Article
Reference
Search for displaced vertices of oppositely charged leptons from decays of long-lived particles in Collisions at TeV with the ATLAS
detector
ATLAS Collaboration
ADORNI BRACCESI CHIASSI, Sofia (Collab.), et al .
Abstract
A search for long-lived particles decaying into an oppositely charged lepton pair, μμ, ee, or eμ, is presented using 32.8−ν (ℓ,ℓ′=e, μ) with a finite lifetime due to the presence of R-parity violating couplings. Cross-section limits are presented for specific squark and neutralino masses. For a 700 GeV squark, neutralinos with masses of 50–500 GeV and mean proper lifetimes corresponding to cτ values between 1 mm to 6 m are excluded. For a 1.6 TeV squark, cτ values between 3 mm to 1 m are excluded for 1.3 TeV neutralinos.
ATLAS Collaboration, ADORNI BRACCESI CHIASSI, Sofia (Collab.), et al . Search for
displaced vertices of oppositely charged leptons from decays of long-lived particles in Collisions at TeV with the ATLAS detector. Physics Letters. B , 2019, vol. nan
DOI : 10.1016/j.physletb.2019.135114
Available at:
http://archive-ouverte.unige.ch/unige:128821
Disclaimer: layout of this document may differ from the published version.
1 / 1
Contents lists available atScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130
Search for displaced vertices of oppositely charged leptons from decays of long-lived particles in pp collisions at √
s = 13 TeV with the ATLAS detector
.TheATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received24July2019
Receivedinrevisedform29October2019 Accepted20November2019
Availableonlinexxxx Editor: M.Doser
A search for long-lived particles decaying into an oppositely charged lepton pair,μμ, ee, oreμ,is presentedusing32.8 fb−1ofppcollisiondatacollectedat√
s=13 TeVbytheATLASdetectorattheLHC.
Candidateleptonsare requiredtoformavertex,withintheinnertracking volumeofATLAS, displaced from theprimary pp interaction region.No leptonpairs withaninvariant mass greaterthan12 GeV areobserved,consistentwiththebackgroundexpectationsderivedfromdata.Thedetectionefficiencies forgenericresonanceswithlifetimes(cτ)of100–1000 mmdecayingintoadileptonpairwithmasses between0.1–1.0 TeVarepresentedasafunctionofpTanddecayradiusoftheresonancestoallowthe extraction of upper limits onthe cross sections for theoretical models.The result is alsointerpreted inasupersymmetricmodelinwhichthelightestneutralino,producedviasquark–antisquarkproduction, decaysinto+ −ν(,=e,μ)withafinitelifetimeduetothepresenceofR-parityviolatingcouplings.
Cross-section limits are presented for specific squark and neutralino masses. For a 700 GeV squark, neutralinoswithmassesof50–500 GeVandmeanproperlifetimescorrespondingtocτvaluesbetween 1 mm to6 mare excluded.Fora1.6 TeV squark,cτ valuesbetween3 mm to1 m are excluded for 1.3 TeVneutralinos.
©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Manyextensionstothe StandardModel (SM)predict the pro- ductionof weakly-coupled,long-livedparticles (LLPs).In particu- lar, several models, including supersymmetry (SUSY) [1–6] with R-parityviolation (RPV) [7,8] or withgauge-mediated supersym- metry breaking(GMSB) [9–11], Hidden Valleymodels [12], dark- photonmodels [13] or modelswithlong-livedright-handedneu- trinos [14],predicttheexistenceofLLPsthatcandecayintoapair ofleptons.IftheLLPhasalifetimeofpicosecondstonanoseconds thenitsdecaymaybeobservedasadisplacedvertexintheinner trackingvolumeoftheATLASdetectorattheLHC.
Thisletterpresentsasearchfordisplaceddileptonverticesorig- inatingfromdecaysofLLPsintoan oppositelycharged μμ,ee,or eμpair,withaninvariantmassofmorethan12 GeV.Theanalysis usesdatafromproton–proton(pp) collisions recordedbythe AT- LASexperimentin2016atacenter-of-massenergyof13 TeV.Two signalmodelsareusedtostudythesensitivityoftheATLASdetec- tortosuchLLPs.ThefirstisasimplifiedRPVSUSYmodelinwhich asquark–antisquark pair isproduced, each decaying intoa long-
E-mailaddress:atlas.publications@cern.ch.
livedneutralino which resultsina pair ofchargedleptons anda neutrino. The secondis atoy modelwhere aLLP, denotedby Z, isproduced inqq¯ annihilations anddecaysintoa pairofcharged leptons. Thesemodels were selected tostudyhow the kinematic properties of a three- or two-body decay affect the signal effi- ciencies. The search is intended to be as model-independent as possibleandisnotoptimizedfortheseparticularsignalmodels.
Previously, the ATLAS Collaboration searched for displaced dilepton vertices in the inner tracking volume of the ATLAS de- tector at √
s=8 TeV [15] and for oppositely charged dimuons usingonlymuon-spectrometer tracksat√
s=13 TeV [16].ATLAS alsosearchedforLLPswithmassoflessthan2 GeVat√
s=8 TeV by considering pairs of highly collimated leptons [17]. The CMS Collaboration searched for displaced dilepton vertices in the in- ner tracking volume of the CMS detector at √
s=8 TeV [18]
and for electrons and muons with large impact parameters at
√s=8 TeV [19].
2. ATLASdetector
The ATLAS experiment [20–22] at the LHC is a multipurpose particledetector withacylindricalgeometryanda near4π cov- https://doi.org/10.1016/j.physletb.2019.135114
0370-2693/©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 erageinsolidangle.1 Itconsistsoftheinnerdetector(ID)tracking
systemsurrounded by a thinsuperconducting solenoidproviding a2 Taxialmagneticfield,electromagneticandhadroniccalorime- ters,andamuonspectrometer(MS).
TheID extendsfromaradiusofabout33to1100 mm andto
|z| of about3100 mm. It provides tracking forcharged particles within thepseudorapidity region |η|<2.5. At smallradii, silicon pixellayersandstereopairsofsiliconmicrostripdetectorsprovide high-resolution positionmeasurements. Thepixel systemconsists of four barrel layers, and three forward discs on either side of the ATLAS detector.The barrel pixellayers, which are positioned atradii of 33.3, 50.5, 88.5, and 122.5 mmare of particular rele- vancetothissearch.Thesiliconmicrostriptracker(SCT)comprises fourdoublelayers inthe barrelandnine forwarddiscson either side. Theradial position ofthe innermost(outermost) SCT barrel layer is 299 mm (514 mm). The final component of the ID, the transition-radiation tracker(TRT), ispositioned ata largerradius, withcoverageupto|η|=2.0.
Lead/liquid-argon(LAr) samplingcalorimeters provide electro- magnetic energy measurements with high granularity. A steel/
scintillator-tilehadroncalorimetercoversthecentralpseudorapid- ityrange(|η|<1.7). The endcapandforwardregions are instru- mented with LAr calorimeters for electromagnetic and hadronic energymeasurementsupto|η|=4.9.
The MS surrounds the calorimeters and is immersed in a toroidal magnetic field. It provides tracking forcharged particles within the pseudorapidity region |η|<2.7 and trigger informa- tionup to|η|=2.4.Threelayers ofmuon detectorsarearranged in concentricshells at radii between5 m and10 m in the bar- relregion,andinwheels perpendicularto thebeamaxisatradii between7.4 mand21.5 mintheendcapregionswhere|η|>1.05.
A two-level trigger system [23] is used to select events. The first-level trigger is implemented in hardware and uses a subset ofthedetectorinformationtoreducetheacceptedratetoatmost 100 kHz. Thisis followed by a software-based, high-level trigger thatreducestheacceptedeventratetoabout1 kHz.
3. Datasetandsimulatedevents
Thisanalysisusestheppcollisiondatarecordedin2016,corre- spondingtoanintegratedluminosityof32.8 fb−1 obtainedwhile all partsofthe detectorwere operational.The uncertaintyinthe integratedluminosityis2.2% [24],obtainedusingtheLUCID-2de- tector [25] fortheprimaryluminositymeasurements.
The sensitivity of the ATLAS detector to LLPs decaying into a pair of leptons was studied using Monte Carlo (MC) simulations oftwodifferentsignals.In theRPVSUSYsimplifiedmodel,apair ofleft- andright-handedsquarksofthefirsttwogenerations was producedvia thestronginteraction.Theseeightsquarkswere as- sumed to be mass-degenerate and decay into their SM partner and the lightest supersymmetric particle (LSP), which is a bino- likeneutralino(χ˜10).Allother SUSYparticles wereassumedtobe decoupled.The LSP decaywasmediated by thefollowinglepton- number-violatingsuperpotentialterm [8]:
WLLE=1
2λi jkLiLjE¯k, (1)
1 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecenter ofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecenter oftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(rxy,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms ofthepolarangleθasη= −ln tan(θ/2).Angulardistanceismeasuredinunitsof
R≡
(η)2+(φ)2.
where λi jk are the coupling strengths of the interactions; i, j,k denotethefermiongeneration;andLandE¯ areSU(2)-doubletand singletsuperfields,respectively,whichcontaintheSMleptonsand their superpartners.Inthepresenceoftheseinteractions,theLSP decaysviaphase spaceintoaleptonandavirtual slepton,whose decayisdescribedbyEq. (1),resultinginthefollowingthree-body decay:
˜
χ10→±k∓i/jνj/i.
Thismodelwasinterpretedintwodifferentscenarios,wherea single dominant λ121 or λ122 coupling was assumed. Decays via a pure λ121 coupling have branching fractions B(χ˜10 →eeν)= B(χ˜10→eμν)=0.5,while decaysvia a pure λ122 couplinghave B(χ˜10→μμν)=B(χ˜10→eμν)=0.5.
The MC samplesoftheRPV SUSYmodelwere generatedwith MadGraph5_aMC@NLO 2.2.3 [26] interfaced toPythia 8.210 [27]
usingtheA14setofunderlyingeventandhadronizationparame- ters(A14tune) [28] andtheNNPDF23LOPDFset [29].Thematrix- element calculationwas performedatthetreelevelandincluded theemissionofuptotwoadditionalpartons.Themergingofma- trix elements and parton showers was done with the CKKW-L algorithm [30], with a matching scale set to one quarter of the squark mass. The cross-sections in the SUSY model were calcu- lated to approximate,next-to-next-to-leading order inthe strong couplingconstant,includingtheresummationofsoftgluon emis- sion atnext-to-next-to-leading-logarithmicaccuracy (approximate NNLO+NNLL) [31–38]. The nominal cross-section and its uncer- tainty were derived using the PDF4LHC15_mc PDF set, following therecommendationsofRef. [39].Thesamplesspanfourhypothe- sesforthesquarkandtheLSP massesandmeanproperlifetimes corresponding to cτ = 10–1000 mm. Instead of producing MC samples foreachofthe twoscenarios separately,combinedsam- ples were produced in whichthe LSP decaysinto the threefinal states (eeν, eμν and μμν) at the same rate. The events were reweightedtomatchthechosenλcoupling.
The events ofeach MC sample of the RPV SUSY model were generated for a specific meanproper lifetime τMC of theLSP. To obtainthesignalefficiencyforadifferentmeanproperlifetime τ, aweightwasgiventoeachLSPwhichdecayedatthetimet:
wLSP=τMC τ exp
t
τMC− t τ
.
Thefinalpredictionofthesignalefficiencyatagivenlifetimeand mass wasdetermined by applyingthisreweighting procedurein- dividually to all MC samples of different lifetimes butthe same mass,andcalculatingtheweightedmeanfortheefficiency.
The second signal is a toy modelof a Z bosonwith massof 100to1000 GeVandmeanproperlifetimecorrespondingtoacτ
value of100, 250,or 500 mm. The natural widthwas based on arelativisticBreit–Wignerdistribution,andwas variedfrom10to 140 GeV.ThesamplesweregeneratedwithPythia8.212usingthe A14 tune andtheNNPDF23LO PDF set.Such a directly produced Z is expectedtobe excluded by searchesfor displacedhadronic jets, since itwoulddecayintoqq¯ witha highbranchingfraction.
Thepurposeofthismodelistoderiveefficiencies foratwo-body decay of an LLP that can be applied to other models of similar kinematics.
In this analysis, all backgrounds were estimated fromdata. A selection of SM MC samples were used to test and validate the background estimationtechniques forrandom crossingsof tracks in two-track vertices and to estimate the systematic uncertain- ties in vertexing and tracking. These included MC samples of t¯t events generated using Sherpa 2.2 [40] with the NNPDF30NNLO
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 Table 1
Requirementsonthemuon,photonandelectroncandidatesthatpassthetriggersusedinthepreselection.e∗ refersto electronsthatarerequiredtopassthe‘loose’electronidentificationcriteria [49].
Trigger Candidate 1 pT[GeV] |η| |d0|[mm] Candidate 2 pT[GeV] |η| |d0|[mm]
μ μ >62 < 1.07 see text
γ γ >150 <2.5 – γ >10 <2.5 –
e >10 <2.5 >2.0
μ >10 <2.5 see text
γ e >150 <2.5 >2.0
γ γ γ >55 <2.5 – γ >55 <2.5 –
γ γ e >55 <2.5 >2.0 e >55 <2.5 >2.0 γ γ e∗ >55 <2.5 – γ >55 <2.5 – γ γ e >55 <2.5 >2.0 γ >55 <2.5 –
PDF set [41], leptonic decays of dibosons (Z Z, W W, W Z) from Sherpa2.1withtheCT10PDFset [42],anddijeteventsgenerated usingPythia8.186 [43,44] withtheA14tuneandtheNNPDF23LO PDF set [29]. These MC samples contain isolated leptons (from gauge boson decays) that have a high transverse momentum pT andleptons and displaced tracks in b-jets, in addition to tracks frompile-upverticesforthestudyofbackground andsystematic uncertainties.
TheATLAS detectorsimulation [45] isbased onGeant4 [46].
The generation of the simulated event samples included the ef- fectofmultiple ppinteractionsperbunchcrossing,aswellasthe effect on the detector response due to interactions from bunch crossingsbeforeor afterthe one containing thehard interaction.
Multipleppcollisionsperbunchcrossingweresimulatedwiththe softQCDprocessesofPythia8.186usingtheA2tune [47] andthe MSTW2008LOPDFset [48].
4. Reconstructionandeventselection
Measurements in the ID are used to reconstruct tracks and vertices, while information from the calorimeters and the muon spectrometerisusedtoidentifyelectronandmuoncandidates.The analysisimposesselection criteriaonthe twoleptons butnot on othercollisionactivity, exceptforrequiringthepresenceofa pri- maryvertex(PV),avertexwithatleasttwotracksandthehighest scalar sum of the p2T of associated tracks. This ensures that the analysisissensitivetoawiderangeofnewphysicsmodels.Events mustalso satisfy the standard data-quality criteriaof ATLAS and musthaveaPVwithzlocationthatsatisfies|zPV|<200 mm.The latterrequirementhasanegligibleimpactonsignalefficiency.
4.1.Triggersandpreselection
InordertoimprovethesensitivitytodecaysofLLPs,dedicated algorithms areused forthereconstruction of tracksandvertices.
As these algorithms are CPU intensive and significantly increase thestoragesizeofarecordedcollisionevent,theyareappliedonly toasubsetofthedataeventsidentifiedusingasetofpreselection criteria.
Thepreselection criteriaare basedontriggers that donotuse ID trackinformation,because the trackreconstruction applied at the trigger stage doesnot reconstruct tracks from particles pro- ducedwithsignificant displacementfromthe pp interactionver- tex.Displacedmuoncandidates are selectedwithamuon trigger requiringaMStrackthathaspT>60 GeVand|η|<1.05.Photon triggersareusedtoselectdisplacedelectroncandidates,triggering ontheelectromagneticshowersproducedby theelectrons.These includea single-photontriggerwithphoton pT>140 GeV anda diphotontriggerrequiringbothphotonstohave pT>50 GeV.The triggerefficiencyis1–90%,dependingonthemassandlifetimeof theLLP.
Table 2
Comparisonoftrackrequirementsbetweenthestandardandlargeradius tracking.
Standard Large radius
Maximum|d0|[mm] 10 300
Maximum|z0|[mm] 250 1500
Maximum|η| 2.7 5
Maximum shared silicon modules 1 2
Minimum unshared silicon hits 6 5
Minimum silicon hits 7 7
Inordertobepreselected,eventsthatpassthetriggersarere- quiredtosatisfyanadditionalsetofcriteria,whichisevaluatedus- ingthedatareconstructedwiththestandardATLASalgorithms,in- cludingtheelectron [49] andmuonreconstructionalgorithms [50].
Thepreselectedeventsarestoredinarawdataformatsothatthe reconstructionoptimizedforLLPdecayscanbeperformed.
Thepreselection criteriaformuonandelectroncandidatesare summarizedin Table1.Ifthe muonis reconstructedusingan ID trackand thereis a good matchwithan MS trackbased onthe
χ2 per degree of freedom, χ2/DoF<5, then the transverse im- pact parameter, calculated relative to the beamline, must satisfy
|d0|>1.5 mm.Thepreselectioncriteriaforelectroncandidatesde- pendonwhetherasingleordiphotontriggerispassed.Ifaphoton insteadofan electronsatisfiesthesingle photon trigger,an addi- tionalcandidateobjectisrequiredtofurther limitthesize ofthe dataset.Thereconstructedphotonsarerequiredtopassthe‘loose’
photonidentificationcriteria [51].
4.2. Reconstructionoflong-livedparticledecays
The standardtracking(ST)andvertexreconstruction inATLAS areoptimizedforthereconstructionofparticlesthatoriginatenear the ppinteraction vertex. The STrequirementson the transverse (d0)andlongitudinal(z0)impactparameters,calculatedrelativeto thebeamlineandbeamspotrespectively,limitthereconstruction efficiencyforhighly displaceddecaysofLLPs.Inordertoimprove theefficiency,asecondtrackreconstructionalgorithm,largeradius tracking (LRT) [52], as well as a dedicated displaced-vertex(DV) reconstructionisused.
ThetrackrequirementsintheSTandLRTare comparedinTa- ble2.Ofparticular note istherelaxation oftherequirements on d0, z0, andthemaximum numberofshared siliconmodules and theminimumnumberofunsharedsiliconhitsbetweentwotracks.
TheLRTreconstructstracksbasedontheremaininghitsthatwere notusedbytheST,andtheresultingtracksfrombothtrackrecon- structionalgorithmsareusedintheanalysis.
The tracksusedinthe DVreconstruction mustsatisfy thefol- lowingrequirements:pT>1.0 GeV,2<|d0|<300 mm,and|z0|<
1500 mm.Inaddition,eachtrackmusthaveatleasttwoSCThits andeitheratleasttwo pixelhitsoratleastoneTRThit,withno
