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Physics Letters B
www.elsevier.com/locate/physletb
Search for pair-produced long-lived neutral particles decaying to jets in the ATLAS hadronic calorimeter in pp collisions at √
s = 8 TeV
.ATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received16January2015
Receivedinrevisedform4February2015 Accepted5February2015
Availableonline11February2015 Editor: W.-D.Schlatter
Keywords:
High-energycolliderexperiment Long-livedneutralparticle Newphysics
The ATLASdetector attheLargeHadronCollider atCERNisused to searchforthe decayofascalar bosontoapairoflong-livedparticles,neutralundertheStandardModelgaugegroup,in20.3 fb−1of datacollectedinproton–protoncollisionsat√
s=8 TeV.Thissearchissensitivetolong-livedparticles thatdecaytoStandardModel particlesproducing jetsattheouteredgeofthe ATLASelectromagnetic calorimeter orinsidethe hadroniccalorimeter.No significantexcessofevents isobserved.Limitsare reportedontheproductofthescalarbosonproductioncrosssectiontimesbranchingratiointolong-lived neutralparticlesasafunctionoftheproperlifetimeoftheparticles.Limitsarereportedforbosonmasses from100GeV to900GeV,andalong-livedneutralparticlemassfrom10GeV to150GeV.
PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
ThediscoveryoftheHiggsboson[1–3]bytheATLASandCMS experiments [4,5] in 2012 identified the last piece ofthe highly successfulStandardModel(SM).Subsequentmeasurementsofthe Higgsbosonbranchingratiosandcouplings,whileconsistentwith theSMexpectations,allowforasubstantialbranchingratiotoex- oticparticles.ThisletterdescribesasearchfordecaysoftheHiggs bosonandotherscalarbosonstonon-SMstatesthatinturndecay toSMparticles.
A number of extensions of the SM involve a hidden sector that isweakly coupled to theSM, with thetwo connected via a communicatorparticle. Thisletter considers models containing a hiddensectorwithaconfininggaugeinteractionthatisotherwise invisibletotheSM.ThecommunicatorischosentobeaSM-sector scalar boson, [6–9]. The communicator mixes with a hidden- sectorscalarboson,hs,whichdecaysintodetectableSMparticles.
Thissearchconsiderscommunicatormassesbetween100 GeV and 900 GeV.AmassclosetothemassofthediscoveredHiggsbo- sonisincludedtosearchforexoticdecaysoftheHiggsboson.
A Hidden Valley (HV) model [8,9] is used as the benchmark model.The lightest HV particles forman isospintriplet of pseu- doscalar particles which are called valley pions (πv) because of theirsimilaritytotheSMtriplet.The πvarepair-produced(hs→ πvπv)andeach decaysto apair ofSM fermions.The πv possess Yukawacouplingstofermionsandthereforepreferentiallydecayto accessibleheavyfermions,primarilybb,cc and τ+τ−.
E-mailaddress:atlas.publications@cern.ch.
Thelifetimeofthe πvisunconstrainedandcouldbequitelong.
A long-lived πv can resultin signatures that traditionalsearches fail todetect.Ifa πv decaysintheinner detectorormuonspec- trometer, itcan be reconstructed asa displacedvertex. However, standard vertex-finding algorithms [10] are not likely to recon- struct it without modification. Likewise, a πv decay deep inside the calorimeteris reconstructed asa jet withan unusual energy signaturethatmosttraditionalsearchesrejectashavingpoordata quality. Thissearch focusses onfinal states whereboth πv decay inthehadroniccalorimeterorneartheouteredgeoftheelectro- magnetic calorimeter. Each heavy fermion pair from a πv decay is reconstructed asa single calorimeterjet withthree character- isticproperties:anarrowradius,notracksfromchargedparticles matchedto thejet,andlittleornoenergydeposited intheelec- tromagneticcalorimeter.
Scalar boson masses ranging from 100 GeV to 900 GeV are considered in addition to the Higgs boson’s mass (generated at mH=126 GeV)and πv massesbetween10 GeV and150 GeV are studied.Othersearchesforpairsofdisplacedverticesgeneratedby pair-produced neutral, long-livedparticleswere performedinAT- LAS [11] and CMS[12] at the LHC andin D0 [13] andCDF [14]
atthe Tevatron.The Tevatronexperiments andCMS searchedfor displaced vertices in their tracking system only, which results in a corresponding proper decay length range of a few meters.
CMSalso lookedatthe multi-leptondecaychannel, anotherpos- sible decay of HV particles. The previous ATLAS analysis, based on 7 TeV data,used the muon spectrometer and is sensitive to proper decay lengths between 0.5 m and 27 m, depending on thebenchmarkmodel.No evidenceofphysicsbeyondtheSMwas found.
http://dx.doi.org/10.1016/j.physletb.2015.02.015
0370-2693/PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
2. TheATLASdetector
TheATLASdetector[15]isamulti-purposedetectorattheLHC, consistingofseveralsub-detectors.Fromtheinteractionpoint(IP) outwards there are an inner detector (ID), electromagnetic and hadroniccalorimeters,andamuonspectrometer(MS).TheID,im- mersed in a 2 T axial magnetic field, provides tracking and ver- tex information for charged particles within the pseudorapidity1 (η) region |η|<2.5. It consistsof three differenttracking detec- tors.From smallradii outwards,thesearea siliconpixeldetector, a siliconmicrostriptracker(SCT)andatransitionradiationtracker (TRT).
The calorimeterprovides coverage overthe range |η|<4.9.It consistsof a lead/liquid-argon electromagneticcalorimeter (ECal) at smaller radii surrounded by a hadronic calorimeter (HCal) at larger radii comprising a steel and scintillator-tile system in the barrel region (|η|<1.7) and a liquid-argon system with copper absorbers in the endcaps (1.5<|η|<3.2). The ECal spans the range1.5 m<r<2.0 m in thebarreland 3.6 m<|z|<4.25 m intheendcaps.TheHCalcovers2.25 m<r<4.25 m inthebarrel and4.3 m<|z|<6.05 m intheendcaps.Thereisalsoa forward calorimeter(FCal), withcoverage between 3.1<|η|<4.9,which usescopperabsorbersinthefirstlayer,andtungstenabsorbersin thesecondandthirdlayers,andliquid-argonastheactivemedium inalllayers.Muonidentificationandmomentummeasurementare provided by the MS, which extends to |η|=2.7. It consistsof a three-layer system of gas-filled precision-tracking chambers. The region|η|<2.4 isalsocoveredbyseparatetriggerchambers.
A sequential three-level trigger system selects events to be recorded for offline analysis. The first level consists of custom hardware that implements selection on jets, electrons, photons,
τ leptons,muons,andmissingtransversemomentumorlargetotal transverseenergy.Thesecondandthirdlevelsaddchargedparticle trackfindingandrefinethefirst-levelselectionswithprogressively moredetailedalgorithms.
3. Dataandsimulationsamples
All data used inthis analysis were collected during the 2012 LHC proton–protonrun ata centre-of-mass energy of 8 TeV. Af- terdataqualityrequirementsareapplied,thesamplecorresponds to an integrated luminosity of 20.3 fb−1. The HV Monte Carlo (MC)samples aregeneratedwithPYTHIA8.165[16] andthePDF MSTW2008[17]tosimulategluon fusiongg→ productionand the hs decay hs→πvπv for different and πv masses (Ta- ble 1).massesbelow300 GeV areconsideredlow-masssamples andtherestareconsideredhigh-masssamples.The πv lifetimeis fixed ineach sample to ensuredecays throughouttheATLAS de- tector.TheissimulatedinPYTHIAbyreplacingtheHiggsboson withtheandhavingthedecayto πv 100% ofthetime.The
samples are produced with cross sections calculated at next- to-next-to-leading-logarithmic accuracy in QCD processes and at next-to-leading-order in electro-weak processes assuming the ateachmasshasthesamepropertiesastheSMHiggsboson[18].
Aftergenerationtheeventsarepassed throughadetailedsimula- tionofthe detectorresponsewithGEANT4 [19,20]andthesame reconstructionalgorithmsasareusedonthedata.GEANT4needed nomodificationtosimulatethesignalasalldecayparticlesareSM
1 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupward.Cylindricalcoordinates (r,φ)areusedinthe transverseplane,φ beingtheazimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedin termsofthepolarangleθasη= −ln tan(θ/2).
Table 1
The massorHiggsbosonmass, gluonfusionproductioncrosssection,and πvmassofeachbenchmarkHiddenValleymodelgenerated.Thecross-sectionsare based onthe assumptioninthebenchmarked modelthatthe bosonproduc- tionmechanismisthesameastheHiggsbosonproductionmechanism.Thedecay branchingratiosoftheπvasafunctionoftheπvmassarelistedinthesecond tableasdeterminedinthesimulationsamples.
mH[GeV] σ[pb] πvMass [GeV]
126 19.0 10, 25, 40
Mass [GeV] σ [pb] πvMass [GeV]
100 29.7 10, 25
140 15.4 10, 20, 40
300 3.59 50
600 0.52 50, 150
900 0.06 50, 150
πvMass [GeV] BR bb [%] BRτ+τ−[%] BR cc [%]
10 70.0 16.4 13.4
20 86.3 8.0 5.6
25 86.6 8.1 5.3
40 86.5 8.5 5.0
50 86.2 8.8 4.9
150 84.8 10.2 4.8
particles.AllMCsamplesarereweightedtoreproducethenumber ofinteractionsperbunchcrossingobservedinthedata.
4. Triggerandeventselection
Candidateeventsarecollected usingadedicatedtrigger,called the CalRatio trigger [21], which looks specifically for long-lived neutral particles that decay near theouter radius of theECal or within the HCal.The trigger is tuned tolook forevents contain- ingatleastonenarrowjetwithlittleenergydepositedintheECal andno chargedtracks pointingtowards thejet. At thefirst level thetriggerselectsonlynarrowjetsbyrequiringatleast40 GeV of transverseenergy(ET)inthecalorimeterina0.2×0.2 (η× φ) region usingtopologicaljets[15,22],incontrasttothedefaultal- gorithminwhichtheenergyina0.4×0.4 regionissummed.The 40 GeV ETthresholdrequirementisfullyefficientatan offlinejet ET of 60 GeV.To selectjets witha highfraction oftheir energy in theHCalthe secondlevel ofthetrigger requiresthesenarrow jetstohavelog10(EH/EEM)>1.2,whereEH/EEMistheratioofthe energydepositedintheHCal(EH)totheenergydepositedinthe ECal (EEM). The trigger also requires no tracks with pT>1 GeV in theregion 0.2×0.2 (η× φ)around thejet axis.The third levelofthetriggerusestheslowerbutmoreaccurateanti-kt algo- rithm[23]withR=0.4 toreconstructthejetandrequiresthejet tohaveaminimumof35 GeV oftransverseenergy.
The probability(επv) forasingle πv tofire thetriggerinsim- ulatedeventsisshowninFig. 1,forthe(a)barreland(b)endcap region of thecalorimeter inseveral differentsignal samples. The averageprobabilityforthelow(high)scalarbosonmassesisabout 20% (55%) for πv decays occurring at radii between 2.0 m and 3.5 m in the barrel, andabout 6% (30%) for πv decays with |z| between4.0 m and5.5 mintheendcaps.Theturn-ontakesplace before the inner edge of the HCAL asthe log10(EH/EEM) cut al- lows for a small amount of energy in the ECal. The probability decreases towards the outer region of the HCalwhere too much of the energy escapes the HCal to pass the jet ET requirement.
The efficiencyislower intheendcapsbecauseeventstendto not satisfy the isolation criteriaduetothe increasedoccupancyfrom extracollisioneventsinthesamebunchcrossingasahard-scatter interaction(pile-up).
Events also contain a reconstructed primary vertex with at leastthree trackswith pT>1 GeV.Events arerejectedifanyre-
Fig. 1. Theprobability(επv)forasingleπvtopassthetriggerasafunctionofthe πv(a)radialdecaylengthinthebarreland(b)thez positionofthedecayvertex intheendcapsforseveralandπvmasses.
constructed jets show evidence of being caused by a beam-halo interaction [21]. A missing transverse momentum requirement, EmissT <50 GeV, is appliedto reject non-collision events,such as cosmicraysorbeam-halointeractions.
Inthe offline selection, jets are reconstructed with an anti-kt algorithm with R=0.4, starting from calorimeter energy clus- terscalibratedusingthelocalclusterweighting method[24]. Jets arethencalibratedusinganenergy- and η-dependentsimulation- basedcalibration scheme. Jets are rejectedif they do not satisfy thestandardATLASgood-jetcriteriawiththeexceptionofrequire- mentsthat reject jetswithsmall electromagneticenergyfraction (EMF)[25].At least one jet must havefired the CalRatio trigger.
The jet matchingthe trigger mustpass an ET>60 GeV require- mentwhileasecondjetmustsatisfyanET>40 GeV requirement.
IfmorethanonejetfiredtheCalRatiotriggerthenonlytheleading jetisrequiredtohaveET>60 GeV.
Individually, all jets must satisfy |η|<2.5, have log10(EH/ EEM)>1.2, andhaveno good tracks inthe ID with pT>1 GeV inaregionR<0.22 centredonthejet axis.Agoodtrackmust haveatleasttwo hitsinthepixeldetectorandatotalofatleast nine hits in the pixel and SCT detectors. Fig. 2(a) compares the distribution of the number of good tracks associated with each jet in the multi-jet sample (described in the next section) with that in jets resulting from simulated πv decays in the HCAL or ID. Fig. 2(b) makes the same comparison forthe distribution of
2 R=
(η)2+ (φ)2.
Fig. 2. Distributionof(a)thenumberofgoodtracks(ntracks)withpT>1 GeV and R<0.2 aroundthejet axisand(b)thedistributionofjetlog10(EH/EEM)with jet |η|<2.5, pT>40 GeV.Thedashed histogramisforπv jetsdecayinginthe hadroniccalorimeter, andthedottedhistogramisforπvjetsdecayingintheID.
BotharefromthemH=126 GeV,mπv=10 GeV sample.Thefilledhistogramisthe multi-jetdatasampleusedtoevaluatethemulti-jetcontributiontothebackground.
EventsarerequiredtosatisfyEmissT <50 GeV.
log10(EH/EEM)ofeach jet.Themulti-jet data was gatheredusing aprescaled,single-jettriggerwitha15 GeV requirement.
Jetscausedbycosmicraysandbeam-halointeractionsareoften out-of-time. The jet timing is calculated by making an energy- weighted average ofthe timing foreach cellin thejet. Each cell isdefinedtohaveatimeof0 nsifitsenergyisrecordedatatime consistentwiththearrivalofaβ=1 particlefromtheIP.Thetim- ing ofeach jet isrequired tosatisfy −1<t<5 ns.This cut will impact the efficiencyforlow β πv.Due to the requirementofa high-ETjetinthisanalysistheβ-distributionispeakednear1for lowmasssamples.Forthehighmasssamplesthedifference betweenm andmπv results in a large boost for the πv at the generatedlifetimes.As aresult,theinefficiencyintroducedby the timingcutisatworst1.5%fortheconsideredsamples.
Theanalysisrequiresthatexactlytwojetssatisfytheserequire- ments. The second jet requirement significantly reduces the SM multi-jet background contribution.Table 3 lists the final number ofexpectedeventsineachsignalMCsample.Thefinalnumberof eventsselectedindatais24.
5. Backgroundestimation
The largest contribution to the expected background comes fromSM multi-jetevents.Cosmic-ray interactionscontribute ata much lower level, and beam-halo interactions make a negligible contribution.
Toestimate the multi-jet backgroundcontribution, amulti-jet datasampleisusedtoderivetheprobability thatajetpassesthe
