Search for standard model production of four top quarks in proton–proton collisions at sqrt(s)=13 TeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Search

for

standard

model

production

of

four

top

quarks

in

proton–proton

collisions

at

√

s

=

13 TeV

.TheCMS Collaboration

CERN,Switzerland

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

Articlehistory:

Received20February2017

Receivedinrevisedform24June2017 Accepted24June2017

Availableonline28June2017 Editor:M.Doser Keywords: CMS Physics Top BSM

Asearchforeventscontainingfourtopquarks(t¯tt¯t)isreportedfromproton–protoncollisionsrecorded by the CMS experiment at √s=13 TeV and corresponding to an integrated luminosity of 2.6 fb−1_. Theanalysisconsidersthesingle-lepton(eorμ)+jetsandtheopposite-signdilepton(μ+μ−,μ±e∓,or e+e−)₊jetschannels.Itusesboosteddecisiontreestocombineinformationontheglobaleventandjet propertiestodistinguishbetweent¯tt¯t andt¯t production.Thenumberofeventsobservedafterallselection requirements is consistentwithexpectations frombackgroundand standard model signalpredictions, andanupperlimitissetonthecrosssectionfort¯tt¯t productioninthestandardmodelof94 fbat95% confidencelevel (10.2×theprediction),withanexpectedlimitof118 fb.Thisiscombined withthe resultsfromthepublishedCMSsearchinthesame-signdileptonchannel,resultinginanimprovedlimit of69 fbat95%confidencelevel (7.4×theprediction),withanexpectedlimitof71 fb.Thesearethe strongestconstraintsontherateoft¯tt¯t productiontodate.

©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

In proton–protoncollisions atthe CERN LHC mosttop quarks areproducedint¯t pairs,withasmallcontributionfromsingletop quarkproduction.Itisalsopossible,however,toproducefourtop quarks(t¯tt¯t)inthestandardmodel(SM)viahigher-orderdiagrams in quantum chromodynamics (QCD), mainly via gluon fusion, as showninFig. 1.

The SM crosssection for t¯tt¯t production isapproximately five orders ofmagnitudesmaller thanthat fort¯t,andt¯tt¯t production has not yet been observed. Observing t¯tt¯t production consistent withpredictionswouldprovideavaluabletestofhigher-order per-turbative QCD calculations. In addition, many models of physics beyond the SM predict an increase in the t¯tt¯t crosssection ow-ingeithertothepresenceofhypotheticalparticlesthatdecayinto top quarks or to modified couplings. These includemodels with massivecoloredbosons,Higgsbosonandtopquarkcompositeness, orextra dimensions, models withextended scalar sector such as 2HDMmodels[1],andsupersymmetricextensionsoftheSM[2–8]. Some ofthesemodels predictenhancements inthe observedt¯tt¯t crosssection,withtheassociated kinematicdistributions remain-ing similar to those from SM production.This is particularlythe

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

casewhennonewparticlesbeyondthoseintheSMareproduced on-shell.

At √s=8 TeV, where the SM predicts a cross section (σ_t¯SM_tt¯t) of 1.3 fb [9],CMS seta 95% confidence level (CL) upperlimit of 32 fb on the production cross section, and comparable results, 23 fb, were obtained by ATLAS [10,11]. At √s=13 TeV, the SM predictionincreasesto9.2 fb[9,12],whereCMSseta95% CL up-per limit of 119 fb usinga same-sign dileptonanalysis [13]. The studiespresentedinthispaperareperformedintwoseparate de-caymodesthatarecomplementarytoandstatisticallyindependent from thesame-sign dilepton analysis. Thefirst analysisexamines thefinal statewhereonlyone ofthefourWbosonsfromthetop quarkdecaysint¯tt¯t productiondecaystoamuonorelectron.This single-lepton final state has the largestbranching fractionin t¯ttt¯

Fig. 1. ArepresentativeFeynmandiagramfort¯tt¯t productionintheSMatlowest

orderinQCD.

http://dx.doi.org/10.1016/j.physletb.2017.06.064

0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

production.Thesecondanalysisfocusesontheopposite-sign dilep-ton channelwith exactlytwo ofany combinationofelectrons or muons.Bothusethe13 TeVdatarecordedbytheCMSexperiment in2015 (corresponding to an integratedluminosity of2.6 fb−1), andapplymultivariatetechniquestodiscriminatebetweenthet¯tt¯t andt¯t processes.Inordertoenhancethesensitivity,thesearchis performedinmultiplejetandbjetmultiplicitycategories.

2. The CMS detector

The central feature of the CMS detector is a superconducting solenoidof 6 m internal diameter, providing a magnetic field of 3.8 T. A silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter, and a brass and scintillator hadron calorimeter,each composed ofa barrelandtwo endcapsections, are located within the solenoid volume. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. A two-tier trigger system selects the rele-vantcollisionsforofflineanalysis[14].Amoredetaileddescription ofthe CMS detector, together witha definition of its coordinate systemandkinematicvariables,canbefoundin[15].

3. Data and simulation

Several Monte Carlo (MC) generators are used to simulate thesignal andbackground processes.The t¯tt¯t signalissimulated using the next-to-leading-order (NLO) mg5_amc@nlo generator (v2.2.2)[12,16],assumingatopquarkmass(mt)of172.5 GeV.The ttt¯¯t crosssectioniscalculatedtobe9.2₋+2₂._.9₄ fb[12],wherethe un-certaintyincludes contributions fromthefactorizationand renor-malizationscale uncertainties andthe dependenceon the choice ofpartondistributionfunctions(PDFs).

The dominant background process is t¯t production, which is simulated at NLO using powheg v2 [17–20]. The events com-ing from Drell–Yan(qq→Z/γ∗→ +_−_{, with =e or μ)+jets} andWboson+jetsproductionaremodeledusing MadGraph[12], withtheMLMmatchingscheme[21],anduptothreejets. Single top quark production and the production of t¯t pairs in conjunc-tionwithaHiggsbosongivesmallbackgroundcontributions,and these are simulated using powheg v1. Lastly, the production of a t¯t pair in association with a W or Z boson is modeled using the mg5_amc@nlo generator.Inall ofthe simulations,the initial-andfinal-stateradiation(ISRandFSR),andthefragmentationand hadronization of quarks are modeled using pythia8.212 [22,23] with the underlying event tune CUETP8M1 [24]. This tune uses a value of 0.137 for the strong coupling αS(MZ) in the parton showersimulation,whichleadstoamismodelingofthejet multi-plicity(Nj) spectrum.Allofthesimulationsare correctedforthis effectusing factors that depend on the number ofobserved jets andwhich are equal to the ratioof the particle-level cross sec-tions calculated assuming αS(MZ)=0.137 and αS(MZ)=0.113. Thelattervaluewasderivedfromacomparisonofthepredictions ofthet¯t simulationanda CMSmeasurement onan independent t¯t datasetat8 TeV[25].

Detailedstudiesshowthatthecontributionsoft¯t+H,Zboson, orWbosondo notsignificantly affectthe sensitivityofthe anal-ysis. Thiscombined backgroundisincluded inthe overwhelming t¯t backgroundforthe restofthispaper,unless mentioned other-wise.Backgrounds containing ZandWbosons butnotop quarks are furtherreferred to as electroweak(EW) backgrounds. Among singletop productionmodes, theonly relevantone isthe contri-butionfromtWproduction.Allofthesebackgroundsareincluded intheanalysisbutareordersofmagnitudesmallerthanthe back-groundoriginatingfromt¯t+jetsproduction.

TheNNPDF3.0andNNPDF_nlo_as_0118[26] PDFsare usedto generateallevents,thelatterbeingusedforsamplescreatedwith NLO generators. The simulated samples already include an esti-mateoftheadditionalppinteractionsperbunchcrossing(pileup), andfurthercorrectionsare appliedtomakethesimulationofthe numberof additionalinteractions representativeof thatobserved in the data. All of the simulated events are propagated through a simulation of the CMS detector, which is based on Geant4 (v.9.4)[27].Thet¯t backgroundprocess isnormalizedtothe next-to-next-to-leading-order crosssection [28].In allother cases, the NLOcrosssectionsareused[29–31].

4. Event selection

Thefinalstatesconsideredinthisanalysisarethesingle-lepton channelwithexactlyonemuonorelectron,andtheopposite-sign dileptonchannel withexactly μ+μ−, μ±e∓,or e+e−.These lep-tonsoriginatefromtheWbosonsfromtopquarkdecaysandtend tobeisolatedfromjets,unlikeleptonsproducedinthedecayofB or other hadronswithin jets. Single-leptonevents were recorded using a trigger that required at least one isolated muon with

pT>18 GeV or one isolated electron with pT>23 GeV. Dilep-toneventswererecordedusingatriggerthatrequiredanelectron ormuonwith pT>17 GeV,incombinationwitha secondlepton wheretherequirementispT>8 GeV foramuonand12 GeVfor anelectron.

Eacheventis requiredto haveatleastone reconstructed ver-tex.Theprimaryvertexischosenastheonewiththelargestvalue ofp2

T ofthetracks associatedwithit.Single-leptoneventsare required to contain exactly one isolated muon or electron with

pT>26 GeV or30 GeV,respectively,andpseudorapidity|η|<2.1. The isolationis ensuredby demandingthe variable Irel to be be-lowthepredefinedthreshold.Therelativeisolation, Irel,isdefined as the scalar pT sumof the additional particles emanating from the same vertex as the lepton, within a cone of angular radius R=(η)2_{+ (φ)}2_{=0.4 around the lepton, divided by the}

pT of thelepton,where η andφ arethe differencesin pseu-dorapidityandazimuthalangle(inradians),respectively,between the directionsof thelepton andthe additional particle.The sum doesnotincludethepTofthemuonandiscorrectedforthe neu-tralparticle contributionfrompileup onan event-by-event basis. Electron candidates are required to satisfy restrictive identifica-tion criteria,including isolation,which aredescribed in Ref.[32]. MuonsarerequiredtosatisfythecriteriadescribedinRef.[33]and havearelativeisolationvariable,Irel,smallerthan0.15.

In the dilepton channel, events are required to contain two isolated leptons of opposite charge with pT>20 GeV or25 GeV for muonsor electrons, respectively, and|η|<2.4. Electron can-didates are required to satisfy the same identificationcriteria as inthesingle-leptonchannel.Becauseofthelowerbackground,the muon isolation requirement is relaxed to Irel<0.25. In the μμ andeechannels,theleptonpairisalsorequiredtohavean invari-ant mass greater than 20 GeV andoutside ofa 30 GeV window centeredontheZbosonmass,toexcludeleptonsfromthedecays oflow-massresonancesandZbosons.Eventscontainingadditional isolatedchargedleptonsarevetoed.

Jetsarereconstructedusingaparticle-flowalgorithminwhich theparticlesareclusteredusingtheanti-kTalgorithm[34,35]with adistanceparameterof0.4.Thejetmomentumisdeterminedfrom the vectorial sum of all particle momenta in the jet, and is re-constructedto within5–10%ofthetruemomentum overthefull range of pT within the detectoracceptance, as determined from simulations.Anoffsetcorrection isappliedtojet energiestotake into account the contributionfrom pileup.Jet energy corrections arederivedforsimulation,andverifiedusinginsitumeasurements

oftheenergybalanceindijetandphoton+jetevents.These correc-tionsareappliedasafunctionofthejet pTand ηtobothdataand simulatedevents[36].

Aminimumofsixorfourjetsareeach requiredtohave pT> 30 GeV and|η|<2.5 inthesingle-leptonordileptonchannel, re-spectively,withtwoormorerequiredtobe taggedasoriginating from the hadronization of b quarks (b jets) using the combined secondary vertex (CSV) algorithm [37]. A working point (“medi-um”) of the algorithm is chosen to give a misidentification rate ofapproximately1%forlight-quarkandgluonjets,withbtagging efficiencies of 40–75% depending on the kinematic properties of thejet. In thedilepton channel, eventsare alsorequired to have

HT>500 GeV, where HT is definedasthe scalar sumof the pT ofall jets. This selectionremoves alarge amount ofthe t¯t back-ground, while not significantly reducing theexpected number of signalevents.

The efficiency of the lepton selection is measured using tag-and-probe techniques [38,39]. The simulation is corrected using

pT- and η-dependent scalefactorsoforder unitytoprovide con-sistency with the data. The distribution of the CSV discriminant insimulationiscorrectedtomatchthatobservedindata [40,41]. Therelative ratesoft¯t+bb andt¯t+light-quarkeventsinthe sim-ulationarecorrectedtomakethemconsistentwithprevious mea-surements[42].Afterimplementing thecomplete eventselection criteriaandallcorrections,goodconsistencyinkinematic distribu-tionsisfound betweenthe dataandsimulationinboth channels. Thetotalnumberofeventsselectedinthesingle-electron(muon) channelis3740(5600)and332eventsareselectedinthedilepton channel.Allareconsistentwiththeexpectedbackground.Selected events are predominantly(over 97% in the single-leptonchannel and95%inthedileptonchannel)expectedtobet¯t+jets,withthe remaining fraction from single top, Drell–Yan+jets, and W+jets production.

5. Multivariate analysis

Two boosted decision trees (BDTs), implemented using the TMVAlibrary[43–45],areusedtoimprovethediscrimination be-tween signal and background. The principal differences between t¯tt¯t and tt production¯ are in the jet multiplicity and the num-berofbjets.Theseandtheassociatedkinematicvariablesfeature stronglyin the choiceof BDT input parameters. Thisis basedon thestrategy developedforthe CMS√s=8 TeV analysis,andthe trainingusessimulatedbackgroundsfromt¯t+jetsandt¯tt¯t produc-tion[10].

ThefirstBDTisusedtoidentifycombinationsofthreejets (tri-jet) consistent with being the product of all-hadronic top quark decays,ratherthan ofother sources suchasISR orFSR.The BDT uses the invariant dijet and trijetmasses, b tagging information, andtheanglesbetweenthethreejetsasinputvariables.All possi-bletrijetpermutationsarerankedaccordingtotheirBDT discrim-inantvalue, fromhighest to lowest. Inthe dilepton channel, the t¯t backgroundcontainsnohadronictopdecays,sotheBDToutput forthe first-ranked trijet (Ttrijet1) is used as the discriminant.In thesingle-leptonchannel,each backgroundeventcontains a gen-uine hadronictopquark decay, sothe jetsincludedin Ttrijet1 are removed andthe highest-ranked BDT discriminant using the re-mainingjets(Ttrijet2)isused.

Thesecond BDT, withdiscriminants Dlj_t¯tt¯t forthe single-lepton channeland Ddil

t¯tt¯t forthedileptonchannel, takesthediscriminant fromthe trijet associations asone of its input parameters. Addi-tionalinputs are then optimizedseparately forthe two channels andare based on the characteristics ofthe lepton andjet activ-ityintheevents.Notall inputsareused byboth analyses.These

are groupedinto three categories: event activity, eventtopology, and b quark multiplicity. Althoughmany ofthese are correlated, each onecontributessome additionaldiscrimination betweenthe t¯t backgroundandthet¯tt¯t signal.

Comparison ofsimulatedt¯t andt¯tt¯t eventsleads tothe selec-tionofthefollowingeventactivityvariables:

1. Thenumberofjetspresentintheevent, Nj. 2. Weighted jet multiplicity (Nw

j ), based on both the jet mul-tiplicity and the pT distribution of the jets. This quantity is sensitivetothedifferencesbetweenthe pT spectraofthejets fromtopquarkdecaysandthoseoriginatingfromgluon radi-ation, havinghighervaluesineventswithmanyhigh-pT jets thanineventswhereonlyafewjetshavehighpT andtherest areclosetotheselectionthreshold.Itisdefinedas

N_jw= 125 30 Nj  pT >pth_T  pth_T dpth_T 125 30 pthT dpthT = 1 14725 GeV2 Nj  i=0 Nj  pT>piT   pth_T2 pi_T+1 pi_T (1)

where the limitsof integration are in GeVand Nj(pT>pthT) is the number of jetswith pT above a threshold pth_T, while

p0

T =30 GeV, piT(p

i+1

T ≥pTi) is the pT of the ith jet and

p_TNj+1=125 GeV.The lowerlimit of30 GeVisdrivenby the minimum pT requirementsonthe jets,whilethe upperlimit of 125 GeV is chosen since above this value, there are few eventsandincreasingtheupperlimitonpth

T wouldnot signif-icantlyaffectthesensitivityofNw_j .

3. ThevariableHb

T,definedasthescalarsumofthepT ofalljets that are identified asbjetsby theCSV algorithm, appliedat itsmediumworkingpoint.

4. Theratio(Hratio

T ) ofthe HT ofthefourhighest-pT jetsinthe event inthe single-lepton, orthe two highest-pT jetsin the dileptonchannel,totheHToftheotherjetsintheevent. 5. Thequantity H2m

T ,definedasthe HT intheeventminus the scalarsumofthepTofthetwohighest-pT bjets.

6. Thetransversemomentaofthejetswiththethird- and fourth-largest pTintheevent(pj3T andp

j4 T). 7. Thereducedeventmass(Mh

red),definedastheinvariantmass of the system comprising all the jets in the reduced event, wherethereducedeventisconstructedbysubtractingthejets contained in Ttrijet1 in single-lepton events.In t¯t events, the reduced event will typically only contain the b jet from the semileptonic top quark decay and jets arising from ISR and FSR. Conversely, a reduced t¯tt¯t event can contain up to two hadronictopquarksand,asaresult,numerousenergeticjets. 8. ThereducedeventHT(HTx).ThisisdefinedastheHTofalljets

inthesingle-leptoneventselectionexcludingthosecontained inTtrijet1.

Theeventtopologyischaracterizedbythevariables:

1. Eventsphericity(S)[46],calculatedfromallofthejetsinthe eventin termsof thetensor Sαβ₌

ipαi p

β

i/



i| pi|2,where

α andβ refer tothethree-componentsofthe momentumof theithjet.ThesphericityisthenS= (3/2)(λ2+ λ3),whereλ2 andλ3 arethetwosmallesteigenvaluesofSαβ.Thesphericity int¯tt¯t eventsshould differfromthat inbackgroundt¯t events of the same energy, since the jets in t¯t events will be less isotropically distributed because of their recoil from sources suchasISR.

Fig. 2. Distributionoftheevent-levelBDTdiscriminantsDlj_t¯tt¯tfortheμ+jets (left)ande+jets(right)finalstatesfromdataandtheestimatedbackgroundcontributionsfrom simulation,intheNj≥9 and3Ntagsm (upperpanels)andtheNj≥9 and≥4Ntagsm categories(lowerpanels).Theverticalbarsshowthestatisticaluncertaintiesinthedata. Thepredictedbackgrounddistributionsfromsimulationareshownbytheshadedhistograms.Thehatchedareashowsthesizeofthedominantsystematicuncertaintyinthe simulation,whichcomesfromthematrix-element(ME)factorizationandrenormalizationscalesusedinthesimulation.TheexpectedSMt¯tt¯t signalcontributionisshown byopenhistogram,multipliedbyafactorof20.

2. Hadroniccentrality(C ),definedasthevalueof HT dividedby thesumoftheenergiesofalljetsintheevent.

Since all the previous variables rely only on the hadronic in-formation in the event, sensitivity to the lepton information is providedthrough the pT and η ofthehighest-pT lepton (oronly lepton for the single-lepton channel) p_T1,η1 _{and the angular} difference(R) betweentheleptons indileptonevents.Finally,

thebjetmultiplicityischaracterizedintermsofthenumberofb jetstaggedbytheCSValgorithmoperatingatitsloose[40](Nltags) andmedium (Nm

tags) operatingpoints, andthe angularseparation Rbbbetween the b-tagged jets with the highest CSV discrimi-nants,andthethird- andfourth-highestdiscriminantvalues.

Theevent-leveldiscriminants Dlj_t¯tt¯t andDdil_t¯tt¯t areoptimized sep-arately,resultinginthechoiceofdifferentsetsofvariables.Forthe single-leptonchannel,theoptimalvariableset,inorderof sensitiv-ity,isfoundtobeNj,Ttrijet2,Hb_T,Hratio_T ,p_T1,Nw_j ,Mh_red,Hx_T,andthe third- andfourth-highestCSVdiscriminants.Inthedilepton chan-nel,theoptimal variableset,inorder ofsensitivity,is Nj, Nw_j , S, Ttrijet1,Ntagsl , Nmtags,Rbb, HbT, p

1 T , η 1_{, H}ratio T , H 2m T ,R,C , p j3 T, andpj4_T.TheMCmodelingoftheindividualobservablesutilized in thediscriminantsDlj_t¯tt¯tandDdil_t¯tt¯t wasverifiedusingcontrolsamples oft¯t eventsandfoundtobeinagreementwiththedatainallthe jetmultiplicitiesandbtagmultiplicities.

Tofurtherimprovethesensitivityoftheanalyses,thedataare split into exclusive jet multiplicity categories. The single-lepton analysis uses categories of Nj=6, 7, 8, and ≥9. The dilepton channel, with fewer events, uses only the 4–5, 6–7, and ≥8 jet categories. A furtherdivision intoexclusive b jet multiplicities is possible only for the single-lepton analysis, where the Nj

cate-gories are subdivided into categories with Nmtags=2, 3, and ≥4. Fig. 2shows Dlj_t¯tt¯t inthe μ +jets ande+jetschannelsfortwo of themostsensitive categories,andFig. 3 showsthe Ddil_t¯tt¯t distribu-tionsforthedileptonchannel.

The distributions of the discriminants Dlj_t¯tt¯t and Ddil_t¯tt¯t are fit-ted simultaneously for each Nj and Nmtags bin. For the single-leptonchannelthefitisalsoperformedseparatelyforthe μ +jets ande+jetsevents.In thethreedileptonchannels(μ+μ−, μ±e∓, e+e−),the Ddil

t¯tt¯t distributions arefoundtobeconsistent,andthey arecombinedtoimprovethestatisticalprecision.Inallcasesgood agreementisobservedbetweenthedataandthesimulated back-ground,andtheresultsfromeachofthechannelsarecombinedto obtainanupperlimitonthet¯tt¯t productioncrosssection.

6. Sources of systematic uncertainty

Thesystematicuncertaintiesaffectingtheanalysesaregrouped into normalization and shape categories, depending on their ef-fect on the event-level BDT discriminant distribution. While all normalization uncertainties apply to both the signal and all the background simulations,the shape uncertainties are only consid-eredforthet¯t backgroundandthet¯tt¯t signal.Theseincludeeffects related to the change in normalization owing to changes in the shape.Thenormalizationuncertaintiesare:

1. Anuncertaintyof2.3%[47]intheintegratedluminosity. 2. The uncertainty inthe theoretical t¯t cross section dominates

theuncertaintyinthepredictedeventyields,sincethet¯t pro-cess dominatesthe selected data samples.This cross section is taken from Ref. [48], and includes uncertainties of +₋2₃._.5%_4%

Fig. 3. Distributionofthe event-level BDTdiscriminants Ddil

t¯tt¯t for the combined dilepton(μ+μ−+μ±e∓+e+e−)eventsamplefor4–5jets(top),6–7jets(middle), and≥8jets(bottom).Theverticalbarsshowthestatisticaluncertaintyinthedata. Thepredictedbackgrounddistributionsfromsimulationareshownbytheshaded histograms.Thehatchedareashowsthesizeofthedominantsystematic uncer-taintyinthesimulation,whichcomesfromthechoiceofthematrix-element(ME) factorizationand renormalizationscalesusedinthe simulation.Theelectroweak (EW)histogramisthesumoftheDrell–YanandWboson+jetsbackgrounds.The expectedSMt¯tt¯t signalcontributionisshownbytheopenhistogram,multipliedby afactorof20.

(renormalizationandfactorizationscale)and+6₋₆._.2%_4% (PDF).The effectofuncertaintiesinthecrosssectionsfortheother back-groundswerecheckedandfoundtobenegligible.

3. Theuncertaintiesfromtrigger,leptonidentification,andlepton isolationcorrections, whichareincludedasnuisance parame-tersindetermining theupperlimit. Combined,thesegive an uncertainty of1.2% in the single-muon channel, 3.7% in the single-electronchannel, 4.3% inthe μμ channel, 4.6% inthe μechannel,and4.8% intheeechannel.

Theshapeuncertaintiesare:

1. Theuncertaintyfromthechoiceofthefactorizationand renor-malizationscales in thecalculation of thematrix element of

the hard-scatteringprocess, whichis estimatedby the maxi-mumvariationintheDdil_t¯tt¯torDlj_t¯tt¯tdistributionobtainedwhen each scale is changed separately by a factor of 1/2 and 2, excluding unphysicalanticorrelatedcombinations.This proce-dureisperformedseparatelyforthet¯ttt signal¯ andthet¯t back-ground.Inaddition,alternativet¯t samplesareusedtoestimate theimpactofachangeinthescaleattheparton-showerlevel, taking intoaccount the uncertaintyin αS forthe

hadroniza-tion [25].The differencesinthe distributionswithrespect to thenominalonesaretakenastheuncertainty.Theuncertainty inthematrix-elementscaleisthedominantsystematic uncer-taintyintheanalysis.

2. Differences in the simulation of t¯t from the choice of the matrix-element generator, which is estimated by comparing thenominalt¯t simulationusingpowheg+pythia8 tosamples generated using MadGraph + pythia 8with MLM matching [16].Thedifferencerelativetothenominalsimulationisused toestimatetheuncertaintyfromthissource.

3. Theuncertaintyinthe fractionofttbb events inthe t¯t back-ground,whichisestimatedusingtheuncertaintyinthe mea-sured cross section ratio σ_ttbb: σt¯tjj [42] that was used to correct the ttbb content of the t¯t simulation. An anticor-related uncertainty in the measured cross section ratio of (σt¯tjj−σttbb): σt¯tjjisappliedsimultaneouslytothelight-quark fractiontomaintainthetotalt¯t crosssection.

4. The uncertainties in the jet energy scale and the jet energy resolution [49], whichare estimatedby varying these within theiruncertaintiesby±1standarddeviation.Asimilarmethod isusedtoestimatetheuncertaintyfromtheinelasticproton– proton cross section and the procedure used in the pileup reweighting. Theseuncertainties havevery littleinfluence on thefinallimit.

5. Theuncertaintyinthecorrectionsto thevaluesoftheb tag-ging CSV discriminator, where three categories ofsystematic uncertaintyareappliedforeachjetflavor:thejetenergyscale, purityofthe datasampleused toderivethe corrections,and the statisticaluncertainties derived fromthe fitsused in the method.Theuncertaintyinthebtaggingcorrectioncausedby thejet energyscaleistreatedasfullycorrelated withthejet energyscaleuncertaintydescribed above.Typical magnitudes ofeachoftheseindividualuncertainties onthecorrectionsto thebtaggingCSVdiscriminatorare10–50%beforethefit, de-pendingon thenumberofjetsandbjetsintheevent.Afull descriptionofthesecorrectionscanbefoundinRef.[41]. Eachsystematicsourcewasattributedanuisanceparameterinthe limitdetermination.

7. Results

No deviationfrom the background-onlysimulation, which in-cludes t¯t production and negligible single top, t¯t+H/Z/W bo-son, Drell–Yan+jets, andW+jetsbackgrounds, isobservedin the

Ddil_t¯tt¯t or Dlj_t¯tt¯t distributions. An upperlimit is derived for the t¯ttt¯ production cross section using the asymptotic approximation of the C Ls method provided in Refs. [50–54]. The signal and back-ground distributions are fitted using a simultaneous maximum-likelihood method. The normalization uncertainties are included using log-normal functions and the shape uncertainties are in-cludedasGaussian-distributednuisanceparameters.Theexpected and observed 95% CL upper limits from the two analyses and their combination are listed in Table 1. For the combination of the single-leptonand opposite-sign dilepton results, the system-atic uncertainties attributedto the integratedluminosity, jet en-ergy scale, andmodeling ofthe pileup contributionare assumed

Table 1

Expectedandobserved95%CLupperlimitsonSMt¯tt¯t productionasamultipleofσSM

t¯tt¯tandinfb.Theresultsforthetwoanalysesfromthispaperareshownseparatelyand combined.TheresultfromapreviousCMSmeasurement[13]isalsogiven,alongwiththeoveralllimitswhenthethreemeasurementsarecombined.Thevaluesquotedfor theuncertaintiesontheexpectedlimitsaretheonestandarddeviationvaluesandincludeallstatisticalandsystematicuncertainties.

Channel Expected limit (×σSM

t¯tt¯t) Observed limit (×σ SM

t¯tt¯t) Expected limit (fb) Observed limit (fb)

Single lepton 16.4₋+95..87 17.2 151+

90

−52 158

Dilepton (opposite sign) 24.7+16.7

−9.2 14.5 227+

154

−84 134

Combined (this analysis) 12.8+8.3

−4.5 10.2 118+

76

−41 94

Dilepton (same sign[13]) 11.0+₋63..28 12.9 101+ 57

−35 119

Combined 7.7₋+42..16 7.4 71+

38

−24 69

to be fully correlated. All other systematic uncertainties are as-sumed to be uncorrelated, but taking them as fully correlated does not modify the expected limit. The likelihood function for the single and opposite-sign dilepton limit has 24 nuisance pa-rameterscorrespondingtothe sourcesofsystematicuncertainties thataredescribedabove.Thecombinationwiththelike-sign dilep-tonanalysis,whichisdescribedbelow,adds19additionalnuisance parametersspecific toRef. [13]. Thedataare ableto significantly constraintheparameterscorrespondingtotheMEgeneratorchoice andthepartonshower scale.Allofthepost-fitnuisance parame-tervalueswere foundtobeconsistent withtheir initialvaluesto wellwithintheirquoteduncertainties.

Thecombinedobserved95% CLupperlimitsonthe cross sec-tionforfour-top-quarkproductionmeasured inthe single-lepton, dilepton, and combined results are shown in Table 1. To cross-checktheincrease insensitivityofthe multivariateapproach,the analysisisperformedinthesingle-leptonchannel usingthesame eventcategorization,butonlytheHTdistributionsinplaceofDlj_t¯tt¯t. Theexpectedlimitincreasesbyapproximately20%,thusjustifying theuseofthemorecomplicatedBDTanalyses.

CMShasalsoproducedanupperlimitonthet¯tt¯t crosssection fromtheanalysisofthelike-signdileptonchannel[13].Thelimit fromtheanalysisisalsoshowninTable 1.Toimprovesensitivity, the resultsfrom this search are combined with the results from thatanalysis.Forthiscombination,inadditiontotheassumed cor-relationsdescribed above,the uncertaintyinthemodeling ofthe responseoftheCMStriggersystemtodileptoneventsistreatedas correlatedbetweentheopposite-signandlike-signdilepton analy-ses.AcombinedupperlimitfortheSM t¯tt¯t cross sectionislisted inTable 1.

8. Summary

Insummary,a search hasbeenperformed forevents contain-ingfourtopquarksusingdatarecordedbytheCMSexperimentin proton–protoncollisionsat√s=13 TeV correspondingtoan inte-gratedluminosityof√s=2.6 fb−1.The finalstatesconsideredin theanalysis are the single-leptonchannel withexactly one elec-tronormuon,andtheopposite-signdileptonchannelwithexactly twoofanycombinationofelectronsormuons.Aboosteddecision treeisusedtodiscriminatebetweenthet¯ttt signal¯ andthet¯t back-ground,andnosignalisobserved.Thisleads toan upperlimiton theSMproductioncrosssection fort¯tt¯t of 94 fb (10.2 σSM

t¯tt¯t), with anexpected limitof 118+₋76₄₁ fb atthe 95% confidencelevel.This resultiscombined witha previous search [13] withsimilar sen-sitivityin thesame-sign dileptonchannel to obtain an improved limit of 69 fb, with an expected limit of 71+₋38₂₄fb. This is the moststringentlimit on t¯tt¯t productionat√s=13 TeV published todate.

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,wegratefullyacknowledgethecomputingcentersand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythe computinginfrastructureessential to ouranalyses. Finally, we acknowledge the enduring support for the construc-tionandoperation oftheLHC andtheCMSdetectorprovidedby thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China); COLCIEN-CIAS(Colombia);MSESandCSF(Croatia);RPF(Cyprus);SENESCYT (Ecuador); MoER, ERC IUT, and ERDF (Estonia); Academy of Fin-land,MEC,andHIP(Finland);CEAandCNRS/IN2P3(France);BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hun-gary);DAEandDST(India);IPM(Iran);SFI(Ireland);INFN(Italy); MSIPandNRF(RepublicofKorea);LAS (Lithuania);MOEandUM (Malaysia); BUAP, CINVESTAV,CONACYT, LNS, SEP, andUASLP-FAI (Mexico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland);FCT(Portugal);JINR(Dubna);MON, RosAtom,RAS,RFBR and RAEP(Russia);MESTD (Serbia); SEIDI,CPAN, PCTI andFEDER (Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEP-Center, IPST, STAR, and NSTDA (Thailand); TUBITAK and TAEK (Turkey);NASUandSFFR(Ukraine); STFC(United Kingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro-gram and the European Research Council and EPLANET (Euro-pean Union); the Leventis Foundation; the A. P. Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technolo-gie (IWT-Belgium); the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Scientific and In-dustrial Research,India;theHOMING PLUSprogramofthe Foun-dation forPolishScience, cofinancedfromEuropean Union, Euro-pean Regional Development Fund, the Mobility Plus program of theMinistryofScienceandHigherEducation,theNationalScience Center (Poland), contracts Harmonia 2014/14/M/ST2/00428, Opus 2014/13/B/ST2/02543, 2014/15/B/ST2/03998, and 2015/19/B/ST2/ 02861,Sonata-bis2012/07/E/ST2/01406;theNationalPriorities Re-search Program by Qatar National Research Fund; the Programa Clarín-COFUND del Principado de Asturias; the Thalis and Aris-teia programs cofinanced by EU-ESF and the Greek NSRF; the Rachadapisek Sompot Fund for Postdoctoral Fellowship, Chula-longkornUniversityandtheChulalongkornAcademic intoIts2nd Century Project Advancement Project (Thailand); and the Welch Foundation,contractC-1845.

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The CMS Collaboration

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. Waltenberger, C.-E. Wulz1

InstitutfürHochenergiephysik,Wien,Austria

O. Dvornikov,V. Makarenko, V. Mossolov,J. Suarez Gonzalez, V. Zykunov

InstituteforNuclearProblems,Minsk,Belarus N. Shumeiko

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, D. Lontkovskyi,S. Lowette,

S. Moortgat, L. Moreels,A. Olbrechts,Q. Python, K. Skovpen, S. Tavernier, W. Van Doninck,

P. Van Mulders,I. Van Parijs

VrijeUniversiteitBrussel,Brussel,Belgium

H. Brun, 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,D. Vannerom, R. Yonamine,F. Zenoni, F. Zhang2

UniversitéLibredeBruxelles,Bruxelles,Belgium

A. Cimmino,T. Cornelis, D. Dobur,A. Fagot, M. Gul, I. Khvastunov,D. Poyraz, S. Salva, R. Schöfbeck,

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

GhentUniversity,Ghent,Belgium

H. Bakhshiansohi,C. Beluffi3,O. Bondu, S. Brochet,G. Bruno, A. Caudron, S. De Visscher, C. Delaere,

M. Delcourt,B. Francois, A. Giammanco, A. Jafari,M. Komm, G. Krintiras,V. Lemaitre, A. Magitteri,

A. Mertens, M. Musich, 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

E. Belchior Batista Das Chagas, W. Carvalho,J. Chinellato4,A. Custódio, E.M. Da Costa, G.G. Da Silveira5,

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. Bernardesa, S. Dograa,T.R. Fernandez Perez Tomeia,E.M. Gregoresb,P.G. Mercadanteb,

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

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

InstituteofHighEnergyPhysics,Beijing,China

Y. Ban, G. Chen, 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, T. Sculac

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic, M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,D. Ferencek, K. Kadija,B. Mesic, T. Susa

InstituteRudjerBoskovic,Zagreb,Croatia

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

UniversityofCyprus,Nicosia,Cyprus

M. Finger8, M. Finger Jr.8

CharlesUniversity,Prague,CzechRepublic E. Carrera Jarrin

UniversidadSanFranciscodeQuito,Quito,Ecuador

A.A. Abdelalim9,10, Y. Mohammed11, E. Salama12,13

M. Kadastik, L. Perrini, M. Raidal, A. Tiko, C. Veelken NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola,J. Pekkanen, M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

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

P. Luukka, 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

IRFU,CEA,UniversitéParis-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, M. Nguyen, C. Ochando, G. Ortona,

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

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, P. Van Hove

InstitutPluridisciplinaireHubertCurien(IPHC),UniversitédeStrasbourg,CNRS-IN2P3,Villearbanne,France S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,C. Bernet, G. Boudoul,C.A. Carrillo Montoya, R. Chierici,D. Contardo, B. Courbon,

P. Depasse,H. El Mamouni, 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. Popov15,D. Sabes, V. Sordini,M. Vander Donckt,

P. Verdier,S. Viret

UniversitédeLyon,UniversitéClaudeBernardLyon1,CNRS-IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France

T. Toriashvili16

GeorgianTechnicalUniversity,Tbilisi,Georgia

Z. Tsamalaidze8

TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann,S. Beranek, L. Feld, M.K. Kiesel, K. Klein, M. Lipinski, M. Preuten,C. Schomakers, J. Schulz,

T. Verlage

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

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

R. Fischer,A. Güth, M. Hamer,T. Hebbeker, C. Heidemann, K. Hoepfner,S. Knutzen, M. Merschmeyer,

A. Meyer,P. Millet, S. Mukherjee,M. Olschewski, K. Padeken, T. Pook,M. Radziej, H. Reithler, M. Rieger,

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

V. Cherepanov, G. Flügge, B. Kargoll, T. Kress, A. Künsken, J. Lingemann, T. Müller,A. Nehrkorn,

A. Nowack,C. Pistone, O. Pooth,A. Stahl17

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

M. Aldaya Martin,T. Arndt, C. Asawatangtrakuldee, 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, T. Eichhorn, E. Eren, E. Gallo19,J. Garay Garcia, A. Geiser, A. Gizhko,

J.M. Grados Luyando, A. Grohsjean, P. Gunnellini, A. Harb,J. Hauk, M. Hempel20,H. Jung,

A. Kalogeropoulos, O. Karacheban20, M. Kasemann,J. Keaveney, C. Kleinwort, I. Korol,D. Krücker,

W. Lange, A. Lelek, T. Lenz,J. Leonard, K. Lipka,A. Lobanov, W. Lohmann20, R. Mankel,

I.-A. Melzer-Pellmann,A.B. Meyer, G. Mittag, J. Mnich, A. Mussgiller, D. Pitzl,R. Placakyte, A. Raspereza,

B. Roland, M.Ö. Sahin,P. Saxena, T. Schoerner-Sadenius,S. Spannagel, N. Stefaniuk,G.P. Van Onsem,

R. Walsh, C. Wissing

DeutschesElektronen-Synchrotron,Hamburg,Germany

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

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

M. Niedziela, D. Nowatschin, F. Pantaleo17,T. Peiffer, A. Perieanu, J. Poehlsen, 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

M. Akbiyik, C. Barth,S. Baur, C. Baus, J. Berger,E. Butz, R. Caspart, T. Chwalek, F. Colombo, W. De Boer,

A. Dierlamm, S. Fink,B. Freund,R. Friese, M. Giffels,A. Gilbert, P. Goldenzweig,D. Haitz, F. Hartmann17,

S.M. Heindl,U. Husemann, I. Katkov15, S. Kudella, H. Mildner,M.U. Mozer, Th. Müller, M. Plagge,

G. Quast, K. Rabbertz,S. Röcker, F. Roscher, M. Schröder,I. Shvetsov, G. Sieber, H.J. Simonis,R. Ulrich,

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

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

UniversityofIoánnina,Ioánnina,Greece

N. Filipovic, G. Pasztor

MTA-ELTELendületCMSParticleandNuclearPhysicsGroup,EötvösLorándUniversity,Budapest,Hungary

G. Bencze,C. Hajdu, 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

J.R. Komaragiri

IndianInstituteofScience(IISc),Bengaluru,India

S. Bahinipati24,S. Bhowmik25, S. Choudhury26,P. Mal, K. Mandal, A. Nayak27, D.K. Sahoo24, N. Sahoo,

S.K. Swain

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

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

P. Kumari,A. Mehta, M. Mittal, J.B. Singh, G. Walia

PanjabUniversity,Chandigarh,India

Ashok Kumar,A. Bhardwaj, B.C. Choudhary, R.B. Garg,S. Keshri, 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. Mohanty17, 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, B. Parida,N. Sur, B. Sutar

TataInstituteofFundamentalResearch-A,Mumbai,India

S. Banerjee, R.K. Dewanjee,S. Ganguly, M. Guchait,Sa. Jain, S. Kumar, M. Maity25, G. Majumder,

K. Mazumdar,T. Sarkar25, N. Wickramage28

TataInstituteofFundamentalResearch-B,Mumbai,India

S. Chauhan,S. Dube, V. Hegde, A. Kapoor, K. Kothekar, S. Pandey, A. Rane, S. Sharma

IndianInstituteofScienceEducationandResearch(IISER),Pune,India

S. Chenarani29, E. Eskandari Tadavani,S.M. Etesami29, M. Khakzad, M. Mohammadi Najafabadi,

M. Naseri, S. Paktinat Mehdiabadi30,F. Rezaei Hosseinabadi, B. Safarzadeh31,M. Zeinali

InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran

M. Felcini,M. Grunewald

UniversityCollegeDublin,Dublin,Ireland

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, A. Sharmaa, L. Silvestrisa,17, 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,17

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

S. Albergoa,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. Civinini}a_{, R. D’Alessandro}a,b_{,E. Focardi}a,b_{,P. Lenzi}a,b_{, M. Meschini}a_,

S. Paolettia,L. Russoa,32, G. Sguazzonia, D. Stroma, L. Viliania,b,17

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

L. Benussi, S. Bianco, F. Fabbri,D. Piccolo, F. Primavera17

INFNLaboratoriNazionalidiFrascati,Frascati,Italy

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

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

L. Brianzaa,b,17,F. Brivioa,b,V. Ciriolo, M.E. Dinardoa,b,S. Fiorendia,b,17,S. Gennaia,A. Ghezzia,b, P. Govonia,b,M. Malbertia,b, S. Malvezzia, R.A. Manzonia,b,D. Menascea,L. Moronia,M. Paganonia,b, D. Pedrinia,S. Pigazzinia,b,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,17, M. Espositoa,b, F. Fabozzia,c,F. Fiengaa,b, A.O.M. Iorioa,b_{, G. Lanza}a_{,L. Lista}a_{, S. Meola}a,d,17_{,P. Paolucci}a,17_{, C. Sciacca}a,b_{, F. Thyssen}a

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

P. Azzia,17, 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,N. Pozzobona,b, P. Ronchesea,b,F. Simonettoa,b, E. Torassaa, M. Zanettia,b,P. Zottoa,b,G. Zumerlea,b

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

A. Braghieria,F. Fallavollitaa,b, 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. Bilei}a_{, D. Ciangottini}a,b_{,L. Fanò}a,b_{, P. Lariccia}a,b_{,R. Leonardi}a,b_,

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

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

K. Androsova,32,P. Azzurria,17,G. Bagliesia,J. Bernardinia,T. Boccalia,R. Castaldia, M.A. Cioccia,32, R. Dell’Orsoa,S. Donatoa,c,G. Fedi, A. Giassia,M.T. Grippoa,32,F. Ligabuea,c, T. Lomtadzea, L. Martinia,b, A. Messineoa,b, F. Pallaa,A. Rizzia,b,A. Savoy-Navarroa,33, P. Spagnoloa,R. Tenchinia, G. Tonellia,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, D. Del Rea,b,17, M. Diemoza, S. Gellia,b, E. Longoa,b, F. Margarolia,b, B. Marzocchia,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,17,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. Montenoa,M.M. Obertinoa,b, L. Pachera,b,N. Pastronea, M. Pelliccionia, G.L. Pinna Angionia,b,F. Raveraa,b,A. Romeroa,b, M. Ruspaa,c, R. Sacchia,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, 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 A. Lee

ChonbukNationalUniversity,Jeonju,RepublicofKorea H. Kim

ChonnamNationalUniversity,InstituteforUniverseandElementaryParticles,Kwangju,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, K. Lee,K.S. Lee,S. Lee,J. Lim, S.K. Park, Y. Roh

KoreaUniversity,Seoul,RepublicofKorea

J. Almond,J. Kim, H. Lee,S.B. Oh, B.C. Radburn-Smith, S.h. Seo, U.K. Yang,H.D. Yoo, G.B. Yu

SeoulNationalUniversity,Seoul,RepublicofKorea

M. Choi,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, J. Lee,I. Yu

SungkyunkwanUniversity,Suwon,RepublicofKorea

V. Dudenas, A. Juodagalvis,J. Vaitkus

I. Ahmed,Z.A. Ibrahim, M.A.B. Md Ali34,F. Mohamad Idris35,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 Cruz36,A. Hernandez-Almada,

R. Lopez-Fernandez, R. Magaña Villalba, 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, A. Saddique,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. Byszuk37, 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, B. Calpas, 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

LaboratóriodeInstrumentaçãoeFísicaExperimentaldePartículas,Lisboa,Portugal

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

A. Malakhov,V. Matveev38,39,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. Kim40, E. Kuznetsova41,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

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

E. Vlasov,A. Zhokin

InstituteforTheoreticalandExperimentalPhysics,Moscow,Russia

A. Bylinkin39

MoscowInstituteofPhysicsandTechnology,Moscow,Russia

M. Chadeeva42, V. Rusinov,E. Tarkovskii

NationalResearchNuclearUniversity‘MoscowEngineeringPhysicsInstitute’(MEPhI),Moscow,Russia

V. Andreev,M. Azarkin39,I. Dremin39, M. Kirakosyan, A. Leonidov39,A. Terkulov

P.N.LebedevPhysicalInstitute,Moscow,Russia

A. Baskakov,A. Belyaev, E. Boos,V. Bunichev, M. Dubinin43, L. Dudko, V. Klyukhin, O. Kodolova,

N. Korneeva,I. Lokhtin,I. Miagkov, S. Obraztsov,M. Perfilov, V. Savrin, A. Snigirev

SkobeltsynInstituteofNuclearPhysics,LomonosovMoscowStateUniversity,Moscow,Russia

V. Blinov44, Y. Skovpen44,D. Shtol44

NovosibirskStateUniversity(NSU),Novosibirsk,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. Adzic45,P. Cirkovic, D. Devetak,M. Dordevic, J. Milosevic,V. Rekovic

UniversityofBelgrade,FacultyofPhysicsandVincaInstituteofNuclearSciences,Belgrade,Serbia

J. Alcaraz Maestre,M. Barrio Luna, 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,I. Suárez Andrés, J.M. Vizan Garcia

UniversidaddeOviedo,Oviedo,Spain

I.J. Cabrillo, A. Calderon, 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

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

D. Abbaneo, E. Auffray, G. Auzinger, P. Baillon, A.H. Ball,D. Barney, P. Bloch, A. Bocci, C. Botta,

T. Camporesi, R. Castello,M. Cepeda, G. Cerminara, Y. Chen,D. d’Enterria, A. Dabrowski,V. Daponte,

A. David,M. De Gruttola, A. De Roeck, E. Di Marco46,M. Dobson, B. Dorney, T. du Pree,D. Duggan,

M. Dünser, N. Dupont, A. Elliott-Peisert,P. Everaerts, S. Fartoukh, G. Franzoni, J. Fulcher,W. Funk, D. Gigi,

K. Gill,M. Girone, F. Glege, D. Gulhan, S. Gundacker, M. Guthoff, P. Harris,J. Hegeman, V. Innocente,