Contents lists available atScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Search for R-parity violating decays of a top squark in proton–proton collisions at √
s = 8 TeV
.CMSCollaboration
CERN,Switzerland
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received13February2016 Receivedinrevisedform7June2016 Accepted17June2016
Availableonline21June2016 Editor:M.Doser
Keywords:
CMS Physics Supersymmetry Leptons
Lowmissingtransverseenergy
The results ofasearchforasupersymmetric partnerofthe top quark(top squark),pair-produced in proton–proton collisionsat√
s=8 TeV,arepresented.Thesearch,whichfocusesonR-parityviolating, chargino-mediateddecays ofthe topsquark, isperformedinfinal stateswithlow missing transverse momentum,twooppositelychargedelectronsormuons,and atleastfivejets.Theanalysisusesadata samplecorrespondingtoanintegratedluminosity of19.7 fb−1 collectedwiththeCMSdetectoratthe LHCin2012. Thedataare foundtobeinagreementwiththe standardmodelexpectation,andupper limits are placedon the top squark pairproduction crosssection at95% confidence level.Assuming a100% branchingfractionfor thetop squarkdecaychain,t→tχ1±,χ1±→ ±+jj,top squarkmasses less than890 (1000) GeVfor the electron(muon)channel are excluded for the firsttime inmodels withasinglenonzeroR-parityviolatingcouplingλi jk(i,j,k≤2),wherei,j,k correspondtothethree generations.
©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Supersymmetry (SUSY) [1,2] is an extension of the standard model (SM) that may provide a solution to the hierarchy prob- lem[3,4].IntheSUSYframework,quadraticallydivergentradiative correctionstotheHiggsbosonmass,dominatedbyloopsinvolving thetopquark,arecanceledbyloopswithasupersymmetricpart- ner ofthe topquark (top squark).The massof thetop squarkis expectedtobewithin afewhundredGeVofthetopquark mass, andthesupersymmetricHiggsbosonpartnersarealsoexpectedto havemasseslessthan1 TeV[5,6].
SearchesforSUSY areperformedin manydecaychannelsand areclassifiedintoR-parityconserving(RPC)andR-parityviolating (RPV)scenarios.Thequantumnumber,R-parity, PR= (−1)3B+L+2s hasavalue +1 forSMparticlesand−1 forsuperpartners, where B,L, ands are baryon number,leptonnumber, andspin, respec- tively[7].InRPC modelsthetopsquarkisexpectedtodecayinto thelightestSUSYparticle,whichescapesdetection.Thisresultsin aneventsignaturewithsubstantialmissingtransversemomentum.
RecentsearchesperformedattheLHCatCERNineventswithhigh missingtransversemomentumhavereducedtheparameter space
E-mailaddress:[email protected].
availableforalowmasstopsquark[8–13].However,R-paritymay notbe conserved,inwhichcasesearchesforSUSYparticledecay- ingtoSMparticleswithoutsubstantialmissingtransversemomen- tumareimportant.
The superpotential terms that result in R-parity violation are givenby:
WRPV=1
2λi jkLiLjEk+ λi jkLiQjDk
+1
2λi jkUiDjDk+μiLiHu; (1)
where λi jk, λi jk, and λi jk are three trilinear Yukawa couplings;
i,j,k=1,2,3 are generation indices; L and Q are the SU(2)L doubletsuperfieldsoftheleptonandquark; Hu istheHiggsfield that givesmass to theup-typequarks; μi are thebilinear terms that mix lepton and Higgssuperfields, and E , D, and U are the SU(2)L singletsuperfieldsofthechargedlepton,down-typequark, and up-type quark. The third term violates the conservation of baryon number, while the first two violate the conservation of lepton number. If baryon number and lepton numberwere both violated, proton decay would proceed at a rate excluded by ex- perimentalobservations[14,15].Toavoidtheseexperimentalcon- straintsandtosimplifytheinterpretationofresults,itiscommonly http://dx.doi.org/10.1016/j.physletb.2016.06.039
0370-2693/©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Fig. 1. DiagramfortheR-parityviolating,chargino-mediateddecayofatopsquark.
Thecharginodecaystoaleptonandtwojetsviaanoff-shellsneutrinowithnonzero λi jkcoupling.
assumedthat onlyoneoftheλi jk,λi jk,orλi jk couplingsisdiffer- entfromzero.Inthisanalysisonlyλi jkcouplingswith(i,j,k)≤2 areconsidered.
InRPVSUSYmodelswiththecharginoχ1±lighterthanthetop squarkandnonzeroλi jk,thetopsquarkt candecayviat→bχ1±, withsubsequentdecayofthecharginotoaleptonandtwojetsvia an off-shellsneutrino (χ1±→ ±+jj)[16],as depictedin Fig. 1.
Thebranchingfractionofdecayχ1±→ν+jj viaanoff-shellslep- ton will be negligible unless the slepton and sneutrino masses are comparable. The decayχ1±→W±χ10 is suppressed formod- elswithχ1± andχ10 almostdegenerateinmass.
We perform a search for top squark decays, as depicted in Fig. 1, using proton–proton (pp) collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb−1,collectedwiththeCMSdetectorattheLHCin2012.As top squarks are expected to be dominantlypair-produced at the LHC[17],the search is performedusingevents withexactly two oppositely charged electrons (e±e∓) or muons (μ±μ∓), at least fivejetsofwhich oneor morejetsare identified asarisingfrom hadronizationofabottomquark(b-taggedjet),andhighST,where ST isdefinedasthescalarsumofthetransversemomentaoflep- tonsandjets. Asa consequenceofthe assumptionthat only one oftheλi jk couplingsisnonzero,thetwoleptonsmusthaveoppo- sitecharge andthesameflavor. Detailsoftheeventselection are describedinSection3.
The sensitivity of the e±e∓ (μ±μ∓) search doesnot depend on which of the four RPV couplings associated with the second operator LQD (LiQjDk) in Eq. (1) are nonzero: λ111, λ112, λ121, andλ122(λ211,λ212,λ221,andλ222), becausethefinal statesand kinematicdistributionsarethesameineach case.Weexpectthat thesearcheshavesomesensitivitytomodelswiththird-generation couplingsλ311, λ312, λ321, andλ322, vialeptonic τ decays;how- ever, we do not include this possible extra contribution in this paper.The difference Mt,χ1± betweentop squark mass Mt, and charginomass Mχ1±,ischosen to be 100 GeV,sincethisvalue is representativeofthebulkoftheMt−Mχ±
1 parameterspacewhere thesignal reconstructionefficiencyisslowlyvarying.Thisanalysis doesnotattempttoquantifythedecreaseinefficiency(andsignal sensitivity)intheregionsofparameterspacewhereeitherMt,χ1± orMχ1± isverysmall(<100 GeV).
Several searches for R-parity violating top squark decays via LQD couplings have been performed by the CMS [18–20] and ATLAS [21] Collaborations. These searches have focused on top squark pairs decaying via λi32 couplings into final states of two leptons(e±or μ±)andtwojetsortwoleptons(e±or μ±)andsix jets,fourofwhichareb-taggedjets[20,21];viaλ3 jkcouplingsinto afinal stateincludingtwotauleptonsandtwob-taggedjets[19];
andviatheλ233 couplingintoafinalstateincludingthreeleptons andadditionaljets[18].Theanalysisdescribedinthispaperisthe firstsearchforR-parityviolatingtopsquarkdecaysviapurelyfirst- orsecond-generation LQD couplings;in thiscase, thefinal states
aretwoleptons(e±or μ±)andsixjets,twoofwhichareb-tagged jets.
2. TheCMSdetector
AdetaileddescriptionoftheCMSdetector,togetherwithadefi- nitionofthecoordinatesystemused,canbefoundelsewhere[22].
A notablefeature ofthe CMSdetectoris its6 m internaldiame- tersuperconductingsolenoidmagnetthatprovidesafieldof3.8 T.
Withinthefieldvolumeareasiliconpixelandstriptracker,alead tungstatecrystalelectromagneticcalorimeter,andabrassandscin- tillatorhadroncalorimeter.Muondetectorsbasedongasionization chambersareembeddedinasteelflux-returnyokelocatedoutside the solenoid. Events are collected by a two-layer trigger system basedonahardware level-1trigger,followedbyasoftware-based high-leveltrigger.
The pseudorapidity range covered by the tracking system is |η|<2.5, the muon detector extends up to |η|<2.4, and the calorimeters cover a region with |η|<3.0. The region of 3<|η|<5 is instrumented with steel and quartz fiber forward calorimeters. The hermeticity of the detector up to large values of |η| permits accurate measurement of the momentum balance transversetothebeamdirection.
3. Triggerandeventselection
Events are selected using a trigger that requires at least one electron (muon) with a transverse momentum (pT) threshold of 27 (24) GeV,and|η|<2.5 (2.1).Allobjectsarereconstructedusing aparticle-flow(PF)algorithm[23,24],whichusesinformationfrom all subsystems to reconstruct photons, electrons, muons, charged hadrons,andneutralhadrons.
Toreducethebackgroundfromjetscontainingleptons,weim- pose isolation constraints on the transverse energy ET,cone from charged-particle tracks or deposits in the calorimeter within a cone R=
(η)2+ (φ)2=0.3 (0.4)around the trajectory of theelectron (muon),whereφ isthe azimuthalangle.The energy fromthe reconstructed lepton andtheaverage transverse energy density frompileup are subtracted from ET,cone, where pileup is definedasadditionalinelasticppcollisionswithinthesameorthe adjacent LHC bunch crossing.Tracking information together with calorimeterinformationis usedto identifyandsubtract hadronic energydepositionsfromchargedparticlesoriginatingfrompileup.
The contributionsto theneutralhadronandphoton energycom- ponents dueto pileup are also computed andsubtracted. In the electronchannel,thecontributionstotheneutralhadronandpho- ton energycomponents dueto pileupinteractions are subtracted from ET,cone using the jet area technique [25], which computes the transverse energy density of neutral particles from the me- dian of the neutral energy distribution in jets with pT>3 GeV onan event-by-eventbasis.Inthemuonchannel,themethodas- sumesthepileup energydensityfromneutralparticlestobe half ofthatfromchargedhadrons, basedonmeasurementsperformed injets[24].
Electrons are reconstructed by matching an energy cluster in the ECAL with a track reconstructed using a Gaussian sum fil- ter[26].ElectronsarerequiredtohavepT>50 GeV and|η|<2.5.
The transitionregionbetween theECALbarrelandendcap isex- cluded (1.444<|η|<1.566) because the calorimeteris not well modeled inthisregion.Electronsare identifiedusingamultivari- ateidentificationalgorithm [26],whoseinput variablesare sensi- tivetobremsstrahlungalongtheelectronpath,matchingbetween tracksandECALenergydeposits,andshower-shapevariables.The algorithm is trained with a sample of simulated Drell–Yan (DY) eventsthatcontainstrueelectrons andadatasampleenriched in
misidentifiedelectrons. Inaddition,thetransverse impactparam- eter of the electron track is required to be lessthan 2 mm. To reducebackgroundsthat arisefromphoton conversionsinthein- ner pixel detector, at least one pixel hit in the innermost pixel layer isrequired andthe electron must be inconsistent withthe hypothesis that it resultedfromphoton pair creation.We ensure thattheelectronisisolatedfromotheractivityintheeventbyre- quiringthat ET,conebelessthan10%oftheelectronpT.
Muontracks arereconstructedusingthe informationfromthe muonchambersandthesilicontrackerandarerequiredtobecon- sistent withthe reconstructed primary vertex. The tracksare re- quiredtohaveatleastonehitinboththepixeltrackerandmuon detector,andatleasttenhitsinthesiliconstriptrackertoensure a precise momentum measurement. Muonsare requiredto have pT>50 GeV and |η|<2.1. Mostcosmic ray muons are rejected by requiring that the transverse (longitudinal) impact parameter be lessthan 2 (5) mmrelative to the primary vertex, definedas thevertexwiththelargestsumofthe pT2 fromalltracks associ- atedwithit.Isolationisimposed bytherequirementthat ET,cone belessthan12%ofthemuonpT[27].
Thedifferencesinleptonreconstructionandtriggerefficiencies betweendataandsimulationarecorrectedinsimulationinbinsof pTand η,usingatag-and-probemethod[28].
Jets are reconstructed from PF objects [29] using the anti-kT clustering algorithm [30] with a distance parameter of 0.5. The tracker and ECAL granularity are exploited to precisely measure thechargedparticles,andhencetodeterminejetdirectionsatthe production vertex. To remove jetsarising frominstrumental and non-collisionbackgrounds,additionalcriteriaonchargedandneu- tralhadronenergyareapplied.
Theenergyandmomentumofeachjetarecorrectedasafunc- tion of the jet pT and η to account for the combined response function of the calorimeters. The average energy from pileup is subtracted from the jet [31]. Only jets within |η|<2.4 are con- sidered. The corrected jet pT must be at least 100 GeV for the leadingjet,50 GeV forthesecond-leadingjet,and30 GeV forthe remainingjets.Atleastfivejetsarerequiredintheevent.
Events with at least one b-tagged jet are selected. The com- bined secondary-vertexalgorithm[32] uses informationfromthe trackimpactparameterandvertexinformationtodiscriminatebe- tweenjetsthat originatefrombquarks andjetsfromlight-flavor quarksandgluons.Thealgorithmcorrectlyidentifiesjetsproduced bythehadronizationofabquark(bjets)withanefficiencyofap- proximately70% andmisidentifies jetsfromlight-flavorquarksor gluons(charm quarks) at a rate of approximately1% (20%) [32].
The b-taggingefficiencyin thesimulation is scaledto matchthe measuredefficiencyindataasafunction of pT, η,andtheflavor ofthejet.
The missing transverse momentum pmissT in the event is de- finedastheprojectionofthenegativevectorsumofthemomenta of all reconstructed PF candidates on the plane perpendicular to thebeams.The magnitudeof pmissT in theeventis referredto as EmissT . To suppress leptonic tt decays that often have significant EmissT becauseofthepresenceofneutrinosinthefinalstate, EmissT isrequiredtobelessthan100 GeV.Thedileptonmass M,com- putedfromthetwoleptonfour-momenta,isrequiredtobegreater than130 GeV,basedonanoptimizationtoreducethecontribution fromlow-massresonancesandZbosondecays.
Toenhancethestatisticalsignificance,foreachleptonflavorthe sampleisdividedintothreeexclusivecategoriesofjetmultiplicity:
Njets=5,6,or≥7.Toimprovethesensitivitytosignaldecays,we computeanSTthresholdSminT optimizedforeachtopsquarkmass hypothesisandforeachNjetsbin.The SminT isdeterminedbymax-
imizingthevalue ofS/√
S+B,whereS andB arethenumberof expectedsignalandbackgroundeventsabove SminT ,respectively.
4. Simulationofbackgroundandsignalevents
MonteCarlo(MC)simulationsofbackgroundandsignalevents are used to optimize the selection criteria for maximum signal sensitivity and to estimate backgrounds. The simulation of the hard-scattering event is performed using the leading-order (LO) matrix element event generator MadGraph 5 [33], unless noted otherwise. The CTEQ6L1 [34] set of partondistribution functions (PDF) is used to describe the proton structure. The simulation of the hard-scattering eventis then passed to pythia 6.426 [35]
with the Z2* tune [36] to model the parton shower, hadroniza- tion,andtheunderlyingevent.A fullsimulationoftheresponseof theCMSdetectorisperformedusing Geant4[37].Additionalsim- ulatedminimumbiaseventsare overlaidtoreproduce theeffects ofpileup.
The main SM backgrounds for this search are DY and tt pair production. Additional SM backgrounds, which include diboson (WW, WZ, and ZZ) and single top quark production, are small.
The tt sample is generated with up to three additional partons, the DY events are produced with up to four additional partons, andthedibosonsamplesaregeneratedwithup totwoadditional partons. Single top quark production (t-, s-, and tW-channels)is simulatedwith powheg v1.0[38–42].Simulatedsamplesoftt and DY arenormalizedusingcrosssectionscomputedatnext-to-next- to-leading-order(NNLO)[43,44].Crosssectionscomputedatnext- to-leading-order(NLO)[45,46]areusedtonormalizethesingletop quarkanddibosonsamples.
The signal samples are generated using MadGraph 5, pythia6.426, andtheCTEQ6L1PDF set.Thetop squarkpairpro- duction cross section is computed at NLO as a function of Mt, including soft gluon resummation at next-to-leading logarithm (NLL) [47–50]. The uncertainty in the cross section includes un- certainties associated with the renormalization and factorization scale,andthePDFset[51].
5. Backgroundestimation
Corrections to the normalizationoftt and DY simulations are estimated by examining background enriched samples in data.
A summary of the selection criteria forthe signal search region andthecontrolregions,includingselectionsonthedileptonmass, is presented in Table 1. Diboson and single top quark produc- tionyieldsmallcontributionstothebackgroundandareestimated fromsimulation.Insimulatedtt sample,eventsarereweightedso thatthe pT ofthetopquark matchesthedatainadedicatedcon- trolsample[52].
The leptonic tt decays contribute to 89% of the total back- ground. Since the signal produces only same-flavor leptons, we estimatethett backgroundfromacontrolsampleofe±μ∓events after correctingitforthe smallcontributionsof DY,diboson,and single top events using simulations. We use this control sample tocomputecorrectionfactorsforthett simulationfordifferentjet multiplicitiesinthesignalregion.Thee±μ∓controlsampleiswell modeledbythesimulation,thuscorrectionfactorsarestatistically consistentwithunity.
The Drell–Yan productionconstitutes approximately8% of the SM background in the signal region, and is reduced by requir- ing atleastoneb-taggedjet.The contributionfromthissourceis estimatedusingacontrolsampleoftwooppositelychargedsame- flavor leptons, which have an invariant mass M in the range 50–130 GeV. We perform a fit to the M distribution to esti- mate the number of DY events. The DY shape is obtained from
