Measurement of the W boson helicity fractions in the decays of top quark pairs to lepton plus jets final states produced in pp collisions at root s=8 TeV

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Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

the

W

boson

helicity

fractions

in

the

decays

of

top

quark

pairs

to

lepton

+

jets

final

states

produced

in

pp

collisions

at

√

s

=

8

TeV

Receivedinrevisedform1October2016 Accepted4October2016

Availableonline11October2016 Editor:M.Doser Keywords: CMS Physics Top Anomalouscouplings Helicity

TheW bosonhelicityfractionsfromtopquarkdecaysintt eventsaremeasuredusingdatafromproton– proton collisions atacentre-of-massenergy of8TeV.Thedata werecollectedin2012 withthe CMS detector atthe LHC, corresponding toan integratedluminosity of19.8fb−1.Eventsare reconstructed with either one muon or one electron, along with four jetsin the final state, with two of the jets beingidentified asoriginating fromb quarks.Themeasured helicityfractionsfrombothchannels are combined,yielding F0=0.681±0.012(stat)±0.023(syst),FL=0.323±0.008(stat)±0.014(syst),and

FR= −0.004±0.005(stat)±0.014(syst) forthelongitudinal,left-,andright-handedcomponentsofthe

helicity,respectively.ThesemeasurementsoftheW bosonhelicityfractionsarethemostaccuratetodate andtheyagreewiththepredictionsfromthestandardmodel.

©2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

The data from proton–proton (pp) collisions produced at the CERNLHCprovideanexcellentenvironmenttoinvestigate proper-tiesofthe topquark, in thecontext ofits productionanddecay, withunprecedentedprecision.Suchmeasurementsenablerigorous testsofthestandardmodel(SM),anddeviationsfromtheSM pre-dictionswouldindicatesignsofpossiblenewphysics[1–4].

In particular, the W boson helicity fractionsin top quark de-cays are very sensitive to the Wtb vertex structure. The W bo-son helicity fractions are defined as the partial decay rate for a given helicity state divided by the total decay rate: FL,R,0≡ L,R,0/ ,where FL, FR,and F0 are theleft-handed, right-handed, andlongitudinal helicity fractions, respectively. The helicity frac-tions areexpectedto be F0=0.687±0.005, FL=0.311±0.005, andFR=0.0017±0.0001 atnext-to-next-to-leadingorder(NNLO) in the SM, including electroweak effects, for a top quark mass mt=172.8±1.3GeV[5].AnomalousWtbcouplings,i.e.thosethat donotariseintheSM,wouldalterthesevalues.

Experimentally,theW bosonhelicitycanbemeasuredthrough thestudyofangulardistributionsofthetopquarkdecayproducts. Thehelicityangleθ∗ isdefinedastheanglebetweenthedirection

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

ofeitherthedown-typequarkorthechargedleptonarisingfrom the W boson decayand thereversed directionof thetop quark, both in the restframe of the W boson. The distribution for the cosineofthehelicityangledependsonthehelicityfractionsinthe followingway, 1  d d cosθ∗= 3 8  1−cosθ∗2FL +3 4(sinθ ∗₎2_F 0 +3 8  1+cosθ∗2FR. (1)

This dependence is shown in Fig. 1 for each contribution sepa-rately, normalised to unity, andfor the SM expectation. Charged leptons (or down-type quarks) from left-handed W bosons are preferentially emitted in the opposite direction of the W boson, and thus tend to havelower momentum andbe closer to the b jet fromthetop quarkdecay,ascomparedtochargedleptons (or down-typequarks)fromlongitudinalorright-handedWbosons.

The measurement of the W bosonhelicity issensitive to the presence of non-SM couplings between the W boson, the top quark,andthebottomquark.AgeneralparametrisationoftheWtb vertexcanbeexpressedas[1,6]

http://dx.doi.org/10.1016/j.physletb.2016.10.007

0370-2693/©2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

Fig. 1. Predictedcosθ∗distributionsforthedifferenthelicityfractions.The distribu-tionsforthefractionsF0,FL,andFRareshownasdashed,dotted,anddash-dotted

lines,respectively,andthesumofthethreecontributionsaccordingtotheSM pre-dictionsisdisplayedasasolidline.

LWtb= − g √ 2b¯γ μ_(V LPL+VRPR)t Wμ− −√g 2 ¯ biσ μν_q ν MW (gLPL+gRPR)t W_μ−+h.c., (2)

where VL, VR, gL, gR are vector and tensor couplings (complex constants),q=pt−pb,and pt (pb) isthefour-momentumofthe top quark (b quark), PL ( PR) is the left (right) projection opera-tor, andh.c.denotes the Hermitian conjugate. Hermiticity condi-tionsonthepossibledimension-sixLagrangiantermsalsoimpose Im(VL)=0 [7]. In the SM and at tree level, VL=Vtb, where Vtb≈1 isthe Cabibbo–Kobayashi–Maskawa matrixelement con-necting the top and the bottom quarks and VR= gL=gR=0. TherelationshipsbetweentheW bosonhelicityfractionsandthe anomalouscouplingsincludingdependencesonthebquark mass aregiveninRef.[8].

The helicity fractions of W bosons in top quark decays were first measured at the Tevatron Collider [9–11]. They have been alsomeasured attheLHC,usingsamplescontainingtt events ob-tainedinpp collisions at 7TeV, andhaving eitherone [12,13]or two[12]chargedleptonsinthefinalstate.TheCMSCollaboration also reported measurements using event topologies that contain onesingle reconstructed topquark [14], inppcollisions at8TeV. Limits on anomalous couplings have also been reported, derived fromWbosonhelicitymeasurements[12–14],andfromsingletop quarkdifferentialcrosssectionproductionmeasurements[15].

This Letter describes a measurement of the W boson helic-ityfractions in tt events involving one lepton and multiple jets, tt→ (W+b) (W−b¯) → (+νb)(qqb),andits charge conjugate,

where is an electron ora muon,including thosefrom leptonic decays of a tau lepton. Final states corresponding to such pro-cessesarereferredtoaslepton+jets.Themeasurementrelieson the analysis strategy described in Ref. [13]. The measurement is performedusing pp collisions atcentre-of-mass energy of8TeV, corresponding to an integratedluminosity of 19.8fb−1, collected during2012bytheCMSdetector.

2. TheCMSdetector

The CMS detector is a multipurpose apparatus of cylindrical design with respect to the proton beams. The main features of the detectorrelevant for this analysisare briefly described here. Charged particle trajectoriesare measured by a silicon pixel and striptracker,covering thepseudorapidity range|η|<2.5.The in-ner tracker is immersed in a 3.8T magnetic field provided by

a superconducting solenoid of 6m in diameterthat also encom-passes several calorimeters. A lead tungstate crystal electromag-netic calorimeter (ECAL), and a brass and scintillator hadronic calorimeter surround the tracking volume and cover the region |η|<3. Quartz fibre and steel hadron forward calorimeters ex-tendthecoverageto|η|≤5.Muonsareidentifiedingasionisation detectors embedded in the steelreturn yoke ofthe magnet. The data forthis analysis are recorded using a two-level trigger sys-tem. A more detailed description of the CMS detector, together witha definitionofthe coordinatesystemused andtherelevant kinematicvariables,canbefoundinRef.[16].

3. Dataandsimulatedsamples

Signal events corresponding to top quark pairs that decay to lepton+jetsfinalstatesareexpectedtocontainoneisolatedlepton (electronormuon)together withatleastfourjets, twoof which originate frombquark fragmentation.Such eventsarereferred to separately ase+jets or μ +jets,respectively,orwhencombined as+jets.Backgroundeventscontaining a single isolatedlepton andfourreconstructedjetsarise mainlyfromprocessesthat pro-duceeventscontaininga singletopquark, processesthat produce multijeteventsinassociationwithaW bosonthatdecays lepton-ically(W+jets),orDrell–Yanprocessesaccompaniedby multiple jets(DY+jets)whenoneoftheleptonsismisidentifiedasajetor goesundetected.Multijetprocessescanalsomimiclepton+jets fi-nalstates,ifajetisreconstructedasanelectromagneticshoweror, moreunlikely,ifanonpromptmuonfromahadrondecayinflight fulfilsallidentificationcriteriaofapromptmuon.

Simulated MonteCarlo (MC) samples,interfaced with Geant4 [17],areusedtoaccountfordetectorresolutionandacceptance ef-fects,aswellastoestimatethecontributionfrombackground pro-cessesthathavecharacteristicssimilartolepton+jetsfinalstates intt decays.Asignaltt sample,whichalsoprovidesareferencefor the SM(see Eq.(5)), issimulatedusing MadGraph v5.1.3.30[18] withmatrix elements havingup to threeextra partonsin the fi-nalstate.Thepartondistributionfunction(PDF)set CTEQ6L1[19] isusedwhen simulatingthisreferencett sample.The MadGraph generatoris interfacedwith pythia v6.426[20],tune Z2∗ [21], to simulate hadronisation and parton fragmentation, and also with tauola v27.121.5 [22] to simulate τ lepton decays. This SM ref-erence tt sample is simulated assuming mt=172.5GeV, which resultsinthe followingleading-order(LO) W boson helicity frac-tionsforthatsample:

F₀SM=0.6902, F_LSM=0.3089, FSM_R =0.0009. (3)

Singletopquarkeventsinthes,t,andtWchannelsaregenerated using powheg v1.0 [23] and pythia interfaced with tauola, with thePDFset CTEQ6M[19].BackgroundW+jets andDY+jets pro-cesses aresimulatedusing MadGraph with thePDF set CTEQ6L1, followed by pythia for fragmentation and hadronisation. Finally, background multijet processes are simulated using the pythia eventgenerator.

Corrections are applied to the simulatedsamples so that res-olutions, energyscales, andefficiencies asfunctions of pT and η ofjets[24] andleptons [25]measuredindataarewelldescribed. The effect ofmultiple pp collisions occurringin the samebunch crossing(pileup)isalsotakenintoaccountinthesimulation.

Thedatasamplesselectedforthismeasurementwererecorded using inclusive single-lepton triggers, which requireat least one isolatedelectron(muon)withpT>27(24)GeV,usedtodefinethe e+jets (μ+jets)datasample.

The decayproducts of candidatetop quarks are reconstructed using the CMS particle-flow (PF) algorithm, described in detail

elsewhere [26,27].Individual chargedparticles identified as com-ing frompileup interactions are removed fromthe event. Effects ofneutralparticles frompileup interactions are mitigatedby ap-plyingcorrectionsbasedoneventproperties.Leptonsarerequired to originate from theprimary vertexof the event[28].A lepton isdeterminedtobeisolatedusingavariablecomputedasthe to-taltransversemomentumofall particles(excepttheleptonitself) containedwithinaconeofradius0.4,centredonthelepton direc-tion,relativetothetransversemomentumofthelepton.Electrons areidentifiedbyusingamultivariateanalysis(MVA)[29]basedon information fromthe inner tracker andthe ECAL.Events are se-lected for the e+jets data sample ifthe identified electron has an MVAdiscriminant value greater than0.9, is determinedto be isolated,has pT>30GeV, and|η|<2.5.Muons areidentified by matchinginformationfromthe innertrackerandthemuon spec-trometer[30].Events are selectedforthe μ +jets datasample if theycontainanisolatedmuon, pT>26GeV,and|η|<2.1.Events with at least one additional isolated electron or muon are ve-toedtorejectbackgroundsfromdileptonicdecaymodesoftt and DY+jetsprocesses. Jetsare reconstructed [24] using the anti-kT clustering algorithm [31], with a distance parameter of 0.5. The selectedorvetoed leptonsdescribed above arenot allowed tobe clusteredintojets,toavoidambiguities.

Theeventselectionrequiresatleastfourreconstructedjets hav-ing |η|<2.4, of which thefour mostenergetic jetsare required to have pT higher than 55, 45,35, and 20GeV.Events with ad-ditionaljetsarenotvetoed.Thetransversemomentumimbalance oftheevent pmiss_T isdeterminedby summingthe negative trans-versemomentumoverall reconstructedparticles,excludingthose chargedparticlesnotassociatedwiththeprimaryvertex.

The transverse mass of the W boson is defined as MT = 

2p

TpmissT (1−cos(φ)), where pT is the transverse momentum ofthelepton, pmiss

T isthemagnitudeof pmissT ,andφistheangle inthe(x,y) planebetweenthedirectionofthelepton and pmiss

T . To reduce the multijet background and suppress dilepton events fromtt processes,eventsarerequiredtohave30<MT<200GeV. Allbackgrounds are further suppressedby requiringthat atleast two jetsbe identified as originatingfromb quarks.All jets with pT>20GeV areconsideredasbquarkcandidates,includingthose thatarenotamongthefourmostenergetic.

Thecombinedsecondaryvertexalgorithm[32,33]tagsbquark jetswith an efficiency ofabout 70% andmistags jets originating fromgluons,u, d,ors quarkswitha probabilityofabout1%, for thetypicalpTranges(30–100 GeV)probedintt events.Charmjets havea probability of ≈20% of beingtagged as bquark jets. The residualmultijetbackgrounds,alreadystronglysuppressed bythe btaggingrequirementdescribedabove,areestimatedby normalis-ing simulatedeventsamplestoyieldsincontroldatasamples.The controlsamplesaredefinedbyselectioncriteriawhicharesimilar tothoseforthesignal,butwhichhavenobtaggingrequirement, andhaveMT<30GeV forthe μ +jets channelorhaveanelectron MVAdiscriminantvaluesmallerthan 0.5forthee+jets channel. The estimated amount of multijet events is ≈2% of the e+jets sample,andlessthan1%ofthe μ +jets sample.Thecontributions ofallotherresidualbackgroundsaredeterminedusingsimulation. 4. Reconstructionofthett systemandreweightingmethod

The reconstruction of the tt system, described in detail in Ref.[13],reliesontestingtheselectedlepton,themeasured pmiss

T , and all selected jets for their compatibility with the top quark decayproducts from the leptonic (t→bW→bν) andhadronic (t→bW→bqq) branches. The unmeasured component of the neutrino momentum pν_z is determined by requiring the tt

sys-temtobeconsistentwiththeinvariantmassesoftwo topquarks andtwoW bosons. Withtheseconstraints,b jetsarecorrectly as-signed to the leptonic (hadronic) branch in about 74% (71%) of signal events. Aftertheassignment, a kinematicfit isperformed, wherethemomenta ofthemeasuredjetsandleptonareallowed to vary within their resolutions to better comply with the mass constraints, leading to an improved determination of pν_z and a more accurate reconstruction of the tt system.In about 5–7% of theselectedevents,thefitfailstofindasolutionthatiscompatible withtheconstraintsandsucheventsarediscarded.Thenumberof data eventspassingall selectioncriteria, includingthefit conver-gence,is71458 inthee+jets sampleand70986inthe μ +jets sample. Astudyusingsimulationsnormalisedtothemostprecise theoretical cross sections available to date[34–37] indicates that the final sample composition is largely dominated by tt events, withabout82%ofeventsfromthe+jets decaymode,≈10% from otherdecaymodes(including τ leptons),and≈3.5% oftheevents fromsingletopquarkprocesses.Theremainingeventscomefrom backgroundsnotcontainingtopquarksinthefinalstate.

The method [13] employed to measure the W boson helicity fractions (FL,F0,FR)≡ F consists of maximising a binned Pois-sonlikelihoodfunctionconstructedusingthenumberofobserved events in data Ndata(i) and expectedevents from MC simulation NMC(i; F),ineachbini ofthereconstructedcosθrec∗ distribution, L( F)= i NMC(i; F)Ndata(i)  Ndata(i)  ! exp[−NMC(i; F)]. (4)

Whilethechargedleptoniseasilyidentifiedintheleptonicbranch oftt decays,thedown-typequarkjetarisingfromtheW boson de-cayinthehadronicbranchoftt decayscannotbeexperimentally distinguished from the up-type quark jet. Due to this ambigu-ity,onlytheabsolutevalue |cosθrec∗ |can bereconstructedforthe hadronicbranch.Hence,onlytheleptonicbranchmeasurementof cosθ_rec∗ is used in thisanalysis. The expectednumbers of events frombackgroundprocesses,NW+jets(i), NDY+jets(i),andNmultijet(i) represent W boson production in association with multiple jets, Drell–Yanproductioninassociationwithmultiplejets,and produc-tionofmultiplejets,whichdonotdependontheW bosonhelicity fractions.Forthe processescontaining topquarks, thenumberof expectedevents inagivenbin i ismodified by reweighting each eventinthatbinbyafactor w,definedforeachdecayingbranch as

wlep/had/single-t(cosθgen∗ ; F)≡

 3 8FL(1−cosθ ∗ gen)2 +3 4F0sin 2_θ∗ gen +3 8FR(1+cosθ ∗ gen)2 /  3 8F SM L (1−cosθgen∗ )2 +3 4F SM 0 sin2θgen∗ +3 8F SM R (1+cosθgen∗ )2 , (5)

whereθ_gen∗ isthehelicityangle(specifiedatmatrixelementlevel) of a particular decay branch, and F_LSM, F₀SM, F_RSM are given in Eq. (3). Therefore, the numberof expected events, asa function ofthehelicityfractionstobemeasured,is

Table 1

SystematicuncertaintiesonthemeasurementsoftheW bosonhelicityfractionsfromlepton+jetsevents.Thecasesinwhichthestatisticalprecisionofthelimitedsample sizewasassignedassystematicuncertaintiesaredenotedbythesymbol(*).

e+jets μ+jets +jets

±F0 ±FL ±F0 ±FL ±F0 ±FL

JES 0.004 0.003 0.005 0.003 0.005 0.003

JER 0.001 0.002 0.004 0.003 0.003 0.003

b tagging eff. 0.001 <10−3 _{0.001 <10}−3 _{0.001 <10}−3

Lepton eff. 0.001 0.002 0.001 0.001 0.001 0.001

Single top normal. 0.002 <10−3 _{0.003 0.001 0.003 0.001}

W+jets bkg. 0.008 0.001 0.007 0.001 0.007 0.001

DY+jets bkg. 0.002 <10−3 _{0.001 <10}−3 _{0.001 <10}−3

Multijet bkg. 0.023 0.007 0.007 0.003 0.008 0.001

Pileup 0.001 0.001 <10−3 _<10−3 _{0.001 <10}−3

Top quark mass 0.012 0.008 0.010 (*) 0.008 (*) 0.010 0.007

tt scales 0.011 0.008 (*) 0.014 0.007 (*) 0.012 0.007 tt match. scale 0.011 (*) 0.007 (*) 0.010 0.007 0.009 0.007 tt MC and hadronisation 0.015 0.009 0.005 0.003 0.006 0.004 tt pTreweight 0.011 0.010 <10−3 0.001 <10−3 0.002 Limited MC size 0.002 0.001 0.002 0.001 0.002 0.001 PDF 0.004 0.001 0.002 0.001 0.002 0.001 Total 0.037 0.020 0.024 0.014 0.023 0.014 NMC(i; F)= Ntt(i; F) +Nsingle-t(i; F) +NW+jets(i) +NDY+jets(i) +Nmultijet(i), (6) where N_tt(i; F)=F_tt ⎡ ⎣  tt events in bin i

wlep(cosθgen∗ ; F)

×whad(cosθgen∗ ; F)



, Nsingle-t(i; F)=

single-t events in bin i

wsingle-t(cosθgen∗ ; F)

(7)

representtheexpectednumberofeventsfulfillingeventselection criteriaforprocessesinvolvingtopquarkpair,andsingletopquark production, respectively. The normalisation factor Ftt for the tt sample is a single free parameter in the fit across all bins. The expected cross section for the simulated reference tt sample is 252.9₋+₁₄13._.3₅pb,calculated atNNLOandnext-to-next-to-leading-log (NNLL) accuracy [34,35], and describes the data well. The fitted values of F_tt in both e+jets and μ +jets channels are com-patible with 1.00 within 3%. The overall normalisation factorfor simulatedsingle top quark events is not modified in thefit and the uncertainty in the assumed cross section is considered as a source of systematic uncertainty. Finally, the unitarity constraint (FL+F0+FR=1)isimposed,sothatoneofthehelicityfractions, namelyFR,isboundbythemeasurementoftheothertwo.

The method was validated using pseudo-experiments, where the fitting procedure was performed on pseudo-data, mimicking alteredhelicityfractions.Linearitytestsshow thatthefitting pro-cedure correctly retrieves the helicity fractions of altered input valuesforF0∈ [0.50,0.85]and FL∈ [0.20,0.50].Likewisethe cor-respondingstatisticaluncertainties wereverifiedusingsetsof sta-tisticallyuncorrelatedpseudo-data.

5. Systematicuncertainties

Systematic effects which could potentially bias the measure-mentoftheW bosonhelicityfractionshavebeeninvestigatedand

their correspondinguncertainties determined,aspresentedin Ta-ble 1.

Residualcorrectionsareappliedinsimulationtothejetenergy scale (JES), to account for differences between data and simula-tion. The momenta of the jets in simulation are also smeared so that the jet energy resolution (JER)in simulation agrees with thatindata.Thesecorrectionsandsmearingsarepropagated into

pmiss_T to correct its momentum scale. The uncertainties [24] as-sociated with the JESandJER corrections are alsopropagated to

pmiss

T ,andthefullanalysis,includingthett reconstructionandthe resulting measurements ofthe W boson helicity fractions, is re-peated.Scalefactorsareusedtocorrectthebtaggingefficiencyin simulation,wherethosecorrectionsareshiftedby theirestimated uncertainties,andthefullanalysisrepeated.Scalefactorsare also used tocorrectleptons fortheir identification, isolation and trig-gerefficiencies, whichare varied within their uncertainties so as tomaximise potentialshapevariationsofthepredictedcosθ∗ dis-tributions.

ToaccountforanypossiblebiasoftheW bosonhelicity mea-surementsdueto uncertainties inthenormalisation ofsimulated backgrounds,theassumedcrosssectionforeachsample isvaried individually [13]. An uncertainty of 30% is used for the normal-isation of DY+jets, single top quark, and W boson production inassociation withlight-quarkorgluon jet production.Since the modellingofthesimulatedheavy-flavourcontent oftheW+jets sample is knownto be inaccurate, an uncertaintyof +₋100%_50% is as-sumedforsimulatedeventsinvolvingaW bosonproducedin asso-ciationwithbquarkjets.TheimpactoftheDY+jetsnormalisation uncertaintyintheanalysisissmall,sinceitcorresponds toonlya few percentofthe sample composition.The normalisationof the multijetbackgroundisestimatedfromcontrolsamplesandresults inan uncertaintyof+₋50%_50% in thee+jets channeland+₋40%_50% inthe

μ+jets channel.Shape uncertainties onthe multijetbackground templateswereinvestigatedbycomparingthedistributionsin sev-eral differentcontrol regions, both in MC andin data, andwere foundtobenegligible,comparedtothemuchlargernormalisation uncertainties.

Several uncertainties from possible systematic effects related to theoretical modelling ofthe signal are estimatedby replacing the default tt samples with alternative tt samples and repeat-ing the entire analysis. Specifically, for the MadGraph interfaced withPYTHIAeventgeneration, thedefaultmt value of 172.5GeV is shifted up and down by 1GeV; the renormalisation and

fac-Table 2

MeasurementsoftheW bosonhelicityfractionsfromlepton+jetsfinalstatesintt decays.Thehelicityfractions F0andFLaremeasuredsimultaneouslyandarestronglyanti-correlated,asindicatedbyacorrelationcoefficient

ρ0,L,becauseFRisderivedfromtheunitaritycondition.

Channel F0±(stat)±(syst) FL±(stat)±(syst) FR±(stat)±(syst) ρ0,L

e+jets 0.705±0.013±0.037 0.304±0.009±0.020 −0.009±0.005±0.021 −0.950 μ+jets 0.685±0.013±0.024 0.328±0.009±0.014 −0.013±0.005±0.017 −0.957

+jets 0.681±0.012±0.023 0.323±0.008±0.014 −0.004±0.005±0.014 −0.959 torisation scales are varied down (up) by afactor of0.5 (2);the

kinematic scale used to matchjets to partons (matching thresh-old) is varied down (up) by factor of 0.5 (2); finally, the parton showerandhadronisationmodellingisstudiedinatt sample sim-ulated withMC@NLO v3.41 [38] usingthe PDF set CTEQ6M and interfacedwith herwig v6.520[39].

Uncertaintiesin thehelicity fractions arising fromthe limited size of the simulated tt samples are takeninto account, both in themain analysisandinthe determinationofthe modelling un-certainties.Intheformercase,theseeffectsareaddedasaseparate sourceofuncertainty.Inthelattercase,thesystematic uncertain-tiesintheW bosonhelicityareassignedtobethelargerofeither (i) the statisticalprecision of the limited sample size or (ii) the systematicshiftofthecentralvalue withrespecttothereference tt sample.

Theshape ofthe pT spectrumfortop quarks,asmeasured by thedifferentialcrosssectionfortopquark pairs[25,40],hasbeen found to be softer than the predictions from MadGraph simula-tion.The effectof thismismodellingis estimatedby reweighting theeventsinthesimulatedtt sample,sothatthetopquark pT at parton level in the MC describes the unfolded data distribution. Further, the systematiceffects due to the PDFs used to simulate the signal and background samples are estimated according to the prescriptions described in [41,42], using NNPDF21 [43] and MSTW2008lo68cl [44] PDF sets as alternatives to those used at generation. Finally, uncertainties related to the modelling of the pileupinsimulatedeventsarealsotakenintoaccount.

Thetotalsystematicuncertaintyisgivenbythesumin quadra-tureofalluncertaintiesdescribedabove.

6. Results

The measurements of the W boson helicity fractions, using cosθ∗fromtheleptonicbranchoftt eventsthatdecayintoe+jets or μ +jets, including the full combination of these two mea-surements, are shown in Table 2. Within an individual channel, the helicity parameters F0 and FL are fit simultaneously, but they are strongly anti-correlated due to the unitarity constraint FL+F0+FR=1,as indicated by thestatistical correlation coef-ficient ρ0,Lgiveninthetable.Theseparatehelicitymeasurements fromthee+jets and μ +jets channelsarecombinedintoasingle +jets measurementusingtheBLUE method[45,46],takinginto accountalluncertaintiesandtheirpossiblecorrelations. Uncertain-tiesrelated to lepton efficiency, multijet backgroundestimations, andstatistical uncertainties are considered uncorrelated between thee+jets and μ +jets analyses,whileallotheruncertaintiesare assumed to be fully correlated. The combined +jets measure-mentofthehelicityfractionsisdominatedbythe μ +jets channel, withweightsmorethandoublethoseofthee+jets channel.The

χ2 _{of the combination is 2.13 for 2 degrees of freedom,} corre-spondingtoaprobabilityof34.5%.Themeasurementuncertainties are dominated by systematic effects that are correlated between both the e+jets and μ +jets channels. The combined F0 and FL values are anti-correlated with a statistical correlation coeffi-cient ρ0,L= −0.959. The total correlation coefficient, considering bothstatisticalandsystematicuncertainties,isfoundtobe−0.870.

ThemeasuredhelicityfractionspresentedinTable 2areconsistent withtheSMpredictionsgivenatNNLOaccuracy[5].Fig. 2 shows, separately for the e+jets and μ +jets channels, the distribu-tionsforthecosineofthehelicityanglesfromtheleptonicbranch, whichareusedinthehelicitymeasurements,andthedistributions ofthecorrespondingabsolutevaluesfromthehadronicbranch,for comparisonpurposes.Thesimulatedsamplesinvolvingtopquarks used in thefigure were produced using the measured valuesfor the W bosonhelicityfractions,asdeterminedfromthecombined +jets fit.Left-handedW bosons tendtopopulatethe regionat cosθ∗≈ −1,wherethechargedleptonoverlapswiththebquark. However,theangularseparationrequirementbetweenleptonsand jetsremovesmostoftheeventsnearcosθ∗= −1.Veryfewevents areexpectedintheregionpreferredbyright-handedbosons,near cosθ∗= +1.However,duetoresolutioneffects,thereconstructed distribution does not fall as rapidly as expected in that region, where the charged lepton and b quark have opposite directions. Forthesereasons,the shapeof thereconstructed cosθ∗ distribu-tion differs fromthat expectedinthe SM (Fig. 1). These features are wellreproduced by thesimulation,andtakeninto accountin themeasurement.

Usingtheseresults,limitsonanomalouscouplingsareobtained by fixing thetwo vector couplingsin Eq.(2) to their SM values, VL=1 and VR=0,andchoosingthetensorcouplings,Re(gL)and Re(gR),asfreeparameters.Thecombined+jets measurementof theW bosonhelicityfractionsF0 andFLisreinterpretedinterms ofthetensorcouplings,Re(gL)andRe(gR),usingtherelationships betweentheW bosonhelicityfractionsgiveninRef.[8].

The W boson helicity measurements are displayed in the (F0,FL)plane inFig. 3(left),together withtheir one-dimensional statistical(inner-tickmark) andtotal(outer-tickmark) errorbars. The full two-dimensional confidence level (CL) contours corre-sponding to 68% (dashed line) and 95% (solid line) probabilities are also displayed for the combined measurement. The SM pre-diction is shown as a star and lies within the 68% CL contour. The corresponding regions in the (Re(gL),Re(gR)) plane, allowed at 68% (dark contour) and95% CL (light contour), are shown in Fig. 3 (right), together withthe SM value.They arederived from Fig. 3(left),usingtherelationshipsbetweentheW bosonhelicity fractions andtheanomalous couplingsgiven inRef. [8].A region near Re(gL)=0 andRe(gR)0, allowed bythe fitbutexcluded by theCMSsingletopquark productionmeasurement[47],isnot shown.

Iftheright-handedcomponentFRisboundtozero,consistently with the SM within the precision of the current measurement, the combined +jets measurement amounts to F0=0.661± 0.006(stat)±0.021(syst).Inthiscase, FL isobtainedviathe uni-tarityconstraintandyields FL=0.339±0.006(stat)±0.021(syst). 7. Summary

AmeasurementoftheW bosonhelicityfractionsintopquark pair events decaying in the e+jets and μ +jets channels has beenpresented,usingproton–protoncollisiondataat√s=8TeV, and corresponding to an integrated luminosity of 19.8fb−1. The helicity fractions F0 and FL are measured with a precision of

Fig. 2. Distributionsforthecosineofthehelicityangleintheleptonic(upperrow)andhadronic(lowerrow)branches,forthee+jets (left)andμ+jets (right)decay channels.Thecombined  +jets post-fitmeasurements ofthehelicityfractionswereusedinthesimulationoftt andsingletopquarkevents.Thedataaredisplayedas solidpoints,simulatedsamplesoftt (signal)processesandthecontributionfrombackgroundprocessesashistograms.Atthebottomofeachplot,theratiobetweenMC simulationanddataisdisplayed.Theerrorbarscorrespondtothestatisticaluncertainties.

Fig. 3. Left:themeasuredW bosonhelicityfractionsinthe (F0, FL)plane.Thedashedandsolidellipsesenclosetheallowedtwo-dimensional68%and95%CLregions,

forthecombined  +jets measurement,takingintoaccountthecorrelationsonthetotal(includingsystematic)uncertainties.Theerrorbarsgivetheone-dimensional68% CLintervalfortheseparateF0and FLmeasurements,withtheinner-tick(outer-tick)markrepresentingthestatistical(total)uncertainty.Right:thecorrespondingallowed

regionsfortherealcomponentsoftheanomalouscouplingsgLandgRat68%and95%CL,forVL=1 andVR=0.AregionnearRe(gL) =0 andRe(gR) 0,allowedbythe

fitbutexcludedbytheCMSsingletopquarkproductionmeasurement,isomitted.TheSMpredictionsareshownasstars.

better than 5%, yielding the most accurate experimental deter-mination of the W boson helicity fractions in tt processes to date.The measured W bosonhelicity fractions are F0=0.681± 0.012(stat)±0.023(syst), FL=0.323±0.008(stat)±0.014(syst), and FR= −0.004±0.005(stat)±0.014(syst), with a correlation coefficient of−0.87 between F0 and FL, andthey are consistent withtheexpectationsfromthestandardmodel.

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 oftheLHCandthe CMSdetectorprovidedby thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES(Bulgaria);CERN;CAS,MOST,andNSFC(China);COLCIENCIAS (Colombia);MSESandCSF(Croatia);RPF(Cyprus);MoER,ERCIUT andERDF(Estonia);Academy ofFinland,MEC,andHIP(Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic of Korea); LAS (Lithuania); MOE and UM (Malaysia); BUAP, CIN-VESTAV,CONACYT, LNS,SEP, andUASLP-FAI(Mexico); MBIE(New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portu-gal);JINR(Dubna);MON,RosAtom,RASandRFBR(Russia);MESTD (Serbia);SEIDIandCPAN(Spain);SwissFundingAgencies (Switzer-land); MST (Taipei); ThEPCenter, IPST, STAR and NSTDA (Thai-land);TÜBITAK andTAEK(Turkey);NASUandSFFR(Ukraine);STFC (UnitedKingdom);DOEandNSF(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 of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Science and Industrial Re-search, India; the HOMING PLUS programme of the Foundation forPolishScience,cofinancedfromEuropean Union,Regional De-velopment Fund; the Mobility Plus programme of the Ministry of Science andHigher Education (Poland); the OPUS programme of the National Science Center (Poland); the Thalis and Aris-teia programmes cofinancedby EU-ESF andthe Greek NSRF;the National Priorities Research Program by Qatar National Research Fund; the Programa Clarín-COFUND del Principado de Asturias; the Rachadapisek Sompot Fund for Postdoctoral Fellowship, Chu-lalongkornUniversity(Thailand);theChulalongkornAcademicinto Its 2nd Century Project Advancement Project (Thailand); andthe WelchFoundation,contractC-1845.

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CMSCollaboration

V. Khachatryan,A.M. Sirunyan, A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam, E. Asilar,T. Bergauer, J. Brandstetter, E. Brondolin, M. Dragicevic, J. Erö,M. Flechl, M. Friedl,

R. Frühwirth1, V.M. Ghete, C. Hartl, N. Hörmann, J. Hrubec,M. Jeitler1,A. König, I. Krätschmer, D. Liko,

T. Matsushita,I. Mikulec, D. Rabady, N. Rad, B. Rahbaran,H. Rohringer, J. Schieck1,J. Strauss,

W. Treberer-Treberspurg,W. Waltenberger, C.-E. Wulz1

InstitutfürHochenergiephysikderOeAW,Wien,Austria

V. Mossolov,N. Shumeiko,J. Suarez Gonzalez

NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt, E.A. De Wolf,X. Janssen,J. Lauwers, M. Van De Klundert, H. Van Haevermaet,

P. Van Mechelen,N. Van Remortel, A. Van Spilbeeck

UniversiteitAntwerpen,Antwerpen,Belgium

S. Abu Zeid,F. Blekman, J. D’Hondt, N. Daci,I. De Bruyn, K. Deroover, N. Heracleous,S. Lowette,

S. Moortgat, L. Moreels,A. Olbrechts,Q. Python, S. Tavernier, W. Van Doninck, P. Van Mulders,

I. Van Parijs

VrijeUniversiteitBrussel,Brussel,Belgium

H. Brun, C. Caillol, B. Clerbaux, G. De Lentdecker, H. Delannoy, G. Fasanella,L. Favart, R. Goldouzian,

A. Grebenyuk,G. Karapostoli,T. Lenzi, A. Léonard,J. Luetic, T. Maerschalk,A. Marinov, A. Randle-conde,

T. Seva,C. Vander Velde, P. Vanlaer,R. Yonamine, F. Zenoni, F. Zhang2

UniversitéLibredeBruxelles,Bruxelles,Belgium

A. Cimmino,T. Cornelis, D. Dobur, A. Fagot, G. Garcia, M. Gul, D. Poyraz,S. Salva, R. Schöfbeck, M. Tytgat,

W. Van Driessche, E. Yazgan,N. Zaganidis

C. Beluffi3,O. Bondu,S. Brochet, G. Bruno, A. Caudron, L. Ceard, S. De Visscher, C. Delaere, M. Delcourt,

L. Forthomme,B. Francois, A. Giammanco, A. Jafari,P. Jez, M. Komm,V. Lemaitre, A. Magitteri,

A. Mertens, M. Musich, C. Nuttens, K. Piotrzkowski,L. Quertenmont, M. Selvaggi, M. Vidal Marono,

S. Wertz

UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

N. Beliy

UniversitédeMons,Mons,Belgium

W.L. Aldá Júnior, F.L. Alves,G.A. Alves, L. Brito,C. Hensel,A. Moraes, M.E. Pol, P. Rebello Teles

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

E. Belchior Batista Das Chagas, W. Carvalho,J. Chinellato4,A. Custódio, E.M. Da Costa, G.G. Da Silveira,

D. De Jesus Damiao,C. De Oliveira Martins, S. Fonseca De Souza, L.M. Huertas Guativa, H. Malbouisson,

D. Matos Figueiredo, C. Mora Herrera,L. Mundim, H. Nogima,W.L. Prado Da Silva, A. Santoro,

A. Sznajder,E.J. Tonelli Manganote4, A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

S. Ahujaa, C.A. Bernardesb, S. Dograa,T.R. Fernandez Perez Tomeia, E.M. Gregoresb, P.G. Mercadanteb,

C.S. Moona,5,S.F. Novaesa,Sandra S. Padulaa, D. Romero Abadb, J.C. Ruiz Vargas

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

A. Aleksandrov, R. Hadjiiska, P. Iaydjiev,M. Rodozov, S. Stoykova, G. Sultanov, M. Vutova

InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria

A. Dimitrov, I. Glushkov,L. Litov, B. Pavlov,P. Petkov

UniversityofSofia,Sofia,Bulgaria

W. Fang6

BeihangUniversity,Beijing,China

M. Ahmad, J.G. Bian, G.M. Chen, H.S. Chen,M. Chen, Y. Chen7,T. Cheng, C.H. Jiang, D. Leggat, Z. Liu,

F. Romeo,S.M. Shaheen, A. Spiezia, J. Tao, C. Wang,Z. Wang, H. Zhang, J. Zhao

InstituteofHighEnergyPhysics,Beijing,China

Y. Ban, Q. Li, S. Liu,Y. Mao, S.J. Qian, D. Wang,Z. Xu

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,A. Cabrera, L.F. Chaparro Sierra, C. Florez, J.P. Gomez, C.F. González Hernández,J.D. Ruiz Alvarez,

J.C. Sanabria

UniversidaddeLosAndes,Bogota,Colombia

N. Godinovic, D. Lelas, I. Puljak,P.M. Ribeiro Cipriano

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic, M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,D. Ferencek, K. Kadija,S. Micanovic, L. Sudic

A. Attikis, G. Mavromanolakis,J. Mousa, C. Nicolaou, F. Ptochos,P.A. Razis, H. Rykaczewski

UniversityofCyprus,Nicosia,Cyprus

M. Finger8,M. Finger Jr.8

CharlesUniversity,Prague,Czechia

E. Carrera Jarrin

UniversidadSanFranciscodeQuito,Quito,Ecuador

Y. Assran9,10, T. Elkafrawy11,A. Ellithi Kamel12,A. Mahrous13

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

B. Calpas,M. Kadastik, M. Murumaa, L. Perrini, M. Raidal, A. Tiko, C. Veelken

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola,J. Pekkanen, M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Härkönen,V. Karimäki, R. Kinnunen, T. Lampén, K. Lassila-Perini,S. Lehti, T. Lindén,P. Luukka,

T. Peltola,J. Tuominiemi,E. Tuovinen, L. Wendland

HelsinkiInstituteofPhysics,Helsinki,Finland

J. Talvitie,T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon,F. Couderc, M. Dejardin, D. Denegri,B. Fabbro, J.L. Faure, C. Favaro, F. Ferri, S. Ganjour,

S. Ghosh,A. Givernaud, P. Gras, G. Hamel de Monchenault, P. Jarry, I. Kucher, E. Locci,M. Machet,

J. Malcles,J. Rander, A. Rosowsky, M. Titov, A. Zghiche

DSM/IRFU,CEA/Saclay,Gif-sur-Yvette,France

A. Abdulsalam,I. Antropov, S. Baffioni, F. Beaudette, P. Busson, L. Cadamuro, E. Chapon,C. Charlot,

O. Davignon,R. Granier de Cassagnac, M. Jo, S. Lisniak,P. Miné, I.N. Naranjo, M. Nguyen,C. Ochando,

G. Ortona,P. Paganini, P. Pigard,S. Regnard, R. Salerno, Y. Sirois, T. Strebler, Y. Yilmaz,A. Zabi

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

J.-L. Agram14,J. Andrea, A. Aubin,D. Bloch, J.-M. Brom,M. Buttignol, E.C. Chabert,N. Chanon, C. Collard,

E. Conte14,X. Coubez, J.-C. Fontaine14, D. Gelé, U. Goerlach,A.-C. Le Bihan, J.A. Merlin15, K. Skovpen,

P. Van Hove

InstitutPluridisciplinaireHubertCurien,UniversitédeStrasbourg,UniversitédeHauteAlsaceMulhouse,CNRS/IN2P3,Strasbourg,France

S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,C. Bernet, G. Boudoul,E. Bouvier, C.A. Carrillo Montoya, R. Chierici,D. Contardo,

B. Courbon,P. Depasse, H. El Mamouni,J. Fan, J. Fay, S. Gascon,M. Gouzevitch, G. Grenier, B. Ille,

F. Lagarde,I.B. Laktineh, M. Lethuillier,L. Mirabito, A.L. Pequegnot, S. Perries,A. Popov16,D. Sabes,

V. Sordini,M. Vander Donckt, P. Verdier, S. Viret

UniversitédeLyon,UniversitéClaudeBernardLyon 1,CNRS–IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France

T. Toriashvili17

D. Lomidze

TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann, S. Beranek,L. Feld, A. Heister, M.K. Kiesel, K. Klein, M. Lipinski, A. Ostapchuk, M. Preuten,

F. Raupach, S. Schael, C. Schomakers,J.F. Schulte, J. Schulz,T. Verlage, H. Weber, V. Zhukov16

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

M. Brodski,E. Dietz-Laursonn, D. Duchardt, M. Endres, M. Erdmann,S. Erdweg, T. Esch, R. Fischer,

A. Güth, T. Hebbeker,C. Heidemann, K. Hoepfner, S. Knutzen,M. Merschmeyer, A. Meyer,P. Millet,

S. Mukherjee,M. Olschewski, K. Padeken, P. Papacz,T. Pook, M. Radziej, H. Reithler,M. Rieger,

F. Scheuch,L. Sonnenschein, D. Teyssier, S. Thüer

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

V. Cherepanov, Y. Erdogan,G. Flügge, F. Hoehle, B. Kargoll, T. Kress, A. Künsken, J. Lingemann,

A. Nehrkorn, A. Nowack,I.M. Nugent, C. Pistone, O. Pooth,A. Stahl15

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

M. Aldaya Martin,C. Asawatangtrakuldee, I. Asin, K. Beernaert, O. Behnke,U. Behrens,A.A. Bin Anuar,

K. Borras18,A. Campbell, P. Connor, C. Contreras-Campana, F. Costanza, C. Diez Pardos,G. Dolinska,

G. Eckerlin, D. Eckstein, E. Gallo19, J. Garay Garcia, A. Geiser, A. Gizhko, J.M. Grados Luyando,

P. Gunnellini, A. Harb, J. Hauk, M. Hempel20,H. Jung,A. Kalogeropoulos,O. Karacheban20,M. Kasemann,

J. Keaveney, J. Kieseler, C. Kleinwort,I. Korol, W. Lange, A. Lelek, J. Leonard,K. Lipka, A. Lobanov,

W. Lohmann20, R. Mankel, I.-A. Melzer-Pellmann,A.B. Meyer, G. Mittag, J. Mnich, A. Mussgiller,

E. Ntomari,D. Pitzl, R. Placakyte, A. Raspereza,B. Roland, M.Ö. Sahin,P. Saxena, T. Schoerner-Sadenius,

C. Seitz, S. Spannagel, N. Stefaniuk,K.D. Trippkewitz, G.P. Van Onsem, R. Walsh, C. Wissing

DeutschesElektronen-Synchrotron,Hamburg,Germany

V. Blobel, M. Centis Vignali, A.R. Draeger,T. Dreyer, E. Garutti, K. Goebel, D. Gonzalez, J. Haller,

M. Hoffmann, A. Junkes,R. Klanner,R. Kogler, N. Kovalchuk,T. Lapsien,T. Lenz,I. Marchesini, D. Marconi,

M. Meyer, M. Niedziela,D. Nowatschin, J. Ott, F. Pantaleo15,T. Peiffer, A. Perieanu, J. Poehlsen, C. Sander,

C. Scharf, P. Schleper, A. Schmidt, S. Schumann,J. Schwandt,H. Stadie, G. Steinbrück, F.M. Stober,

M. Stöver, H. Tholen, D. Troendle,E. Usai, L. Vanelderen,A. Vanhoefer, B. Vormwald

UniversityofHamburg,Hamburg,Germany

C. Barth,C. Baus, J. Berger,E. Butz, T. Chwalek, F. Colombo, W. De Boer, A. Dierlamm,S. Fink, R. Friese,

M. Giffels,A. Gilbert, D. Haitz, F. Hartmann15, S.M. Heindl,U. Husemann, I. Katkov16, P. Lobelle Pardo,

B. Maier, H. Mildner, M.U. Mozer, T. Müller,Th. Müller, M. Plagge, G. Quast, K. Rabbertz, S. Röcker,

F. Roscher,M. Schröder, G. Sieber, H.J. Simonis,R. Ulrich, J. Wagner-Kuhr, S. Wayand, M. Weber,

T. Weiler, S. Williamson,C. Wöhrmann, R. Wolf

InstitutfürExperimentelleKernphysik,Karlsruhe,Germany

G. Anagnostou, G. Daskalakis,T. Geralis,V.A. Giakoumopoulou, A. Kyriakis, D. Loukas, I. Topsis-Giotis

InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece

A. Agapitos, S. Kesisoglou, A. Panagiotou, N. Saoulidou, E. Tziaferi

NationalandKapodistrianUniversityofAthens,Athens,Greece

I. Evangelou, G. Flouris,C. Foudas, P. Kokkas, N. Loukas, N. Manthos, I. Papadopoulos,E. Paradas

N. Filipovic

MTA-ELTELendületCMSParticleandNuclearPhysicsGroup,EötvösLorándUniversity,Hungary

G. Bencze,C. Hajdu, P. Hidas,D. Horvath21, F. Sikler,V. Veszpremi, G. Vesztergombi22, A.J. Zsigmond

WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni,S. Czellar, J. Karancsi23,A. Makovec,J. Molnar, Z. Szillasi

InstituteofNuclearResearchATOMKI,Debrecen,Hungary

M. Bartók22,P. Raics, Z.L. Trocsanyi, B. Ujvari

UniversityofDebrecen,Debrecen,Hungary

S. Bahinipati, S. Choudhury24,P. Mal, K. Mandal, A. Nayak25,D.K. Sahoo, N. Sahoo,S.K. Swain

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S. Bansal,S.B. Beri, V. Bhatnagar, R. Chawla, R. Gupta,U. Bhawandeep, A.K. Kalsi, A. Kaur, M. Kaur,

R. Kumar,A. Mehta,M. Mittal, J.B. Singh, G. Walia

PanjabUniversity,Chandigarh,India

Ashok Kumar,A. Bhardwaj, B.C. Choudhary, R.B. Garg,S. Keshri, A. Kumar,S. Malhotra, M. Naimuddin,

N. Nishu, K. Ranjan,R. Sharma, V. Sharma

UniversityofDelhi,Delhi,India

R. Bhattacharya,S. Bhattacharya, K. Chatterjee, S. Dey,S. Dutt, S. Dutta, S. Ghosh, N. Majumdar,

A. Modak, K. Mondal,S. Mukhopadhyay, S. Nandan,A. Purohit, A. Roy, D. Roy, S. Roy Chowdhury,

S. Sarkar,M. Sharan, S. Thakur

SahaInstituteofNuclearPhysics,Kolkata,India

P.K. Behera

IndianInstituteofTechnologyMadras,Madras,India

R. Chudasama,D. Dutta, V. Jha, V. Kumar, A.K. Mohanty15, P.K. Netrakanti,L.M. Pant, P. Shukla,A. Topkar

BhabhaAtomicResearchCentre,Mumbai,India

T. Aziz,S. Dugad, G. Kole, B. Mahakud, S. Mitra, G.B. Mohanty, N. Sur, B. Sutar

TataInstituteofFundamentalResearch-A,Mumbai,India

S. Banerjee, S. Bhowmik26,R.K. Dewanjee, S. Ganguly,M. Guchait, Sa. Jain, S. Kumar, M. Maity26,

G. Majumder,K. Mazumdar, B. Parida,T. Sarkar26, N. Wickramage27

TataInstituteofFundamentalResearch-B,Mumbai,India

S. Chauhan,S. Dube, A. Kapoor, K. Kothekar, A. Rane, S. Sharma

IndianInstituteofScienceEducationandResearch(IISER),Pune,India

H. Bakhshiansohi,H. Behnamian,S. Chenarani28,E. Eskandari Tadavani, S.M. Etesami28, A. Fahim29,

M. Khakzad, M. Mohammadi Najafabadi, M. Naseri, S. Paktinat Mehdiabadi,F. Rezaei Hosseinabadi,

B. Safarzadeh30, M. Zeinali

InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran

M. Felcini,M. Grunewald

M. Abbresciaa,b, C. Calabriaa,b,C. Caputoa,b,A. Colaleoa,D. Creanzaa,c, L. Cristellaa,b,N. De Filippisa,c, M. De Palmaa,b, L. Fiorea,G. Iasellia,c, G. Maggia,c,M. Maggia, G. Minielloa,b, S. Mya,b, S. Nuzzoa,b, A. Pompilia,b,G. Pugliesea,c,R. Radognaa,b,A. Ranieria,G. Selvaggia,b,L. Silvestrisa,15, R. Vendittia,b,

P. Verwilligena

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

G. Abbiendia,C. Battilana, D. Bonacorsia,b, S. Braibant-Giacomellia,b, L. Brigliadoria,b, R. Campaninia,b, P. Capiluppia,b,A. Castroa,b,F.R. Cavalloa,S.S. Chhibraa,b, 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, A. Montanaria, F.L. Navarriaa,b,A. Perrottaa, A.M. Rossia,b,T. Rovellia,b, G.P. Sirolia,b,N. Tosia,b,15

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

S. Albergoa,b,M. Chiorbolia,b,S. Costaa,b,A. Di Mattiaa,F. Giordanoa,b,R. Potenzaa,b,A. Tricomia,b, C. Tuvea,b

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

G. Barbaglia, V. Ciullia,b,C. Civininia, R. D’Alessandroa,b,E. Focardia,b,V. Goria,b, P. Lenzia,b, M. Meschinia, S. Paolettia,G. Sguazzonia,L. Viliania,b,15

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

L. Benussi, S. Bianco, F. Fabbri,D. Piccolo, F. Primavera15

INFNLaboratoriNazionalidiFrascati,Frascati,Italy

V. Calvellia,b, F. Ferroa, M. Lo Veterea,b, M.R. Mongea,b,E. Robuttia,S. Tosia,b

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

L. Brianza,M.E. Dinardoa,b, S. Fiorendia,b,S. Gennaia,A. Ghezzia,b, P. Govonia,b,S. Malvezzia,

R.A. Manzonia,b,15, B. Marzocchia,b,D. Menascea,L. Moronia, M. Paganonia,b,D. Pedrinia, S. Pigazzini,

S. Ragazzia,b, T. Tabarelli de Fatisa,b

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

S. Buontempoa, N. Cavalloa,c, G. De Nardo, S. Di Guidaa,d,15, M. Espositoa,b, F. Fabozzia,c,

A.O.M. Iorioa,b, G. Lanzaa,L. Listaa, S. Meolaa,d,15,M. Merolaa,P. Paoluccia,15, C. Sciaccaa,b, F. Thyssen

a_{INFNSezionediNapoli,Napoli,Italy} b_{UniversitàdiNapoli‘Federico II’,Napoli,Italy} c_{UniversitàdellaBasilicata,Potenza,Italy} d_{UniversitàG.Marconi,Roma,Italy}

P. Azzia,15, N. Bacchettaa, L. Benatoa,b,D. Biselloa,b, A. Bolettia,b,R. Carlina,b,

A. Carvalho Antunes De Oliveiraa,b, P. Checchiaa,M. Dall’Ossoa,b,P. De Castro Manzanoa,T. Dorigoa,

U. Dossellia,F. Gasparinia,b,U. Gasparinia,b,A. Gozzelinoa,S. Lacapraraa, M. Margonia,b, A.T. Meneguzzoa,b,J. Pazzinia,b,15, N. Pozzobona,b,P. Ronchesea,b, F. Simonettoa,b,E. Torassaa, M. Zanetti,P. Zottoa,b,A. Zucchettaa,b,G. Zumerlea,b

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

A. Braghieria, A. Magnania,b, P. Montagnaa,b,S.P. Rattia,b,V. Rea, C. Riccardia,b, P. Salvinia, I. Vaia,b, P. Vituloa,b

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

L. Alunni Solestizia,b, G.M. Bileia,D. Ciangottinia,b,L. Fanòa,b, P. Laricciaa,b, R. Leonardia,b,

G. Mantovania,b, M. Menichellia,A. Sahaa,A. Santocchiaa,b

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

K. Androsova,31,P. Azzurria,15,G. Bagliesia,J. Bernardinia,T. Boccalia,R. Castaldia, M.A. Cioccia,31, R. Dell’Orsoa,S. Donatoa,c,G. Fedi, A. Giassia,M.T. Grippoa,31,F. Ligabuea,c, T. Lomtadzea, L. Martinia,b, A. Messineoa,b_{, F. Palla}a_{,A. Rizzi}a,b_{,A. Savoy-Navarro}a,32_{, P. Spagnolo}a_{,R. Tenchini}a_{, G. Tonelli}a,b_,

A. Venturia,P.G. Verdinia

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

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

L. Baronea,b, F. Cavallaria,M. Cipriania,b, G. D’imperioa,b,15,D. Del Rea,b,15,M. Diemoza, S. Gellia,b, C. Jordaa, E. Longoa,b, F. Margarolia,b,P. Meridiania,G. Organtinia,b, R. Paramattia,F. Preiatoa,b, S. Rahatloua,b,C. Rovellia, F. Santanastasioa,b

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

N. Amapanea,b,R. Arcidiaconoa,c,15,S. Argiroa,b,M. Arneodoa,c,N. Bartosika,R. Bellana,b, C. Biinoa, N. Cartigliaa,F. Cennaa,b, M. Costaa,b,R. Covarellia,b,A. Deganoa,b,N. Demariaa, L. Fincoa,b, B. Kiania,b, C. Mariottia, S. Masellia,E. Migliorea,b, V. Monacoa,b, E. Monteila,b, M.M. Obertinoa,b,L. Pachera,b, N. Pastronea,M. Pelliccionia,G.L. Pinna Angionia,b_{, F. Ravera}a,b_{,A. Romero}a,b_{, M. Ruspa}a,c_{,R. Sacchi}a,b_, K. Shchelinaa,b, V. Solaa,A. Solanoa,b,A. Staianoa,P. Traczyka,b

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

c_{UniversitàdelPiemonteOrientale,Novara,Italy}

S. Belfortea,M. Casarsaa, F. Cossuttia,G. Della Riccaa,b, C. La Licataa,b, A. Schizzia,b, A. Zanettia

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

D.H. Kim,G.N. Kim, M.S. Kim, S. Lee, S.W. Lee, Y.D. Oh,S. Sekmen, D.C. Son,Y.C. Yang

KyungpookNationalUniversity,Daegu,RepublicofKorea

H. Kim,A. Lee

ChonbukNationalUniversity,Jeonju,RepublicofKorea

J.A. Brochero Cifuentes,T.J. Kim

HanyangUniversity,Seoul,RepublicofKorea

S. Cho,S. Choi, Y. Go, D. Gyun,S. Ha,B. Hong, Y. Jo,Y. Kim, B. Lee, K. Lee,K.S. Lee, S. Lee, J. Lim,

S.K. Park,Y. Roh

KoreaUniversity,Seoul,RepublicofKorea

J. Almond,J. Kim, S.B. Oh,S.h. Seo, U.K. Yang,H.D. Yoo, G.B. Yu

M. Choi,H. Kim, H. Kim,J.H. Kim, J.S.H. Lee, I.C. Park, G. Ryu, M.S. Ryu

UniversityofSeoul,Seoul,RepublicofKorea

Y. Choi,J. Goh, C. Hwang, D. Kim, J. Lee,I. Yu

SungkyunkwanUniversity,Suwon,RepublicofKorea

V. Dudenas, A. Juodagalvis,J. Vaitkus

VilniusUniversity,Vilnius,Lithuania

I. Ahmed,Z.A. Ibrahim, J.R. Komaragiri, M.A.B. Md Ali33,F. Mohamad Idris34,W.A.T. Wan Abdullah,

M.N. Yusli,Z. Zolkapli

NationalCentreforParticlePhysics,UniversitiMalaya,KualaLumpur,Malaysia

H. Castilla-Valdez, E. De La Cruz-Burelo,I. Heredia-De La Cruz35,A. Hernandez-Almada,

R. Lopez-Fernandez, J. Mejia Guisao, A. Sanchez-Hernandez

CentrodeInvestigacionydeEstudiosAvanzadosdelIPN,MexicoCity,Mexico

S. Carrillo Moreno, C. Oropeza Barrera, F. Vazquez Valencia

UniversidadIberoamericana,MexicoCity,Mexico

S. Carpinteyro, I. Pedraza, H.A. Salazar Ibarguen, C. Uribe Estrada

BenemeritaUniversidadAutonomadePuebla,Puebla,Mexico

A. Morelos Pineda

UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico

D. Krofcheck

UniversityofAuckland,Auckland,NewZealand

P.H. Butler

UniversityofCanterbury,Christchurch,NewZealand

A. Ahmad, M. Ahmad, Q. Hassan,H.R. Hoorani, W.A. Khan, M.A. Shah, M. Shoaib, M. Waqas

NationalCentreforPhysics,Quaid-I-AzamUniversity,Islamabad,Pakistan

H. Bialkowska, M. Bluj,B. Boimska, T. Frueboes,M. Górski, M. Kazana, K. Nawrocki,

K. Romanowska-Rybinska, M. Szleper,P. Zalewski

NationalCentreforNuclearResearch,Swierk,Poland

K. Bunkowski,A. Byszuk36, K. Doroba,A. Kalinowski, M. Konecki,J. Krolikowski, M. Misiura,

M. Olszewski, M. Walczak

InstituteofExperimentalPhysics,FacultyofPhysics,UniversityofWarsaw,Warsaw,Poland

P. Bargassa,C. Beirão Da Cruz E Silva, A. Di Francesco, P. Faccioli, P.G. Ferreira Parracho,M. Gallinaro,

J. Hollar, N. Leonardo,L. Lloret Iglesias, M.V. Nemallapudi, J. Rodrigues Antunes, J. Seixas,O. Toldaiev,

D. Vadruccio,J. Varela, P. Vischia

S. Afanasiev,P. Bunin, M. Gavrilenko,I. Golutvin, I. Gorbunov, A. Kamenev,V. Karjavin, A. Lanev,

A. Malakhov,V. Matveev37,38, P. Moisenz, V. Palichik,V. Perelygin, S. Shmatov, S. Shulha,N. Skatchkov,

V. Smirnov,N. Voytishin, A. Zarubin

JointInstituteforNuclearResearch,Dubna,Russia

L. Chtchipounov,V. Golovtsov, Y. Ivanov, V. Kim39, E. Kuznetsova40,V. Murzin, V. Oreshkin, V. Sulimov,

A. Vorobyev

PetersburgNuclearPhysicsInstitute,Gatchina(St.Petersburg),Russia

Yu. Andreev,A. Dermenev,S. Gninenko, N. Golubev, A. Karneyeu, M. Kirsanov,N. Krasnikov,

A. Pashenkov,D. Tlisov, A. Toropin

InstituteforNuclearResearch,Moscow,Russia

V. Epshteyn,V. Gavrilov, N. Lychkovskaya,V. Popov, I. Pozdnyakov,G. Safronov, A. Spiridonov, M. Toms,

E. Vlasov,A. Zhokin

InstituteforTheoreticalandExperimentalPhysics,Moscow,Russia

R. Chistov41, V. Rusinov,E. Tarkovskii

NationalResearchNuclearUniversity‘MoscowEngineeringPhysicsInstitute’(MEPhI),Moscow,Russia

V. Andreev,M. Azarkin38,I. Dremin38, M. Kirakosyan, A. Leonidov38,S.V. Rusakov, A. Terkulov

P.N.LebedevPhysicalInstitute,Moscow,Russia

A. Baskakov,A. Belyaev, E. Boos,V. Bunichev, M. Dubinin42, L. Dudko, V. Klyukhin, O. Kodolova,

N. Korneeva,I. Lokhtin,I. Miagkov, S. Obraztsov,M. Perfilov, V. Savrin, P. Volkov

SkobeltsynInstituteofNuclearPhysics,LomonosovMoscowStateUniversity,Moscow,Russia

I. Azhgirey,I. Bayshev,S. Bitioukov, D. Elumakhov, V. Kachanov, A. Kalinin, D. Konstantinov,

V. Krychkine, V. Petrov, R. Ryutin, A. Sobol,S. Troshin, N. Tyurin,A. Uzunian, A. Volkov

StateResearchCenterofRussianFederation,InstituteforHighEnergyPhysics,Protvino,Russia

P. Adzic43,P. Cirkovic, D. Devetak,J. Milosevic,V. Rekovic

UniversityofBelgrade,FacultyofPhysicsandVincaInstituteofNuclearSciences,Belgrade,Serbia

J. Alcaraz Maestre,E. Calvo, M. Cerrada,M. Chamizo Llatas, N. Colino, B. De La Cruz, A. Delgado Peris,

A. Escalante Del Valle,C. Fernandez Bedoya, J.P. Fernández Ramos,J. Flix, M.C. Fouz,P. Garcia-Abia,

O. Gonzalez Lopez,S. Goy Lopez, J.M. Hernandez, M.I. Josa, E. Navarro De Martino,

A. Pérez-Calero Yzquierdo,J. Puerta Pelayo, A. Quintario Olmeda,I. Redondo, L. Romero,M.S. Soares

CentrodeInvestigacionesEnergéticasMedioambientalesyTecnológicas(CIEMAT),Madrid,Spain

J.F. de Trocóniz,M. Missiroli, D. Moran

UniversidadAutónomadeMadrid,Madrid,Spain

J. Cuevas,J. Fernandez Menendez, I. Gonzalez Caballero, J.R. González Fernández,E. Palencia Cortezon,

S. Sanchez Cruz,J.M. Vizan Garcia

UniversidaddeOviedo,Oviedo,Spain

I.J. Cabrillo, A. Calderon, J.R. Castiñeiras De Saa, E. Curras, M. Fernandez,J. Garcia-Ferrero,G. Gomez,

A. Lopez Virto,J. Marco, C. Martinez Rivero,F. Matorras, J. Piedra Gomez, T. Rodrigo, A. Ruiz-Jimeno,

L. Scodellaro,N. Trevisani, I. Vila, R. Vilar Cortabitarte