaylengthsigni cance.hAnothercorrectionisfortheUUpresenceoflonglivedparticlesinlightjets(SFl `l).qBothfactorsarederivedfromMonteCarlo.!C%ELIV.xBACCKGROUNDSTwomaindistinctivefeaturesofthesignaldeterminewhichbackgroundsareimpGortant.InordertobGerelevqant,theproGcessmusthaveahigh(>3)numbGerofjetsaswellasasizeable(>15GeV)6qȵETL.MTAllofthecandidateproGcessesareUUlistedinT*ableIV.qThecrosssectionslistedincludethebranchingfractionsintowleptons.W*e.canconcludethatthetwo.dominantbackgroundsourcesareQCD.{(\fake!Dz")andWc+4jets.dThesetwosourcesweretakenintoaccountinthisanalysis.wQCDisderivedfromthedata,-,whileMonteCarlosimulationisusedforWc+jets.This|simulation|utilizesevents|generatedatp !fe3荵s=1:96T*eV|withtheALPGEN1.2[10 ]matrixelementgenerator,assumingatopmassof175GeV/c^2\andthepartondistributionfunctionsetCTEQ6.1M[11 ].[NTheseeventsarethenproGcessedtthroughF- cmcsc10FPythias6.2[12 ].$Thispackqagetakestintoaccountthegluonradiationsandfragmentatione ectsand)also*pGerformstheshortlivedparticledecays,eexceptforbhadronsandleptons.=EvtGen[13 ]isusedtomoGdelthedecays'of'bhadrons.bIòleptonsaredecayed'usingT*auola[14 ].bThegeneratedevents'arethenproGcessedthroughafullFgeantBj[15 ]BksimulationoftheDdetectorprovidingtrackingBjhits,F3calorimetercellenergyandmuonhitinformation.Multiple interactions areaddedtoalleventsaccording toaPoissondistributionwithameanthatisdeterminedfromtheaverageinstantaneousluminosity*.b]Thesamereconstructionandob8jectIDisappliedtodataandMonteCarloevents.qTheUUsignal(t'#btr)MonteCarloispreparedinthesameway*.V.>EVENTSELECTIONF*orthisanalysis,$Gweusedthedatacollectedwitha4-jettrigger,$Gwhichrequired4jetcandidateswithj[ٸj<3:6andETL>10GeV}usingasimpleconealgorithm.aThistriggerwasdesignedfortheall-hadronictopdecaymoGdeandworks!well!forourpurpGoses,,AsinceanenergetichadronicCòleptonisalways!foundasajetcandidate.`TheeciencyofthistriggerhadbGeenparametrizedandcross-checkedondataandisappliedasaweightingfactortotheMonteCarloevents.TheUUanalysisproGcedureinvolvedUUseveralstages:Preselectionqg(sectionqhVA).Atleast4jetsand6ET signi cance>3arerequired.653,727eventsqgareselectedintheUUdata,withapredictionof109.97.3t'#btDzevents,UUforaS:B1:6,000.IDzcutsz(sectionVB).nAtleastonegoGodzTleptonzcandidateandatleastonetightSVTztagarezrequired.oW*ealsorequired2jetswithj[ٸj<2:4andpT ε>20GeV/c.K216eventsareselectedinthedata,~9.30.6t'#bt~amongthemUUareexpGected.qS:B1:58.T*opGologicalNN(sectionVD).5VAsequenceoftwofeed-forwardNN'shavebGeentrainedandapplied.5VTheoptimalcutoofe?eventsrecordedbye>ourtriggerinthis349pb^1!datasetis17million.Themaingoalofpreselectionwastoreducethisdataset,&whileimpGosingthemostobviousandstraightforwardrequirementsthatcharacterizeUUthesignalsignature.qSuchcharacteristicfeaturesincludethefollowing:@ۍMoGderateUU6qȵET arisingfromboththeWvertexandwdecay*.fAtUUleast4jetsmustbGepresent.AtUUleast1wleptonand2bjetsarepresent.@ڍ6qȵETsigni canceK[17 ]Lisde nedasmeasureofthelikelihoGodofL6qȵET~arisingfromLphysicalsources, Iratherthan uc- tuationsHinGdetectormeasurements.*These uctuationsarepredictedfrommeasuringandparametrizingtheob8jectresolutionsinthedata. AscanbGeobservedinFig. 1,acutonthisvqariableprovestobGeane ectivewaytoreducetheUUdataset.qAcutof3wasusedforpreselection.Nowweneedtoscaletheoriginal10KeventsoftheMCsampleto349pb^1 t.Z0:95andatleastoneSVTtag(asintableIGII).ThisUUisthemainsampleusedforthemeasurement.qItcontains268events.Q26T:ABLE IAII:b-taggingandID.IntheMC,w9eusetheb-taggingcerti edparameterizationratherthanactualb-tagging,that is,Uw9eappliedtheb-taggingweight. W:ealsousedthetriggeringweightascomputedbythetriggereciencyparameterization. &iffffffffYdata 1taggingMC kF1ȴwithj@Lj<2:4andpmTJ>20GeV/chF1ȴwithj@Lj<2:4andpmTJ>20GeV/c»F1SVT3Tr7ig@LWeight,bTagPr7obgF2jetswithj@Lj<2:4andpmTJ>20GeV/cdF2jetswithj@Lj<2:4andpmTJ>20GeV/cΉffffffRîT:ABLEIV:b-taggingandIDresults.GSho9wnarethetotalacceptances(includingpreselection)andthenumbAerofevents scaledTto349pb-=1 .pAnestimateofQCDbac9kgroundisnotincluded.0)(=ffffffffYH(#Tpassed)/(total#)>3Acceptancel#TpassedscaledΉff!ndatam216/653,7279216 􍍑 tSaHWt!ƻ7+8jetsٹ524.0/614140.0480T0.0020 B9.320T0.620 WHbbjj׽!T+8bbjj ع54.5/232140.0150T0.0024 B0.012T0.002ֻWHccjj׽!T+8ccjj ع13.3/228940.0039T0.0012 B0.034T0.005JWHcjjj׽!T+8cjjj]׹8.0/216940.0025T0.0010 B0.160T0.020WHjjjj׽!T+8jjjj]׹3.3/268340.0009T0.0006 B0.860T0.100Ήffffff()The\fvetosample"-Sameselection,butinsteadofNNo>o0:95,0o0:8)~rleptonswereallowed.Thissampleisused\forthetopGologicalNNUUtraining.qItcontains21,022events.TheD\bCveto"sample-atleast1 leptonwithNN>p0:95,ԿbutNO)SVT*tags.ThissampleistobGeusedfortheUUQCDprediction.qItcontains4,642events.+C%ZC. ^QCDmoKdelingThedi erencebGetweenthetotalnumbGeroft'#bt]andW$eventsanddatahastobGeaccountedforbyQCD-initiatedevents,+wheregah.candidateisajet,mistakenlyidenti edhasa.lepton.Inordertoestimatethisbackgroundcontribution,UUthefollowingstrategywasemployed.W*estartedwiththe\bveto"sample.EThissampleisdominatedbymultijeteventsinwhichajetismisidenti edasJahadronic!Dz.R Sincethelcandidatesherearereallyjets,%wecanJsimplydividevspT ~distributionsofthecandidatesbythesamedistributionsforjetsbinbybintoparameterizethe?0:80forthe(typGe3,3Gwhichagreeswiththeobserveddata(inT*ableV)fairlywell.rOnecanalsoobserve(seeWAppGendix)WthatthepredicteddistributionsofthemaintopGologicalvqariables(sectionVD)areinfairlygoGodagreementUUwithwhatisobservedinthedata. 粍D. ^TopKologicalNNSimilarlyGtoHtheall-jetsanalysis[5],wede ne2neuralnetworksG(NN)+(theMultiLayerGPerceptron[19 ]GprogramwasUUused).qThetwoNNscontain'1.3UUtopGological(aplanarity*,sphericityandcentrality)and2energy-based(HT andp fe3荵sZ)vqariables.'2.theUUoutputofthe rstNN,WandtmasslikelihoGods,UUpT anddecaylengthsigni canceofthebjets.lTheUUkinematicandtopGologicalvqariablesusedare:HT -UUthescalarsumofalljet'spT(andwleptoncandidates).SphericityأandآAplanarity[5]آ-thesevqariablesareformedfromtheeigenvqaluesofthenormalizedMomentum T*ensorpofothejetsintheevent./ThesearepexpGectedtobGehigherinthetoppaireventsthanpinatypicalQCDevent.Centrality*,UUde nedasHmT7fe WQHmEha,whereHE ²issumofenergiesofthejets.)ˍT*op$and$W$masslikelihoGod-a$^2|s-likevqariable.aZL`텍 /M3j#mt /ŭfemʟ t*,`0ݱ26ֲ+T`텍 kM2j#MmW kŭfe$gx ̴mW.`4ݱ29m,-wheremtV;MW ;t;W 20GeV/cQandj[ٸj<2:5.pPlotsofsomeofthesevqariables,RwhichalsoservesasanUUadditionalcheckUUoftheagreementbGetweenthedataandprediction,areincludedintheappGendix.The5resultofapplyingthese5NN5todataisshowninFig.g<4.The5maximumsignal5signi canceisusedtodeterminetheoptimalNN{cut.8oThesignalsigni canceisde nedasENmS7fe$Ti捍pɟW  noNmS+NmB*R\,whereNS bandNB yaretheexpGectednumbersof bgsignalandbackgroundeventsrespGectivelyandisshowninFig.=5.Thissigni cancereachesitsmaximumatNN3>0:9forΐbGothtype2and3.DTherefore,thisselectionisusedfortheΏcrosssectionmeasurement.TheresultsaresummarizedinUUT*ableVI.|[28*:t (a) TÎypƹ0:9cut. (tVۍ %tW)isthetotalsignalacceptance. Uncertain9tiesarestatistical only:.,xMخffffffffn ChannelN N-=obs{ =BUzi cmex9ŖLdt;pb-=1B{Bac9kgroundsQ)&h #ff^؉"(tVۍ %tW)T(%)sT(7pb)6s8+bΉffY 1+jetsTt9ypAe2VV5xK0.1~349.3RWک!ƻ0.60T0.03Yh1.57T0.01A3.83+0:468Q0:516.84+0:468Q0:51 Yfak9es2.41T0.09+jetsTt9ypAe3VV5xK0.1~349.3RWک!ƻ0.27T0.01Yh0.73T0.01A1.80+0:228Q0:234.39+0:228Q0:23Yfak9es2.33T0.09Ήffffff ˠ29 񍟜 }ӂT:ABLETVIAI:Systematicuncertain9tiesonR(tVۍ %tW)(inpb).6&kffffffffY-ChannelPr+jetsXtÎypfromQCD.=Thesingleandtripleprongedleptonchannelswere>treatedasindepGendentchannels>andthencombined.TheUUcrosssectionwasmeasuredtobGen@[ٲ(t7ferYɍtr)=5:05䍑0+4:3103:46 1(stat)䍑0+0:6800:67(syst;)80:330(lumiq)pb:Thisresultisinagreementwiththetheoreticallypredictedvqalueof6.80.4pb[4]aswellasotherDandCDF measurementsUU[5,16 UW]. Ǡ210j)@AcCknowledgmentsW*evthankvthesta satF*ermilabandcollabGoratinginstitutions,~andacknowledgevsuppGortfromtheDOEvandNSF (USA);1CEAandCNRS/IN2P3(F*rance);FASI,RosatomandRFBR(Russia);CAPES,0CNPq,hFAPERJ,FAPESPand/FUNDUNESP(Brazil); cmmi10 0ercmmi7O \cmmi5K`y cmr10ٓRcmr7Zcmr5u cmex10