Measurement of the production cross section for a W boson and two b jets in pp collisions at √√s=7 TeV

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Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

the

production

cross

section

for

a

W boson

and

two

b jets

in

pp collisions

at

√

s

=

7 TeV

.CMSCollaboration

CERN,Switzerland

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received23December2013 Receivedinrevisedform14June2014 Accepted14June2014

Availableonline18June2014 Editor:M.Doser

Keywords: CMS Physics SMP

TheproductioncrosssectionforaW bosonandtwobjetsismeasuredusingproton–protoncollisions at √s=7 TeV inadata samplecollected withthe CMS experiment atthe LHCcorresponding to an integrated luminosity of 5.0 fb−1_{. The W+bb events are selectedin the W→μν decay mode by}

requiring amuonwith transversemomentum pT>25 GeV andpseudorapidity|η|<2.1,and exactly

two b-tagged jets with pT>25 GeV and |η|<2.4. The measured W+bb production cross section

in the fiducial region, calculated at the level of final-state particles, is σ(pp→W+bb)× B(W→

μν)=0.53±0.05(stat.)±0.09 (syst.)±0.06(theo.)±0.01(lum.) pb,inagreementwiththestandard modelprediction.Inaddition,kinematicdistributionsoftheW+bb systemareinagreementwiththe predictionsofasimulationusing MadGraph and pythia.

©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP3_.

1. Introduction

ThisLetterreportsastudyoftheproductionofaW bosonand twobjetsinproton–protoncollisions,wherethe W bosonis ob-served viaits decayto amuon anda neutrino, andeachb jet is identified by the presence ofa b hadron with a displaced decay vertex. The production mechanism of bb pairs together with W orZ bosons hasbeenthe subjectofextensivetheoretical studies andisincludedindifferentsimulationprograms[1–3],butisstill not thoroughlyunderstood. Previousmeasurements of vector bo-son production withassociated b-quark jets haveshown varying levelsofagreementwiththeoreticalcalculations[4–6].

According to the standard model (SM), the primary contribu-tion for bb production in association with a W boson is due to the splitting ofa gluon intoa bb pair. Twodifferent models for b-quarkproductionareavailable,dependingonwhetherthereare fouror fivequark flavorsin theproton partondistribution func-tions(PDFs)[7].Therefore,apreciseexperimentalmeasurementof theW+bb productioncross section providesimportant inputto therefinementoftheoreticalcalculationsinperturbativequantum chromodynamics (QCD),aswell as thevalidation ofMonte Carlo (MC)techniques.

Akeyfeatureofthisanalysiscomparedtoothers[4–6]isthebb phasespacethatiscovered.Previousmeasurementshave concen-tratedonW-bosonproductionwithatleastoneobservedb-quark jet,forwhichthepredictionsdifferfromtheexperimentalresults.

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

Thisdifferenceislargerintheproductionofeventswithacollinear bb pair that isreconstructed asonejet [8,9],atopology afflicted bysignificanttheoreticaluncertainties.Focusingontheobservation ofW-bosonproductionwithtwowell-separatedb-quarkjets,this analysis provides a complementary approach by probing a kine-maticregimethatisbetterunderstoodtheoretically.

The productionof W+bb eventsisan irreduciblebackground inanalysesinvolvingtwoseparatedandwell-identifiedbjets,such asSMHiggsbosonproductioninassociationwithanelectroweak gaugebosonandsubsequentdecaytobb.ThediscoveryofaHiggs boson with amass of approximately125 GeV by the ATLAS and CMSCollaborations[10–12]motivatesfurtherstudiestodetermine thecouplingofthisnewbosontobquarks.

Other SM processesproduce eventswithan experimental sig-naturesimilartotheonestudiedhere.Theseincludeproductionof topquark–antiquarkpairs(tt),associatedproductionofaW boson withlightjetsmisidentifiedasb-quark jets,single-top-quark pro-duction, multijet production (henceforth labeled “QCD multijet”), Drell–Yan productionassociated withjets, and electroweak dibo-sonproduction.

2. CMSdetectorandeventsamples

This analysis uses a sample of proton–proton collisions at a center-of-mass energyof √s=7 TeV, collected in2011 withthe Compact MuonSolenoid(CMS)experimentattheLHC,and corre-spondingtoanintegratedluminosityLdt of5.0 fb−1.Whilethe CMSdetectorisdescribedindetailelsewhere[13],thekey compo-nentsforthisanalysisaresummarizedbelow.TheCMSexperiment

http://dx.doi.org/10.1016/j.physletb.2014.06.041

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

usesaright-handedcoordinatesystem,withtheoriginatthe nom-inalinteractionpoint,the x axispointingtothecenteroftheLHC ring, the y axis pointing up (perpendicular to the plane of the LHC ring), and the z axis along the counterclockwise-beam di-rection. The polar angle θ is measured from the positive z axis andthe azimuthal angle φ is measured in the x– y plane in ra-dians. The magnitude of the transverse momentum pT is calcu-latedas pT=

 p2

x+p2y. A superconducting solenoid is the cen-tralfeatureoftheCMSdetector,providingan axialmagneticfield of3.8 T parallel to the beam direction. A silicon pixel and strip tracker,acrystalelectromagneticcalorimeter,anda brass/scintilla-torhadron calorimeterarelocated withinthe solenoid.A quartz-fiber Cherenkov calorimeter extends the coverage to |η| <5.0, where η= −ln[tan(θ/2)].Muons are measuredin gas-ionization detectors embedded in the steel flux return yoke outside the solenoid. The first level of the CMS trigger system, composed of customhardwareprocessors, isdesignedtoselectthemost inter-esting eventsusing informationfromthe calorimetersand muon detectors.Ahigh-leveltriggerprocessor farmdecreasesthe event ratetoafewhundredhertz,beforedatastorage.

A number of MC event generators are used to simulate the signal and background event samples. Vector boson + jets and tt + jets productions are generated at leading order (LO) using MadGraph5.1[3]interfacedwith pythia 6.4.24[14]for hadroniza-tion. The W+jets sample was generated using the five-flavor scheme, which includes massless b quarks in the initial state. Single-top-quark event samples are generated at next-to-leading order(NLO)with powheg 2.0[15–17].Diboson (W+W−,WZ,ZZ) samplesare generatedwith pythia 6.4.24. ForLO generators,the defaultPDFsetusedisCTEQ6L[18],whileforNLOgeneratorsthe showeringofpartonsandhadronizationaresimulatedwith pythia usingtheZ2tune [19].Forallprocesses,thedetectorresponseis simulatedusingadetaileddescription oftheCMSdetectorbased on Geant4 [20]. The reconstruction of simulated events is per-formedwiththesamealgorithmsusedfortheanalyzeddata sam-ple.Thesimulatedeventsamplesincludeadditionalminimum-bias interactionsperbunchcrossing(pileup).

3. Eventreconstruction

Individual particles emerging from each collision are recon-structed with the particle-flow (PF) technique [21,22]. This ap-proachuses theinformationfromall subdetectorstoidentifyand reconstructindividualparticle candidatesinthe event, classifying themintomutuallyexclusivecategories:chargedhadrons,neutral hadrons,photons,electrons,andmuons.

Muons are reconstructed by combining the information from thetrackerandthemuonspectrometer[23].Themuoncandidates are required to originate from the primary vertex of the event, chosen asthe vertex withthe highest p2

T ofthe charged par-ticlesassociatedwithit.The muonrelativeisolation isdefinedas Irel=_ipT(i)/pT(μ),withi runningoverPFcandidates(hadrons, electrons,photons) ina cone around themuon directiondefined by R <0.4,where R=(η)2_{+ (φ)}2_.

The pT of a muon passing the identificationand isolation re-quirementsiscombinedwiththemissingtransverseenergy Emiss_T oftheeventtoforma W candidate.We define Emiss_T asthe neg-ativevector sum ofthe transverse momenta of all reconstructed particle candidates in the event. The value of Emiss_T is corrected fornoiseintheelectromagneticandhadroncalorimetersusingthe proceduredescribedinRef.[24],whichisbasedona parametriza-tion of the recoil energy measured in Z→μμ events. The re-constructedtransversemassofthesystem, MT,iscalculatedfrom the pT of the isolated muon and the EmissT of the event; MT=



2pT(μ)|Emiss_T |(1−cosφ), where φ is the difference in az-imuthbetweenEmiss_T and pT(μ).InW→μν decays,theMT dis-tributionexhibitsaJacobianpeakwithakinematicendpointatthe W mass.Itisthereforeanaturaldiscriminatoragainstnon-W final states,such asQCD multijetevents,that havea lepton candidate andE_TmissandarelativelylowvalueofMT.

Jetsareconstructedusingtheanti-kT clusteringalgorithm[25], asimplementedinthe fastjet package[26,27],withadistance pa-rameterof0.5.Jetclusteringisperformedusingindividualparticle candidates reconstructedwiththe PFtechnique. Jetsare required to pass identificationcriteria that eliminate jets originatingfrom noisychannelsinthehadroncalorimeter[28].Jetsoriginatingfrom pileup interactions are rejected by requiring consistency of the jets withthe primary interaction vertex. Small correctionsto jet energy—relativeandabsolutecalibrationsofthedetector—are ap-pliedasafunctionofthepTand ηofthejet[29].

Thecombinedsecondaryvertex(CSV)b-taggingalgorithm[30] exploitsthelonglifetimeandrelativelylargemassofbhadronsto provide optimized b-quark jet discrimination. The CSV algorithm combines information about impact parameter significance, sec-ondaryvertex(SV)kinematicproperties,andjetkinematic proper-tiesinalikelihood-ratiotechnique. Jetsareb-taggedbyimposing aminimumthresholdontheCSVdiscriminatorvalue.This thresh-oldprovidesanefficiencyofapproximately50%foridentifyingjets containingb-flavoredhadrons,whilelimitingthemisidentification probability forlight-quarkandgluon jetsto 0.1%andforc-quark jets to 3%. Furthermore, to increase the purity of the sample, a selected jet is required to have a reconstructed SV. This addi-tionalrequirementhasasmallimpactontheselectedb-quarkjets (93%efficiencywithrespecttotheb-tagselection)whilereducing thecombinedmisidentificationprobabilityforc-quark,light-quark, andgluonjetsto <0.1%.

4. W+bb eventselection

Candidate events are selected online by a single-muon trig-ger that requires a reconstructed muon with pT>24 GeV and |η| <2.1.The offline W+bb eventselection requiresan isolated muon with Irel<0.12, pT>25 GeV, |η| <2.1, and exactly two jets with pT>25 GeV and |η| <2.4, where both selected jets mustcontainan SVandpasstheb-taggingrequirement.Toreduce the contribution from Z-boson production, the event is rejected if a second muon forms an invariant mass mμμ>60 GeV with theisolated muon.There arenorequirements ontheisolation or pT ofthe second muon.The tt background is reducedby requir-ing that there be noadditional isolatedelectrons or muonswith pT>20 GeV intheeventandnoadditionaljetswithpT>25 GeV and2.4 <|η| <4.5. Toreduce the contribution fromQCD multi-jetevents,MT>45 GeV isrequired.Thetotalnumberofobserved eventsinthedatasampleafterall selectionrequirementsare ap-pliedis1230.

We first estimate the normalized distributions for the signal and each type of background and then perform a global fit to determinethe fractionofeachbackgroundin thecandidate sam-ple.Theshapesofthesignalandbackgrounddistributionsforthe variables weusein thefitare evaluatedusing simulation,except fortheQCDmultijetbackground,whichisderivedfromdata.The crosssectionsfortheW+jets andZ+jets processesarecalculated with the predictions from fewz [31] evaluated at next-to-next-to-leading order (NNLO) using the MSTW08 NNLO PDF set [32]. Single-top-quark and diboson production cross sections are nor-malized to the NLO cross section predictions from mcfm [33,34] using theMSTW08 NLO PDF set.The tt crosssection is takenat NNLO as calculated inRef. [35]. For each background simulation

we apply the same selection requirements as for the candidate sampletogeneratetherelevantdistributionsforfitting.

Toprovethat thesimulation describesthedataboth inshape andnormalization,andcanbeused inthefitasdescribed inthe followingsection,weselectspecificdatasamplesthatareenriched withtherelevantbackgrounds andverifytheperformance ofthe simulation.These controlregions arenot used inthe final signal extractionandserveonlyforthisverificationtask,withthe excep-tionoftheQCDmultijetcontrolregion.

The shapes of the distributions for multijet events are taken directlyfroma multijetenriched data sample obtainedusingthe signal selection requirements and, in addition, requiring a non-isolatedmuon: Irel_{>0.2.Theyield ofmultijeteventsisobtained} fromafitperformedoneventswithMT<40 GeV,whichisbelow theJacobian peakofthe W→μν.The resultingnormalizationis extrapolatedto the signal region, MT>45 GeV. The relative un-certainty in the yield of QCD multijet events is estimated to be ±50%,takingintoaccountboththefitresultandtheextrapolation tothe high-MT range.Thisrelative uncertaintyalso coversshape mismodelingsofthe smallmultijetcontributioninthefinal sam-ple.

The W+light-quark jets process,wherethe jetsarenot initi-atedbyborcquarks,isthedominantbackgroundbeforeapplying the selection requirements on the SV and on b-tagging. The b-tagging algorithm reduces the contamination of light-quark and c-quarkjetsintheselectedsampletoapproximately2%ofthetotal expectedyield.Thecontributionofeventswithasingleb-quarkjet intheinitialstateandamisidentifiedsecondlight-quarkorgluon jetisnegligible.

Att backgroundcontroldatasampleisformedbyrequiringtwo jetsinadditiontothetwohighestpTb-taggedjets.Thishigherjet multiplicity requirementselectsasample thatis dominatedbytt events.Fig. 1(top)showstheinvariantmassmJ3J4ofthethird- and

fourth-highest pT jets in the event. In tt events this observable is correlated to the mass ofthe hadronically decaying W boson. Thistt controlregionisusedinthefinalfitforthesignalyieldto constrainthett backgroundnormalizationinthesignalregion.The simulationdescribestheobserveddistributionswell,bothinterms ofshapeandnormalization.

AZ+jets backgrounddatasample isdefinedbyrequiringthe standard selection criteria with the additional requirement of a second muon with opposite charge such that the invariant mass ofthedimuon systemisconsistent witha Z boson(70 <mμμ<

100 GeV).ThissampleisusedtovalidatetheZ+jets background estimate,asdocumentedinRef.[36].Thesimulationdescribesthe experimentaldistributionswellinthiscontroldatasample.

A single-top-quark backgroundsample is definedby selecting events inwhich the W boson is accompanied by exactly one b-quark jet, which passes the tagging criteria, and an additional forwardjetwith|η| >2.8.No furtherrejectionofadditionallight jetsorleptonsisimposed.Thesimulationdescribesthe single-top-quark background data sample well, as documented in Ref. [37], andthereforeitisusedtoestimatetheyieldandshapeofthe dis-tributionsofkinematicvariablesinthesignalregion.

TheexpectedyieldinthesignalregionfortheSMHiggsboson ofMH=125 GeV associatedwithaW bosonwheretheHiggs bo-sondecaystobb pairs andtheW bosondecaystoamuon anda neutrinohasbeencomputedusingthe powheg eventgenerator.It wouldaccountfor <0.2%ofthetotalexpectedyieldinthesignal regionandisnotconsideredforthismeasurement.

5. Signalextraction

After all the selection requirements, the largest background contributionsaretheproductionoftt pairsandsingletop quarks.

Fig. 1. ThedistributionoftheinvariantmassmJ3J4 ofthetwoadditionallightjets

inthett controlregion(top).ThepTdistributionofthehighest-pTjet,pT,J1,inthe

signalregion(bottom).Signalandbackgroundyieldsaretakenfromthe maximum-likelihoodfittoCMSdatadescribedinthetext.Theuncertaintybandcorresponds tothetotaluncertaintyonthefittedyields.Thelastbininbothfiguresincludes overflowevents.Thelowerpanelsshowtheratioofobserveddataeventstothe totalfittedyield.

Contributions tothe backgroundfromW+jets, Z+jets, diboson production,andQCD multijeteventsare much smaller,asshown inthemiddlecolumnofTable 1.

The compositionofthe candidatedatasample isextractedvia a binned extended maximum-likelihood fit. Because the tt back-ground is large (largerin fact than the signal), it is essential to constrain tightlyboththesignalandthett backgroundwitha si-multaneous fit to the pT of the leading jet (pT,J1) in the signal

Table 1

ComparisonoftheyieldsexpectedfromtheSMandobtainedfromthefittothe analyzeddatasample.Thepredictedyielduncertaintytakesintoaccountthe uncer-taintiesinthemeasurementofthecrosssectionforeachoftheprocesses,exceptfor themultijetcontribution,whichisestimatedusingamultijetenrichedbackground datasample.Theuncertaintyinthefittedyieldscombinesthestatisticaland sys-tematicuncertaintiesobtainedfromtheextendedbinnedmaximum-likelihoodfit technique.

Process Predicted yield Fitted yield

W+bb 332±99 301±59

tt 621±36 653±37

Single top quark 160±13 167±13

QCD multijet 33±17 33±16 W+c, W+cc 21±4 20±10 Z+jets 31±3 32±3 WW, WZ 19±3 19±3 W+light-quark jets 1.5±0.2 1±1 Total 1219±79 1226±73 Observed events 1230

distributionobtained fromthe tt control sample. The normaliza-tionofeachbackgroundcontributionisallowed tovaryinthefit withinits uncertainty. The pT ofthe leading jet ischosen asthe final fitvariable becauseof itsdiscrimination power against top-relatedbackgrounds.Fig. 1 showsthefitteddistributions: pT,J1 in

thesignalregion(bottom)andmJ3J4 inthett controlsample(top);

the yields shown for the different processes are those resulting fromthefit.The χ2 _{of thefitis 16.9,for29 degreesof freedom} (χ2_{/dof=0.58).Thefittedyieldsforalltheprocessesarelistedin} Table 1,andcomparedto thepredictions.All observedyields are foundtobeinagreementwiththeexpectations.

The systematicuncertainties, including those in thepredicted backgroundyields, are introduced asnuisance parameters in the fitwithconstraintsaround theestimatedcentralvalue. Anycross section oracceptanceuncertainty inthe backgroundprocesses is introducedasa log-normal constrainton therateofthe process. Alternatebinned templates are obtainedby varying thedifferent sourcesofsystematicuncertainty;thenominalandalternate tem-platesaretheninterpolateddependingonthenuisanceparameter values.Oneofthelargestsystematicuncertaintiescomesfromthe relative uncertainty in the b-tagging efficiency (6% per jet). This andother uncertaintiesinthelight-quarkandc-quarkjet mistag-gingefficienciesaretakenfromRef.[30].Thejetenergyandmuon pTscalesareallowedtovarywithintheiruncertainties(1–3%and 0.2%, respectively). Relative uncertainties in the muon efficiency (duetotrigger,reconstruction,identification,andisolation)are es-timatedtobe1%.Theaveragenumberofpileupeventsinthedata sample analyzedis 9.The uncertaintyassociatedwiththe pileup in the simulation is studied by shifting the overall mean num-berof interactions perbunch crossing up ordownby 0.6, which has a negligible effect on the measurement. To account for the uncertaintyinthe descriptionofthe Emiss

T spectrum,the compo-nentof Emiss

T that isnotclusteredinjetsisshifted by±10%.The normalizationsofthebackgroundprocessesarealsotakeninto ac-count,withan uncertainty assignedtoeach process accordingto thetheoreticalpredictions,thepreviousCMSmeasurementswhen available,oran estimatefromthemultijetdatasample.The over-all relative uncertainty in the signal selection efficiency due to thechoiceofPDFset isestimatedby followingthePDF4LHC rec-ommendationandfound tobe approximately 1% [32,38–41]. The varyingofthe factorization(μF)andrenormalization(μR) scales, alsobased on the PDF4LHC recommendation, leads to an uncer-taintyof10%.Asimilarprocedureisfollowedtoestimatetheeffect ofscalevariations onthesignal shape,yielding anuncertaintyin

thecrosssectionsmallerthan1%.Therelativeintegrated luminos-ityuncertaintyis2.2%[42].

The number of events in the candidate sample, 1230, is in agreementwiththeexpectedandfittedtotalyields,althoughitis not explicitly includedin the fittingprocess. The number of sig-nal events obtained from the binned maximum-likelihood fit is NS=301±30(stat.)±51(syst.).

Tocross-checktheseresults,anindependentstudyisperformed with looser b-tagging criteria, corresponding to an efficiency of 70% for selecting a jet containing b-flavored hadrons, while the misidentification probability for light-quark andgluon jets is 1% andforc-quarkjetsis11%.Allotherselection criteriaforthe sig-nalandcontrol samplesremain unchanged. Sincethe c-quarkjet contribution becomes significant with these looser criteria, it is essential to use variables in thefit that can discriminateagainst bothW+cc andtop-quark-initiatedprocesses.Theinvariantmass measured using all particles originating at the SVs of the high-est pT (mSV,J1) andsecond-highest pT (mSV,J2)jetscan distinguish

between W+bb and W+cc. The scalar sum of the transverse momenta of the jets, HT, is used to distinguish W+jets from top-quarkcontributions.TheW+bb signal isextractedina two-dimensionalfitusingthetwovariablesmSV,J1+mSV,J2 andHTand

constraining the tt contribution in the tt background data sam-pleasdescribedabove.Thedistributions ofthevariablesmSV,J1+

mSV,J2 and HT,which areprojectionsofthetwo-dimensional

dis-tributionsfittedinthiscross-check,areshowninFig. 2,withyields asgivenbythefit.Thecentralvalueofthecrosssectioncomputed withthismethoddiffersbylessthan3% fromtheprimary fit re-sult.

6. Results

The W+bb cross section is measured within a fiducial vol-umedefinedbyrequiringafinal-statemuonwithpT>25 GeV and |η|<2.1 andexactlytwofinal-stateparticlejets,reconstructed us-ingtheanti-kTjetalgorithmwithadistanceparameterof0.5,with pT>25 GeV and|η|<2.4 andwitheachcontainingatleastoneb hadronwithpT>5 GeV.Events withextrajetswithpT>25 GeV and|η| <4.5 arevetoed.

Within thisfiducial phase space, the W+bb cross section is obtainedusingtheexpression

σ(pp→W+bb)× B(W→μν)= NS

L dtsel

,

wherethe efficiencyoftheselection requirements, sel= (11.2± 1.0)%,iscomputedusingthe MadGraph+pythia_MCsample.The uncertainty in thisselection efficiency comes from the PDF and scale variation uncertainties mentioned above. The experimental uncertaintiesareincludedinthedeterminationofNS.

Themeasuredfiducialcrosssectionis

σ(pp→W+bb)× B(W→μν)

=0.53±0.05 (stat.)±0.09 (syst.) ±0.06 (theo.)±0.01 (lum.) pb.

This measured value cannot be directly compared to the SM NLO cross section calculated with mcfm [33,34] because the lat-terpertains to jetsof partons,not jetsofhadrons, anddoesnot includethe productionof bb pairsfromdouble-parton scattering (DPS).

mcfmpredicts a crosssection of0.52±0.03 pb attheparton level,usingthe MSTW2008NNLOPDF setandsettingthe factor-ization andrenormalizationscales to μF=μR=mW+2mb [34].

Fig. 2. Thedistributionofthesumofthemassesreconstructedfromalltheparticles originatingattheSVs,mSV,J1+mSV,J2(top)andthedistributionforthevariableHT

(bottom)inthealternativelooserb-tagselection.Thesetwodistributionsarethe projectionsofthe two-dimensionalfitperformedasacross-checkanddescribed inthetext. Signaland backgroundyieldscorrespondtothe post-fitresults.The uncertaintybandcorrespondstothetotaluncertaintyinthefittedyields.Thelast bininbothfiguresincludesoverflowevents.Thelowerpanelsshowtheratioof observeddataeventstothetotalfittedyield.

Theuncertaintyinthetheoreticalcrosssectionquotedaboveis es-timatedbyvaryingthescales μF, μRsimultaneouslyupanddown by a factorof two. It alsotakes into account thePDF uncertain-tiesfollowingthePDF4LHCrecommendation.Thescaleuncertainty in the theoretical cross section may be underestimated because of the requirement of exactly two jets in the final state, which introducesavetooneventswithextrajets.Therefore,amore con-servativeestimate ofthisuncertaintyinthetheoreticalprediction

is computed, following the procedure described in Ref. [43],and thetotaltheoreticaluncertaintyisfoundtobe30%.

Two corrections are needed to link the theoretical prediction to the measurement, a hadronization correction and a DPS cor-rection. At the parton level, the events are required to have a muon of pT>25 GeV and|η| <2.1 and exactly two partonjets of pT>25 GeV and |η| <2.4, each containing a b quark. The hadronizationcorrectionfactorCb→B=0.92±0.01,calculated us-ing a five-flavor MadGraph + pythia reference MC, is used to extrapolate the cross section computed at the level of parton jets to the level of final-state particle jets. The uncertainty as-signedtothiscorrectionisobtainedbycomparingthe correspond-ing factors computed with a four-flavored MadGraph MC sim-ulation. The simulated MadGraph + pythia events include DPS production of bb pairs and they reproduce theseprocesses ade-quatelyasmeasured byCMS[44].ThecontributionofDPSevents to the cross section at the parton-jet level is estimated to be

σDPS= (σW×σ_bb)/σeff=0.08±0.05 pb. The value of the effec-tive cross section, σeff, is taken from Ref. [45], and is assumed to be independent of the process and interaction scale. The un-certainty in σDPS takes into account both the uncertainty in the measurement of σeff andtheuncertainty inthe fiducialbb cross section. The theoretical cross section at hadron level can be ex-trapolated from the MCFMparton-jet prediction by applying the hadronization correctionandaddingtheDPScontribution, result-ingin0.55±0.03 (MCFM) ±0.01 (had.) ±0.05 (DPS)pb.Thisvalue isinagreementwiththemeasuredvalue.

In addition to this measurement of the production cross sec-tion, we have explored the kinematics of the W+bb system. The angulardistance between the two selected b jets, RJ1,J2 =



(ηJ1,J2)2+ (φJ1,J2)

2

,iscomparedtotheSMpredictioninFig. 3 (top). Signal and background yields are taken from the binned maximum-likelihoodfit,andtheir shapesfromMonteCarlo simu-lationsordataasdescribedinSection4.Theminimumseparation of 0.5betweenthe two jetsis an important aspectof the phase spacedefinition, asdiscussed inthe introduction.Fig. 3(bottom) compares theMT distributiontotheSMpredictions.Fig. 4shows theinvariantmassofthetwoselectedb-quarkjets(mJ1J2)aswell

as the transverse momentum of the system formed by the two b-quarkjets(pT,J1J2).Thesimulationdescribestheobserved

distri-butionswell.

7. Summary

Insummary,wehavepresentedameasurementoftheW+bb production cross section in proton–proton collisions at 7 TeV. The W+bb events have been selected in the W→μν de-cay mode with a muon of pT>25 GeV and |η| <2.1, and two b jets of pT>25 GeV and |η| <2.4. The data sample corre-sponds to an integrated luminosity of 5.0 fb−1_{. The measured} fiducialcrosssectionforproductionofaW bosonandtwobjets,

σ(pp→W+bb) ×B(W→μν) =0.53±0.05(stat.)±0.09(syst.)± 0.06(theo.)±0.01(lum.) pb,isinagreementwiththeSM predic-tion of0.55±0.03 (MCFM) ±0.01 (had.) ±0.05 (DPS)pb,which accounts for double-parton scattering production and hadroniza-tioneffects.

This study provides the first measurement for pp→W+bb productionat7 TeVinthisparticularphasespace,thereby comple-menting previousmeasurementsperformedattheLHC[6],which focusedontheproductionofW bosonsaccompaniedbyone iden-tifiedbjet.Theprecisionofthemeasuredcrosssectionapproaches that of theoretical predictions at NNLO, thus enabling sensitive testsofperturbativecalculationsintheSM.

Fig. 3. Thedistributionoftheangular distanceR between the twoselectedb jets(top)andthedistributionofthetransversemassMT ofthemuon-EmissT

sys-tem(bottom).Signalandbackgroundyieldsaretakenfromthebinned maximum-likelihoodfitdescribedinthetext.Theuncertaintybandcorrespondstothe uncer-taintyintheyieldsasgivenbythefit.Thelastbininbothplotsincludesoverflow events.Thelowerpanelsshowtheratioofobserveddataeventstothetotalfitted yield.

Acknowledgements

We congratulate our colleagues in the CERN accelerator de-partments for the excellent performance of the LHC and thank thetechnicalandadministrativestaffsatCERN andatotherCMS institutes for their contributions to the success of the CMS ef-fort.Inaddition,wegratefullyacknowledgethecomputingcenters andpersonnel of the Worldwide LHC Computing Grid for deliv-eringso effectively the computinginfrastructure essential to our

Fig. 4. ThedistributionoftheinvariantmassmJ1J2 ofthetwoselectedbjets(top)

andthedistributionofthetransversemomentumofthedijetsystem,pT,J1J2

(bot-tom).Signalandbackgroundyieldsaretakenfromthebinnedmaximum-likelihood fitdescribedinthetext.Theuncertaintybandcorrespondstothetotaluncertainty inthefittedyields.Thelastbininbothfiguresincludesoverflowevents.Thelower panelsshowtheratioofobserveddataeventstothetotalfittedyield.

analyses. Finally, we acknowledge the enduring support for the constructionandoperationoftheLHCandtheCMSdetector pro-vided by the following funding agencies: BMWF and FWF (Aus-tria);FNRSandFWO(Belgium);CNPq,CAPES,FAPERJ,andFAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China); COLCIENCIAS (Colombia); MSES andCSF (Croatia); RPF (Cyprus); MoER, SF0690030s09 and ERDF (Estonia); Academy of Finland, MEC, and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hun-gary);DAEandDST(India);IPM(Iran);SFI(Ireland);INFN(Italy);

NRF and WCU (Republic of Korea); LAS (Lithuania); CINVESTAV, CONACYT,SEP,andUASLP-FAI(Mexico);MBIE(NewZealand);PAEC (Pakistan);MSHEandNSC(Poland);FCT(Portugal);JINR(Dubna); MON,RosAtom,RASandRFBR(Russia);MESTD(Serbia);SEIDIand CPAN(Spain);SwissFundingAgencies(Switzerland);NSC(Taipei); ThEPCenter,IPST,STARandNSTDA(Thailand);TUBITAKandTAEK (Turkey);NASU (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

S. Chatrchyan,V. Khachatryan,A.M. Sirunyan, A. Tumasyan YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam, T. Bergauer, M. Dragicevic,J. Erö,C. Fabjan1,M. Friedl,R. Frühwirth1, V.M. Ghete,

N. Hörmann, J. Hrubec, M. Jeitler1,W. Kiesenhofer, V. Knünz,M. Krammer1, I. Krätschmer,D. Liko, I. Mikulec,D. Rabady2, B. Rahbaran,H. Rohringer, R. Schöfbeck, J. Strauss, A. Taurok,

W. Treberer-Treberspurg,W. Waltenberger, C.-E. Wulz1 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, L. Mucibello,S. Ochesanu, B. Roland,R. Rougny, Z. Staykova, H. Van Haevermaet,P. Van Mechelen, N. Van Remortel,A. Van Spilbeeck

UniversiteitAntwerpen,Antwerpen,Belgium

F. Blekman, S. Blyweert,J. D’Hondt, N. Heracleous,A. Kalogeropoulos, J. Keaveney, S. Lowette,M. Maes, A. Olbrechts,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, T. Hreus, 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. Dildick,G. Garcia,B. Klein, J. Lellouch, A. Marinov,J. Mccartin, A.A. Ocampo Rios, D. Ryckbosch, M. Sigamani,N. Strobbe, F. Thyssen, M. Tytgat, S. Walsh,E. Yazgan, N. Zaganidis

GhentUniversity,Ghent,Belgium

S. Basegmez, C. Beluffi3, G. Bruno,R. Castello, A. Caudron, L. Ceard, G.G. Da Silveira,C. Delaere, T. du Pree,D. Favart,L. Forthomme, A. Giammanco4,J. Hollar, P. Jez, V. Lemaitre,J. Liao, O. Militaru, C. Nuttens,D. Pagano,A. Pin, K. Piotrzkowski, A. Popov5, M. Selvaggi, M. Vidal Marono,J.M. Vizan Garcia UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

N. Beliy,T. Caebergs, E. Daubie, G.H. Hammad UniversitédeMons,Mons,Belgium

G.A. Alves,M. Correa Martins Junior,T. Martins, M.E. Pol, M.H.G. Souza CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

W.L. Aldá Júnior, W. Carvalho,J. Chinellato6, A. Custódio, E.M. Da Costa, D. De Jesus Damiao,

C. De Oliveira Martins, S. Fonseca De Souza, H. Malbouisson,M. Malek, D. Matos Figueiredo, L. Mundim, H. Nogima,W.L. Prado Da Silva, J. Santaolalla, A. Santoro, A. Sznajder,E.J. Tonelli Manganote6,

A. Vilela Pereira

C.A. Bernardesb, F.A. Diasa,7,T.R. Fernandez Perez Tomeia, E.M. Gregoresb, C. Laganaa, P.G. Mercadanteb,S.F. Novaesa,Sandra S. Padulaa

a_{UniversidadeEstadualPaulista,SãoPaulo,Brazil} b_{UniversidadeFederaldoABC,SãoPaulo,Brazil}

V. Genchev2, P. Iaydjiev2,S. Piperov, M. Rodozov,G. Sultanov, M. Vutova InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria

A. Dimitrov, I. Glushkov,R. Hadjiiska, V. Kozhuharov, L. Litov,B. Pavlov, P. Petkov UniversityofSofia,Sofia,Bulgaria

J.G. Bian, G.M. Chen,H.S. Chen, M. Chen, C.H. Jiang, D. Liang, S. Liang,X. Meng, R. Plestina8,J. Tao, X. Wang,Z. Wang

InstituteofHighEnergyPhysics,Beijing,China

C. Asawatangtrakuldee, Y. Ban, Y. Guo, Q. Li, W. Li, S. Liu, Y. Mao,S.J. Qian, D. Wang,L. Zhang, W. Zou StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,C.A. Carrillo Montoya, L.F. Chaparro Sierra,C. Florez, J.P. Gomez, B. Gomez Moreno,J.C. Sanabria UniversidaddeLosAndes,Bogota,Colombia

N. Godinovic, D. Lelas, D. Polic, I. Puljak TechnicalUniversityofSplit,Split,Croatia

Z. Antunovic, M. Kovac UniversityofSplit,Split,Croatia

V. Brigljevic,K. Kadija, J. Luetic, D. Mekterovic, S. Morovic, L. Tikvica InstituteRudjerBoskovic,Zagreb,Croatia

A. Attikis, G. Mavromanolakis, J. Mousa,C. Nicolaou, F. Ptochos, P.A. Razis UniversityofCyprus,Nicosia,Cyprus

M. Finger,M. Finger Jr. CharlesUniversity,Prague,CzechRepublic

A.A. Abdelalim9,Y. Assran10,S. Elgammal9, A. Ellithi Kamel11,M.A. Mahmoud12,A. Radi13,14 AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

M. Kadastik, M. Müntel, M. Murumaa, M. Raidal, L. Rebane, A. Tiko NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola, G. Fedi,M. Voutilainen DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Härkönen,V. Karimäki, R. Kinnunen, M.J. Kortelainen, T. Lampén, K. Lassila-Perini,S. Lehti, T. Lindén, P. Luukka, T. Mäenpää,T. Peltola, E. Tuominen, J. Tuominiemi,E. Tuovinen, L. Wendland

HelsinkiInstituteofPhysics,Helsinki,Finland T. Tuuva

M. Besancon,F. Couderc, M. Dejardin, D. Denegri,B. Fabbro, J.L. Faure, F. Ferri, S. Ganjour, A. Givernaud, P. Gras, G. Hamel de Monchenault,P. Jarry,E. Locci, J. Malcles,A. Nayak,J. Rander, A. Rosowsky, M. Titov DSM/IRFU,CEA/Saclay,Gif-sur-Yvette,France

S. Baffioni,F. Beaudette, L. Benhabib, M. Bluj15,P. Busson, C. Charlot, N. Daci,T. Dahms, M. Dalchenko, L. Dobrzynski,A. Florent, R. Granier de Cassagnac, M. Haguenauer, P. Miné, C. Mironov, I.N. Naranjo, M. Nguyen,C. Ochando, P. Paganini, D. Sabes, R. Salerno,Y. Sirois, C. Veelken, A. Zabi

LaboratoireLeprince-Ringuet,EcolePolytechnique,IN2P3–CNRS,Palaiseau,France

J.-L. Agram16,J. Andrea, D. Bloch,J.-M. Brom, E.C. Chabert,C. Collard, E. Conte16,F. Drouhin16, J.-C. Fontaine16,D. Gelé, U. Goerlach,C. Goetzmann, P. Juillot, A.-C. Le Bihan, P. Van Hove InstitutPluridisciplinaireHubertCurien,UniversitédeStrasbourg,UniversitédeHauteAlsaceMulhouse,CNRS/IN2P3,Strasbourg,France

S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,N. Beaupere, G. Boudoul,S. Brochet, J. Chasserat, R. Chierici,D. Contardo, P. Depasse, H. El Mamouni,J. Fan, J. Fay, S. Gascon,M. Gouzevitch, B. Ille, T. Kurca, M. Lethuillier,L. Mirabito, S. Perries,J.D. Ruiz Alvarez, L. Sgandurra,V. Sordini, M. Vander Donckt, P. Verdier,S. Viret, H. Xiao UniversitédeLyon,UniversitéClaudeBernardLyon1,CNRS–IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France

Z. Tsamalaidze17

InstituteofHighEnergyPhysicsandInformatization,TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann,S. Beranek, M. Bontenackels, B. Calpas,M. Edelhoff, L. Feld, O. Hindrichs, K. Klein, A. Ostapchuk,A. Perieanu, F. Raupach, J. Sammet, S. Schael, D. Sprenger, H. Weber, B. Wittmer, V. Zhukov5

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

M. Ata, J. Caudron,E. Dietz-Laursonn, D. Duchardt, M. Erdmann,R. Fischer,A. Güth, T. Hebbeker, C. Heidemann,K. Hoepfner,D. Klingebiel, S. Knutzen,P. Kreuzer, M. Merschmeyer,A. Meyer,

M. Olschewski, K. Padeken,P. Papacz, H. Pieta,H. Reithler, S.A. Schmitz,L. Sonnenschein, J. Steggemann, D. Teyssier,S. Thüer, M. Weber

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

V. Cherepanov, Y. Erdogan,G. Flügge, H. Geenen, M. Geisler, W. Haj Ahmad, F. Hoehle,B. Kargoll, T. Kress,Y. Kuessel, J. Lingemann2,A. Nowack, I.M. Nugent,L. Perchalla, O. Pooth, A. Stahl

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

I. Asin,N. Bartosik, J. Behr,W. Behrenhoff, U. Behrens,A.J. Bell, M. Bergholz18,A. Bethani, K. Borras, A. Burgmeier,A. Cakir,L. Calligaris, A. Campbell, S. Choudhury, F. Costanza, C. Diez Pardos,S. Dooling, T. Dorland,G. Eckerlin, D. Eckstein, G. Flucke,A. Geiser, A. Grebenyuk, P. Gunnellini, S. Habib,J. Hauk, G. Hellwig,M. Hempel, D. Horton, H. Jung, M. Kasemann,P. Katsas, C. Kleinwort, H. Kluge, M. Krämer, D. Krücker,W. Lange, J. Leonard, K. Lipka,W. Lohmann18, B. Lutz,R. Mankel, I. Marfin,

I.-A. Melzer-Pellmann,A.B. Meyer, J. Mnich, A. Mussgiller, S. Naumann-Emme, O. Novgorodova, F. Nowak,J. Olzem, H. Perrey,A. Petrukhin, D. Pitzl,R. Placakyte, A. Raspereza, P.M. Ribeiro Cipriano, C. Riedl,E. Ron, M.Ö. Sahin,J. Salfeld-Nebgen, R. Schmidt18, T. Schoerner-Sadenius,M. Schröder, N. Sen, M. Stein, R. Walsh, C. Wissing

DeutschesElektronen-Synchrotron,Hamburg,Germany

M. Aldaya Martin,V. Blobel, H. Enderle, J. Erfle,E. Garutti, U. Gebbert,M. Görner, M. Gosselink, J. Haller, K. Heine, R.S. Höing,G. Kaussen, H. Kirschenmann, R. Klanner,R. Kogler, J. Lange,I. Marchesini,

T. Peiffer, N. Pietsch,D. Rathjens, C. Sander, H. Schettler, P. Schleper, E. Schlieckau, A. Schmidt,T. Schum, M. Seidel,J. Sibille19, V. Sola, H. Stadie,G. Steinbrück, J. Thomsen, D. Troendle,E. Usai, L. Vanelderen UniversityofHamburg,Hamburg,Germany

C. Barth,C. Baus, J. Berger,C. Böser, E. Butz, T. Chwalek, W. De Boer,A. Descroix, A. Dierlamm, M. Feindt,M. Guthoff2, F. Hartmann2,T. Hauth2,H. Held, K.H. Hoffmann, U. Husemann,I. Katkov5, J.R. Komaragiri, A. Kornmayer2,E. Kuznetsova, P. Lobelle Pardo, D. Martschei, M.U. Mozer, Th. Müller, M. Niegel, A. Nürnberg, O. Oberst,J. Ott, G. Quast, K. Rabbertz, F. Ratnikov,S. Röcker, F.-P. Schilling, G. Schott, H.J. Simonis,F.M. Stober, R. Ulrich, J. Wagner-Kuhr, S. Wayand,T. Weiler, M. Zeise

InstitutfürExperimentelleKernphysik,Karlsruhe,Germany

G. Anagnostou, G. Daskalakis,T. Geralis,S. Kesisoglou, A. Kyriakis, D. Loukas, A. Markou, C. Markou, E. Ntomari,I. Topsis-giotis

InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece L. Gouskos,A. Panagiotou, N. Saoulidou, E. Stiliaris UniversityofAthens,Athens,Greece

X. Aslanoglou,I. Evangelou, G. Flouris, C. Foudas,P. Kokkas, N. Manthos, I. Papadopoulos, E. Paradas UniversityofIoánnina,Ioánnina,Greece

G. Bencze,C. Hajdu, P. Hidas, D. Horvath20, F. Sikler,V. Veszpremi, G. Vesztergombi21,A.J. Zsigmond WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni, S. Czellar, J. Molnar, J. Palinkas, Z. Szillasi InstituteofNuclearResearchATOMKI,Debrecen,Hungary

J. Karancsi, P. Raics, Z.L. Trocsanyi,B. Ujvari UniversityofDebrecen,Debrecen,Hungary

S.K. Swain22

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S.B. Beri, V. Bhatnagar,N. Dhingra, R. Gupta, M. Kaur, M.Z. Mehta, M. Mittal, N. Nishu, A. Sharma, J.B. Singh

PanjabUniversity,Chandigarh,India

Ashok Kumar, Arun Kumar,S. Ahuja, A. Bhardwaj, B.C. Choudhary,A. Kumar, S. Malhotra,M. Naimuddin, K. Ranjan,P. Saxena, V. Sharma, R.K. Shivpuri

UniversityofDelhi,Delhi,India

S. Banerjee, S. Bhattacharya, K. Chatterjee,S. Dutta, B. Gomber, Sa. Jain, Sh. Jain,R. Khurana, A. Modak, S. Mukherjee,D. Roy, S. Sarkar, M. Sharan, A.P. Singh

SahaInstituteofNuclearPhysics,Kolkata,India

A. Abdulsalam, D. Dutta, S. Kailas,V. Kumar, A.K. Mohanty2, L.M. Pant, P. Shukla,A. Topkar BhabhaAtomicResearchCentre,Mumbai,India

T. Aziz, R.M. Chatterjee,S. Ganguly, S. Ghosh,M. Guchait23,A. Gurtu24,G. Kole, S. Kumar, M. Maity25, G. Majumder, K. Mazumdar,G.B. Mohanty, B. Parida, K. Sudhakar,N. Wickramage26

S. Banerjee, S. Dugad

TataInstituteofFundamentalResearch–HECR,Mumbai,India

H. Arfaei, H. Bakhshiansohi,S.M. Etesami27,A. Fahim28,A. Jafari, M. Khakzad, M. Mohammadi Najafabadi,S. Paktinat Mehdiabadi, B. Safarzadeh29,M. Zeinali InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran

M. Grunewald

UniversityCollegeDublin,Dublin,Ireland

M. Abbresciaa,b, L. Barbonea,b,C. Calabriaa,b,S.S. Chhibraa,b,A. Colaleoa,D. Creanzaa,c,

N. De Filippisa,c, M. De Palmaa,b, L. Fiorea, G. Iasellia,c, G. Maggia,c, M. Maggia,B. Marangellia,b, S. Mya,c,S. Nuzzoa,b, N. Pacificoa, A. Pompilia,b,G. Pugliesea,c, R. Radognaa,b,G. Selvaggia,b, L. Silvestrisa, G. Singha,b,R. Vendittia,b,P. Verwilligena,G. Zitoa

a_{INFNSezionediBari,Bari,Italy} b_{UniversitàdiBari,Bari,Italy} c_{PolitecnicodiBari,Bari,Italy}

G. Abbiendia,A.C. Benvenutia, D. Bonacorsia,b, S. Braibant-Giacomellia,b,L. Brigliadoria,b, R. Campaninia,b,P. Capiluppia,b,A. Castroa,b, F.R. Cavalloa,G. Codispotia,b, M. Cuffiania,b,

G.M. Dallavallea,F. Fabbria,A. Fanfania,b,D. Fasanellaa,b, P. Giacomellia, C. Grandia, L. Guiduccia,b, S. Marcellinia, G. Masettia,M. Meneghellia,b,A. Montanaria,F.L. Navarriaa,b,F. Odoricia,A. Perrottaa, F. Primaveraa,b, A.M. Rossia,b, T. Rovellia,b, G.P. Sirolia,b,N. Tosia,b, R. Travaglinia,b

a_{INFNSezionediBologna,Bologna,Italy} b_{UniversitàdiBologna,Bologna,Italy}

S. Albergoa,b, G. Cappelloa,M. Chiorbolia,b,S. Costaa,b,F. Giordanoa,c,2,R. Potenzaa,b, A. Tricomia,b, C. Tuvea,b

a_{INFNSezionediCatania,Catania,Italy} b_{UniversitàdiCatania,Catania,Italy} c_{CSFNSM,Catania,Italy}

G. Barbaglia,V. Ciullia,b,C. Civininia, R. D’Alessandroa,b, E. Focardia,b,E. Galloa, S. Gonzia,b, V. Goria,b, P. Lenzia,b, M. Meschinia, S. Paolettia,G. Sguazzonia,A. Tropianoa,b

a_{INFNSezionediFirenze,Firenze,Italy} b_{UniversitàdiFirenze,Firenze,Italy}

L. Benussi,S. Bianco, F. Fabbri, D. Piccolo INFNLaboratoriNazionalidiFrascati,Frascati,Italy

P. Fabbricatorea,R. Ferrettia,b,F. Ferroa,M. Lo Veterea,b,R. Musenicha, E. Robuttia,S. Tosia,b

a_{INFNSezionediGenova,Genova,Italy} b_{UniversitàdiGenova,Genova,Italy}

A. Benagliaa,M.E. Dinardoa,b, S. Fiorendia,b,2, S. Gennaia,A. Ghezzia,b,P. Govonia,b,M.T. Lucchinia,b,2, S. Malvezzia, R.A. Manzonia,b,2,A. Martellia,b,2, D. Menascea, L. Moronia, M. Paganonia,b,D. Pedrinia, S. Ragazzia,b,N. Redaellia,T. Tabarelli de Fatisa,b

a_{INFNSezionediMilano-Bicocca,Milano,Italy} b_{UniversitàdiMilano-Bicocca,Milano,Italy}

S. Buontempoa, N. Cavalloa,c,F. Fabozzia,c,A.O.M. Iorioa,b, L. Listaa,S. Meolaa,d,2,M. Merolaa, P. Paoluccia,2

a_{INFNSezionediNapoli,Napoli,Italy} b_{UniversitàdiNapoli‘FedericoII’,Napoli,Italy} c_{UniversitàdellaBasilicata(Potenza),Napoli,Italy} d_{UniversitàG.Marconi(Roma),Napoli,Italy}

P. Azzia,N. Bacchettaa,D. Biselloa,b,A. Brancaa,b,R. Carlina,b, P. Checchiaa,T. Dorigoa, U. Dossellia, M. Galantia,b,2, F. Gasparinia,b, U. Gasparinia,b, P. Giubilatoa,b,A. Gozzelinoa,K. Kanishcheva,c, S. Lacapraraa,I. Lazzizzeraa,c, M. Margonia,b, A.T. Meneguzzoa,b,F. Montecassianoa,J. Pazzinia,b, M. Pegoraroa, N. Pozzobona,b, P. Ronchesea,b,F. Simonettoa,b, E. Torassaa,M. Tosia,b,A. Triossia, P. Zottoa,b, A. Zucchettaa,b,G. Zumerlea,b

a_{INFNSezionediPadova,Padova,Italy} b_{UniversitàdiPadova,Padova,Italy} c_{UniversitàdiTrento(Trento),Padova,Italy}

M. Gabusia,b, S.P. Rattia,b,C. Riccardia,b,P. Vituloa,b

a_{INFNSezionediPavia,Pavia,Italy} b_{UniversitàdiPavia,Pavia,Italy}

M. Biasinia,b, G.M. Bileia,L. Fanòa,b, P. Laricciaa,b, G. Mantovania,b,M. Menichellia, A. Nappia,b,†, F. Romeoa,b, A. Sahaa, A. Santocchiaa,b,A. Spieziaa,b

a_{INFNSezionediPerugia,Perugia,Italy} b_{UniversitàdiPerugia,Perugia,Italy}

K. Androsova,30,P. Azzurria,G. Bagliesia,J. Bernardinia,T. Boccalia,G. Broccoloa,c,R. Castaldia, M.A. Cioccia,30, R. Dell’Orsoa, F. Fioria,c,L. Foàa,c,A. Giassia, M.T. Grippoa,30, A. Kraana, F. Ligabuea,c, T. Lomtadzea, L. Martinia,b, A. Messineoa,b,C.S. Moona,31, F. Pallaa,A. Rizzia,b,A. Savoy-Navarroa,32, A.T. Serbana,P. Spagnoloa,P. Squillaciotia,30,R. Tenchinia, G. Tonellia,b,A. Venturia, P.G. Verdinia, C. Vernieria,c

a_{INFNSezionediPisa,Pisa,Italy} b_{UniversitàdiPisa,Pisa,Italy}

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

L. Baronea,b,F. Cavallaria, D. Del Rea,b,M. Diemoza, M. Grassia,b,C. Jordaa, E. Longoa,b, F. Margarolia,b, P. Meridiania,F. Michelia,b,S. Nourbakhsha,b, G. Organtinia,b, R. Paramattia,S. Rahatloua,b,C. Rovellia, L. Soffia,b

a_{INFNSezionediRoma,Roma,Italy} b_{UniversitàdiRoma,Roma,Italy}

N. Amapanea,b, R. Arcidiaconoa,c,S. Argiroa,b,M. Arneodoa,c,R. Bellana,b, C. Biinoa, N. Cartigliaa, S. Casassoa,b, M. Costaa,b,A. Deganoa,b,N. Demariaa, C. Mariottia, S. Masellia,E. Migliorea,b,

V. Monacoa,b, M. Musicha, M.M. Obertinoa,c, G. Ortonaa,b, L. Pachera,b,N. Pastronea,M. Pelliccionia,2, A. Potenzaa,b,A. Romeroa,b, M. Ruspaa,c, R. Sacchia,b,A. Solanoa,b,A. Staianoa, U. Tamponia

a_{INFNSezionediTorino,Torino,Italy} b_{UniversitàdiTorino,Torino,Italy}

c_{UniversitàdelPiemonteOrientale(Novara),Torino,Italy}

S. Belfortea,V. Candelisea,b,M. Casarsaa, F. Cossuttia,2, G. Della Riccaa,b,B. Gobboa,C. La Licataa,b, M. Maronea,b,D. Montaninoa,b,A. Penzoa,A. Schizzia,b,T. Umera,b, A. Zanettia

a_{INFNSezionediTrieste,Trieste,Italy} b_{UniversitàdiTrieste,Trieste,Italy}

S. Chang, T.Y. Kim,S.K. Nam KangwonNationalUniversity,Chunchon,RepublicofKorea

D.H. Kim,G.N. Kim, J.E. Kim, D.J. Kong, S. Lee, Y.D. Oh,H. Park, D.C. Son KyungpookNationalUniversity,Daegu,RepublicofKorea

J.Y. Kim, Zero J. Kim, S. Song

S. Choi, D. Gyun,B. Hong,M. Jo, H. Kim, T.J. Kim, K.S. Lee, S.K. Park,Y. Roh KoreaUniversity,Seoul,RepublicofKorea

M. Choi,J.H. Kim, C. Park, I.C. Park, S. Park,G. Ryu UniversityofSeoul,Seoul,RepublicofKorea

Y. Choi,Y.K. Choi,J. Goh, M.S. Kim, E. Kwon, B. Lee, J. Lee,S. Lee, H. Seo,I. Yu SungkyunkwanUniversity,Suwon,RepublicofKorea

I. Grigelionis,A. Juodagalvis VilniusUniversity,Vilnius,Lithuania

H. Castilla-Valdez,E. De La Cruz-Burelo, I. Heredia-de La Cruz33,R. Lopez-Fernandez, J. Martínez-Ortega, A. Sanchez-Hernandez,L.M. Villasenor-Cendejas

CentrodeInvestigacionydeEstudiosAvanzadosdelIPN,MexicoCity,Mexico S. Carrillo Moreno, F. Vazquez Valencia UniversidadIberoamericana,MexicoCity,Mexico

H.A. Salazar Ibarguen

BenemeritaUniversidadAutonomadePuebla,Puebla,Mexico E. Casimiro Linares, A. Morelos Pineda UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico D. Krofcheck

UniversityofAuckland,Auckland,NewZealand

P.H. Butler,R. Doesburg, S. Reucroft, H. Silverwood UniversityofCanterbury,Christchurch,NewZealand

M. Ahmad, M.I. Asghar,J. Butt, H.R. Hoorani, S. Khalid, W.A. Khan, T. Khurshid, S. Qazi,M.A. Shah, M. Shoaib

NationalCentreforPhysics,Quaid-I-AzamUniversity,Islamabad,Pakistan

H. Bialkowska,B. Boimska, T. Frueboes,M. Górski, M. Kazana, K. Nawrocki, K. Romanowska-Rybinska, M. Szleper,G. Wrochna, P. Zalewski

NationalCentreforNuclearResearch,Swierk,Poland

G. Brona,K. Bunkowski, M. Cwiok,W. Dominik,K. Doroba, A. Kalinowski,M. Konecki, J. Krolikowski, M. Misiura, W. Wolszczak

InstituteofExperimentalPhysics,FacultyofPhysics,UniversityofWarsaw,Warsaw,Poland

N. Almeida, P. Bargassa,C. Beirão Da Cruz E Silva, P. Faccioli, P.G. Ferreira Parracho,M. Gallinaro, F. Nguyen, J. Rodrigues Antunes,J. Seixas2,J. Varela, P. Vischia

LaboratóriodeInstrumentaçãoeFísicaExperimentaldePartículas,Lisboa,Portugal

P. Bunin,M. Gavrilenko, I. Golutvin, A. Kamenev, V. Karjavin, V. Konoplyanikov,G. Kozlov, A. Lanev, A. Malakhov,V. Matveev, P. Moisenz, V. Palichik,V. Perelygin, S. Shmatov, S. Shulha,N. Skatchkov, V. Smirnov,A. Zarubin

S. Evstyukhin,V. Golovtsov, Y. Ivanov, V. Kim, P. Levchenko,V. Murzin, V. Oreshkin, I. Smirnov, V. Sulimov, L. Uvarov, S. Vavilov, A. Vorobyev,An. Vorobyev

PetersburgNuclearPhysicsInstitute,Gatchina(St.Petersburg),Russia

Yu. Andreev,A. Dermenev, S. Gninenko, N. Golubev, M. Kirsanov,N. Krasnikov, A. Pashenkov, D. Tlisov, A. Toropin

InstituteforNuclearResearch,Moscow,Russia

V. Epshteyn, V. Gavrilov, N. Lychkovskaya,V. Popov, G. Safronov, S. Semenov, A. Spiridonov, V. Stolin, E. Vlasov, A. Zhokin

InstituteforTheoreticalandExperimentalPhysics,Moscow,Russia

V. Andreev,M. Azarkin, I. Dremin, M. Kirakosyan, A. Leonidov, G. Mesyats, S.V. Rusakov,A. Vinogradov P.N.LebedevPhysicalInstitute,Moscow,Russia

A. Belyaev,E. Boos, V. Bunichev, M. Dubinin7, L. Dudko,A. Ershov, A. Gribushin,V. Klyukhin, O. Kodolova,I. Lokhtin, A. Markina,S. Obraztsov,V. Savrin, A. Snigirev

SkobeltsynInstituteofNuclearPhysics,LomonosovMoscowStateUniversity,Moscow,Russia

I. Azhgirey,I. Bayshev,S. Bitioukov, V. Kachanov, A. Kalinin, D. Konstantinov, V. Krychkine, V. Petrov, R. Ryutin, A. Sobol, L. Tourtchanovitch,S. Troshin, N. Tyurin, A. Uzunian,A. Volkov

StateResearchCenterofRussianFederation,InstituteforHighEnergyPhysics,Protvino,Russia P. Adzic34, M. Djordjevic,M. Ekmedzic, J. Milosevic UniversityofBelgrade,FacultyofPhysicsandVincaInstituteofNuclearSciences,Belgrade,Serbia

M. Aguilar-Benitez,J. Alcaraz Maestre, C. Battilana,E. Calvo, M. Cerrada,M. Chamizo Llatas2,N. Colino, B. De La Cruz,A. Delgado Peris,D. Domínguez Vázquez, C. Fernandez Bedoya,J.P. Fernández Ramos, A. Ferrando, J. Flix, M.C. Fouz,P. Garcia-Abia, O. Gonzalez Lopez,S. Goy Lopez, J.M. Hernandez, M.I. Josa, G. Merino, E. Navarro De Martino,J. Puerta Pelayo, A. Quintario Olmeda, I. Redondo,L. Romero,

M.S. Soares, C. Willmott

CentrodeInvestigacionesEnergéticasMedioambientalesyTecnológicas(CIEMAT),Madrid,Spain C. Albajar, J.F. de Trocóniz

UniversidadAutónomadeMadrid,Madrid,Spain

H. Brun, J. Cuevas,J. Fernandez Menendez, S. Folgueras, I. Gonzalez Caballero, L. Lloret Iglesias UniversidaddeOviedo,Oviedo,Spain

J.A. Brochero Cifuentes, I.J. Cabrillo, A. Calderon, S.H. Chuang,J. Duarte Campderros, M. Fernandez, G. Gomez, J. Gonzalez Sanchez, A. Graziano, A. Lopez Virto,J. Marco, R. Marco,C. Martinez Rivero, F. Matorras, F.J. Munoz Sanchez,J. Piedra Gomez, T. Rodrigo, A.Y. Rodríguez-Marrero, A. Ruiz-Jimeno, L. Scodellaro,I. Vila, R. Vilar Cortabitarte

InstitutodeFísicadeCantabria(IFCA),CSIC–UniversidaddeCantabria,Santander,Spain

D. Abbaneo, E. Auffray, G. Auzinger, M. Bachtis,P. Baillon, A.H. Ball, D. Barney, J. Bendavid,J.F. Benitez, C. Bernet8,G. Bianchi, P. Bloch, A. Bocci, A. Bonato, O. Bondu,C. Botta, H. Breuker, T. Camporesi, G. Cerminara, T. Christiansen, J.A. Coarasa Perez, S. Colafranceschi35,M. D’Alfonso, D. d’Enterria, A. Dabrowski,A. David, F. De Guio, A. De Roeck, S. De Visscher, S. Di Guida, M. Dobson,

N. Dupont-Sagorin, A. Elliott-Peisert,J. Eugster, G. Franzoni, W. Funk, M. Giffels,D. Gigi, K. Gill, D. Giordano, M. Girone,M. Giunta, F. Glege, R. Gomez-Reino Garrido, S. Gowdy, R. Guida,J. Hammer,

M. Hansen,P. Harris,C. Hartl, A. Hinzmann, V. Innocente, P. Janot, E. Karavakis,K. Kousouris, K. Krajczar, P. Lecoq,Y.-J. Lee,C. Lourenço, N. Magini, L. Malgeri, M. Mannelli,L. Masetti, F. Meijers, S. Mersi,

E. Meschi,M. Mulders, P. Musella,L. Orsini, E. Palencia Cortezon, E. Perez,L. Perrozzi,A. Petrilli,

G. Petrucciani,A. Pfeiffer,M. Pierini, M. Pimiä, D. Piparo, M. Plagge, L. Quertenmont,A. Racz, W. Reece, G. Rolandi36,M. Rovere, H. Sakulin,F. Santanastasio, C. Schäfer, C. Schwick, S. Sekmen,A. Sharma, P. Siegrist,P. Silva, M. Simon, P. Sphicas37, D. Spiga,B. Stieger, M. Stoye, A. Tsirou,G.I. Veres21, J.R. Vlimant,H.K. Wöhri, W.D. Zeuner

CERN,EuropeanOrganizationforNuclearResearch,Geneva,Switzerland

W. Bertl,K. Deiters,W. Erdmann, K. Gabathuler,R. Horisberger, Q. Ingram, H.C. Kaestli, S. König, D. Kotlinski,U. Langenegger, D. Renker, T. Rohe

PaulScherrerInstitut,Villigen,Switzerland

F. Bachmair, L. Bäni, L. Bianchini, P. Bortignon, M.A. Buchmann,B. Casal, N. Chanon, A. Deisher, G. Dissertori, M. Dittmar, M. Donegà, M. Dünser, P. Eller, K. Freudenreich,C. Grab, D. Hits,P. Lecomte, W. Lustermann,B. Mangano,A.C. Marini, P. Martinez Ruiz del Arbol, D. Meister,N. Mohr, F. Moortgat, C. Nägeli38, P. Nef,F. Nessi-Tedaldi, F. Pandolfi, L. Pape, F. Pauss,M. Peruzzi, M. Quittnat,F.J. Ronga, M. Rossini,L. Sala,A.K. Sanchez, A. Starodumov39, M. Takahashi, L. Tauscher†, K. Theofilatos,D. Treille, R. Wallny,H.A. Weber

InstituteforParticlePhysics,ETHZurich,Zurich,Switzerland

C. Amsler40,V. Chiochia, A. De Cosa, C. Favaro,M. Ivova Rikova, B. Kilminster, B. Millan Mejias, J. Ngadiuba,P. Robmann, H. Snoek, S. Taroni, M. Verzetti, Y. Yang

UniversitätZürich,Zurich,Switzerland

M. Cardaci,K.H. Chen, C. Ferro, C.M. Kuo, S.W. Li, W. Lin, Y.J. Lu, R. Volpe,S.S. Yu NationalCentralUniversity,Chung-Li,Taiwan

P. Bartalini,P. Chang, Y.H. Chang, Y.W. Chang,Y. Chao, K.F. Chen, C. Dietz, U. Grundler,W.-S. Hou, Y. Hsiung, K.Y. Kao,Y.J. Lei, Y.F. Liu,R.-S. Lu, D. Majumder, E. Petrakou,X. Shi, J.G. Shiu, Y.M. Tzeng, M. Wang

NationalTaiwanUniversity(NTU),Taipei,Taiwan B. Asavapibhop,N. Suwonjandee ChulalongkornUniversity,Bangkok,Thailand

A. Adiguzel, M.N. Bakirci41,S. Cerci42, C. Dozen,I. Dumanoglu, E. Eskut, S. Girgis, G. Gokbulut, E. Gurpinar,I. Hos, E.E. Kangal, A. Kayis Topaksu,G. Onengut43, K. Ozdemir, S. Ozturk41, A. Polatoz, K. Sogut44, D. Sunar Cerci42,B. Tali42, H. Topakli41,M. Vergili

CukurovaUniversity,Adana,Turkey

I.V. Akin,T. Aliev, B. Bilin, S. Bilmis, M. Deniz,H. Gamsizkan, A.M. Guler, G. Karapinar45,K. Ocalan, A. Ozpineci,M. Serin, R. Sever, U.E. Surat,M. Yalvac, M. Zeyrek

MiddleEastTechnicalUniversity,PhysicsDepartment,Ankara,Turkey

E. Gülmez,B. Isildak46,M. Kaya47,O. Kaya47, S. Ozkorucuklu48, N. Sonmez49 BogaziciUniversity,Istanbul,Turkey

H. Bahtiyar50,E. Barlas, K. Cankocak, Y.O. Günaydin51, F.I. Vardarlı, M. Yücel IstanbulTechnicalUniversity,Istanbul,Turkey

L. Levchuk,P. Sorokin

NationalScientificCenter,KharkovInstituteofPhysicsandTechnology,Kharkov,Ukraine

J.J. Brooke,E. Clement, D. Cussans, H. Flacher,R. Frazier, J. Goldstein,M. Grimes, G.P. Heath, H.F. Heath, J. Jacob, L. Kreczko,C. Lucas, Z. Meng, S. Metson, D.M. Newbold52,K. Nirunpong, S. Paramesvaran, A. Poll, S. Senkin, V.J. Smith,T. Williams

UniversityofBristol,Bristol,UnitedKingdom

K.W. Bell, A. Belyaev53, C. Brew, R.M. Brown, D.J.A. Cockerill, J.A. Coughlan, K. Harder, S. Harper, J. Ilic, E. Olaiya,D. Petyt, B.C. Radburn-Smith, C.H. Shepherd-Themistocleous, A. Thea, I.R. Tomalin,

W.J. Womersley, S.D. Worm RutherfordAppletonLaboratory,Didcot,UnitedKingdom

R. Bainbridge,O. Buchmuller, D. Burton,D. Colling, N. Cripps,M. Cutajar, P. Dauncey,G. Davies,

M. Della Negra, W. Ferguson,J. Fulcher, D. Futyan, A. Gilbert,A. Guneratne Bryer, G. Hall,Z. Hatherell, J. Hays, G. Iles, M. Jarvis, G. Karapostoli, M. Kenzie,R. Lane, R. Lucas52,L. Lyons, A.-M. Magnan,

J. Marrouche, B. Mathias,R. Nandi, J. Nash, A. Nikitenko39,J. Pela, M. Pesaresi, K. Petridis,M. Pioppi54, D.M. Raymond, S. Rogerson,A. Rose, C. Seez,P. Sharp†,A. Sparrow, A. Tapper, M. Vazquez Acosta, T. Virdee,S. Wakefield, N. Wardle

ImperialCollege,London,UnitedKingdom

M. Chadwick, J.E. Cole, P.R. Hobson, A. Khan,P. Kyberd, D. Leggat,D. Leslie, W. Martin,I.D. Reid, P. Symonds, L. Teodorescu, M. Turner

BrunelUniversity,Uxbridge,UnitedKingdom

J. Dittmann, K. Hatakeyama,A. Kasmi, H. Liu, T. Scarborough BaylorUniversity,Waco,USA

O. Charaf,S.I. Cooper, C. Henderson, P. Rumerio TheUniversityofAlabama,Tuscaloosa,USA

A. Avetisyan, T. Bose,C. Fantasia, A. Heister, P. Lawson, D. Lazic, J. Rohlf, D. Sperka, J. St. John,L. Sulak BostonUniversity,Boston,USA

J. Alimena,S. Bhattacharya, G. Christopher,D. Cutts, Z. Demiragli,A. Ferapontov, A. Garabedian,

U. Heintz, S. Jabeen, G. Kukartsev, E. Laird,G. Landsberg, M. Luk, M. Narain, M. Segala, T. Sinthuprasith, T. Speer

BrownUniversity,Providence,USA

R. Breedon,G. Breto, M. Calderon De La Barca Sanchez, S. Chauhan,M. Chertok, J. Conway,R. Conway, P.T. Cox, R. Erbacher,M. Gardner, W. Ko, A. Kopecky, R. Lander,T. Miceli, D. Pellett, J. Pilot, F. Ricci-Tam, B. Rutherford, M. Searle, S. Shalhout, J. Smith, M. Squires, M. Tripathi,S. Wilbur, R. Yohay

UniversityofCalifornia,Davis,Davis,USA

V. Andreev,D. Cline, R. Cousins, S. Erhan, P. Everaerts,C. Farrell, M. Felcini,J. Hauser, M. Ignatenko, C. Jarvis,G. Rakness, P. Schlein†, E. Takasugi, P. Traczyk,V. Valuev,M. Weber

UniversityofCalifornia,LosAngeles,USA

J. Babb, R. Clare, J. Ellison,J.W. Gary, G. Hanson, J. Heilman, P. Jandir,F. Lacroix, H. Liu, O.R. Long, A. Luthra, M. Malberti,H. Nguyen, A. Shrinivas, J. Sturdy,S. Sumowidagdo, R. Wilken, S. Wimpenny UniversityofCalifornia,Riverside,Riverside,USA