Measurement of the ratio B(t→wb)/b(t→wq) In pp collisions at S=8 Tev

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DOI: 10.1016/j.physletb.2014.06.076

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©2014

by Elsevier. All rights reserved.

DIRETORIA DE TRATAMENTO DA INFORMAÇÃO

Cidade Universitária Zeferino Vaz Barão Geraldo

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http://www.repositorio.unicamp.br

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

the

ratio

B

(

t

→

Wb

)/

B

(

t

→

Wq

)

in

pp

collisions

at

√

s

=

8 TeV

.

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received8April2014

Receivedinrevisedform5June2014 Accepted29June2014

Availableonline3July2014 Editor: M.Doser Keywords: CMS Top CKM Width

The ratio of the top-quark branching fractions _R=B(t_→Wb)/B(t_→Wq),where the denominator includesthesumoveralldown-typequarks(q=b,s,d),ismeasuredinthet¯t dileptonfinalstatewith proton–protoncollisiondataat√s=8 TeV fromanintegratedluminosityof19.7 fb−1_{,collectedwiththe}

CMSdetector.Inordertoquantifythepurityofthesignalsample,thecrosssectionismeasuredbyfitting theobservedjetmultiplicity,therebyconstrainingthesignalandbackgroundcontributions.Bycounting thenumber ofbjetsperevent,anunconstrained valueofR=1.014±0.003(stat.)±0.032(syst.) is measured,inagoodagreementwithcurrentprecisionmeasurementsinelectroweakandflavoursectors. AlowerlimitR>0.955 atthe95%confidencelevelisobtainedafterrequiringR≤1,andalowerlimit onthe Cabibbo–Kobayashi–Maskawamatrix element_|Vtb|>0.975 isset at95% confidencelevel. The

resultiscombinedwithapreviousCMSmeasurementofthet-channelsingle-top-quarkcrosssectionto determinethetop-quarktotaldecaywidth,Γt=1.36±0.02(stat.)+₋00..1411 (syst.) GeV.

©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP3.

1. Introduction

Because of its large mass [1], the top quark decays before fragmentingor forminga hadronic bound state [2].According to thestandard model (SM), thetop quark decaysthrough an elec-troweak interaction almost exclusively to an on-shell W boson andab quark. Themagnitudeofthetop–bottom chargedcurrent is proportional to

|

Vtb

|

, an element of the Cabibbo–Kobayashi–

Maskawa(CKM)matrix.Undertheassumptionthatthe CKM ma-trixisunitaryandgiventhemeasuredvaluesforVubandVcb(or

Vts and Vtd),

|

Vtb

|

isexpectedtobe closetounityanddominate

over the off-diagonal elements, i.e.

|

Vtb

|

 |

Vts

|,

|

Vtd

|

. Thus, the

decaymodesofthetopquarktolighterdown-typequarks(d or s) are allowed, but highly suppressed. The indirect measurement of

|

Vtb

|

,fromtheunitarityconstraintoftheCKMmatrix,is

|

Vtb

|

=

0

.

999146+₋0_0..₀₀₀₀₄₆000021 [3].Any deviationfromthisvalueorinthe par-tialdecaywidthofthetopquarktob quarks,wouldindicatenew physics contributions such as those from new heavy up- and/or down-typequarksora chargedHiggs boson,amongst others [4]. DirectsearchesattheLargeHadronCollider(LHC)havesetlower limitsonthemassofthesehypotheticalnewparticles[5–15],and the observation of a SM Higgs boson candidate [16–18] places stringentconstraintson theexistence ofa fourth sequential gen-eration of quarks. These results support the validity of both the

 _{E-mailaddress:cms-publication-committee-chair@cern.ch.}

unitarityhypothesisandthe3

×

3 structureoftheCKMmatrixfor theenergy scaleprobed bythe LHCexperiments.However, other newphysicscontributions,includingthosedescribedabove,could invalidatetheboundsestablishedsofaron

|

Vtb

|

[3].

In this Letter, we present a measurement of

R

=

B(

t

→

Wb

)/B(

t

→

Wq

)

, wherethe denominator includes the sum over the branching fractions of the top quark to a W boson and a down-type quark (q

=

b

,

s

,

d). Under the assumption of the uni-tarity of the 3

×

3 CKM matrix,

R

= |

Vtb

|

2, and thus to

indi-rectly measure

|

Vtb

|

. In addition, the combination of a

determi-nation of

R

andameasurementofthet-channelsingle-topcross section can provide an indirect measurement of the top-quark width (

Γ

t) [19]. The most recent measurement of

Γ

t based on

this approach [20] is found to be compatible with the SM pre-dictions with a relative uncertainty of approximately 22%. The value of

R

has been measured at the Tevatron, and the most precise result is obtained by the D0 Collaboration, where

R

=

0

.

90

±

0

.

04

(

stat.

+

syst.

)

[21] indicates a tension with the SM prediction.Thistensionisenhancedforthemeasurementinthet

¯

t dilepton decaychannel, whereboth W bosonsdecayleptonically and

R

=

0

.

86₋+₀0._.041₀₄₂(stat.)

±

0

.

035 (syst.) isobtained.Themost re-centmeasurementsbytheCDFCollaborationaregivenin[22,23].

Owing toits purity, the t

¯

t dilepton channel is chosen forthis measurement.Events are selectedfromthe datasample acquired in proton–protoncollisions at

√

s

=

8 TeV bythe Compact Muon Solenoid(CMS)experimentattheLHCduring2012.Theintegrated luminosity of the analysed data sample is 19

.

7

±

0

.

5 fb−1 [24]. http://dx.doi.org/10.1016/j.physletb.2014.06.076

0370-2693/©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/3.0/).Fundedby SCOAP3_.

The selected eventsare used to measure the t

¯

t production cross section by fitting the observed jet multiplicity distribution, con-straining the signal and background contributions. The b-quark contentoftheeventsisinferredfromthedistributionofthe num-ber of b-tagged jets per event as a function of jet multiplicity foreach of thedilepton channels. Data-based strategiesare used toconstrainthe mainbackgroundsandthecontributionsofextra jetsfrom gluon radiation in t

¯

t events. The

R

value is measured by fittingthe observedb-tagged jet distributionwith a paramet-ric modelthat dependson theobserved cross section,correcting forthe fractionofjets that cannot be matchedto a t

→

Wq de-cay.Themodelalsodependsontheefficiencyforidentifyingb jets anddiscriminatingthem fromother jets.Lastly,themeasurement of

R

iscombined witha previously published CMSresultofthe

t-channelproductioncrosssectionofsingletopquarksinpp colli-sions[25]toyieldanindirectdeterminationofthetop-quarktotal decaywidth.

2. TheCMSdetector

The central feature ofthe Compact Muon Solenoid (CMS) ap-paratus is a superconducting solenoid of 6 m internal diame-ter, providing a magnetic field of 3.8 T. Within the supercon-ducting solenoid volume are a silicon pixel and strip tracker, a leadtungstatecrystalelectromagneticcalorimeter(ECAL), anda brass/scintillator hadron calorimeter(HCAL), each composed of a barrelandtwo endcapsections.Muonsare measured in gas-ion-isation detectors embedded in the steel flux-return yoke outside thesolenoid. Extensiveforwardcalorimetrycomplementsthe cov-erageprovidedbythebarrelandendcapdetectors.

Thesilicontrackermeasureschargedparticleswithinthe pseu-dorapidityrange

|

η

|

<

2

.

5,wherethepseudorapidity

η

isdefined as

η

= −

ln

[

tan

(θ/

2

)

]

and

θ

isthepolarangleofthetrajectory of theparticlewithrespecttotheanticlockwise-beamdirection.The trackerconsistsof 1440siliconpixel and15 148 siliconstrip de-tectormodules andislocated inthefield ofthesuperconducting solenoid. It provides an impact parameter resolution of

∼

15 μm and a transverse momentum (pT) resolution of about 1.5% for

100 GeV particles. The electron energyis measured by the ECAL anditsdirectionis measuredby thetracker.The massresolution forZ

→

ee decaysis1.6%whenbothelectronsareintheECAL bar-rel, and2.6% when both electrons are in the ECAL endcap [26]. Matchingmuonsto tracksmeasured inthesilicontrackerresults ina pT resolutionbetween1and10%,for pT valuesupto 1 TeV.

Thejetenergyresolution(JER)amountstypicallyto15%at10 GeV, 8%at100 GeV,and4%at1 TeV[27].

A more detailed description of the detector can be found in Ref.[28].

3. Simulationofsignalandbackgroundevents

The top-quark pair production cross section has been calcu-latedatnext-to-next-to-leadingorder(NNLO)and next-to-next-to-leadinglogarithmicsoftgluonterms(NNLL)[29].Inproton–proton collisions at

√

s

=

8 TeV,andforatop-quarkmass of172.5 GeV, the expected cross section is

σ

NNLO+NNLL

(

t

¯

t

)

=

253+₋6₈ (scale)

±

6 (PDF) pb, where the first uncertainty is from the factorisation and renormalisation scales, and the second is from the parton distribution functions (PDFs). Signal events are simulated for a top-quarkmass of172.5 GeV with theleading-order (LO) Monte Carlo(MC)generator MadGraph (v5.1.3.30)[30]matchedto pythia (v6.426)[31],where the

τ

lepton decays are simulatedwiththe tauola package (v27.121.5) [32]. The CTEQ6L1 PDF set is used in the event generation [33]. Matrix elements describing up to three partons, andincluding b quarks, in addition to the t

¯

t pair

are includedin thegenerator usedto producethesimulated sig-nal samples. An alternative simulation at next-to-leading order (NLO) basedon powheg (v1.0,r1380)[34–36],usingtheCTEQ6M PDF set [33] andinterfaced with pythia, is used to evaluate the signal description uncertainty. Acorrection to thesimulated top-quark pT is applied, based on the approximate NNLO

computa-tion [37]: the events are reweighted at the generator level to matchthetop-quarkpTprediction,andthefulldifferencebetween

the reweighted andunweighted simulationsis assignedasa sys-tematicuncertainty.

Themostrelevantbackgroundprocessesforthedilepton chan-nel arefromtheproductionoftwo genuineisolated leptonswith large pT.This includesDrell–Yan(DY) productionofcharged

lep-tons, i.e.froma Z

/

γ

∗ decay,which is modelledwith MadGraph fordileptoninvariant massesabove 10 GeV,andisnormalised to aNNLOcrosssectionof4.393 nb,computedusing fewz[38].The Z

+

γ

processisalsosimulatedwith MadGraph andnormalised to the LOpredictedcrosssection of123.9 pb. Single-top-quark pro-cesses aremodelledatNLO with powheg[39,40]andnormalised tocrosssectionsof22

±

2 pb, 86

±

3 pb,and5

.

6

±

0

.

2 pb forthe tW,t-,ands-channelproduction,respectively[37].Thetheory un-certainties are dueto the variation ofthe PDFs andfactorisation and renormalisationscales. Diboson processes are modelledwith MadGraph and normalised to the NLO cross section computed with mcfm [41].ThegenerationofWW,WZ,andZZ pairsis nor-malisedtoinclusivecrosssectionsof54.8 pb,33.2 pb,and17.7 pb, respectively. For WZ and ZZ pairs a minimum dilepton invari-ant massof12 GeVisrequired.AssociatedproductionofW orZ bosonswitht

¯

t pairsismodelledwith MadGraph,andnormalised to the LO cross sections of 232 fb and208 fb, respectively. The productionofa W bosonin associationwithjets, whichincludes misreconstructedandnon-promptleptons,ismodelledwith Mad-Graphandnormalisedtoatotalcrosssectionof36.3 nbcomputed with fewz. Multijet processes are also studied in simulation but arefoundtoyieldnegligiblecontributionstotheselectedsample.

A detector simulation based on Geant4 (v.9.4p03) [42,43] is applied after the generator step for both signal and background samples.Thepresenceofmultipleinteractions(pileup)per bunch crossingisincorporatedbysimulatingadditionalinteractions(both in-time and out-of-time with the collision) with a multiplicity matchingthatobservedinthedata.Theaveragenumberofpileup eventsinthedatais21interactionsperbunchcrossing.

4. Eventselectionandbackgrounddetermination

The event selection is optimised for t

¯

t dilepton final states that contain two isolated oppositely charged leptons

(electrons or muons), missingtransverse energy (Emiss_T ) definedbelow, and at least two jets. Events in which the electrons or muons are fromintermediate

τ

leptondecaysareconsideredassignalevents. Dilepton triggers are used to acquire the data samples, where a minimumtransverse momentumof8 GeVisrequiredforeachof theleptons,and17 GeVisrequiredforatleastoneoftheleptons. Electron-based triggers include additional isolation requirements, bothinthetrackerandcalorimeterdetectors.

Allobjectsintheeventsarereconstructedwithaparticle-flow (PF) algorithm [44,45]. Reconstructed electron and muon candi-dates are requiredtohave pT

>

20 GeV andto bein thefiducial

region

|

η

|

≤

2

.

4 of the detector. A particle-based relative isola-tion parameter iscomputed foreach lepton andcorrected onan event-by-event basis forthecontribution frompileup events.We require that the scalar sum of the pT of all particle candidates

reconstructed in an isolation cone built around the lepton’s mo-mentum vector is less than 15% (12%) of the electron (muon) transverse momentum. The isolation cone is defined using the

Table 1

Predictedandobservedeventyieldsafterthefulleventselection.Thecombinationofstatisticaluncertainties withexperimentalandtheoretical systematicuncertaintiesisreported.Non-dileptonic t¯t channels,identified usingagenerator-levelmatching,aswellasassociatedproductionwithvectorbosons(W orZ),isdesignated as“other tt”¯ andgroupedwiththeexpectedcontributionfromsingleW bosonandmultijetsproductions.The expectedcontributionfromvectorbosonpairprocessesisdesignatedas“VV”.

Source ee μμ eμ

W→ ν, multijets, other t¯t 134±91 43±10 (38±20)×10

VV 292±15 333±16 995±39

Z/γ∗→ (297±63)×10 (374±79)×10 (184±39)×10 Single top quark 526±26 583±26 1834±64 t¯t dileptons (signal) (1003±50)×10 (1104±54)×10 (349±17)×102

Total (1395±81)×10 (1574±96)×10 (400±17)×102

Data 13 723 15 596 38 892

radius R

=



(

η

)

2

_{+ (φ)}

2

₌

₀

_.

_4,where



_η

_and

_φ

_arethe

dif-ferencesinpseudorapidityandazimuthalanglebetweenthe parti-clecandidate andthe lepton. Foreach eventwe require atleast two lepton candidates originating from a single primary vertex. Among the vertices identified in the event, the vertex with the largest



p2_T,wherethesumrunsoverall tracksassociatedwith thevertex,ischosenastheprimaryvertex. Thetwoleptons with thehighest pT arechosen toformthedileptonpair.Same-flavour

dileptonpairs(eeor

μμ

)compatiblewithZ

→

decaysare re-movedbyrequiring

|

MZ

−

M

|

>

15 GeV,whereMZistheZ boson

mass[3]andM istheinvariantmassofthedileptonsystem.For all dileptonchannels it isfurther required that M

>

12 GeV in ordertovetolow-massdilepton resonances,andthatthe leptons haveoppositeelectriccharge.

Jetsare reconstructed by clustering all the PF candidates us-ingtheanti-kTalgorithm[46]withadistanceparameterof0.5.Jet

momentumisdefinedasthevectorsumofallparticlemomentain thejet,andinthesimulationitisfoundtobewithin 5to10%of thehadron-levelmomentumovertheentire pT spectrumand

de-tectoracceptance.Acorrection isapplied bysubtractingtheextra energyclusteredinjetsduetopileup,followingtheprocedure de-scribedinRefs.[47,48].Theenergiesofcharged-particlecandidates associatedwithother reconstructedprimary verticesinthe event are alsosubtracted. Jet energyscale (JES)corrections are derived fromsimulation, andare validated within-situ measurements of theenergybalanceofdijetandphoton

+

jetevents[27].Additional selectioncriteriaareappliedtoeventstoremove spuriousjet-like features originatingfrom isolated noise patterns in certain HCAL regions.In theselection oft

¯

t events,atleasttwo jets, each with acorrectedtransversemomentum pT

>

30 GeV and

|

η

|

≤

2

.

4,are

required.The jetsmustbe separatedfromtheselectedleptons by



R

( ,

jet

)

=



(

η

)

2

_{+ (φ)}

2

_≥

₀

_.

_{3. Events with up to four jets,}

selectedunderthesecriteria,areused.

Themagnitudeofthevectorsumofthetransversemomentaof allparticlesreconstructedintheeventisusedastheestimatorfor the momentum imbalance inthe transverse plane, Emiss_T .All JES correctionsappliedtotheeventarealsopropagatedintothe Emiss_T

estimate.Fortheeeand

μμ

channels, Emiss_T

>

40 GeV isrequired inordertoreduce thecontamination fromlepton pairsproduced throughtheDYmechanisminassociationwithatleasttwojets.

The DY contribution to the same-flavour dilepton channels is estimatedfromthedataafterthefulleventselectionthroughthe modellingoftheangle

Θ

betweenthetwoleptons.The

Θ

dis-tributiondiscriminatesbetweenleptonsproducedinDYprocesses and leptons from the top-quark pair decay cascade. In the first caseanangular correlationis expected, whileinthe secondcase the leptons are nearly uncorrelated. The probability distribution function for

Θ

is derived from data using a DY-enriched con-trol region selected after inverting the Emiss

T requirement of the

standard selection. Studies of simulated events indicate that the shapeofthe

Θ

distributioniswell describedwiththismethod, and that the contamination from other processes in the control region can be neglected. Compatibility testsperformed in simu-lations usingdifferent channelsandjet multiplicities are usedto estimate an intrinsic10%uncertainty inthefinal DY background. Theother sourcesofuncertaintyinthemethodarerelatedtothe simulation-based description ofthe probability distribution func-tion for the

Θ

distribution fromother processes. Uncertainties areestimatedeitherbypropagatingtheuncertaintiesinpileupor JESandJER,orbytrying alternativefunctionsforthet

¯

t contribu-tionwithvariedfactorisation/renormalisationscales(

μ

R

/

μ

F)with

respect to their nominal values given by the momentum trans-ferintheevent,matrixelement/partonshower(ME-PS)matching threshold,orgeneratorchoice(powheg vs. MadGraph).Theshapes ofkinematicdistributionsforDYandotherprocessesareusedina maximum-likelihoodfittoestimatetheamountofDYbackground in the selected sample. A total uncertainty of 21% is estimated fromthedataintherateofDYeventsforthesame-flavour chan-nels.

Forthee

μ

channel,a similarfit procedureisadoptedusinga differentvariable:thetransversemassMT

=



2Emiss

T pT

(

1

−

cos

φ)

of each lepton, where

φ

is the difference in azimuthal an-gle between the lepton and the missing transverse momentum. The distribution of the sum



MT is used as the distribution

in the fit. In this case the probability distribution function for Z

/

γ

∗

→

τ τ

→

e

μ

is derived fromsimulation. The determination of the uncertainty associatedwith thismethod follows a similar prescription to that described above for the same-flavour chan-nels. Atotal uncertainty of21% isassigned tothe amount ofDY contaminationinthee

μ

channel.

Thesecond-largestbackgroundcontributionisfrom single-top-quark processes (in particular the tW channel) that is relevant for this measurement since the decay products of a single top quark(insteadofapair)areselected.Thecontributionofthis pro-cessisestimatedfromsimulation.Otherbackgroundprocessesare alsoestimatedfromsimulation.Uncertaintiesinthenormalisation stemmingfrominstrumentaluncertainties intheintegrated lumi-nosity,triggerandselectionefficiencies,andenergyscales,aswell asgenerator-specificuncertainties,aretakenintoaccount.

Table 1 shows the yields in the data andthose predicted for signal and background events after the full event selection. The systematicuncertainties assignedtothe predictionsofsignal and background events include the uncertainties in the JES and JER, pileupmodelling,crosssectioncalculations,integratedluminosity, and trigger and selection efficiencies. A conservative uncertainty isassignedto thepredictedyields ofmultijetandW

→

ν

back-groundeventssincethesecontributionsarefrommisidentified lep-tonsandhavebeenestimatedsolelyfromsimulation.Goodoverall

agreement is observed for all three dilepton categories between theyieldsindataandthesumofexpectedyields.

5. Crosssectionmeasurement

Theselectedeventsarecategorised bythedileptonchanneland thenumberofobservedjets. Fig. 1showstheexpected composi-tionforeacheventcategory.Goodagreementisobservedbetween thedistributionsfromthedataandtheexpectations,includingthe control regions, definedas events with fewer than two ormore than four jets. The chosen categorisation not only allows one to study the contamination from initial- and final-state gluon radi-ation (ISR/FSR) in the sample, but also to constrain some of the uncertaintiesfromthedata.

The t

¯

t dilepton signal strength,

μ

, definedas theratio ofthe observed to the expected signal rate, is measured from the jet multiplicitydistributionbyusingaprofilelikelihoodmethod[49]. A likelihoodis calculatedfromtheobservednumberofeventsin thek dileptonchannelsandjetmultiplicitycategoriesas

L

(μ, θ )

=



k

P



Nk

, ˆ

Nk

(μ, θ

i

)



·



i

ρ

(θ

i

),

(1)

where

P

is the Poisson probability density function, Nk is the

number of events observed in the k-th category, N

ˆ

k is the total

number of expected events from signal and background, and

θ

i

arethenuisanceparameters,distributedaccordingtoaprobability densityfunction

ρ

.The nuisance parameters are used to modify theexpectednumberofeventsaccordingtothedifferent system-aticuncertaintysources,which includeinstrumentaleffects(such asintegratedluminosity,pileup,energyscaleandresolution, lep-tontriggerandselectionefficiencies)andsignalmodelling(

μ

R

/

μ

F,

ME-PS scale, top-quark mass,leptonic branching fractions of the W boson)amongstothers.ThePDFuncertaintyisestimatedusing thePDF4LHCprescription[50,51].Theuncertaintyfromthechoice ofthet

¯

t signalgeneratorisestimatedbyassigning thedifference betweenthe MadGraph-based andthe powheg-basedpredictions asanextrauncertaintyinthefit.Thenuisanceparametersare as-sumed tobe unbiased anddistributed accordingto a log-normal function.Based onthelikelihood expressedinEq.(1),theprofile likelihoodratio(PLR)

λ

isdefinedas

λ(μ)

=

L

(μ, ˆˆ

θ )

L

(

μ, ˆ

ˆ

θ )

,

(2)

wherethedenominatorhasestimators

μ

ˆ

and

ˆθ

thatmaximisethe likelihood,andthenumeratorhasestimators

ˆˆθ

thatmaximisethe likelihoodforthe specifiedsignal strength

μ

.Thesignal strength isobtainedaftermaximising

λ(

μ

)

inEq.(2).Thisapproachallows ustoparameterisetheeffectofthesystematicuncertaintiesinthe fit.

The signal strength

μ

is determined independently in each category, i.e. for each dilepton channel and jet multiplicity. For each category, the purity of the selected sample ( ft¯t) is defined

asthe fraction of “true” t

¯

t signal events in the selected sample,

ft¯t

=

μ

·

Nt¯texp

/

Nobs, where Nt¯texp is the number of expected t

¯

t

events,and Nobs isthe totalnumberof observedevents.By

per-formingthe fitforeach category,the purityofthe sampleis ob-tained.Theresultsaresummarised inTable 2.Asexpected,thee

μ

categoryhasthehighestpurity(

≈

90%).Becauseofthe contamina-tionfromDY events,thesame-flavourchannelshavelowerpurity (

≈

70%).Overall, thesignalpurityincreaseswithhigherjet multi-plicity.

Asa cross-check,a fitincludingall categories, givesthe range 0

.

909

<

μ

<

1

.

043 atthe68%confidencelevel(CL).Thisleadstoa t

¯

t productioncrosssectionof

Fig. 1. Theupperplotsshowtheobservedjetmultiplicityafterthefullevent se-lection,exceptfortherequirementonthenumberofjets,inthesame-flavour(top) anddifferent-flavour(bottom)channels.Theexpectationsareshownasstacked his-tograms,whilethe observeddatadistributionsarerepresentedasclosedcircles. Thepredicteddistributionsforthesimulatedt¯t andsingle-top-quarkevents corre-spondtoascenariowithR=1.Thelowerpanelsshowtheratioofthedatatothe expectations.Theshadedbandsrepresentthesystematicuncertaintyinthe deter-minationofthemainbackground(DY)andtheintegratedluminosity,andvaryfrom 31%(16%)to5%(3%)inthesame- (different-)flavour channelswhengoingfromthe 0 jetsto≥5 jetsbin.

σ

(

t¯t

)

=

238

±

1 (stat.)

±

15 (syst.) pb

,

in good agreement withNNLO

+

NNLL expectation [29] and the latestCMSmeasurement [52].Theresultisalsofoundto be con-sistentwiththeindividualresultsobtainedineacheventcategory. An extrauncertaintyis assignedintheextrapolationof thecross section tothe fullphase spacebecauseofthe dependenceofthe

Fig. 2. Theupperplotshowsthenumberofb-taggedjetspereventforthedifferentt¯t dileptonchannels.Foreachfinalstate,separatesubsetsareshowncorresponding toeventswithtwo,three,orfourjets.Thesimulatedt¯t andsingle-top-quarkeventscorrespondtoascenariowithR=1.Thelowerpanelshowstheratioofthedatato theexpectations.Theshadedbandsrepresenttheuncertaintyowingtothefinitesizeofthesimulationsamples,themainbackgroundcontribution(DY),andtheintegrated luminosity.

Table 2

Fractionoft¯t events( ft¯t)andrelativecontributionfromsingle-top-quarkprocesses

(kst)for variousjet multiplicitiesanddileptonchannels,asdeterminedfromthe

profilelikelihoodfit.Thetotaluncertaintyisshown. Parameter Jet multiplicity Dilepton channel ee μμ eμ ft¯t 2 0.67±0.07 0.65±0.08 0.85±0.06 3 0.79±0.06 0.78±0.07 0.90±0.07 4 0.81±0.11 0.82±0.11 0.94±0.10 kst 2 0.062±0.004 0.063±0.004 0.062±0.003 3 0.040±0.003 0.040±0.003 0.041±0.002 4 0.036±0.004 0.036±0.006 0.029±0.003

acceptanceon

μ

R

/

μ

F,ME-PSthresholdchoices,andthetop-quark

mass.

Therelative single-top-quark contribution(kst), definedasthe

ratiooftheexpectednumberofsingle-top-quarkeventstothe es-timatednumberofinclusivet

¯

t events,isalsoshowninTable 2for eachcategory.Forthisdeterminationweusetheexpectednumber of single-top-quark events obtained after maximising the PLR in Eq.(2).Thecontributionduetosingle-top-quarkeventstendstobe mostsignificantinthetwo-jet category(

<

7%relativetoinclusive t

¯

t events).Sincetheestimateisobtainedforaspecificscenarioin which

R

=

1,anextralineardependenceofkst on

R

isintroduced

inordertoaccountfortheincreaseinthetW crosssectionas

|

Vtb

|

becomes smaller while

|

Vtd

|

and

|

Vts

|

become larger [4]. In this

parameterisation,themeasuredratio

|

Vtd

|/|

Vts

|

=

0

.

211

±

0

.

006 is

used[3],andtheuncertaintyisconsideredasanintrinsic system-aticuncertaintyinthemeasurementof

R

.

6. Probingtheb-flavourcontent

Inthissectiontheb-flavourcontentoftheselectedevents(both signalandbackground)isdetermined fromthe b-taggedjet mul-tiplicitydistribution.Theprobability ofincorrectly assigning a jet

mustbe evaluated(Section6.1)inordertocorrectlyestimate the heavy-flavourcontentoftop-quarkdecays(Section6.2).

Theb-taggingalgorithmthat isused(thecombinedsecondary vertex,CSVmethoddescribedinRef.[53])isamultivariate proce-dure inwhichboth informationonthe transverseimpact param-eter withrespect to the primary vertexof the associated tracks, andthereconstructedsecondaryverticesisusedtodiscriminateb jetsfromc,light-flavour(u,d,s),andgluonjets.Theb-tagging ef-ficiency(

ε

b)ismeasured[54]usingmultijeteventswhereamuon

is reconstructed inside a jet; a data-to-simulation scale factor is derived and is used to correct the predicted

ε

b value in the t

¯

t

dileptonsamplefromsimulation.Aftercorrection,theexpected ef-ficiencyintheselectedt

¯

t sampleis

≈

84%,andtheuncertaintyin thescalefactorfromthedatais1–3%, dependingonthe kinemat-icsofthejets[54].Thesamescalefactorisappliedtotheexpected c-taggingefficiencybutwithadoubleduncertaintywithrespectto theoneassignedtob jetsowingtothefactthatnodirect measure-mentofthec-taggingefficiencyisperformed. Forjetsoriginating from the hadronisation of light-flavourjets, the misidentification efficiency(

ε

q)isevaluated[53]fromso-callednegative tagsinjet

samples,whichareselectedusingtracksthat haveanegative im-pactparameterorsecondaryverticeswithanegativedecaylength. The scalar product of the jet direction with the vector pointing from the primary vertex to the point of closest approach of a trackwithnegative impactparameterhastheoppositesignofthe scalarproducttakenwithrespecttothepointofclosestapproach. The data-to-simulation correction factor for the misidentification efficiency is known with an uncertainty of about 11%, and the expectedmisidentificationefficiencyintheselectedsampleis ap-proximately12%[54].

Fig. 2showsthenumberofb-taggedjetsintheselected dilep-ton data sample, compared to the expectations from simulation. Themultiplicityisshownseparatelyforeachdileptonchanneland jet multiplicity.The expectedeventyields are correctedafter the PLR fitforthe signalstrength (describedin theprevious section)

and also incorporate the data-to-simulation scale factors for

ε

b

and

ε

q.Data andsimulation agreewithin 5%.The residual

differ-encescanberelatedtothedifferentnumberofjetsselectedfrom top-quarkdecays in dataand simulation,the modellingof gluon radiation(ISR/FSR) and if

R

is differentfromunity (which is an assumptionusedinthesimulation).

6.1. Jetmisassignment

Thereisa non-negligibleprobability thatatleastonejet from at

¯

t decayismissed,eitherbecauseitfallsoutsideofthedetector acceptanceoris notreconstructed, andanotherjet froma radia-tive processis chosen instead.Inthe followingdiscussion,thisis referredtoasa“misassignedjet”.Conversely,jetsthatcomefrom atop-quarkdecaywill bereferred toas“correctlyassigned”.The rateofcorrectjetassignmentsisestimatedfromthedatausinga combinationofthreedifferentcategories:

•

eventswithnojetsselectedfromtop-quarkdecays,whichalso include backgroundeventswithnotopquarks;

•

events withonly one jet froma top-quarkdecay, which in-cludes some t

¯

t events and single-top-quark events (mainly producedthroughthetW channel);

•

eventswithtwojetsproducedfromthetwotop-quarkdecays. Inorder toavoidmodel uncertainties,the numberofselected jetsfromtop-quarkdecaysisderivedfromthelepton-jet invariant-mass(M j)distribution,reconstructedbypairingeachleptonwith

all selected jets. For lepton-jet pairs originating from the same top-quark decay, the endpoint of the spectrum occurs at M j

≈



M2_t

−

M2_W

≈

153 GeV [55], where Mt (MW) is the top-quark

(W boson) mass(Fig. 3, top, open histogram).The predicted dis-tribution forcorrectpairings is obtainedaftermatching the sim-ulated reconstructed jets to the b quarks from t

→

Wb at the generator level using a cone of radius R

=

0

.

3. The same quan-titycalculatedfora leptonfroma top-quarkdecaypairedwitha jetfromthetopantiquarkdecayandviceversa(“wrong”pairing) showsadistributionwithalongtail(Fig. 3,top,filledhistogram), which can be used as a discriminating feature. A similar tail is observed for “unmatched” pairings: either background processes withouttopquarks,orleptons matchedto otherjets. The combi-nationswith M j

>

180 GeV are dominatedby incorrectly paired

jets,andthiscontrolregionisusedtonormalisethecontribution frombackground.

Inordertomodelthelepton-jet invariant-massdistributionof the misassigned jets, an empirical method is used based on the assumptionofuncorrelatedkinematics.Thevalidityofthemethod hasbeentestedusingsimulation.Foreacheventindata,the mo-mentum vectorofthe selectedlepton is“randomlyrotated” with uniform probability in the

(

cos

(θ ),

φ)

phase space, and the M j

is recomputed. This generates a combinatorial distribution that is used to describe the true distribution of M j for misassigned

jets. Fig. 3 (bottom)compares the data distributionwiththe two componentsof the M j spectrum, i.e.“correct assignments”from

simulationand“wrongassignments”modelledfromthedata.The background modelprovides a goodestimate of theshape ofthe spectrumofthemisassignedlepton-jetpairs.Afterfittingthe frac-tions ofthetwo components tothe data,the“misassigned” con-tributionissubtractedfromtheinclusivespectrum,andtheresult is compared to the expectedcontribution from the correctly as-signed lepton-jet pairs. The resultof this procedure is shown in theinsetofFig. 3(bottom).Thismethodisusedtodeterminethe fraction( fcorrect)ofselectedjetsfromtop-quarkdecaysintheM j

spectrum.Consequently,bymeasuring fcorrect,weestimatedirectly

Fig. 3. Thetopplot showsthecorrectandmisassignedlepton-jetinvariant-mass spectrainsimulatedt¯t dileptonevents.Bothdistributionsarenormalisedtounity. Theendpointofthespectrumforcorrectlyassignedpairsisshownbythedashed line.Inthebottomplottheobserveddataiscomparedwiththecorrect(from sim-ulation)andmisassigned(fromthedata)componentsforthelepton-jet invariant-massspectraineμeventswithexactlytwojets.Thelepton-jetmassdistributionis shownintheinset,afterthemisassignedpairsaresubtracted.

Table 3

Fractionoflepton-jetpairscorrectlyassignedintheselectedeventsestimatedfrom the dataandpredictedfromsimulation.Thelastcolumnshowsthe ratioofthe fractionmeasuredindatatothepredictionfromsimulation.Thetotaluncertainty isshown.

Dilepton channel # jets fdata

correct fcorrectMC data/MC

ee 2 0.28±0.05 0.277±0.001 1.03±0.19 3 0.22±0.07 0.223±0.001 0.99±0.29 4 0.19±0.07 0.175±0.001 1.09±0.43 μμ 2 0.28±0.06 0.276±0.001 1.00±0.21 3 0.24±0.06 0.227±0.001 1.05±0.25 4 0.20±0.07 0.181±0.001 1.08±0.37 eμ 2 0.36±0.06 0.3577±0.0007 1.01±0.16 3 0.26±0.05 0.2625±0.0007 1.00±0.18 4 0.21±0.06 0.2047±0.0008 1.00±0.27

fromthedata thenumberoftop-quarkdecaysreconstructedand selected. Noticethat fcorrect cannot belarger than 1

/

n for events

withn jets,asitincludesthecombinatorialcontributionby defini-tion.

In Table 3 the values of fcorrect found in the data are

Fig. 4. Fractionofeventswith0,1,or2top-quarkdecaysselected,asdetermined fromthedata:thesefractions,shownfordifferenteventcategories,arelabelledα0, α1,andα2,respectively.

contamination from background events as well as the effect of missingoneortwojetsfromtop-quarkdecaysafterselection.The systematic uncertainties affecting the estimate of fcorrect can be

splitintotwosources:

•

distortion ofthe M jshape duetotheJESandJERofthe

re-constructedobjects[27];

•

calibrationuncertainties(derivedintheprevioussection) ow-ing tothe uncertainty inthe

μ

R

/

μ

F scale, the simulationof

gluon radiationandtheunderlyingevent,thetop-quarkmass value used in simulation, and the contributions from back-groundprocesses.

Foreachcasethefitisrepeatedwithdifferentsignalprobability distributionfunctions. The systematicuncertainty is estimatedto be 3–10%,depending on the jet multiplicity in the event, andis dominatedbytheME-PSmatchingthresholdandthe

μ

R

/

μ

Fscale

uncertainties.

By combining the measured fcorrect from the data with the

fraction of tt and

¯

single-top-quark events, a parameterisation of thethree classesofevents isobtained, i.e.the numberofevents with0,1,or2selectedtop-quarkdecays.Therelativeamountsof thethreeeventclasses areparameterised by theprobabilities

α

i,

where i corresponds to the number of jets from top-quark de-cays selectedin an event.The probabilities

α

i are constrainedto



i

α

i

=

1.Fig. 4summarises thevaluesof

α

i obtainedforthe

in-dividualeventcategories, wherethedifferencesaredominatedby theeventselectionefficienciesandthebackgroundcontributionin eachcategory.

6.2.Heavy-flavourcontent

Foragivennumberofcorrectlyreconstructedandselectedjets, theexpectedb-taggedjet multiplicitycanbemodelledasa func-tionof

R

and theb-tagging andmisidentificationefficiencies. In the parameterisation, we distinguish events containing jetsfrom 0,1,or2top-quarkdecays.Themodelisanextension oftheone proposedinRef. [56].Forillustration, themostsignificantcaseis considered,i.e.modellingtheobservationoftwo b-taggedjetsin aneventwithtworeconstructedjets.Forthecasewheretwojets fromtop-quarkdecaysareselectedintheevent,theprobabilityto observetwob-taggedjetscanbewrittenas

P2j,2t,2d

=

R

2

ε

2b

+

2

R

(

1

−

R

)ε

b

ε

q

+ (

1

−

R

)

2

ε

q2

,

(3)

wherethesubscripts(2j,2t,2d)indicateatwo-jetevent,withtwo b-tagged jets, and two top-quark decays.If instead, only one jet fromatop-quarkdecayispresentintheevent, theprobability is modified totake thesecondjet intoaccountinthemeasurement of

R

. Inthiscase, theprobability ofobserving two b-taggedjets is

P2j,2t,1d

=

R

2

ε

b

ε

q∗

+

R

(

1

−

R

)(ε

b

+

ε

q

)ε

q∗

+ (1

−

R

)

2

ε

q

ε

q∗

,

(4)

where

ε

q∗ istheeffectivemisidentificationrate, andiscomputed by taking into account the expected flavour composition of the “extra”jetsintheevents(i.e.thosenotmatchedtoatop-quark de-cay). Theeffectivemisidentificationrateisderived specificallyfor eacheventcategory.Fromsimulation,theseextrajetsareexpected to comemostly fromlight-flavourjets(

≈

87%). For completeness, forthecaseinwhichnojetfromtop-quarkdecayisreconstructed, theprobabilityofobservingtwob-taggedjetsis

P2j,2t,0d

=

ε

2q∗

.

(5)

For each dilepton channel and jet multiplicity, analogous ex-pressions are derived and combined using the probabilities

α

i

of having i reconstructed jetsfrom top-quark decays. Additional terms are added to extend the modelto events with morethan two jets. All efficiencies are determinedper event category, after convolvingthe correctionsfromdijeteventsinthedata withthe expected efficiencies (

ε

q and

ε

b) andthe simulated jet pT

spec-trum.

For the measurement of

R

, a binned-likelihood function is constructed using the model described above and the observed b-taggingmultiplicity ineventswithtwo, three,orfourobserved jets in the different dilepton channels. A total of 36event cate-gories, corresponding to different permutations of three lepton-flavour pairs, three jet multiplicities, and up to four observed b-tagged jets are used (see Fig. 2). The likelihood is generically writtenas

L

(

R

,

f_t¯t

,

kst

,

fcorrect

,

ε

b

,

ε

q

,

ε

q∗

, θ

i

)

=





Njets=2...4 N



jets k=0

P



N ,evNjets

(

k

), ˆ

N ,Njets ev

(

k

)

 

i

G



θ

_i0

, θ

i

,

1

,

(6) where N ,evNjets (N

ˆ

,Njets

ev ) is the number of observed (expected)

events with k b-tagged jets in a given dilepton channel (

=

ee

,

μμ

,

e

μ

) with a given jet multiplicity (Njets),

θ

i are the

nui-sance parameters (a total of 33, which will be discussed later), and

G

isaGaussiandistribution.Forthenominalfit,thenuisance parameters are assumed to be unbiased (

θ

_i0

=

0) and normally distributed. The nuisance parameters parameterise the effect of uncertainties,such asJESandJER,b-taggingandmisidentification rates,and

μ

R

/

μ

Fscales,amongst others,ontheinputparameters

of the likelihood function. The mostlikely value for

R

is found after profilingthe likelihood usingthe same technique described in Section 5. The result of the fit is verified to be unbiased in simulation,byperformingpseudo-experimentswithdedicatedMC sampleswhere

R

isvariedinthe

[

0

,

1

]

interval.Theresidual dif-ference found fromthesetestsis assignedasamodelcalibration uncertainty.

6.3. Measurementof

R

In the fit,

R

is allowed to vary without constraints. The pa-rametersofthemodelarealltakenfromthedata: f_t¯t andkst are

Fig. 5. Expectedeventfractionsofdifferentb-taggedjetmultiplicitiesindilepton eventsasafunctionofR.

Fig. 6. Variationofthelogoftheprofilelikelihoodratio(λ)usedtoextractRfrom thedata.Thevariationsobservedinthecombinedfitandintheexclusiveee,μμ, andeμchannels,areshown.Theinsetshowstheinclusiveb-taggedjetmultiplicity distributionandthefitdistribution.

takenfromTable 2, fcorrect istakenfromTable 3,

ε

b and

ε

q from

dijet-based measurements [53],and

ε

q∗ is derived following the

methoddescribed intheprevioussection.Fig. 5showsthe result-ingpredictionforthefractionofeventswithdifferentnumbersof observedb-tagged jetsasa functionof

R

.The individual predic-tionsforallcategoriesaresummedtobuildtheinclusivemodelfor theobservationofuptofourb-taggedjetsintheselectedevents.

Fig. 6 shows the results obtained by maximising the profile likelihood. The combined measurement of

R

gives

R

=

1

.

014

±

0

.

003(stat.)

±

0

.

032 (syst.),ingood agreementwiththeSM pre-diction. Fits to the individual channels give consistent results. For these, we obtain values of

R

ee

=

0

.

997

±

0

.

007 (stat.)

±

0

.

035 (syst.),

R

μμ

=

0

.

996

±

0

.

007 (stat.)

±

0

.

034 (syst.), and

R

eμ

=

1

.

015

±

0

.

003 (stat.)

±

0

.

031 (syst.) for the ee,

μμ

, and

e

μ

channels, respectively. The measurement in the e

μ

channel dominatesinthefinalcombinationsincethemainsystematic un-certainties are fully correlated and this channel has the lowest statisticaluncertainty.

Thetotalrelativeuncertaintyinthemeasurementof

R

is3.2%, andisdominatedbythesystematicuncertainty,whoseindividual contributionsare summarised inTable 4.Thelargest contribution

Table 4

SummaryofthesystematicuncertaintiesaffectingthemeasurementofR.The val-uesoftheuncertaintiesarerelativetothevalueofRobtainedfromthefit.

Source Uncertainty (%) Experimental uncertainties: εb 2.4 εq 0.4 ft¯t 0.1 DY 0.2 misidentified lepton 0.1 JER 0.5 JES 0.5 unclustered Emiss T 0.5 integrated luminosity 0.2 pileup 0.5 simulation statistics 0.5 fcorrect 0.5 model calibration 0.2 selection efficiency 0.1 Theoretical uncertainties: top-quark mass 0.9 top-quark pT 0.5 ME-PS 0.5 μR/μF 0.5 signal generator 0.5 underlying event 0.1 colour reconnection 0.1 hadronisation 0.5 PDF 0.1 t→Wq flavour 0.4 |Vtd|/|Vts| <0.01

relative single-top-quark fraction (tW) 0.1 VV (theoretical cross section) 0.1 extra sources of heavy flavour 0.4

Total systematic 3.2

tothesystematicuncertaintyisfromtheb-taggingefficiency mea-surement. Additionalsourcesofuncertaintyarerelatedtothe de-termination ofthe purity of thesample ( f_t¯t) and the fractionof correct assignments ( fcorrect) from the data; thesequantities are

affected by theoretical uncertainties relatedto the description of t

¯

t events, which have similar impact on the final measurement, suchas

μ

R

/

μ

F,ME-PS,signalgenerator,top-quarkmass,and

top-quark pT.Instrumental contributionsfromJESandJER,modelling

oftheunclusteredEmiss

T componentinsimulation,andthe

contri-butionfromtheDYandmisidentified-leptonbackgroundsareeach estimated to contribute a relative systematic uncertainty

<

0.6%. Anothersourceofuncertaintyisduetothecontributionfrom ex-tra sources of heavy-flavour production,either fromgluon split-tinginradiatedjetsorfromdecaysinbackgroundeventssuchas W

→

c

¯

s.Thiseffecthasbeenestimatedinthecomputationof

ε

q∗

by assigning a conservative uncertainty of 100% to the c and b contributions.Theeffectoftheuncertaintyinthemisidentification efficiencyisestimatedtobesmall(

<

1%),aswellasothersources ofuncertainty,suchaspileupandintegratedluminosity.Afterthe fit isperformedno nuisanceparameter isobservedto changeby morethan 1

.

5

σ

.The mostrelevantsystematicuncertainty(

ε

b) is

movedby

∼

0

.

5

σ

asaresultofthefit.

If the three-generationCKM matrix is assumedto be unitary, then

R

= |

Vtb

|

2[4].Byperformingthefitintermsof

|

Vtb

|

,avalue

of

|

Vtb

|

=

1

.

007

±

0

.

016

(

stat.

+

syst.

)

ismeasured.Upperandlower

endpointsofthe95%CLintervalfor

R

areextractedbyusingthe Feldman–Cousins(FC)frequentistapproach[57].The implementa-tion of the FC methodin RooStats [58] is used to compute the interval.Allthenuisanceparameters(including

ε

b)areprofiledin

ordertotakeintoaccountthecorrespondinguncertainties (statis-ticalandsystematic).Ifthecondition

R

≤

1 isimposed,weobtain

Fig. 7. Expectedlimitbandsatdifferentconfidencelevelsasafunctionofthe mea-suredRvalue.TherangeofmeasuredvaluesofRthatareallowedforeachtrue valueofRis shownascolouredbandsfordifferentconfidencelevels.Theobserved valueofRisshownasthedashedline.

bands for 68% CL, 95% CL, and 99.7% CL, obtained from the FC method.Theexpectedlimitbandsaredeterminedfromthe distri-butionof the profilelikelihood obtainedfrom simulated pseudo-experiments.Theupperandloweracceptanceregionsconstructed in this procedure are used to determine the endpoints on the allowed interval for

R

. In the pseudo-experiments the expected signalandbackgroundyields are variedusing Poissonprobability distributions for the statistical uncertainties and Gaussian distri-butionsforthesystematicuncertainties.Byconstraining

|

Vtb

|

≤

1,

a similarprocedureisusedtoobtain

|

Vtb

|

>

0

.

975 atthe95% CL.

6.4.Indirectmeasurementofthetop-quarktotaldecaywidth

The result obtained for

R

can be combined with a mea-surementofthe single-top-quark production crosssection in the

t-channeltoyieldanindirectdeterminationofthetop-quarktotal width

Γ

t.Assumingthat



q

B(

t

→

Wq

)

=

1,then

R

=

B(

t

→

Wb

)

and

Γ

t

=

σ

t-ch.

B

(

t

→

Wb

)

·

Γ (

t

→

Wb

)

σ

theor. t-ch.

,

(7)

where

σ

t-ch. (

σ

ttheor-ch..) is the measured (theoretical) t-channel single-top-quark cross section and

Γ (

t

→

Wb

)

is the top-quark partial decay width to Wb. If we assume a top-quark mass of 172.5 GeV,thenthetheoreticalpartialwidthofthetopquark de-cayingtoWbis

Γ (

t

→

Wb

)

=

1

.

329 GeV[3].Afittotheb-tagged jetmultiplicity distributioninthedataisperformed,leaving

Γ

t as

a free parameter. Inthe likelihood function we use the theoreti-calpredictionforthe t-channelcrosssection at

√

s

=

7 TeV from Ref.[59]andthecorrespondingCMSmeasurementfromRef.[25]. The uncertainties in the predicted and measured cross sections are taken into account as extra nuisance parameters in the fit. The uncertainty in the theoretical cross section is parameterised by convolving a Gaussian function for the PDF uncertainty with a uniform prior describing the factorisation and renormalisation scale uncertainties. Some uncertainties in the experimental cross sectionmeasurementsuchasthosefromJESandJER,b-tagging ef-ficiency,

μ

R

/

μ

F scales,andME-PSthresholdfort

¯

t productionare

fullycorrelatedwiththeonesassignedtothemeasurementof

R

. All othersare summed in quadratureand assumedto be uncor-related.Afterperformingthemaximum-likelihoodfit,wemeasure

Table 5

SummaryofthesystematicuncertaintiesinthemeasurementofΓt.Thevaluesof

theuncertaintiesarerelativetothevalueofΓtobtainedfromthefit.The“other

sources”categorycombinesalltheindividualcontributionsbelow0.5%.

Source Uncertainty (%)

Single-top quark t-channel cross section 9.2

εb 4.3 JES 0.7 pileup 0.8 ME-PS 0.8 μR/μF 0.8 top-quark mass 0.6 other sources 1.5 Total systematic 10.4

Γ

t

=

1

.

36

±

0

.

02 (stat.)₋+00..1411 (syst.) GeV, in good agreement with

the theoretical expectation[3]. The dominant uncertainty comes from the measurement of the t-channel cross section, as sum-marised inTable 5.

7. Summary

A measurement of the ratio ofthe top-quark branching frac-tions

R

=

B(

t

→

Wb

)/B(

t

→

Wq

)

, where the denominator in-cludesthesumoverthebranchingfractionsofthetopquarktoa W bosonandadown-typequark(q

=

b

,

s

,

d),hasbeenperformed using a sample of t

¯

t dilepton events. The sample has been se-lected fromproton–proton collision data at

√

s

=

8 TeV from an integrated luminosity of 19.7 fb−1_{, collected with the CMS}

de-tector.The b-taggingandmisidentificationefficienciesarederived from multijet control samples. The fractions of events with0, 1, or 2 selected jets from top-quark decays are determined using the lepton-jet invariant-mass spectrum and an empirical model forthe misassignment contribution.The unconstrained measured value of

R

=

1

.

014

±

0

.

003 (stat.)

±

0

.

032 (syst.) is consistent with the SM prediction, and the main systematic uncertainty is fromthe b-tagging efficiency(

≈

2.4%). All other uncertainties are

<

1%.A lower limit of

R

>

0

.

955 at 95% CLis obtainedafter re-quiring

R

≤

1 and taking into account both statistical and sys-tematical uncertainties. This result translates into a lower limit

|

Vtb

|

>

0

.

975 at95%CLwhenassumingtheunitarityofthe

three-generationCKM matrix.Bycombiningthisresultwithaprevious CMS measurement of the t-channel production cross section for single topquarks,anindirectmeasurementofthetop-quarktotal decay width

Γ

t

=

1

.

36

±

0

.

02 (stat.)+₋0₀._.14₁₁ (syst.) GeV is obtained,

inagreementwiththeSM expectation.Thesemeasurementsof

R

and

Γ

t arethe mostprecise todateandthe firstobtainedatthe

LHC.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technicalandadministrative staffsatCERN andatother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentresand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythe computinginfrastructureessential to ouranalyses. Finally, we acknowledge the enduring support for the construc-tionandoperation oftheLHC andtheCMSdetectorprovidedby thefollowingfundingagencies:BMWFWandFWF(Austria);Fonds De La Recherche Scientifique - FNRS andFWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST,andNSFC(China);COLCIENCIAS(Colombia);MSESandCSF (Croatia);RPF (Cyprus);MoER,SF0690030s09andERDF(Estonia);

AcademyofFinland,MEC,andHIP(Finland);CEAandCNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ire-land);INFN(Italy);NRFandWCU(RepublicofKorea);LAS (Lithua-nia); MOE and UM (Malaysia); CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mexico);MBIE(New Zealand);PAEC(Pakistan);MSHE andNSC (Poland);FCT (Portugal); JINR (Dubna); MON, RosAtom, RASandRFBR(Russia); MESTD(Serbia); SEIDI andCPAN(Spain); Swiss Funding Agencies (Switzerland); MST (Taipei); ThEPCenter, IPST, STAR and NSTDA (Thailand); TUBITAK and TAEK (Turkey); NASU andSFFR(Ukraine); STFC (United Kingdom);DOE andNSF (USA).

Individuals have received support from the Marie-Curie pro-grammeandthe European ResearchCouncil andEPLANET (Euro-peanUnion);theLeventisFoundation;theAlfredP.Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherchedansl’Industrieetdansl’Agriculture(FRIA-Belgium);the AgentschapvoorInnovatiedoorWetenschapenTechnologie (IWT-Belgium);the Ministry ofEducation,Youth andSports (MEYS) of Czech Republic; the Council of Science and Industrial Research, India; the Compagnia di San Paolo (Torino); the HOMING PLUS programmeofFoundationForPolishScience,cofinancedbyEU, Re-gionalDevelopmentFund;andtheThalisandAristeiaprogrammes cofinancedbyEU–ESFandtheGreekNSRF.

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CMSCollaboration

V. Khachatryan,

A.M. Sirunyan,

A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam,

T. Bergauer,

M. Dragicevic,

J. Erö,

C. Fabjan

1

,

M. Friedl,

R. Frühwirth

1

,

V.M. Ghete,

C. Hartl,

N. Hörmann,

J. Hrubec,

M. Jeitler

1

,

W. Kiesenhofer,

V. Knünz,

M. Krammer

1

,

I. Krätschmer,

D. Liko,

I. Mikulec,

D. Rabady

2

,

B. Rahbaran,

H. Rohringer,

R. Schöfbeck,

J. Strauss,

A. Taurok,

W. Treberer-Treberspurg,

W. Waltenberger,

C.-E. Wulz

1

InstitutfürHochenergiephysikderOeAW,Wien,Austria

V. Mossolov,

N. Shumeiko,

J. Suarez Gonzalez

NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt,

M. Bansal,

S. Bansal,

T. Cornelis,

E.A. De Wolf,

X. Janssen,

A. Knutsson,

S. Luyckx,

S. Ochesanu,

B. Roland,

R. Rougny,

M. Van De Klundert,

H. Van Haevermaet,

P. Van Mechelen,

N. Van Remortel,

A. Van Spilbeeck

UniversiteitAntwerpen,Antwerpen,Belgium

F. Blekman,

S. Blyweert,

J. D’Hondt,

N. Daci,

N. Heracleous,

A. Kalogeropoulos,

J. Keaveney,

T.J. Kim,

S. Lowette,

M. Maes,

A. Olbrechts,

Q. Python,

D. Strom,

S. Tavernier,

W. Van Doninck,

P. Van Mulders,

G.P. Van Onsem,

I. Villella

VrijeUniversiteitBrussel,Brussel,Belgium

C. Caillol,

B. Clerbaux,

G. De Lentdecker,

L. Favart,

A.P.R. Gay,

A. Grebenyuk,

A. Léonard,

P.E. Marage,

A. Mohammadi,

L. Perniè,

T. Reis,

T. Seva,

L. Thomas,

C. Vander Velde,

P. Vanlaer,

J. Wang

UniversitéLibredeBruxelles,Bruxelles,Belgium

V. Adler,

K. Beernaert,

L. Benucci,

A. Cimmino,

S. Costantini,

S. Crucy,

S. Dildick,

A. Fagot,

G. Garcia,

B. Klein,

J. Mccartin,

A.A. Ocampo Rios,

D. Ryckbosch,

S. Salva Diblen,

M. Sigamani,

N. Strobbe,

F. Thyssen,

M. Tytgat,

E. Yazgan,

N. Zaganidis