Search for new phenomena in final states with large jet multiplicities and missing transverse momentum with ATLAS using $\sqrt{s}=13$ TeV proton–proton collisions

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Physics Letters B

www.elsevier.com/locate/physletb

Search for new phenomena in final states with large jet multiplicities and missing transverse momentum with ATLAS using √

s = ^{13 TeV} proton–proton collisions

.ATLASCollaboration^

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

Articlehistory:

Received22February2016 Accepted1April2016 Availableonline6April2016 Editor:W.-D.Schlatter

Results arereportedofasearchfornewphenomena,suchassupersymmetricparticleproduction,that could beobservedinhigh-energyproton–protoncollisions.Eventswithlargenumbersofjets,together with missing transverse momentum fromunobserved particles, are selected. The data analysed were recordedbytheATLASexperimentduring2015usingthe13 TeV centre-of-massproton–protoncollisions at the Large Hadron Collider, and correspond to an integrated luminosity of 3.2 fb⁻¹. The search selectedeventswithvariousjetmultiplicitiesfrom≥^{7 to}≥^{10 jets,andwithvariousb-jet}multiplicity requirements to enhancesensitivity. No excess above Standard Model expectations is observed. The results are interpreted withintwo supersymmetry models,where gluino massesupto 1400 GeV are excludedat95%confidencelevel,significantlyextendingpreviouslimits.

©^{2016CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Newstronglyinteractingparticles,ifpresentattheTeV energy scale, maybe produced in high-energy proton–proton(pp) colli- sionsanddecaytofinalstateswithlargejetmultiplicities.Iftheir decayproduces stableparticleswhichonlyinteractweakly,itwill alsoresult ina momentum imbalancein theplane transverse to thebeam(E^miss_T ).

Such particles are present in supersymmetry (SUSY) [1–6], a theoretically favouredextension of theStandard Model(SM) that predicts partner fields for each of the SM particles. These fields combineintophysicalsuperpartnersoftheSMparticles.Thescalar partnersofquarksandleptonsareknownassquarks(^q)˜ ^andslep- tons(˜).ThefermionicpartnersofgaugeandHiggsbosonsarethe gluinos(˜^{g),thecharginos (}χ˜_i^±,withi=¹,2) andtheneutralinos (χ˜_i⁰ with i=¹,2,3,4), with χ˜_i^± and χ˜_i⁰ beingthe mass eigen- states,ordered fromthe lightesttotheheaviest,formed fromthe linearsuperpositionsofthe SUSYpartnersofthe Higgsandelec- troweakgaugebosons.

Underthe hypothesis of R-parity conservation[7],SUSY part- nersareproducedinpairsanddecaytothelightestsupersymmet- ricparticle(LSP),whichisstableandinalargevarietyofmodels isassumedto be thelightest neutralino (χ˜₁⁰), whichescapes de- tection.Theundetectedχ˜₁⁰wouldresultinmissingtransversemo- mentum,whilethe restofthe cascadecan yieldfinal stateswith

 E-mailaddress:atlas.publications@cern.ch.

multiple jets and possibly leptons and/or photons. The strongly interacting gluinos and squarks can have large production cross- sections at the Large Hadron Collider (LHC), butno evidence of theirexistencehasbeenobservedtodate.

Thispaperpresentstheresultsofasearchfornewphenomena, suchassupersymmetry,infinal stateswithlargejet multiplicities (from ≥^7to≥^10jets)inassociationwith E^miss_T .Thissignature is exhibited,forexample,by squarkandgluinoproductionfollowed bycascadedecaychains,and/ordecaystoheavySMparticles,such astopquarksorW ,Z orHiggsbosons,eachofwhichcanproduce multiplejetsintheirdecays.Incontrasttomanyothersearchesfor the productionofstrongly interactingSUSYparticles,therequire- ment madehereoflarge jet multiplicitymeans that therequire- mentonE^miss_T canbemodest.

Previous searches[8–10] insimilarfinal stateshavebeenper- formed by the ATLAS Collaboration at the lower centre-of-mass energies of √

s=^{7 TeV and 8 TeV, with integrated} luminosities upto20.3 fb⁻¹.Thelargerenergyofthepresentdatasetprovides increased sensitivity,particularlyto particles withhighermasses.

Thispapercloselyfollowsthestrategyofthosepreviousstudies.In particular,dataarecollectedusinganonlineselectionrelyingonly on high jet multiplicity andthe signal regions (SR)are designed such that the dominant multijet background can be determined fromthedatausingregionsoflower E^miss_T and/orlowerjetmulti- plicity.

The datawerecollected bythe ATLASdetector[11] in pp col- lisions at the LHC at a centre-of-mass energy of 13 TeV, from 16th August to 3rd November 2015. The detector covers the

http://dx.doi.org/10.1016/j.physletb.2016.04.005

0370-2693/©^{2016CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

pseudorapidity¹ rangeof |η|<4.9 andis hermetic inazimuth. It consistsofan inner trackingdetectorsurrounded by a supercon- ductingsolenoid, electromagneticand hadroniccalorimeters, and anexternalmuon spectrometerincorporatinglargesuperconduct- ing toroidal magnets. After applying beam-, data- and detector- qualitycriteria, theintegratedluminositywas 3.2±⁰.2 fb⁻¹.The uncertaintywas derived usingbeam-separationscans,followinga methodologysimilartothatdetailedinRef.[12].

2. Physicsobjectdefinition

Jetsarereconstructedusingtheanti-kt clusteringalgorithm[13, 14]with jet radius parameter R=⁰.4 and starting from clusters ofcalorimetercells [15]. Theeffectsofcoincident pp interactions (‘pileup’) on jet energies are accountedfor by an event-by-event p_T-densitycorrection[16].Theenergyresolutionofthejetsisim- provedbyusingglobalsequentialcalibrations[17,18].Eventswith jets originating from cosmic rays, beam background and detec- tor noise are vetoed using the ‘loose’ requirements of Ref. [19].

Jetscontainingb-hadrons(b-jets)areidentifiedusinganalgorithm exploitingthelonglifetime,highdecaymultiplicity,hardfragmen- tationandlargemass ofb-hadrons [20].The b-taggingalgorithm tags b-jets with an efficiency ofapproximately 70% in simulated t¯t events, and mis-tags c-jets, τ-jets and light-quark or gluon jetswithprobabilities ofapproximately10%, 4%and0.2% respec- tively[21].

The primary vertex(PV) ineach eventis the vertexwiththe largestvalueofp²_T foralltracksassociatedwithit.Toreducethe effectofpileup,ajet having20 GeV<pT<50 GeV and|η_|<2.4 isdisregardedwhenthe pT-weightedsumofitsassociatedtracks indicatesthatitoriginatedfromapileupcollisionandnotthePV, basedonajetvertextaggerasdescribedinRef.[16].

Electroncandidates are identified accordingto the likelihood- based ‘loose’ criterion described in Ref. [22], formed from e.g.

calorimeter shower shape and inner-detector track properties.

Muon candidates are identified accordingto the ‘medium’ crite- rion described in Ref. [23], based on combined tracks from the inner detector and muon spectrometer. These candidates (which may cause an event to be rejectedfrom the signal regions) are requiredtohavep_T>10 GeV,|η|<2.47 fore and|η|<2.5 for μ. Toavoiddouble-countingofreconstructedobjects,electroncan- didatessharinganinner-detectortrackwithamuoncandidateare removed.Next,jetcandidatesseparatedfromanelectroncandidate by R_y<0.2 are removed, where R_y=

(y)²+ (φ)^{2. Jet} candidateswithfewerthanthreetracksandwithRy<0.4 from a muon candidate are then removed. Following this, any lepton candidateseparatedfromasurviving jetcandidate byR_y<0.4 isremoved.

Themissingtransversemomentum, E^miss_T ,isthenegative two- vector sum of the calibrated ^pT of reconstructed jetswith p_T>

20 GeV and |η|<4.5, electrons, muons and photons [24]. It includes an additional contribution from inner-detector tracks, matched to the PV, that are not associated with these recon- structedobjects. Photons arenot considered beyondtheir contri- butiontotheE^miss_T unlesstheyarereconstructedasjets.Toreduce the effect of pileup, jets do not contribute to the E^miss_T calcula- tionwhenthey aredisregarded basedonthejet vertextaggeras described above. Additionally, when a jet having 50 GeV<p_T<

1 ATLAS uses a right-handed coordinate systemwith itsorigin at the nomi- nalinteractionpoint (IP)inthecentreofthedetector andthe z-axisalongthe beampipe.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeing theazimuthalanglearoundthebeampipe.Thetransversemomentumofafour- momentumis^pT= (^px,py),itsrapidityisy=¹2ln^E_E⁺₋^p_p^z

z,andthepseudorapidityis definedintermsofthepolarangleθasη= −^{ln tan}(θ/2).

70 GeV,|η|<2.0 and azimuthrelativetothemissingmomentum

φ (pT,E^miss_T )>2.2 meets thesame vertex-tagging criterion, the eventisdiscarded.EventsinwhichthejetclosestinφtotheE^miss_T isfoundinornearaninactiveregioninthehadroniccalorimeter barrel(i.e.−⁰.1<η<1.0,0.8< φ <1.1)arealsodiscarded,inor- dertoreducetheimpactofthissourceofE^miss_T mismeasurement.

Thesedata-qualityrequirementsreducetheexpectedacceptanceof typicalSUSYmodelsbyapproximately 5%.

Whendefiningleptonsforcontrolregions(Section5),thecan- didates definedabove are requiredto be isolated, to havea lon- gitudinal impactparameter z0 (withrespectto thePV) satisfying

|^z0sinθ|<0.5 mm,andtohavethesignificanceoftheirtransverse impactparameter|^d0/σ(d0)|^{(withrespecttothemeasuredbeam} position) be less than five for electrons and less than three for muons. Additionally, electrons must satisfy the‘tight’ criterion of Ref.[22].

3. Eventselection

Thesignalregionsaredefinedusingtwojetmultiplicitycounts:

eithern50, thenumberofjetshaving pT>50 GeV and |η_|<2.0, or n80, the number of such jets which additionally satisfy the higherrequirement pT>80 GeV.Theonlineselection(trigger)for n₅₀-based regions requires events to have at least six jets each with pT>45 GeV and |η_|<2.4,whilethat forn80-basedregions requires atleast fivejetseach with pT>70 GeV. The triggeref- ficiency isgreater than 99.5% for eventssatisfying the signal se- lectiondescribedbelow.Jetswithalooserdefinition–thosehav- ing pT>40 GeV and|η_|<2.8 – areused toconstructthe scalar sum H_T=

p^jet_T , while thosehaving p_T>40 GeV and |η|<2.5 are candidatesforb-tagging, contributingto the numbern_b-jet of b-taggedjets.

The signal selection requires large jet multiplicity, which de- pendsonthesignalregion(SR),asshowninTable 1.Fifteendiffer- entSRs aredefined, providingwide-ranging sensitivitytomodels with differentfinal states andmass spectra. There are three dif- ferent triplets of regions defined in terms of the jet multiplicity n₅₀ andtwo differenttripletsof regions definedinterms ofn₈₀. Withineachtriplet,differentrequirementsaremadeonn_b-jet,from no requirement to the requirement of at least two b-jets. In all casesthefinalselection isontheratioof E^miss_T to√

HT,withthe choice of athreshold at4 GeV^1/2 being a goodbalance between background rejectionand signal efficiency while maintainingthe effectivenessofthebackgroundestimation.Eventscontainingelec- tron ormuon candidateswith pT>10 GeV arevetoed toreduce backgroundfromSMprocesses.

The SRs have events in common, for example all events in 9j50-1balsoappearin9j50,whichdoesnot requiretheb-jet, andin8j50and8j50-1b,which havealooserrequirementon n₅₀.Eventsmayalsoappearinboththen₅₀andthen₈₀categories.

4. Backgroundandsimulation

StandardModelprocessescontributetotheeventcountsinthe SRs.The dominantbackgroundcontributionsare multijetproduc- tion,includingthosefrompurelystronginteractionprocessesand fullyhadronicdecaysoft¯t;partiallyleptonicdecaysoftt;¯ andlep- tonically decaying W or Z bosons produced in association with jets.Top-quark,W - and Z -bosondecaysthatarenotfullyhadronic arecollectivelyreferredtoas‘leptonic’backgrounds.Theycancon- tributetothesignalregionswhennoe or μleptonsareproduced, forexample Z→νν orhadronic W→τ ν decays, orwhenthey areproducedbutareoutofacceptance,liewithinjets, orarenot reconstructed.

Table 1

Definitionofthesignalregions.TheselectionvariablesaredescribedinSections2and3.Alongdash‘—’indicatesthatnorequirementismade.Eventswithleptonsare vetoed.

(a) Signal regions using n50

8j50 8j50-1b 8j50-2b 9j50 9j50-1b 9j50-2b 10j50 10j50-1b 10j50-2b

n50 ≥8 ≥9 ≥10

nb-jet – ≥¹ ≥^{2 –} ≥¹ ≥^{2 –} ≥¹ ≥²

E_T^miss/√

HT >4 GeV^1/2

(b) Signal regions using n80

7j80 7j80-1b 7j80-2b 8j80 8j80-1b 8j80-2b

n80 ≥⁷ ≥⁸

nb-jet – ≥1 ≥2 — ≥1 ≥2

E^miss_T /√

HT >4 GeV^1/2

The most significant leptonic backgrounds are t¯t and W bo- sonproductioninassociation withjets.The contributionofthese twobackgroundstothesignalregionsisdeterminedfromacom- binedfit asdescribedlater inSection 5.The yields forthe other, generally subdominant, leptonic backgrounds are taken fromthe simulationsasdescribedbelow.

MonteCarlosimulations are used inthe determinationofthe leptonic backgrounds and to assess sensitivity to specific SUSY signal models. All simulated events are overlaid with multiple pp collisions simulatedwith the soft QCD processes of PYTHIA 8.186 [25] using the A2 set of parameters (tune) [26] and the MSTW2008LO parton distribution functions (PDF) [27]. The sim- ulationsareweightedsuchthatthepileupconditionsmatchthose ofthedata.The responseofthedetectorto particlesismodelled withanATLASdetectorsimulation[28]basedfullyon Geant4[29], orusing fast simulationbased on a parameterisation of the per- formanceoftheATLASelectromagneticandhadroniccalorimeters [30] andon Geant4elsewhere. Leptonicbackgroundsamples use fullsimulation,whilesignalsamples(describedbelow)usethefast simulation option.Corrections are applied to thesimulated sam- plestoaccountfordifferencesbetweendataandsimulationforthe leptonidentificationandreconstructionefficiencies,andfortheef- ficiencyandmisidentificationrateoftheb-taggingalgorithm.

4.1. Leptonicbackgroundsimulation

Forthegeneration oft¯t and single top quarksinthe W t and s-channels[31] Powheg-Boxv2[32] isusedwiththeCT10PDF sets[33]inthematrixelementcalculations.Electroweakt-channel single-top-quarkeventsaregeneratedusingPowheg-Boxv1.This generatorusesthefour-flavourschemeforthenext-to-leadingor- der (NLO) matrix element calculations together with the fixed four-flavour PDF setCT10f4[33]. Forthisprocess,the topquarks aredecayed usingMadSpin[34] preserving allspin correlations, whileforallprocessesthe partonshower, fragmentation, andthe underlying event are simulated using PYTHIA v6.428 [35] with the CTEQ6L1 PDF sets [36] and the corresponding Perugia 2012 tune (P2012) [37]. The top quark mass is set to 172.5 GeV. The EvtGenv1.2.0program[38]modelsthebottomandcharmhadron decays,as it doesforall non-SHERPA-simulatedprocesses men- tionedbelow.Thet¯t simulationisnormalisedtothecross-section calculatedtonext-to-next-to-leadingorder(NNLO)inperturbative QCD,includingsoft-gluonresummationtonext-to-next-to-leading- log(NNLL)accuracy[39].

Eventscontaining t¯t andadditionalheavyparticles –compris- ing three-top, four-top,t¯t+^{W , t}¯t+^{Z andt}t¯+W W production [40] –aresimulatedatleading orderinthe strongcouplingcon- stant αs, usingMadGraphv2.2.2 [41] withup to two additional partons in the matrix element, interfaced to the PYTHIA 8.186

[25,35] partonshower model.The A14 tune of the PYTHIA pa- rametersisused[42],togetherwiththeNNPDF2.3LOPDFset[43].

The predicted productioncross-sectionsare calculated to NLO as described in Ref. [41] for all processes other than three-top, for whichitiscalculatedtoLO.

Events containing W bosonsor Z bosonswithassociatedjets [44] arelikewisesimulatedusingMadGraph,butwithuptofour additionalfinal-statepartonsinthematrixelement,andinterfaced to PYTHIA, using the same tunes and particle decay programs.

The W +^jetsandZ +^{jetseventsarenormalisedtoNNLOcross-} sections[45].Dibosonprocesseswithatleastonebosondecaying leptonically [46] are simulated using the SHERPA v2.1.1 genera- tor[47].Thematrixelementcalculationscontainalldiagramswith fourelectroweakvertices.Theyarecalculatedforuptoone(for4, 2+²ν,semileptonic Z Z ) orno additionalpartons (for3+¹ν, other semileptonic processes) at NLO and up to three additional partons at LO using the Comix [48] and OpenLoops [49] ma- trix element generators and interfacedwith the SHERPA parton shower [50] using the ME+^PS@NLO prescription [51]. The CT10 PDFsetisusedinconjunctionwithdedicatedpartonshowertun- ingdevelopedbytheSHERPAauthors.

Theoretical uncertaintiesareconsideredforall thesesimulated samples. Production of tt is¯ by far the most important process simulatedinthisanalysisandtoevaluate theuncertaintyonthis background severalsamplesarecompared. Samplesare produced withthefactorisationandrenormalisationscalesvariedcoherently, alongwithvariations oftheresummationdampingparameterand with more/less radiation tunes ofthe partonshower [52]. Addi- tionallythenominalsampleiscomparedtoonewithPowhegin- terfacedwithHerwig++[53]andSHERPAv2.1.1sampleswithup to one additionaljet at next-to-leading orderusing OpenLoops anduptofouradditionaljetsatleadingorder,toaccount forun- certainties in the parton shower and the generator respectively.

The comparison withthe SHERPA sample dominates the uncer- taintyinthesignalregionprediction.

4.2. SUSYsignalmodels

TwoclassesofSUSY signalmodelsareused wheninterpreting the results. The first is a simplified model,in which gluinos are pair-producedandthendecayviathecascade

˜

g→^q+ ¯^q+ ˜χ₁^± (q=^u,d,s,c)

˜

χ₁^±→W^±+ ˜χ₂⁰

˜

χ₂⁰→^Z+ ˜χ₁⁰.

Theparametersofthemodelarethemassesofthegluino,m_g˜,and thelightestneutralino,mχ_˜1⁰.Themassoftheχ˜₁^±isconstrainedto