Search for supersymmetric partners of electrons and muons in proton-proton collisions at √s=13 TeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Search

for

supersymmetric

partners

of

electrons

and

muons

in

proton–proton

collisions

at

√

s

=

13

TeV

.

The

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received13June2018

Receivedinrevisedform15December2018 Accepted7January2019

Availableonline16January2019 Editor:M.Doser

Keywords: CMS Physics SUSY

Asearchfordirectproductionofthesupersymmetric(SUSY)partnersofelectronsormuonsispresented infinalstateswithtwoopposite-charge,same-flavourleptons(electronsandmuons),nojets,andlarge missingtransversemomentum.Thedatasamplecorrespondstoanintegratedluminosityof35.9 fb−1_of proton–protoncollisionsat√s₌13TeV,collectedwiththeCMSdetectorattheLHCin2016.Thesearch usesthe

M

T2variable,whichgeneralisesthetransversemassforsystemswithtwoinvisibleobjectsand providesadiscriminationagainststandardmodelbackgroundscontainingW bosons.Theobservedyields areconsistentwiththeexpectationsfromthestandardmodel.Thesearchisinterpretedinthecontextof simplifiedSUSYmodelsandprobessleptonmassesuptoapproximately290,400,and450 GeV,assuming right-handedonly,left-handedonly,andbothright- andleft-handedsleptons(massdegenerateselectrons and smuons), and a masslesslightest supersymmetric particle. Limits are also set onselectrons and smuonsseparately.Theselimitsshowanimprovementontheexistinglimitsofapproximately150 GeV.

©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

The standard model (SM) of particle physics provides a de-scription of the fundamental particles and their interactions, andits predictions have been confirmedexperimentally with in-creasing precision over the last several decades. Supersymmetry (SUSY) [1–8],oneofthemostpromisingextensionsoftheSM, ad-dresses severalopen questions forwhich the SM hasno answer, suchasthehierarchyproblemandtheoriginofdarkmatter.The theory postulates a new fundamental symmetry that assigns to each SM particle a SUSY partner whose spin differs by one half, causingtheSUSYpartnerofanSMfermion(boson)tobeaboson (fermion). Inadditiontostabilising theHiggsboson(H) massvia cancellationsbetweenquantumloop correctionsincludingthetop quark and its superpartner, SUSY provides a naturaldark matter candidate,ifR-parity [9] isconserved,intheformofthelightest SUSYparticle(LSP),whichisassumedtobemassiveandstable.

SUSY particles (sparticles) that are coloured, the squarks and gluinos,are producedvia thestronginteractionwithsignificantly larger cross sections than colourless sparticles of equal masses, at the LargeHadron Collider (LHC). However, ifthe squarks and gluinosaretooheavytobeproducedattheLHC,thedirect produc-tionofcolourlesssparticles,suchastheelectroweaksuperpartners

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

(charginos(

χ



₁±),neutralinos(

χ



₂0),andsleptons(



)),

wouldbethe dominantobservableSUSYprocess.

Supersymmetric models predict charged sleptons (



eL,

μ



L,



τ

L,



eR,



μ

R,



τ

R), the superpartners of the charged left-handed and

right-handedSMleptons,whichcanbeproducedatproton–proton (pp) collidersindirectelectroweakpairproduction.Atsufficiently heavysleptonmasses,thesleptonsundergoatwo-bodydecayinto oneoftheheavierneutralinosorachargino,whiledirectdecaysto aneutralino LSParefavoured forlightslepton masses.ThisLetter presentsasearchfordirectlyproducedselectronsandsmuons(



eL,



μ

L,



eR,

μ



R),undertheassumptionofdirectdecays





→ 

χ

10 with

100% branching ratio, as sketched in Fig. 1. The final state con-tainslittle ornohadronicactivity andprovidesa cleansignature composed oftwo opposite-charge(OC), same-flavour (SF)leptons

Fig. 1. Diagramofsleptonpairproductionwithdirectdecaysintoleptonsandthe lightestneutralino.

https://doi.org/10.1016/j.physletb.2019.01.005

0370-2693/©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

(dielectronordimuonpairs)andlargemissingtransverse momen-tum(pmiss_T )fromthetwoLSPsthatescapedetection.

ThemainSM backgroundsresulting intwoOCSF leptons and no reconstructed jetsare pp

→

tt (if both jets fromthe top de-caysareoutofacceptance)andpp

→

WW

→

22

ν

,bothofwhich involve W bosons that decay into an electron or a muon with equalprobability,resultinginthesamenumberofdielectronand dimuoneventsaselectron-muonevents(differentflavour,DF).This flavoursymmetryisusedintheanalysistopredictthenumberof backgroundSF leptons basedonthenumberofDFleptons inthe signalregion(SR)indata,aftercorrectingfordifferencesintrigger andlepton reconstructionefficiencies. TheDrell–Yan(DY) process wouldalso be a main background in the analysis, but is greatly suppressed by the SR requirements. The pp

→

ZZ

→

22

ν

and pp

→

WZ

→

3

ν

processes can alsoresultin twoOC SF leptons. Thesecontributions are takenfromMonte Carlo(MC) simulation aftercomparingdataandsimulationpredictionsincontrolregions (CR).

The data set of proton–proton collisions used for this search wascollected in2016withtheCMSdetectorata centre-of-mass energyof

√

s

=

13TeV,andcorrespondstoanintegrated luminos-ityof35.9 fb−1.Interpretations ofthe search resultsare givenin termsofsimplifiedSUSYmodelspectra [10,11].SearchesforSUSY inthesefinalstateswere performedpreviouslybytheATLAS [12] andCMS [13] Collaborationsat

√

s

=

8TeV,bytheATLAS [14] Col-laborationat

√

s

=

13TeV,anda complementary searchtargeting scenarioswherethemassdifferencebetweentheLSPandthe slep-tonissmallhas beenperformedbytheATLAS Collaboration [15] at

√

s

=

13TeV.

2. TheCMSdetector

ThecentralfeatureoftheCMSapparatusisasuperconducting solenoid,13 m inlengthand6 m indiameter,thatprovidesan ax-ialmagneticfieldof3.8 T.Withinthesolenoidvolumearevarious particle detectionsystems. Charged-particle trajectories are mea-suredby silicon pixel andstrip trackers, covering 0

< φ

≤

2

π

in azimuthand

|

η

|

<

2.5, wherethe pseudorapidity

η

isdefined as

−

log

[

tan(θ/2)

]

, with

θ

beingthepolar angleofthe trajectoryof theparticlewithrespect tothe counterclockwise-beam direction. A lead tungstate crystal electromagnetic calorimeter (ECAL), and abrass andscintillator hadron calorimetersurroundthe tracking volume. The calorimeters provideenergy and direction measure-mentsofelectrons andhadronicjets. Muonsaredetected in gas-ionisationdetectorsembeddedinthesteelflux-returnyokeoutside thesolenoid. Thedetector isnearly hermetic, allowing for trans-verse momentum (pT) balance measurements, in the plane

per-pendiculartothebeamdirection.Atwo-tiertriggersystemselects eventsofinterestforphysicsanalysis.Amoredetaileddescription oftheCMS detector,together witha definitionof thecoordinate systemusedandtherelevantkinematicvariables,canbefoundin Ref. [16].

3. Eventsamples

The search is based on samples of dielectron and dimuon events.Asmentioned inSection 1,DFeventsare usedto predict thecontributionofbackgroundSFeventsintheSR.TheSFandDF samplesare collected witha variety ofisolated andnon-isolated dilepton triggers. Triggers that include loose isolation criteriaon bothleptons require pT

>

23GeV (electron) or17 GeV (muon)on

thehighest pT lepton.The other lepton is then required tohave pT

>

12GeV (electrons) or 8 GeV (muons). In addition, dilepton

triggers without isolation requirements are used to increase the signalefficiency.TheserequirepT

>

33 (30) GeV forbothleptonsin

the dielectron(electron-muon) case. The dimuon trigger requires either pT

>

27 (8) GeV forthe highest(next-to-highest) pT muon

duringearlydatatakingperiods,withanincreaseofthethresholds topT

>

30 (11) GeV forthehighest(next-to-highest)pTmuon

dur-ing remaining data takingperiods. The datacollected withthese triggersareusedforthedata-drivenbackgroundpredictionaswell astocollecttheeventsintheSRwithahigherleading lepton pT

requirementof50 GeV.Theleptonpseudorapiditycoverageforthe triggers is

|

η

|

<

2.5 (2.4) forelectrons (muons). The trigger effi-cienciesare measuredin datausingeventsselectedby a suiteof jettriggersandarefoundtobe90–96%.

The main SM backgrounds are estimated using data control samples,while simulatedeventsare usedto predictbackgrounds fromdiboson(ZZ andWZ) production.Simulatedeventsare also usedextensivelyintheanalysistoestimate systematic uncertain-ties. Next-to-leading order (NLO) and next-to-NLO (NNLO) cross sections [17–28] areusedtonormalise thesimulatedbackground samples.ForthesignalsamplesweuseNLO plus next-to-leading-logarithmic (NLL) calculations for left- or right-handed sleptons, withall theother sparticlesexcepttheLSP assumedtobe heavy anddecoupled [29–31].

The gg

→

ZZ process is generated atLO with mcfm 7.0 [32], and all other diboson production processes [33,34], and tt [35] andthe production ofsingle top quark associated witha W bo-son [36], are generated at NLO withno additional partons with powheg_{v2.SimulatedsamplesofDYprocessesaregeneratedwith} MadGraph5_amc@nlo 2.3.3 program [17] to leading order preci-sionwithuptofouradditionalpartonsinthematrixelement cal-culation.SimulatedVVV andttV (V

=

W,Z) eventsare simulated withthesamegeneratorbutatNLOprecision.TheNNPDF3.0 [37] LO (NLO) parton distribution functions (PDFs) are used for the samples generated at LO(NLO). The matrix element calculations performedwiththesegenerators are interfacedwith pythia [38], includingtheCUETP8M1tune [39,40] forthesimulationofparton showering and hadronisation. Double counting of partons gener-atedwith MadGraph5_amc@nlo and pythia isremovedusingthe MLM [41] and FxFx [42] matching schemes in the LO and NLO samples, respectively. The detector response is simulated with a Geant4model [43] of the CMSdetector. The simulation of new-physicssignalsisperformedusingthe MadGraph5_amc@nlo 2.2.2 programatLOprecision,withuptotwoadditionalpartonsinthe matrixelementcalculation.Eventsaretheninterfacedwith pythia forfragmentationandhadronisationandsimulatedusingtheCMS fast simulation package [44]. The slepton decays are also simu-latedwith pythia.Multiplepp interactions,alsoknownaspileup, are superimposed on the hard collision, andthe simulated sam-ples are reweighed in such a way that the number of collisions perbunchcrossingaccuratelyreflectsthedistributionobservedin data.Correctionsareappliedtothesimulatedsamplestoaccount fordifferencesbetweensimulationanddatainthetriggerand re-constructionefficiencies.

4. Objectselection

Theparticle-flow(PF)algorithm [45] reconstructsandidentifies particle candidatesin the event,referred toas PFobjects. To se-lect collisioneventswe requireatleastone reconstructedvertex, andtheonewiththelargestvalueofsummedphysicsobjectp2

T is

takentobetheprimarypp interactionvertex. Thephysicsobjects usedforthe primaryvertexselection arethe objectsreturnedby a jetfinding algorithm [46,47] appliedtoall chargedtracks asso-ciatedwiththevertex,plusthecorrespondingassociatedpmiss_T .Its vector



pmiss_T isdefinedastheprojectionontotheplane perpendic-ulartothebeamaxisofthenegativevector sumofthemomenta ofall reconstructed PFobjectsin theevent, andits magnitudeis

pmiss_T .Electronsarereconstructedby associatingtrackswithECAL clusters. They are identified using a multivariate approach based oninformationonECALclustershapes, trackreconstruction qual-ity,andthematchingbetweenthetrackandtheECALcluster [48]. Electrons coming from reconstructed photon conversions are re-jected. Muonsare reconstructedfromtracks inthe muon system associated with tracks in the tracker. The identification uses the quality of thetrack fit andthe number ofassociated hitsin the trackingdetectors [49].Forbothelectronsandmuons,theimpact parameter with respect to the primary vertex is required to be within 0.5 mm inthe transverseplane andlessthan1 mm along the beam direction. A lepton isolation variable is defined asthe scalar pT sum ofall PF objects in acone around the lepton,

ex-cludingidentified electronsormuons. Theeffectofadditional pp interactionsinthesameornearbybunchcrossings(pileup)canbe mitigatedbyonlyconsideringchargedPFobjectsthatare compat-ible withthe primary vertexandthe per-eventaverage expected pileup contribution issubtracted fromthe neutral componentof theisolation.The isolation sumisrequiredto besmallerthan10 (20)% of the electron (muon) pT.A shrinking cone-size with

in-creasingpT ischosenthatensureshighefficiencyforleptonsfrom

Lorentz-boostedbosondecays [50]. Thisvaryingconesize is cho-senasthefollowing



R

=



(φ)

2

_{+ (}

_η

₎

2

₌

_{0.2 forp}

T

<

50GeV,

=

10GeV/pTfor50

<

pT

<

200GeV,and0.05 for pT

>

200GeV.

Isolated chargedparticle tracksidentified by the PFalgorithm areselectedwithaloosercriteriathantheleptons definedabove, andareusedasavetoonthepresenceofadditionalcharged lep-tonsfromvectorbosondecays.Isolationisevaluatedbysumming the pT ofall charged PF objects within a cone of



R

=

0.3 and

with the longitudinal impact parameter

|

z

|

<

1 mm relative to theprimaryvertex.PFobjectsidentifiedaschargedhadrons (elec-trons or muons) are required to have pT

>

10 (5) GeV and an

isolationvaluelessthan10(20)%oftheobject pT.

Jetsare clusteredfrom PF objects, excluding charged hadrons not associated with the primary vertex that are assumed to be the result of pileup interactions, using the anti-kT clustering

al-gorithm [46] with adistanceparameterof0.4asimplementedin the FastJet package [47,51].Jetsarerequiredtohave

|

η

|

<

2.4 and

pT

>

25GeV, where the pT is corrected for non-uniform

detec-tor responseand pileupeffects [52,53]. Jetsreconstructed within



R

<

0.4 of any of the selected leptons are removed from the event. Corrections to the jet energy are propagated to pmiss_T us-ing the proceduredeveloped inRef. [52].At least two jetsof pT

above35 GeV areselected forCRsofthisanalysis,andeventsare vetoedthatcontainjetswithpT above25 GeV intheSR.

Eventsare selectedforthe SRbyrequiringtwo OCSF leptons (e±e∓ or

μ

±

μ

∓) withpT

>

50 (20) GeV for thehighest

(next-to-highest) pT lepton and

|

η

|

<

2.4 for both leptons. For the

back-ground prediction methods a sample of lepton pairs is selected, with a pT threshold of 25 (20) GeV for the leading (subleading)

lepton. The highest minimum pT value is chosen because it

ef-ficientlysuppresses backgrounds while maintaining signal accep-tanceefficiency.Eventswithadditionalleptons,identifiedwiththe looserrequirementthattheisolationsumshouldbelessthan40% ofthelepton pT,are vetoed.Leptons mustbe spatially separated

by



R

>

0.1 toavoidreconstructionefficiencydifferencesbetween electrons and muonsin events withcollinear leptons. All events containingleptonsinthetransitionregionbetweenthebarreland endcapof theECAL, 1.4

<

|

η

|

<

1.6,are rejectedto ensure simi-laracceptanceforelectronsandmuons.Thesameleptonselection criteriaareused fora controlsample ofOC DFpairs,e±

μ

∓.The selectionrequirementshavebeenchoseninordertomaximisethe leptonselectionefficiencywhilemaintainingasimilaritybetween electronandmuonefficiencies.

5. Searchstrategy

ThesleptonSRsaredesignedtosuppressexpectedbackgrounds from SM processes, while maintainingsensitivity to different as-sumptions on the masses of the





and

χ



0

1. To suppress

back-groundsduetolow-massresonancesandZ bosonproduction,the dileptoninvariantmassisrequiredtobeabove 20 GeV,andtobe either below 76 or above 106 GeV. Little orno hadronic activity is expected in the direct production of sleptons at pp colliders when assuming a 100%branching ratiofor





→ 

χ

₁0.As a result, eventsarerejectediftheycontainjetswithpT above25 GeV.

Fur-thermore,eventswithtwo leptonsandanadditionalisolatedand chargedPFcandidatepassingtheselectionsdescribedinSection4

are vetoed in order to reduce the background from events with morethantwoisolatedleptons.

The kinematic variable MT2 [54,55] is used to reduce

back-groundsfromtt and WW processes.Thisvariablewas first intro-ducedtomeasurethemassofpair-producedparticles,each decay-ing tothesamefinal state,consistingofavisibleandaninvisible particle.Itisdefinedas:

MT2

=

min  pmiss T (1)+pmissT (2)=pmissT



max



M(_T1)

,

M(_T2)



,

₍₁₎ where



pmiss

T (i)(i

=

1,2)aretrialvectorsobtainedbydecomposing



pmiss

T . The transverse masses M (i)

T

=



2pvis

T pmissT (i)

[

1

−

cos(φ)

]

are obtainedby pairing eitherof these trial vectorswith one of thetwoleptons.

The

φ

isthe angle between the pT ofthe lepton (noted as pvis_T )and



pmiss_T (i).Theminimisationisperformedoveralltrial mo-mentasatisfyingthe



pmiss_T constraint.WhenbuildingMT2fromthe

two selected leptons and



pmiss

T , denoted as MT2

(),

its

distribu-tionexhibitsasharpdecreaseabovethemassoftheW bosonfor tt and WW eventsandisthereforewell suitedtosuppressthese backgrounds.ForthisreasonarequirementofMT2

()

>

90GeV is

imposedinthissearch.

The SRis divided into four bins of pmiss_T : 100–150, 150–225, 225–300,and

≥

300 GeV.Theselectionresultsinasignalselection efficiency that ranges from 20 to 30% assuming a massless LSP. The simplified models do not assume that smuonand selectron massesshouldbethesame,sotheresultsarepresentedfor dielec-tronanddimuonpairsseparately.SincethesearchforcombinedSF dileptonpairs(i.e.dielectrons

+

dimuons)isabletoemploy addi-tional background estimationtechniques which lower theoverall backgrounduncertainty,thecorrespondingresultsarealsoquoted separately.

6. Standardmodelbackgroundpredictions

The backgrounds fromtheSM processesare divided intofour categories. Flavour-symmetric(FS)backgroundprocessesarethose processes that result in DF pairs (e±

μ

∓) as often as SF pairs (

μ

±

μ

∓, e±e∓). The dominant contributions to this category are due to top quark pair production and WW production, but also processessuchasZ

→

τ

±

τ

∓ areestimatedwiththismethod.

Diboson production,ZZ andWZ, can yieldOC SF leptons,and thiscontributionisestimatedfromsimulation.TheZZ processcan resultinafinalstatewithtwoleptonsoriginatingfromone Z bo-son decay andtwo neutrinosfrom theother Z bosondecay. The WZ process can give rise to a final state withthree leptons and

pmiss

T ,whichcan satisfy the signal selectioncriteriaif oneof the

leptonsfailstheidentificationoracceptancerequirements. ThecontributionfromDY(Z

→

e±e∓ andZ

→

μ

±

μ

∓) issmall in theSRduetothe large pmiss

estimatedusingsimulatedeventsafter relaxingthe Z bosonveto andatransferfactorrout/in thatgivesthecontributionofDYevents

outsideoftheZ bosonmasswindowof76–106 GeV.Furthermore, leptons from Z

→

τ

±

τ

∓ decays are vetoed, as this background is FS.The transferfactor rout/in ismeasured in aDY enriched CR

astheratioofeventsoutsideoftheZ bosonmassovertheevents compatiblewiththeZ bosonmass.Fromsimulationstudiesa sys-tematicuncertaintyof50%isfound tocoveranydependenciesof thetransferfactorrout/in onthepTmissandtheMT2,andisassigned

tothemethod.

Finally,a very minorbackground, referred to inthe following asRarebackgrounds,originatesfromtribosonproduction,or pro-cessesresultingin non-FSleptons,such asttZ,tZq and tWZ.The simulationisalsoused toestimate thiscontribution,witha con-servative systematicuncertaintyof50% assignedin placeof QCD scaleandPDFvariations.

6.1.Flavour-symmetricbackgrounds

This paper presents limits on the direct production of slep-tons,selectronsandsmuonsintheSF,dielectronanddimuonfinal states.Fortheresultsinthedielectronanddimuonfinalstates,the SMdielectron(Ne+e−) anddimuon (Nμ+μ−) backgroundsare

ob-tainedusingeventcountsintheDFsample(ND F)multipliedbya

translationfactorRee/DFandRμμ/DF respectively,accordingto

Ne+e−

=

Ree/DF

×

ND F

,

Nμ+μ−

=

Rμμ/DF

×

ND F

.

(2)

Fortheresultsinthe SFfinal state, predictionofSF backgrounds (NS F) is similarly obtained usingevent countsin the DFsample

(ND F),multipliedbyatranslationfactor, RSF/DF,accordingto

NS F

=

RSF/DF

×

ND F

.

(3)

The translation factors Ree/DF and Rμμ/DF are estimated through

a measurement of the rate of dielectron and dimuon events to DFeventsinadedicatedCR.The translationfactor RSF/DF is

mea-sured,similarlytothe Ree/DF andRμμ/DF,astherateofSFevents

toDFeventsinadedicatedCR.AnothermethodtoestimatetheSF yieldsismeasuring thedifference forelectronsandmuonsin re-construction,identification andtriggerefficiencies. As the second methoduses informationof both electrons andmuons, it cannot beusedfortheestimationoftheRee/DFandRμμ/DF.However,itis

combinedwiththeresultsfromtheinitialmeasurementof RSF/DF

usingtheweightedaverageaccordingtotheir uncertaintiesas de-scribed inRef. [56], andresults in a reduction in thesystematic uncertaintythatcomesfromthecombinationofthetwomethods. Thefirst method estimatesdirectly thetranslation factors Ree/DF,

Rμμ/DFandRSF/DFinadataCRenrichedintt events,requiring

ex-actlytwo jets, 100

<

pmiss_T

<

150GeV,andexcluding thedilepton invariantmass range70

<

m

<

110GeV to reduce contributions

fromDYproduction.The RSF/DF, Ree/DF and Rμμ/DF are computed

usingtheobservedyield oftheSF, dielectronanddimuonevents compared to the observed yield of DF events, RSF/DF

=

NSF

/

NDF, Ree/DF

=

Ne+e−

/

NDF and Rμμ/DF

=

Nμ+μ−

/

NDF respectively. Data

and simulation agree within 2% in this region. A 4% systematic uncertaintyon the translation factoris assigned fromsimulation studies, as the maximal magnitude of the systematic needed to coverdiscrepanciesinthetranslationfactorasafunctionofsome SRvariables.ThemainSFbackgroundsestimatedwiththemethod describedabovearett andWW.Simulationstudiesshowthatthe WW is the dominating FS process at high pmiss_T and that there isnodependenceonthe RSF/DF, Ree/DF andRμμ/DF factorsarising

fromthedifferentprocesses.

The second method utilises a factorised approach. The ratio ofmuon toelectron reconstruction andidentificationefficiencies,

rμ/e, is measured ina CRenriched in DY events by requiring at

least two jets, pmiss_T

<

50GeV, and 60

<

m

<

120GeV.

Assum-ing factorisation forthe efficiencies of the two leptons, the ratio of efficiencies for muons and electrons is measured as rμ/e

=



Nμ+_μ−

/

Ne+e−.This ratiodependson the lepton pT dueto the

triggerandreconstruction efficiencydifferences, especiallyatlow leptonpT,andaparametrisationasafunctionofthepToftheless

energeticleptonisused:

rμ/e

=

rμ/e,c

+

α

pT

.

(4)

Hererμ/e,c and

α

areconstants thataredetermined froma fitto

dataandcross-checkedusingsimulation.Thesefitparametersare determinedtoberμ/e,c

=

1.140

±

0.005 and

α

=

5.20

±

0.16GeV.

Inaddition tothe fituncertainty,a 10%systematicuncertainty is assignedtoaccountforvariationsobservedwhenstudyingthe de-pendence of rμ/e on pmiss_T andon the pT of the more energetic

lepton.

The trigger efficiencies for thethree flavour combinationsare used to define the factor RT

=



T μ±μ∓

T e±e∓

/

T e±μ∓,which takes

intoaccountthedifferencebetweenSFandDFchannels. Theefficiencies,

T

μ±μ∓,

eT±e∓ and

T

e±μ∓,arecalculatedasthe

fractionofeventsinacontrolsample recordedwithnon-leptonic triggersthatwouldalsopassthedimuon,dielectronand electron-muontriggerselection,respectively.Theefficienciesaremeasured to range between90–96% depending on the flavour composition of thedilepton trigger, anda systematicuncertainty of3% is as-signed to each triggerefficiency, which isthe maximal deviation betweenthe efficienciesin dataandMC. Thisresultsin thefinal value of RT

=

1.052

±

0.043,wheretheuncertaintyisdueto the

errorpropagationoftheuncertaintiesontheindividualefficiencies to RT.

Thefactorised approachmeasures RSF/DF accordingto RSF/DF

=

0.5(rμ/e

+

r−_μ/1_e

)

RT wherethefactor of0.5isdueto the

assump-tionthatthenumberofproducedDFeventsistwicethenumberof producedeventsineach SFsample (ee and

μμ

).The summation oftherμ/e withitsinverseleads toareduction intheassociated

uncertainty. Asthe parameterisation ofrμ/e inthe factorised

ap-proachhastobe appliedonan event-by-eventbasis,noconstant resultfor RSF/DF canbe given. However, the RSF/DF fromthe first

methodcan be comparedto theresultsfromthe second method byestimatingtheRSF/DFthroughdividingthenumberofpredicted

SFeventsbytheobservedDFeventsineachSR.Bothfactorsrange from1.08to1.1overallSRsandsincethepredictionsfromthetwo methodsagreewelltheyarecombinedusingaweightedaverage.

6.2. Dibosonbackgrounds

Although a Z boson veto is applied, the ZZ process can still entertheSRthroughanoff-shellZ boson.Thiscontributionis es-timated from simulatedevents, validatedin a data CRwith four identified leptons.The selectionsforthe CRandSRare exclusive, and thephysics process in the CR(ZZ where both Z bosons de-caytochargedleptons)hassimilarkinematicsastheprocessitis designedto validate.In orderforthe CRto accuratelyreflect the kinematicsintheSR, thesamejetvetoasintheSRisapplied in theCR.Inaddition,fortheCRtheZ bosoncandidatewiththe in-variant massbest (nextbest) compatiblewiththe Z boson mass is required to have 76

<

m

<

106GeV (50

<

m

<

130GeV).

A generator-levelpT dependentNNLO/NLOKfactorof1.1–1.3,

tak-ing into account missing electroweak corrections [57–59], is ap-plied to the qq

→

ZZ process cross-sections. The smaller contri-bution from the gg

→

ZZ process is normalised to the NLO cal-culation [21].AftersubtractingcontributionstotheCRfromother

Fig. 2. Upper:DistributionofMT2fortheZZ (left)andWZ (right)CRs,insimulation(colouredhistograms)andwiththecorrespondingeventcountsobservedindata(black points).Lower:Ratioofdatatosimulation,withthefilledbandrepresentingthestatisticaluncertaintyonthedataandthesimulations.

processes,asdeterminedbysimulation,asimulation-to-datascale factorof0.94

±

0.07 isobtained.Thisscalefactorisusedtocorrect the ZZ background prediction from simulation in the SR, where oneZ bosondecaystochargedleptons,andtheotherZ boson de-caystoneutrinos.Asystematicuncertaintyof7%resultsfromthe limitednumberofeventsintheCR.ThedistributionofMT2inthe

ZZ CRis showninFig. 2,where the pT ofthe two leptons most

compatiblewiththeZ bosonis addedtothe pmiss_T andtheother twoleptonsareusedtoformtheMT2,andshowagoodagreement

betweendataandsimulation.

Adifferenceinthe pmiss_T andMT2

()

distributionsisobserved

afterapplyingtheqq

→

ZZ NNLO/NLOKfactorasafunctionof dif-ferentgenerator levelkinematic variables.An uncertainty isthen assignedtothemethodbasedonthedifferenceinthepmiss_T shape for MC events in the SR before andafter the applicationof the Kfactor.Additionalsystematicuncertainties areconsideredinthe background prediction,originatingfrom the jet energy scale,the variation ofthe renormalisation andfactorisation scales, the PDF choice,andtheuncertainties intheleptonreconstructionand iso-lationefficiencies,andinthetriggermodelling.

TheWZ processresultinSFeventswhenoneofthethree lep-tonsisnotreconstructed(lost).Thetwodetectedleptonsareofthe SF whenthelepton fromW decayislost,butthey canbe either SForDF,withequalprobability,whenthelostleptonisfromthe Z bosondecay.Inthefirstcasethebackgroundcontributionis es-timatedfromsimulation,whereasinthesecondcaseitiscovered bythedata-drivenFSpredictionmethod.

Justasfor theZZ background, the predictionfromsimulation isvalidatedinaCRenrichedinWZ events.Weselecteventswith three leptons, the same jet veto asapplied in the SR and a re-quirement of pmiss_T

>

70GeV. The invariant mass of the two SF leptons must be within 76

<

m

<

106GeV. Toincrease the

pu-rityof the WZ, eventsare required to have MT

>

50GeV, where MTiscalculatedfrompmissT andtheleptonfromtheW boson.The

distribution of MT2 inthe WZ CR is shown inFig. 2, wherethe MT2 is constructed with the two leptons compatiblewith the Z

bosonandshow agoodagreement betweendata andsimulation. Aftersubtractingcontributionsfromotherprocesses,a simulation-to-data scale factor of 1.06 with a systematic uncertainty of 6%

Fig. 3. Upper:DistributionofpmissT fortheresultingSMbackgroundyieldsestimated intheanalysisSR(colouredhistograms)withthecorrespondingeventcounts ob-servedindata(blackpoints),selectingonlySFevents.Lower:RatioofdatatoSM prediction,withthefilledbandrepresentingthestatisticaluncertaintyonthedata and theestimatedbackgroundsandthe systematicuncertaintyontheestimated backgrounds.

resultingfromthelimitednumberofeventsintheCRisobtained andappliedtothepredictionfromsimulationintheSR. An addi-tionaluncertaintyof5%isadded(inquadrature)tocoverpossible differencesintheidentificationandisolation efficienciesbetween data and simulation in the third lepton low pT region. Finally,

uncertainties due to the jet energy scale, the lepton efficiencies, thetriggermodelling,thePDFchoice,andtherenormalisationand factorisationscalesaretakenintoaccountwhencomputingthe ex-pectedWZ yieldsintheSR.

Table 1

ThepredictedSMbackgroundcontributions,theirsumandtheobservednumberofSFeventsindata. Theyieldsexpectedforseveralsignalscenariosareprovidedasareference.Theuncertaintiesassociated withthebackgroundyieldsstemfromstatisticalandsystematicsources.Thelastbinisinclusiveabove 300 GeV. pmiss T [GeV] 100–150 150–225 225–300 ≥300 FS bkg. 96+₋1312 15.3+ 5.6 −4.5 4.4+ 3.6 −2.3 1.1+ 2.5 −1.0 ZZ 13.5±1.5 9.78±1.19 2.84±0.56 1.86±0.12 WZ 6.04±1.19 2.69±0.88 0.86±0.45 0.21±0.20 DY+jets 2.01+0.39 −0.23 0.00+0.28 0.00+0.28 0.00+0.28 Rare processes 0.69±0.44 0.68±0.47 0.00+0.20 0.05±0.12 Total prediction 118+13 −12 28.4+ 5.9 −4.8 7.9+ 3.7 −2.4 3.2+ 2.6 −1.1 Data 101 31 7 7 m=450 GeV, mχ0 1=20 GeV 1.03±0.09 2.67±0.15 2.67±0.15 8.09±0.26 m=400 GeV, mχ0 1=20 GeV 2.25±0.21 5.05±0.31 6.28±0.35 13.0±0.50 m=350 GeV, mχ0 1=20 GeV 3.97±0.30 10.9±0.49 11.2±0.50 15.9±0.59 Table 2

ThepredictedSMbackgroundcontributions,theirsumandtheobservednumberof dielectron(upper)anddimuon(lower)eventsindata.Theuncertaintiesassociated withtheyieldsstemfromstatisticalandsystematicsources.Thelastbinisinclusive above300 GeV. Dielectron events pmissT [GeV] 100–150 150–225 225–300 ≥300 FS bkg. 36.1+6.6 −6.3 5.7+ 2.5 −2.1 1.6+ 1.5 −1.1 0.41+ 1 −0.5 ZZ 5.17±0.68 3.79±0.58 1.18±0.31 0.69±0.07 WZ 2.65±0.68 1.16±0.45 0.39±0.33 0.21±0.20 DY+jets 0.98+0.14 −0.15 0.00+0.28 0.00+0.28 0.00+0.28 Rare processes 0.02±0.14 0.26±0.21 0.00+0.11 0.06±0.04 Total prediction 45+₋66..74 11.0+ 2.6 −2.3 3.2+ 1.6 −1.2 1.4+ 1.1 −0.6 Data 45 10 2 2 Dimuon events pmissT [GeV] 100–150 150–225 225–300 ≥300 FS bkg. 61.3+₋98..15 9.8+ 3.9 −3.2 2.8+ 2.4 −1.7 0.70+ 1.7 −0.8 ZZ 8.33±0.99 5.98±0.80 1.67±0.42 1.17±0.10 WZ 3.40±0.91 1.53±0.73 0.47±0.30 0.00+0.06 DY+jets 1.03+0.33 −0.14 0.00+0.28 0.00+0.28 0.00+0.28 Rare processes 0.66±0.41 0.42±0.35 0.00+0.16 0.00+0.11 Total prediction 75+₋98..27 17.7+ 4.1 −3.4 4.8+ 2.5 −1.8 1.9+ 1.7 −0.8 Data 56 21 5 5 7.Results

TheobservednumberofeventsindataintheSRarecompared withthestackedSM backgroundestimatesasshowninFig.3(SF events),andsummarised inTable 1 forSF eventsandin Table2

for dielectron and dimuon events, separately. The MT2 shape of

thestackedSMbackgroundestimates,theobserveddataandthree signal scenarios are shown in Fig. 4, for SF events, with all SR selection applied except the MT2 requirement. Applying the MT2

requirementintheSRisgreatly suppressingthe tt andDrell–Yan contributions.

Athigh pmiss

T values,theuncertainties inthe background

pre-diction aredriven by thestatisticaluncertaintyin thenumberof eventsintheDFsampleused toderivethe FSbackground.There isagreement betweenobservation andSM expectation giventhe systematicandstatisticaluncertainties.

8. Interpretation

The results are interpreted in terms of the simplified model described inSection 1.Upperlimitson thecrosssection, assum-ing branching ratios of100%, have beencalculated at 95% confi-dencelevel(CL)usingtheCLs criterionandan asymptotic

formu-Fig. 4. Upper:DistributionofMT2fortheresultingSMbackgroundyieldsestimated intheanalysisSR(colouredhistograms)withthecorrespondingeventcounts ob-servedindata(blackpoints),andthreesignalscenarios(hatchedlines),selecting onlySFeventsandassumingtheproductionofbothleftandright-handedsleptons. Lower:RatioofdatatoSMprediction,withthefilled bandrepresentingthe sta-tisticaluncertaintyonthedataandtheestimatedbackgroundsandthesystematic uncertaintyontheestimatedbackgrounds.

lation [60–63], taking into account the statistical and systematic uncertainties inthesignal yieldsandthebackground predictions. Systematicuncertaintiesaremodelledusingalog-normal distribu-tioninthefit,exceptfortheFSuncertaintywherethedistribution ofthenuisanceparameterismodelledusingthegammafunction. TheuncertaintiesarecorrelatedamongtheSRsandareconsidered assuchinacombinedfittoallSRssimultaneously.

8.1. Systematicuncertaintyinthesignalyield

The systematic uncertainties associated with the signal are shown in Table 3, and described below. The uncertainty in the measurementofthe integratedluminosity is2.5% [64].Aflat un-certainty of 5% isassociated to thelepton identification and iso-lation efficiency in the signal acceptance [48,49], an uncertainty of up to 2.5 (3)% in the electron (muon) efficiency of the signal using fast simulation, and dedicated corrections for fast

simula-Fig. 5. Crosssectionupperlimitandexclusioncontoursat95%CL fordirectsleptonproductionoftwoflavours,selectronsandsmuons,asafunctionoftheχ0

1 andmasses, assumingtheproductionofbothleft- andright-handedsleptons(upper)orproductionofonlyleft- (lowerleft)orright-handed(lowerright).Theregionunderthethickred dotted(blacksolid)lineisexcludedbytheexpected(observed)limit.Thethinreddottedcurvesindicatetheregionscontaining95%ofthedistributionoflimitsexpected underthebackground-onlyhypothesis.Thethinsolidblackcurvesshowthechangeintheobservedlimitduetovariationofthesignalcrosssectionswithintheirtheoretical uncertainties.

Table 3

Listofsystematicuncertaintiestakenintoaccountforthe signalyields.

Source of uncertainty Uncertainty (%)

Integrated luminosity 2.5

Lepton reconstruction/isolation eff. 5

Trigger modelling 3

Fast simulation electron efficiency 1–2.5

Fast simulation muon efficiency 1–3

Jet energy scale 1–15

Pileup 0.5–7

Fast simulation pmiss

T modelling 0.5–20

Unclustered energy shifted pmiss

T 0.5–8

Muon energy scale shifted pmiss

T 0.5–20

Electron energy scale shifted pmiss

T 0.5–4

Renormalisation/factorisation scales 1–11

PDF 3

MC statistical uncertainty 0.5–20

tiontomatchthedataareapplied.Theuncertaintyonthetrigger efficiency is measured to be 3%, asdescribed in Section 6.1. The uncertainty inthe jetenergy scaleis assessedby shifting thejet energycorrectionfactorsforeachjetbyonestandarddeviationup and down and recalculating the kinematic quantities. The result variesbetween1and16%dependingonthesignalkinematics.The uncertainty due to the simulation of pileup for simulated back-ground processes is taken into account by varying the expected cross section of inelastic collisions by 5% [65], and amounts to 0.5–7%depending onthesignal scenario.Varyingtheunclustered energy, andtheelectronandmuon scales,up anddown bytheir 1

σ

variations and evaluating the effect on the pmiss_T , results in systematic uncertainties of 0.5–8%, 0.5–4%, and 0.5–20% respec-tively. The large variation associated to the muon energy scale is due to the poor muon momentum resolution at high pT,and

thequoted valueisdriven byonesignal scenariocontainingsuch high pT muon events.The theoretical uncertainties are those

Fig. 6. Crosssectionupperlimitandexclusioncontoursat95%CL fordirectselectronproductionasafunctionoftheχ0

1andmasses,assumingtheproductionofboth left-andright-handedselectrons(upper),orproductionofonlyleft- (lowerleft)orright-handed(lowerright)selectrons.Theregionunderthethickreddotted(blacksolid)line isexcludedbytheexpected(observed)limit.Thethinreddottedcurvesindicatetheregionscontaining95%ofthedistributionoflimitsexpectedunderthebackground-only hypothesis.Fortheright-handedselectrons,onlythe+1σ expectedline(thinreddottedcurve)isshownasnoexclusioncanbemadeat−1σ.Thethinsolidblackcurves showthechangeintheobservedlimitduetovariationofthesignalcrosssectionswithintheirtheoreticaluncertainties.

factorisation (

μ

F) scales, and of the PDF. The systematic

uncer-taintiesassociatedwiththe

μ

Rand

μ

F scalesareevaluatedusing

weights derived from the SysCalccode applied to simulated sig-nal events [66]. For renormalisation and factorisation scales the Stewart–Tackmann prescription [67] is followed, that treats the theory uncertainties in analyses with a jet selection. This pro-cedure results in an uncertainty of 1–11%. Finally the statistical uncertaintyinthenumberofsimulatedeventsis alsoconsidered andfound to be in the range 0.5–20%, depending on the signal scenario.

8.2.Interpretationsusingsimplifiedmodels

Upperlimitsonthedirectsleptonpairproductioncrosssection aredisplayedinFig.5forthreescenarios:assumingtheexistence ofboth flavour mass degenerate left- and right-handedsleptons, foronly left-handedsleptons,andforonlyright-handedsleptons. Similarly,thelimitsondirectselectron andsmuonproductionare

displayedinFigs.6and7,respectively.TheFigs.5–7alsoshowthe 95% CL exclusioncontours,asa functionofthe





and

χ



0

1 masses.

Notethatthecrosssection atagivenmassforright-handed slep-tons is expected to be about one third of that for left-handed sleptons.Theanalysisprobes sleptonmassesup toapproximately 450,400, or290 GeV, assuming bothleft- and right-handed, left-handed only, or right-handed sleptons, and a massless LSP. For models with high slepton masses and light LSPs the sensitivity is driven by the highest pmiss_T bin. The sensitivity is reduced at higher LSP massesdueto the effectof thelepton acceptance. In thecaseofselectrons(smuons),thelimitscorresponding tothese 3 scenarios are 350, 310 and 250 GeV (310, 280, and 210 GeV). Since the dimuon data yield in the highest pmiss_T bin is some-what higher than predicted, the observed limits in this channel are weaker than expectedin theabsence ofsignal. These results improvetheprevious8 TeV exclusionlimitsby100–150 GeV inthe sleptonmass [13].

Fig. 7. Crosssectionupperlimitandexclusioncontoursat95%CL fordirectsmuonproductionasafunctionoftheχ0

1andmasses,assumingtheproductionofboth left-andright-handedsmuons(upper),orproductionofonlyleft- (lowerleft)orright-handed(lowerright)smuons.Theregionunderthethickreddotted(blacksolid)lineis excludedbytheexpected(observed)limit.Thethinreddottedcurvesindicatetheregionscontaining95%ofthedistributionoflimitsexpectedunderthebackground-only hypothesis.Fortheright-handedsmuons,onlythe+1σ expectedline(thinreddottedcurve)isshownasnoexclusioncanbemadeat−1σ.Thethinsolidblackcurves showthechangeintheobservedlimitduetovariationofthesignalcrosssectionswithintheirtheoreticaluncertainties.

9. Summary

A search for direct slepton (selectron or smuon) production, ineventswithopposite-charge,same-flavourleptons,nojets, and missingtransversemomentumhasbeenpresented.Thedata com-priseasampleofproton–protoncollisionscollectedwiththeCMS detector in 2016 at a centre-of-mass energy of 13 TeV, corre-sponding to an integrated luminosity of 35.9 fb−1_{. Observations}

are in agreement with Standard Model expectations within the statistical and systematic uncertainties. Exclusion limits are pro-videdassumingright-handedonly,left-handedonlyandright-and left-handed two flavour slepton production scenarios (mass de-generateselectrons andsmuons). Slepton massesup to 290,400 and 450 GeV respectively are excluded at 95% confidence level, assuming a massless LSP. Exclusion limits are also provided as-suming a massless LSP and right-handed only, left-handed only andright-and left-handedsingle flavour productionscenarios, ex-cluding selectron (smuon) masses up to 250, 310 and 350 GeV

(210, 280 and 310 GeV), respectively. These results improve the previous exclusion limits measured by the CMS experiment at a centre-of-mass energy of 8 TeV by 100–150 GeV in slepton masses.

Acknowledgements

We congratulate our colleagues in the CERN accelerator de-partments for the excellent performance of the LHC and thank the technicalandadministrativestaffsatCERNandatother CMS institutes for their contributions to the success of the CMS ef-fort.Inaddition,wegratefullyacknowledgethecomputingcentres and personnel of the Worldwide LHC Computing Grid for deliv-ering so effectivelythe computing infrastructure essential to our analyses. Finally, we acknowledge the enduring support for the construction andoperationoftheLHCandtheCMSdetector pro-videdbythefollowingfundingagencies:BMWFWandFWF (Aus-tria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, FAPERGS,

andFAPESP(Brazil); MES(Bulgaria); CERN;CAS,MOST, andNSFC (China); COLCIENCIAS (Colombia); MSES and CSF (Croatia); RPF (Cyprus);SENESCYT(Ecuador);MoER,ERCIUT,andERDF(Estonia); AcademyofFinland,MEC,andHIP(Finland);CEAandCNRS/IN2P3 (France);BMBF, DFG, andHGF (Germany); GSRT(Greece); NKFIA (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP andNRF (Republic of Korea); LAS (Lithuania); MOE and UM (Malaysia); BUAP, CINVESTAV, CONACYT, LNS, SEP, and UASLP-FAI(Mexico); MBIE(NewZealand);PAEC (Pakistan);MSHE andNSC (Poland);FCT(Portugal);JINR(Dubna);MON, ROSATOM, RAS,RFBR,andNRCKI(Russia);MESTD(Serbia);SEIDI,CPAN,PCTI, and FEDER (Spain); Swiss Funding Agencies (Switzerland); MST (Taipei);ThEPCenter, IPST, STAR, and NSTDA(Thailand); TUBITAK andTAEK(Turkey);NASUandSFFR(Ukraine);STFC (United King-dom);DOEandNSF(USA).Individualshavereceivedsupportfrom theMarie-Curie programandthe European ResearchCouncil and Horizon 2020 Grant, contract No. 675440 (European Union); the Leventis Foundation;the Alfred P. Sloan Foundation; the Alexan-dervon HumboldtFoundation;theBelgianFederal SciencePolicy Office; the Fonds pour la Formation à la Recherche dans l’In-dustrieetdansl’Agriculture (FRIA-Belgium); theAgentschapvoor Innovatie door Wetenschap en Technologie (IWT-Belgium); the F.R.S.-FNRS andFWO (Belgium) under the “Excellence of Science – EOS” – be.h project n. 30820817; the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Lendület (“Momentum”)ProgramandtheJánosBolyaiResearchScholarship of the Hungarian Academy of Sciences, the New National Excel-lenceProgram ÚNKP,the NKFIA research grants 123842, 123959, 124845, 124850 and 125105 (Hungary); the Council of Science andIndustrial Research,India;the HOMING PLUSprogramofthe Foundation for Polish Science, cofinanced from European Union, Regional Development Fund, the Mobility Plus program of the Ministry of Science and Higher Education, the National Science Centre (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 Estatalde Fomentode laInvestigaciónCientíficayTécnicade Ex-celencia María de Maeztu, grant MDM-2015-0509 and the Pro-grama Severo Ochoa del Principado de Asturias; the Thalis and Aristeia programs cofinanced by EU-ESF and the Greek NSRF; theRachadapisekSompotFundforPostdoctoralFellowship, Chula-longkornUniversity andthe ChulalongkornAcademicintoIts 2nd CenturyProjectAdvancementProject (Thailand);theWelch Foun-dation,contractC-1845;andtheWestonHavensFoundation(USA).

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TheCMSCollaboration

A.M. Sirunyan,

A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam,

F. Ambrogi,

E. Asilar,

T. Bergauer,

J. Brandstetter,

E. Brondolin,

M. Dragicevic,

J. Erö,

A. Escalante Del Valle,

M. Flechl,

R. Frühwirth

1

,

V.M. Ghete,

J. Hrubec,

M. Jeitler

1

,

N. Krammer,

I. Krätschmer,

D. Liko,

T. Madlener,

I. Mikulec,

N. Rad,

H. Rohringer,

J. Schieck

1

,

R. Schöfbeck,

M. Spanring,

D. Spitzbart,

A. Taurok,

W. Waltenberger,

J. Wittmann,

C.-E. Wulz

1

,

M. Zarucki

InstitutfürHochenergiephysik,Wien,Austria

V. Chekhovsky,

V. Mossolov,

J. Suarez Gonzalez

E.A. De Wolf,

D. Di Croce,

X. Janssen,

J. Lauwers,

M. Pieters,

M. Van De Klundert,

H. Van Haevermaet,

P. Van Mechelen,

N. Van Remortel

UniversiteitAntwerpen,Antwerpen,Belgium

S. Abu Zeid,

F. Blekman,

J. D’Hondt,

I. De Bruyn,

J. De Clercq,

K. Deroover,

G. Flouris,

D. Lontkovskyi,

S. Lowette,

I. Marchesini,

S. Moortgat,

L. Moreels,

Q. Python,

K. Skovpen,

S. Tavernier,

W. Van Doninck,

P. Van Mulders,

I. Van Parijs

VrijeUniversiteitBrussel,Brussel,Belgium

D. Beghin,

B. Bilin,

H. Brun,

B. Clerbaux,

G. De Lentdecker,

H. Delannoy,

B. Dorney,

G. Fasanella,

L. Favart,

R. Goldouzian,

A. Grebenyuk,

A.K. Kalsi,

T. Lenzi,

J. Luetic,

N. Postiau,

E. Starling,

L. Thomas,

C. Vander Velde,

P. Vanlaer,

D. Vannerom,

Q. Wang

UniversitéLibredeBruxelles,Bruxelles,Belgium

T. Cornelis,

D. Dobur,

A. Fagot,

M. Gul,

I. Khvastunov

2

,

D. Poyraz,

C. Roskas,

D. Trocino,

M. Tytgat,

W. Verbeke,

B. Vermassen,

M. Vit,

N. Zaganidis

GhentUniversity,Ghent,Belgium

H. Bakhshiansohi,

O. Bondu,

S. Brochet,

G. Bruno,

C. Caputo,

P. David,

C. Delaere,

M. Delcourt,

B. Francois,

A. Giammanco,

G. Krintiras,

V. Lemaitre,

A. Magitteri,

A. Mertens,

M. Musich,

K. Piotrzkowski,

A. Saggio,

M. Vidal Marono,

S. Wertz,

J. Zobec

UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

F.L. Alves,

G.A. Alves,

L. Brito,

G. Correia Silva,

C. Hensel,

A. Moraes,

M.E. Pol,

P. Rebello Teles

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

E. Belchior Batista Das Chagas,

W. Carvalho,

J. Chinellato

3

,

E. Coelho,

E.M. Da Costa,

G.G. Da Silveira

4

,

D. De Jesus Damiao,

C. De Oliveira Martins,

S. Fonseca De Souza,

H. Malbouisson,

D. Matos Figueiredo,

M. Melo De Almeida,

C. Mora Herrera,

L. Mundim,

H. Nogima,

W.L. Prado Da Silva,

L.J. Sanchez Rosas,

A. Santoro,

A. Sznajder,

M. Thiel,

E.J. Tonelli Manganote

3

,

F. Torres Da Silva De Araujo,

A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

S. Ahuja

a

,

C.A. Bernardes

a

,

L. Calligaris

a

,

T.R. Fernandez Perez Tomei

a

,

E.M. Gregores

b

,

P.G. Mercadante

b

,

S.F. Novaes

a

,

Sandra S. Padula

a

,

D. Romero Abad

b

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

A. Aleksandrov,

R. Hadjiiska,

P. Iaydjiev,

A. Marinov,

M. Misheva,

M. Rodozov,

M. Shopova,

G. Sultanov

InstituteforNuclearResearchandNuclearEnergy,BulgarianAcademyofSciences,Sofia,Bulgaria

A. Dimitrov,

L. Litov,

B. Pavlov,

P. Petkov

UniversityofSofia,Sofia,Bulgaria

W. Fang

5

,

X. Gao

5

,

L. Yuan

BeihangUniversity,Beijing,China

M. Ahmad,

J.G. Bian,

G.M. Chen,

H.S. Chen,

M. Chen,

Y. Chen,

C.H. Jiang,

D. Leggat,

H. Liao,

Z. Liu,

F. Romeo,

S.M. Shaheen,

A. Spiezia,

J. Tao,

C. Wang,

Z. Wang,

E. Yazgan,

H. Zhang,

J. Zhao

InstituteofHighEnergyPhysics,Beijing,China

Y. Ban,

G. Chen,

J. Li,

L. Li,

Q. Li,

Y. Mao,

S.J. Qian,

D. Wang,

Z. Xu

Y. Wang

TsinghuaUniversity,Beijing,China

C. Avila,

A. Cabrera,

C.A. Carrillo Montoya,

L.F. Chaparro Sierra,

C. Florez,

C.F. González Hernández,

M.A. Segura Delgado

UniversidaddeLosAndes,Bogota,Colombia

B. Courbon,

N. Godinovic,

D. Lelas,

I. Puljak,

T. Sculac

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic,

M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,

D. Ferencek,

K. Kadija,

B. Mesic,

A. Starodumov

6

,

T. Susa

InstituteRudjerBoskovic,Zagreb,Croatia

M.W. Ather,

A. Attikis,

G. Mavromanolakis,

J. Mousa,

C. Nicolaou,

F. Ptochos,

P.A. Razis,

H. Rykaczewski

UniversityofCyprus,Nicosia,Cyprus

M. Finger

7

,

M. Finger Jr.

7

CharlesUniversity,Prague,CzechRepublic

E. Ayala

EscuelaPolitecnicaNacional,Quito,Ecuador

E. Carrera Jarrin

UniversidadSanFranciscodeQuito,Quito,Ecuador

S. Elgammal

8

,

S. Khalil

9

,

A. Mahrous

10

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

S. Bhowmik,

A. Carvalho Antunes De Oliveira,

R.K. Dewanjee,

K. Ehataht,

M. Kadastik,

M. Raidal,

C. Veelken

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola,

H. Kirschenmann,

J. Pekkanen,

M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Havukainen,

J.K. Heikkilä,

T. Järvinen,

V. Karimäki,

R. Kinnunen,

T. Lampén,

K. Lassila-Perini,

S. Laurila,

S. Lehti,

T. Lindén,

P. Luukka,

T. Mäenpää,

H. Siikonen,

E. Tuominen,

J. Tuominiemi

HelsinkiInstituteofPhysics,Helsinki,Finland

T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon,

F. Couderc,

M. Dejardin,

D. Denegri,

J.L. Faure,

F. Ferri,

S. Ganjour,

A. Givernaud,

P. Gras,

G. Hamel de Monchenault,

P. Jarry,

C. Leloup,

E. Locci,

J. Malcles,

G. Negro,

J. Rander,

A. Rosowsky,

M.Ö. Sahin,

M. Titov