Search for Single Top tW Associated Production in the Dilepton Channel at CMS

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by  Jochen Ott on behalf of the CMS Collaboration

Search for Single Top tW Associated Production in the Dilepton Channel at
CMS
JochenOttonbehalfoftheCMSCollaboration
KarlsruheInstituteofTechnology
Abstract. We present a first study of the single top quark W-associated production (tW) in proton-proton
collisionsattheLHCatacentre-of-massenergyof7TeV,usingdatacollectedwiththeCMSexperiment.The
2 searchisperformedinthedileptonicfinalstatesee/eµ/µµwithaselectionbasedonkinematicalpropertiesand
1 b-tagginginformation.ThecontributionoftheZ+jetsprocessestothebackgroundisestimatedfromasideband
0 indata.Twot¯tdominatedcontrolregionsareusedtoconstrainthenormalizationoftopquarkpairproductionin
2
thesignalregion.Anexcessofeventsovertheexpectedbackgroundisobserved.Assigningthisexcesstoevents

n fromtWproduction,theextractedtWcrosssectionisinagreementwiththeStandardModelexpectation.
a
J

4 1 Single Top tW Production ThisanalysisconsiderstWdileptoneventsinwhichthe
2 final state W boson and the W boson from the top quark
Single top quark associated production (tW) is character- decay both decay into a charged lepton (e/µ) and a neu-
]
x izedbyafinalstatewithatopquarkandaWboson.Acon- trino.Theeventsignatureconsistsof2oppositelycharged
e tributingleading-orderFeynmandiagramisshowninFig. leptons,missingtransverseenergyfromtheneutrinos,and
- 1 (a). At next-to-leading order, real contributions with an oneb-jetfromthetopquarkdecay.
p
additional bottom quark in the final state lead to ambigu-
e
h itiesintheconceptualdistinctionbetweenthetWprocess Events are selected online by a corresponding dilep-
[ andtopquarkpairproduction,asshowninFig.1(b).Two ton trigger (ee/eµ/µµ). In the offline analysis, events are
possibilitiestoresolvethisambiguity[1]havebeenconsid- required to have exactly two isolated, oppositely charged
1 ered:inthediagramremovalmethod,alldiagramswhich leptons with pT > 20 GeV and |η| < 2.4 (2.5) for muons
v can also arise in top quark pair production are removed (electrons).ToreducethenumberofselectedZ+jetevents
7
for the calculation. This is used as default scheme for the in the ee and µµ channels, events with invariant dilepton
9
9 simulationusedinthisanalysis.Thesecondpossibilityis massm(cid:96)(cid:96)between81and101GeVarerejected.Tofurther
4 the diagram subtraction method which locally subtracts reduceeventswithnopromptneutrinos(Z+jets,QCD),a
. resonant contributions. The difference between these two cutonmissingtransverseenergy,Emiss >30GeVisplaced
1 T
methodsisconsideredasasystematicuncertainty. intheeeandµµchannels.
0
2 ThepredictedStandardModelcrosssectionevaluated
As tW signal has a b-quark in its final state, signal
1 atapproximateNNLOis15.6±1.2pb[2].
eventsareexpectedtohaveexactlyoneb-taggedjetwhile
:
v forthedominatingbackgroundprocess,topquarkpairpro-
i duction,twob-taggedjetsareexpected.Tosimultaneously
X 2 Event Selection
extractsignalandconstrainthetopquarkpairbackground,
r
a Thepresentanalysisusesadatasetofproton-protoncolli- three event categories are defined based on jet and b-tag
√
sions at s = 7 TeV which corresponds to an integrated multiplicity:the“1jet1tag”categorycontainseventswith
exactlyonejetwith p > 30GeVand|η| < 2.4whichhas
luminosityof2.1fb−1,recordedwiththeCMSdetector[3]. T
been identified by a b-tagging algorithm based on the re-
constructionofasecondaryvertex[7].Similary,the“2jet
1tag” and “2jet 2tag” categories contain events with ex-
actly 2 jets and where either one or both of them are b-
tagged.
MostofthetWsignalisinthe1jet1tagcategory.Inthis
selection, the dominating background are top quark pair
eventsinthedileptonchannelinwhichoneofthetwoex-
pectedjetsisoutsidethekinematicacceptance.Thisleads
toamomentumimbalanceinthevariablePsystem whichis
(a) (b) T
definedasthethevector-sumoftransversemomentaofthe
Fig. 1. (a) Leading order Feynman diagram for tW production. leptons, missing transverse energy, and the jet: while for
(b) Next-to-leading order Feynman diagram for tW production tW,thisisthetransversemomentumofthecompletefinal
whichalsoappearsasaleadingorderdiagramfortopquarkpair statewhichisexpectedtobesmall,itisexpectedtohave
production(andpartialdecay). largervaluesfort¯tduetothemissingjet.ThePsystem vari-
T
EPJWebofConferences
TodeterminetheremainingnumberofDrell-Yanback-
250 CMS Preliminary, s = 7 TeV data groundevents,atechniquebasedonadatasidebandisused
2.1 fb-1, ee/em /m m tW
tt whichusesthenumberofeventsintheZbosonmasspeak,
200 Z/g *+jets Nobs ,definedasthenumberofeventsinthem cutregion
Other (cid:96)(cid:96),in (cid:96)(cid:96)
-1b from 81 to 101 GeV. From this number, the non-resonant
1 f150 contributionfromprocesseswithtwofinalstateWbosons
2.
s / (such as t¯t, tW, WW), is subtracted which is estimated in
vent100 theeµchannel.Finally,thepredictedratioofeventsinside
e
andoutsidethevetoregionisusedtoestimatetheremain-
ingnumberofDrell-Yaneventspassingtheselection:
50
NMC 1
NDY = (cid:96)(cid:96),out ·(Nobs − kN )
0 100 200 300 400 500 600 est Nobs (cid:96)(cid:96),in 2 eµ
H [GeV] (cid:96)(cid:96),in
T
wherek correctsfordifferencesine/µreconstructioneffi-
(a)
ciencyandthefactor 1 accountsfortheratioofbranching
2
ratiosofsame-flavortodifferent-flavorleptonchannelsin
250 CMS Preliminary, s = 7 TeV data dileptontWandtopquarkpairevents,BR((cid:96)(cid:96))/BR(eµ).
2.1 fb-1, ee/em /m m tW
tt
Z/g *+jets
200
Other 4 Systematic Uncertainties
-1b
1 f150 Anumberofdifferentsourcesofuncertaintyaffecttheex-
2.
s / pectedbackgroundandsignalyield.Theconsidereduncer-
nt
ve100 taintiesare
e
– Pileupmodeling:additionalproton-protoninteractions
50 inthesamebunchcrossing(pileup)shiftleptonisola-
tionandjetenergies.Apossiblyimperfectmodelingof
pileupintroducesthisuncertaintywhichturnsouttobe
0 20 40 60 80 100 120 140 160 180 200
below1%forallyields.
P system [GeV]
T – Trigger efficiency: the efficiency of the online selec-
(b)
tionisknownto1.5%.
– Leptonreconstructionandidentificationefficiency:
Fig.2.Variablesusedintheeventselectionforeventswithone
theefficienciesareknownto1%(2%)formuons(elec-
b-taggedjet:(a)Psystem,definedasthetransversemomentumsum
T trons).
ofjets,leptonsandmissingtransverseenergyforeventswithex-
– Emissmodeling: to account for the uncertainty of ca-
actly. (b) HT, defined as the scalar sum of the leptons, jets and T
lorimeter response of energy not included in jets and
missingtransverseenergy.
leptons,thisenergyissmearedby10%whichchanges
thesignalacceptanceby1–2%.
ableisshowninFig.2(a).Eventswith Psystem < 60GeV – Jet energy scale and resolution: the absolute jet en-
T ergycalibrationisknowntoabout2–3%[5],thejeten-
areselectedforfurtheranalysis.
In the eµ channel, where no invariant mass and Emiss ergyresolutiontoabout10%[6].Varyingjetenergies
T according to these uncertainties yields to acceptance
requirements are applied, H (Fig. 2 (b)), defined as the
T differencesofabout1–2%fortWsignaland4–6%for
scalarsumofthetransversemomentaoftheleptons,jets,
andEmiss,isrequiredtobelargerthan160GeV. t¯tbackground.
T – Background normalization: the estimate for Drell-
After this selection, the dominating background pro-
Yan background events is assigned an uncertainty of
cess is top quark pair production. In the same-flavor lep-
50%, for other backgrounds, the cross section uncer-
tonchannelseeandµµ,thereisanon-negligiblecontribu-
taintiesfromtheoryareused.
tion from Drell-Yan events. Other background processes
include W+jets with a fake lepton and diboson processes – tW and t¯t modeling: a number of parameters in the
(WW/WZ/ZZ). simulation, such as the factorization and renormaliza-
tion scale, matrix-element / parton shower matching
parameters,partondistributionfunctions,diagramsub-
tractionandremovalmethods,havebeenvariedinthe
3 Background Estimation simulationoftWsignalandt¯t.
– Luminosity:thecalibrationoftheabsoluteintegrated
luminosityis4.5%.
The background contributions from W+jets and diboson
– B-tagging:theefficiencyoftheb-taggingalgorithmto
processes are expected to be very small and taken from
correctlytagab-jetisknowntoabout10%.[7]
simulation.
Toestimatethebackgroundcontributionfromtopquark
pair production, simulated events are used, scaled to the 5 Statistical Analysis
approximateNNLOcrosssectioncalculationfrom[4].The
topquarkpaircrosssectionisallowedtovaryinthestatis- To extract the cross section and significance, a Poisson
ticalevaluationwithinthesystematicuncertainties. countingmodelisusedin9channels:3leptonfinalstates
HadronColliderPhysicsSymposium2011
(ee/eµ/µµ), each with 3 jet / b-tag multiplicities shown in
700
Fig. 3. The likelihood function is the product of Poisson C2.M1 Sfb P-1,r eelmim cihnaanrny,e l s = 7 TeV dtWata
probabilitiesoverall9channels: 600 tt
Z/g*+jets
500 Other
L(p|n)=(cid:89)9 µniieµi -11 fb400
i=1 ni! s / 2.
nt300
e
v
where n is the number of observed events in channel i e
i
200
and µ is the predicted event yield which depends on the
i
modelparametersp.Systematicuncertaintiesareincluded 100
asnuisanceparametersaslog-normaluncertaintieswhich
change the predicted yield µi correlated across all chan- 1 jet 1 tag 2 jet 1 tag 2 jet 2 tag
nels.
Thecrosssectionisextractedusingaprofilelikelihood (a)
technique.Thesignificanceisevaluatedusingthedistribu-
tionofaprofilelikelihoodratioteststatisticfortoyexperi-
mentsincludingnosignal.Fortoygeneration,thenuisance 350 C2.M1 Sfb P-1,r memlim chinaanrny,e l s = 7 TeV dtWata
parametersaredrawnrandomlyaccordingtotheirpriors. tt
300 Z/g*+jets
Other
-1b250
6 Result 2.1 f200
s /
nt
e150
TheextractedsingletoptWcrosssectionis ev
100
σ =22+9pb
tW −7 50
which is consistent with the Standard Model prediction.
1 jet 1 tag 2 jet 1 tag 2 jet 2 tag
The uncertainty includes the statistical and all systematic
uncertaintiesdiscussedinSection4.
(b)
The probability that the observed excess of events is
merelyanupwardfluctuationofbackgroundwithavanish-
ingtWcrosssectioncorrespondsto2.7σ,theexpectedsig- 200 CMS Preliminary, s = 7 TeV data
nificancefortheStandardModelcrosssectionis1.8±0.9σ. 2.1 fb-1, ee channel tW
180 tt
Moredetailsaboutthisanalysiscanbefoundin[8]. Z/g*+jets
160
Other
-1b140
2.1 f120
References s / 100
nt
ve 80
e
1. Frixione, Laenen, Motylinski et al., JHEP 07 (2008) 60
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2. Kidonakis,Phys.Rev.D82(2010)054018
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1 jet 1 tag 2 jet 1 tag 2 jet 2 tag
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5. The CMS Collaboration, Physics Analysis Summary
(c)
JME-10-010(2010)
6. The CMS Collaboration, Physics Analysis Summary
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tt
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TOP-11-022(2011) -11 fb800
2.
s / 600
nt
e
v
e
400
200
1 jet 1 tag 2 jet 1 tag 2 jet 2 tag

(d)
Fig.3.Obervedandpredictednumberofeventsinallleptonand
jet/b-taggingcategories.Thepredictionisevaluatedatthemax-
imumofthelikelihoodfunction.