Search for supersymmetry in electroweak production with photons and large missing transverse energy in pp collisions at √s=8TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Search for supersymmetry in electroweak production with photons and large missing transverse energy in pp collisions at √

s = ^{8 TeV}

.TheCMS Collaboration^

CERN,Switzerland

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

Articlehistory:

Received28February2016 Receivedinrevisedform4May2016 Accepted27May2016

Availableonline1June2016 Editor:M.Doser

Keywords:

CMS Physics Supersymmetry

Results are reported fromasearchfor supersymmetry withgauge-mediated supersymmetry breaking inelectroweakproduction. Finalstateswithphotons andlarge missingtransverse energy(E^miss_T )were examined. Thedatasamplewascollectedinppcollisions at√

s=^{8TeV with theCMSdetectoratthe} LHCandcorrespondsto7.4 fb⁻¹.Theanalysisfocusesonscenariosinwhichthelightestneutralinohas bino- orwino-likecomponents,resultingindecaystophotonsandgravitinos,wherethegravitinosescape undetected. Thedata wereobtainedusingaspecially designedtrigger withdedicatedlow thresholds, providinggoodsensitivitytosignatureswithphotons,E_T^miss,andlowhadronicenergy.Noexcessofevents overthestandardmodelexpectationisobserved.Theresultsareinterpretedusingthemodelofgeneral gaugemediation.Withthewinomassfixedat10 GeV abovethatofthebino,winomassesbelow710 GeV areexcludedat95%confidencelevel.Constraintsarealsosetinthecontextoftwosimplifiedmodels,for whichtheanalysissetsthelowestcrosssectionlimitsontheelectroweakproductionofsupersymmetric particles.

©^{2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Supersymmetry [1–14] (SUSY) can stabilize the mass of the Higgsboson,recentlymeasuredtobearound125 GeV[15,16],and hence the electroweak scale against large quantum corrections, thus providing a solution to the gauge hierarchy problem [17].

Searchesforsupersymmetricpartnersofstandardmodel(SM)par- ticles with photons in the final state are already probing SUSY parameterspaceatthe TeV scale[18–21].Thereisastronginter- estinprobingso-callednaturalSUSYscenarios,whereasubsetof SUSYpartnerscan remain light,while manyother SUSY partners can have large masses that are inaccessible to present searches.

TheseregionsofSUSYparameterspacearestilllargelyunexplored.

Inthisanalysis, R-parity[22,23]isassumedtobeconserved,so thatSUSYparticles arealways producedinpairs.In SUSYmodels ofgauge-mediatedSUSYbreaking(GMSB)[24–30]thegravitino(G) isthelightestSUSYparticle(LSP)andescapesundetected,leading tomissing transverseenergy (E^miss_T ) inthe detector.In thestud- iedcases,thenext-to-lightest SUSY particle(NLSP)is thelightest neutralino(χ₁⁰).Depending onits composition,theχ₁⁰ candecay accordingtoχ₁⁰_→NG,whereN iseitheraphoton γ,aSMHiggs

 E-mailaddress:[email protected].

bosonH,oraZ boson.Ifthegauginosarenearlymass-degenerate, chargino(χ₁^±)decaysaccordingtoχ₁^±→^W±G arealsopossible.

The ATLAS and CMS Collaborations have searched for direct electroweakproductionofgauginos.Finalstateswithatleastone photonandoneelectronormuonhavebeenexamined[21,31],re- quiringone gaugino decayingto γ^G andone to W^±G. The NLSP masses below 540 GeV in the simplified model spectra TChiWg scenario, introducedbelow, were excluded atthe 95% confidence level(CL).Indecaysintoanyoftheheavystandardmodelbosons (H, Z,W^±), higgsino(chargino) masses up to 380 GeV (210 GeV) have been excluded at the 95% CL [32–34]. Other analyses re- quiring two photons in the final state [19–21] probe bino-like neutralinos and within the context of general gauge mediation (GGM)[35–40]excludeelectroweaklyproducedwinoswithmasses up to 740 GeV, depending on the bino mass. A previous single- photon analysis [20] has set limits on bino- and wino-like neu- tralinos for strong production, but the search is insensitive to electroweakproductionbecausethe chosentrigger requires H_T>

500GeV,whereH_Tisthescalarsumoftransverseenergyclustered injets.

To provide sensitivity to GMSB scenarios with low gaugino masses and mass differences, this analysis uses signatures with at least one photon together with large E^miss_T . In signal events, hadronicenergyarisesonlyfrominitial-stateradiationorfromthe http://dx.doi.org/10.1016/j.physletb.2016.05.088

0370-2693/©^{2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

Fig. 1. Scenariosfortheproductionanddecayofcharginosandneutralinosconsideredinthisanalysis.IntheTChiNgscenario(toprow),thecharginosareonlyslightly heavierthantheneutralinos,leadingtocharginotoneutralinodecaysaccompaniedbysoftradiation.Oneneutralinodecaystoaphotonandagravitino,whiletheother decaysintoaZ oranH bosonandagravitinowithequalprobability.IntheTChiWgscenario(bottomleft),thegauginosaremass-degenerateandtheχ₁⁰decaysareas shown.WithinGGMmodels,theχ₁⁰→γG toχ₁⁰→ZG branchingfractiondependsontheneutralinomass. ThedominantprocessforelectroweakGGMproductionisshown inbottomright.AsmalleramountofhadronicenergycomparedtostrongproductionandatleastonephotonandE^miss_T arecommonfeaturesofallscenarios.

decaysofaW^± orZ boson,collectivelydenotedasV bosons.We concentrateonfinalstateswithonlyamoderateamountofHTdue todirectelectroweakproductionofgauginos.Thelightestgauginos areassumedtobeeitherbino- orwino-like,leadingtofinalstates withE^miss_T and γ γ, γV,orVV.

Weconsiderthreesignalscenarios:ThescenarioTChiNgmodels the electroweak pair and associated production of nearly mass- degenerate charginos andneutralinos, whichthen decay intothe NLSP, as shown in Fig. 1 (top row). The branching fractions of the NLSP decay correspond here to a wino-like χ₁⁰ of similar mass.TheTChiWgscenariomodelsassociativeproductionofmass- degeneratecharginosandneutralinos,whichthendecayasshown inFig. 1 (bottomleft).The third scenario iselectroweak produc- tion within the GGM context; the dominant production channel isshowninFig. 1 (bottomright).Massesofthe bino- and wino- like neutralinos involved in thisscenario are scanned, while the squarkandgluinomassesaredecoupled.TheamountofE^miss_T and thephotontransversemomentum(pT)isdeterminedbythemass ofthe χ₁⁰, while themass ofthe χ^± determines the production crosssection.IntheGGMframework,wherethegauginosarenot mass-degeneratebyconstruction,alarger χ^±–χ₁⁰massdifference increasesthehadronicenergyinthefinalstate.

The analysisuses aspecial dataset corresponding to an inte- grated luminosity of 7.4 fb⁻¹ recorded witha trigger requiringa photon candidate with pT>30GeV measured within |η_|<1.44 andE^miss_T >25GeV,wherethe E^miss_T iscalculatedusingcalorime- ter information without correcting for muons. Compared to the available triggers inthe full 2012data set,which requirea pho- toncandidatewith pT>135GeV oreventswith E^miss_T >120GeV, these low trigger thresholds enable a high signal sensitivity to electroweakproductionandcompressedmassspectrumscenarios.

Thedatasetwasrecordedduringthesecondhalfofthe2012data- takingperiodbutonlyreconstructedduringtheLongShutdown1 oftheLHCaspartoftheso-called“parked-data”program[41].

2. TheCMSdetector

The central feature of the CMS apparatus is a superconduct- ing solenoid of6 m internal diameter,providing a magnetic field of 3.8 T. Withinthesolenoidvolume are a siliconpixelandstrip tracker,aleadtungstatecrystalelectromagneticcalorimeter(ECAL), andabrassandscintillatorhadron calorimeter(HCAL),eachcom- posed of a barrel and two endcap sections. Extensive forward calorimetrycomplementsthecoverageprovidedbythebarreland endcapdetectors.Muonsaremeasuredingas-ionizationdetectors embeddedinthesteelflux-returnyokeoutsidethesolenoid.

In the barrelsection ofthe ECAL,an energy resolutionof ap- proximately1%isachievedforunconvertedorlate-convertingpho- tonsarisingfromtheH→γ γ decay.Theremainingbarrelphotons haveanenergyresolutionofabout1.3%uptoapseudorapidityof

|η_|=1,risingtoabout2.5%at|η_|=1.4[42].

AmoredetaileddescriptionoftheCMSdetector,togetherwith a definitionofthecoordinatesystemusedandtherelevantkine- maticvariables,canbefoundinRef.[43].

3. Objectreconstructionandsimulation

Photons are reconstructed [42] fromclustersin theECAL bar- rel with |η_|<1.44 and are required to be isolated. The energy deposit in the HCAL tower closest to the seed of the ECAL su- percluster assignedto the photon divided by the energy deposit in theECALisrequiredto be lessthan 5%. Aphoton-like shower shape isrequired.Thephoton isolationisdeterminedby comput- ing the transverse energy in a cone centered around the pho- ton momentum vector. The cone has an outer radius of 0.3 in

R=√

(φ)²+ (η)², where φ is azimuthal angle in radians, andthecontributionofthephotonisremoved.Correctionsforthe effectsofmultipleinteractionsinthesamebunchcrossing(pileup) are applied to all isolation energies, depending on the η of the photon. Discrimination against electrons is achieved by requiring that photonshavenomatchingpatternofhitsinthepixeldetec- tor.

Allobjects used in thissearch, i.e.photons, electrons, muons, and jets, are reconstructed using the particle-flow (PF) algo- rithm [44,45]. Jets are reconstructed with the anti-kT clustering algorithm[46]asimplementedinthe FastJet[47]package,usinga distanceparameterof0.5.Thejetsarerequiredtohavetransverse momentaabove 30 GeV and to satisfy the requirement|η_|<2.4.

Pileupcorrectionsandcorrectionsfortheresponseofthedetector areappliedtothemomentaofthejets[48,49].

The missing pT vector is defined as the projection onto the plane perpendicular to the beams of the negative vector sumof themomenta of all reconstructed particles in an event. Its mag- nitude is referred to as E^miss_T . The E^miss_T is calculated using all particlesidentifiedbythePFalgorithm.Filtersagainstanomalously highE^miss_T frominstrumentaleffectsareapplied[50].

The dimensionless variable E^miss_T -significance (Emiss,signif

T ) [50]

is a measure of the probability that the E^miss_T in a given event arisesfrom genuine noninteracting stable particles, such asneu- trinosor gravitinos, andnotasa consequenceof thelimiteden- ergy resolution of objects, such asjets, photons, or leptons. The Emiss,signif

T is proportional to the logarithm of the likelihood ra- tio of both hypotheses. Energy and angular resolutions of the relevant objects, determined by data and simulation studies, are takeninto account. For processes where the reconstructed E^miss_T onlyarisesfromthelimitedenergyresolution,the Emiss,signif

T isan

exponentially falling distribution, falling by threeorders ofmag- nitude for 0<Emiss,signif

T <25. SM processes withgenuine E^miss_T haveapproximatelythe samedecrease between0 and125.Elec- troweakSUSY processes intrinsically havehighvaluesof genuine E^miss_T alongwithasignificantlylower presenceofjets,whichoth- erwiseincrease theenergyuncertaintyused inthe calculationof Emiss,signif

T .Therefore,the Emiss,signif

T distributionofthoseprocesses doesnot decreasemonotonically, and aconsiderable fractionhas Emiss,signif

T >200.

The SM ttγ andQCD multijet productionsamples, aswell as theTChiNg and TChiWg signal scenarios, are simulated withthe MadGraph5.1.3generator[51].Thedibosoneventsamplesandthe electroweakGGMsignal scanaregeneratedusing pythia 6.4[52].

AllMonteCarlo(MC)samplesincorporatetheCTEQ6L1[53]parton distribution functions (PDF) and use the pythia program to de- scribe the parton showering and the hadronization. The Geant4 [54] package is used to model the detector and detector re- sponse. The cross sections of the electroweak GGM signal scan are calculated at next-to-leading-order (NLO) accuracy using the prospino2.1.[55] program,thecrosssectionsfortheTChiNgand TChiWgsignalpointsarecalculatedatNLO+^NLL(next-to-leading logarithm)accuracy[56–58].

4. Analysis

The data are selected by a trigger with E^miss_T and photon pT requirements.The eventsare subsequentlyrequiredto containat leastonetightlyisolatedphotonmeasuredintheECALbarrelwith apTofatleast40 GeV.TheE^miss_T isrequiredtoexceed100 GeV.In addition, HT isrequiredtoexceed100 GeV, improvingthesignal- to-backgroundratio.Withthisselectiontheparkeddatasettrigger efficiencyisuniformandmeasured to be 86.5⁺₋¹₁^._.⁰₃%. No selection criteria were applied on the presence or absence of an identi- fiedlepton.All eventsare requiredto have Emiss,signif

T >80 and a transversemassMT(E^miss_T ,pγ1

T )>300GeV,wherepγ1

T isthetrans- versemomentum ofthe photon withthe highest pT.The cut on Emiss,signif

T strongly reducesthecontributionofbackgrounds with- out genuine E^miss_T , whereas the cut on MT(E^miss_T ,pγ1

T ) affects all backgrounds butretains mostof the signal events, since only in

signalprocessesthephotonandthesourceforE^miss_T areexpected tooriginatefromthesamemotherparticle.Toincreasethesensi- tivity tohigher gauginomasses, the signal region is divided into four exclusive bins defined by regions in the variables Sγ

T and Emiss,signif

T ,whereSγ

T isdefinedbySγ

T =

ipγi

T +^Emiss_T ^.Thebound- ariesofthebinsareatSγ

T =^{600GeV and E}miss,signif

T =^200.

The dominant SM backgrounds are vector-boson production withinitial- orfinal-statephotonradiation(Vγ)anddirectphoton production(γ+^jets).Thenormalizationofthesetwobackgrounds isdeterminedsimultaneouslybya χ²-fitinthecontrolregionse- lection definedby Emiss,signif

T >10 and M_T(E^miss_T ,pγ1

T )>100GeV, butexcludingthesignal regiondefinedabove.The distributionof E^miss_T /√

HT is chosen as template variable for the χ²-fit, which sufficientlyseparatestheshapesofVγ and γ₊jets,so thatscal- ing one background cannot compensate the other. The shape of both backgrounds is simulated with MadGraph 5.1.3. Under the constraintof afixed total yield, thescale factors fortheVγ and

γ ₊jets simulations are given by the minimum of the χ²/ndf distribution and found to be fVγ =⁰.94±⁰.23 and fγ+^jets= 2.20±⁰.31,respectively.Beforeperformingthenormalization,the Vγ backgroundis scaled to theNLO cross section [59], whereas

γ ₊jets is used with LO cross section calculated by the event generator. The upper and lower uncertainty is given by the dif- ference of the best estimate and the scale factor corresponding to the χ²/ndf values at the minimum ofthe parabola increased by unity.Themeasured scalefactorsandtheir uncertaintieswere studied andfoundstablewithrespect to systematicvariations in thebackgroundpredictionoverdifferentcontrolregions, template variables,andbinningsofthetemplatevariables.The anticorrela- tion of the Vγ and γ +jets systematic uncertainties due to the fixed total normalization is takeninto account in the interpreta- tion. Signal contamination becomes relevant if the gauginos are light because thesignal kinematics for light gauginosare similar tothatofVγ production.Intheexaminedphasespace,signalcon- taminationisnegligible.

Asubdominantbackgroundarisesfromelectronsmisidentified as photons (e→γ). The misidentification rate f_e→γ = (¹.46± 0.16)% is determined from Z→^{e+e− decays in data [20]. The} backgroundisestimatedfromadatacontrolsamplewiththesame event selection, but containing an identified electron instead of a photon. The predictionof electrons misidentified asphotons is thenobtainedbyscalingthiscontrolsampleby fe→γ/(1−^fe→γ). The uncertaintyofthisestimationis11%, whichisdominatedby themisidentificationrateuncertainty.Furtherminorcontributions fromttγ,diboson,andQCD multijetproductionareestimatedus- ing MC simulations andare corrected for electrons misidentified asphotonsatthe generatorlevelto avoidoverlaps.Forthe cross sections,26%,50%,and100%systematicuncertaintiesareassigned tottγ,diboson,andQCDmultijetbackgrounds,respectively.Based onsimulationstudies,thebackgroundfromQCDmultijeteventsis foundtobenegligible.

The systematicuncertaintieswithrespect tothe choiceof the PDF inthe signalacceptance aredetermined by thedifference in acceptance using different sets of PDFs [60–64] and vary from less than 1% to 11%. Further systematic uncertainties arise from the jet energy correction (0.1–2.4% for the signal, 1.3% for the backgroundestimation) andfromthe integratedluminosity mea- surement(2.6%)[65].When evaluatingthe exclusioncontoursfor SUSYparticlemassesinspecificmodels,signalcrosssections(σs) are conservativelyloweredby one standarddeviation(4–8%) cor- responding to thecombined theoretical uncertainty in σs due to thechoicesoftherenormalizationandfactorizationscalesandthe PDFs.AllsystematicuncertaintiesaresummarizedinTable 1.

Table 1

Summarytableofsystematicuncertaintiesrelevantfortheanalysis.Uncertaintiesduetotheluminosityandtriggerefficiencymea- surementapply onlytothebackgroundsestimatedusingMCsimulationwithoutdatanormalization,namelyttγ,diboson,and multijet,andforthesignal.ThetotaluncertaintyisdominatedbytheuncertaintyintheVγbackground.

Source Sample Relative uncert. (%)

In sample In total bkg

Vγnormalization Vγ 24 19

γ+jets normalization γ+jets 14 1

Z→e⁺e⁻fit e→γ 11 0.3

Cross section measurement ttγ 26 3

Cross section Diboson 50 1

Cross section Multijet 100 0

Integrated luminosity Diboson, multijet, and signal 2.6 —

Trigger efficiency Diboson, multijet, and signal 1.2 —

Jet energy scale Diboson, multijet, and signal 1–2 —

PDF uncertainty in acceptance Signal <1–11 —

PDF and scale uncertainty Signal 4–8 —

Fig. 2. TheEmiss,signif

T (left)andS^γ_T (right)variablesareshowninthesignalselectionandusedtodefinefoursearchregionswithEmiss,signif

T =^{200 andSγ}T=⁶⁰⁰GeV partitions.

AbenchmarkTChiNgsignalpointwithanNLSPmassof500 GeV isshownforcomparison.

5. Resultsandinterpretation

AsshowninFig. 2,theobserveddataareinagreementwiththe totalstandardmodelbackgroundexpectationwithinthecombined statistical and systematic uncertainties. Shown are the distribu- tionsofEmiss,signif

T (Fig. 2,left)andSγ

T (Fig. 2,right)usedtodefine thefoursearchregionsdescribedinSection4.Theresultsaresum- marizedinTable 2.Nosignofnewphysicsisobserved.

Cross section limits are calculated combining the results of all four search regions defined inthe Sγ

T–Emiss,signif

T plane atthe 95%CL, usingthemodified frequentistCLs criterion[66–68] with a test statistic corresponding to a profile likelihood ratio of the background-only andsignal-plus-background hypotheses. Asymp- toticformulae[69]areusedinthecalculation.

The interpretation of the TChiNg and TChiWg scenarios is shown in Fig. 3. The analysis excludes NLSP masses below 570 (680) GeV atthe95%CLintheTChiNg(TChiWg)scenario.

The95% CL observedupper crosssection limit,aswell asthe observedandexpectedexclusioncontours,fortheGGMsignalscan in the Mwino–Mbino plane are shown in Fig. 4.For nearly mass- degenerategauginos,i.e. forM_wino=^Mbino+^{10GeV,winomasses} uptoapproximatelyM_wino=^{710GeV areexcluded.}

6. Conclusion

We have searched for electroweak production of gauginos in the framework ofgauge mediated supersymmetrybreaking infi- nalstateswithphotonsandE^miss_T .Adataset,correspondingtoan integrated luminosity of 7.4 fb⁻¹, recorded witha special trigger with low thresholds is used. The data are found to agree with the SM expectation. The analysis issensitive to electroweakpro- duction andcompressedmassspectrawhich arecharacterizedby minimalhadronicactivityinthefinalstate,complementingprevi- ouslypublishedsearches.LimitsintheTChiNgscenarioaresetfor the first time, excluding NLSP masses below570GeV at 95% CL.

IntheTChiWgscenario,NLSPmassesbelow680GeV areexcluded at95%CL,increasingthepreviousmasslimitinthisscenario[31]

by140 GeV.Inthegeneralgaugemediationmodelforcompressed mass spectrumscenarios withe.g. M_wino−^Mbino=^10GeV,wino masses below 710GeV can be excluded, increasing the previous CMSlimit[19]byabout220 GeV.

Acknowledgements

WecongratulateourcolleaguesintheCERNacceleratordepart- ments for the excellent performance of the LHC and thank the