Contents lists available atScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Search for dark matter in association with a Higgs boson decaying to b-quarks 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:
Received16September2016
Receivedinrevisedform17November2016 Accepted21November2016
Availableonline24November2016 Editor:W.-D.Schlatter
AsearchfordarkmatterpairproductioninassociationwithaHiggsbosondecayingtoapairofbottom quarks ispresented,using3.2 fb−1 ofpp collisions atacentre-of-massenergyof13 TeVcollectedby theATLASdetectorattheLHC.ThedecayoftheHiggsbosonisreconstructedasahigh-momentumbb¯ systemwitheitherapairofsmall-radiusjets,orasinglelarge-radiusjetwithsubstructure.Theobserved dataarefoundtobeconsistentwiththeexpectedbackgrounds.Resultsareinterpretedusingasimplified modelwithaZgaugebosonmediatingtheinteractionbetweendarkmatterandtheStandardModelas wellasatwo-Higgs-doubletmodelcontaininganadditional ZbosonwhichdecaystoaStandardModel HiggsbosonandanewpseudoscalarHiggsboson,thelatterdecayingintoapairofdarkmatterparticles.
©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Althoughdark matter (DM)constitutes the dominant compo- nentofmatterintheuniverse,littleisknownaboutitsproperties andparticlecontent[1].Theleadinghypothesissuggeststhatmost DMisintheformofstable,electrically neutral, massiveparticles withcosmologicalconstraintsindicatingthatDMinteractionswith StandardModel(SM)particlesoccurataweakscaleorbelow[2].
Collider-basedsearchesfortheparticlecontentofDMprovideim- portantinformationcomplementary to thatfromdirect andindi- rectdetectionexperiments[3].
Atraditionaldark-mattersignature ata proton–protoncollider is one where one or more SM particles, X , are produced and detected, recoiling against missingtransverse momentum –with magnitudeEmissT –associatedwiththenon-interactingDMcandi- date.AnumberofsearchesattheLargeHadronCollider(LHC)[4]
have been performed recently, where X is considered to be a hadronic jet [5,6], b- or t-quarks [7–9], a photon [10–13], or a W/Z boson [14–17]. The discovery of a Higgs boson, h [18,19], providesa new opportunityto search forDM productionvia the h+EmissT signature [20–22].Incontrastto mostoftheaforemen- tionedprobes,Higgsbosonradiationfroman initial-statequarkis Yukawa-suppressed.Asaresult,inapotentialsignaltheHiggsbo- sonwouldbepartoftheinteractionproducingtheDM,providing uniqueinsightintothestructureoftheDMcouplingtoSM parti- cles.Recently,theATLASCollaborationhaspublishedsuchsearches using20.3fb−1 ofproton–protoncollision dataat√
s=8TeV, ex-
E-mailaddress:[email protected].
ploitingtheHiggsbosondecaystotwophotonsorapairofbottom quarks[23,24].
This Letter presents an update on the search for h+EmissT , wheretheHiggsbosondecaystoapairofbottomquarks(h→bb),¯ using 3.2fb−1 of pp collision datacollected by the ATLAS detec- toratacentre-of-massenergyof13TeVduring2015. Theresults are interpreted in the context of simplified models of DM, char- acterised by a minimal particle content and the corresponding renormalisableinteractions[25].
Many simplified models of DM production contain a massive particle which can be a vector, an axial-vector, a scalar or a pseudoscalar,andmediatestheinteractionbetweenDMandStan- dardModelparticles.Inthissearch,simplifiedmodels involvinga vector mediator are consideredfollowing the recommendationin Ref.[26].
In the first model [21], a vector mediator, Z, is exchanged in the s-channel, radiates the Higgs boson and decays into two DM particles. A diagram for this process is shown in Fig. 1(a).
The vector mediator has an associated baryon number B, which isassumedtobegaugeinvariant underU(1)B thusallowing itto coupleto quarks[27].Thissymmetry isspontaneously brokento generatethe Z mass.However,thereisno Z couplingtoleptons assuchcouplingsaretightlyconstrainedbydileptonsearches.Fi- nally, the dark-matter candidate carries a baryon number, which allows it to coupleto quarks through the Z. The parameters of thismodelareasfollows:thecouplingof Ztodarkmatter(gχ );
the coupling of Z to quarks (gq);the coupling of Z to the SM Higgs boson(gZ);the mixinganglebetween thebaryonic Higgs boson,introduced inthemodelto generatethe Z mass,andthe http://dx.doi.org/10.1016/j.physletb.2016.11.035
0370-2693/©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Fig. 1. Diagramsshowingthesimplifiedmodelswhere(a)aZdecaystoapairof DMcandidatesχχ¯afteremittingaHiggsbosonh,andwhere(b)aZdecaystoa Higgsbosonh andthepseudoscalarA ofatwo-Higgs-doubletmodel,andthelatter decaystoapairofDMcandidatesχχ¯.
SM Higgs boson (sinθ);the Z mass (mZ); andthe DM particle mass(mχ ).
In thesecond model, apart fromthe vector mediator, the SM isextended byan additional Higgsfield doublet,resultingin five physical Higgs bosons [22]: a light scalar h associated with the observed Higgs boson, a heavy scalar H , a pseudoscalar A, and two charged scalars H±. The vector mediator is produced res- onantly and decays as Z→h A in a Type-II two-Higgs-doublet model(2HDM)[28].Thepseudoscalar A subsequentlydecaysinto twoDMparticleswithalarge branchingratio.Adiagramforthis process is shown in Fig. 1(b). To define the model, the ratio of theup- anddown-typevacuumexpectationvalues,tanβ,mustbe specified along with the Z gauge coupling, gZ, the DM particle mass, mχ , and the Z and A masses, mZ and mA, respectively.
The results presented are for the alignment limit, in which the h–H mixing angle α is related to β by α= β −π/2. Only re- gions of parameter space consistent with precision electroweak constraints[29] andwithconstraints fromdirectsearches fordi- jetresonances[30–32]areconsidered.AstheA bosonisproduced on-shellanddecaysintoDM,themassoftheDMparticledoesnot affectthekinematicpropertiesorcross-section ofthesignal pro- cessifitisbelowhalfofthe A bosonmass.Hence,the Z-2HDM model is interpreted in the parameter spaces of Z mass (mZ), A mass(mA)andtanβ.
2. ATLASdetector
ATLAS isa multi-purposeparticle physics detector[33] atthe LHC,withanapproximatelyforward-backwardsymmetricandher- metic cylindrical geometry.1 At its innermost part lies the inner detector (ID), immersed in a 2 T axial magnetic field provided byathinsuperconductingsolenoid,consistingofsiliconpixeland microstripdetectors,whichprovideprecisiontrackinginthepseu- dorapidity range |η|<2.5. It is complemented by a transition radiationtrackerproviding trackingandparticleidentification in- formation for |η|<2.0. Between Run 1 and Run 2 of the LHC, the pixel detector was upgraded by the addition of a new in- nermostlayer[34] thatsignificantlyimprovestheidentificationof heavy-flavourjets[35,36].Thesolenoidissurroundedbysampling calorimeters: a lead/liquid-argon (LAr) electromagnetic calorime- terfor |η|<3.2 and a steel/scintillator tile hadronic calorimeter for |η|<1.7. Additional LAr calorimeters withcopper and tung- stenabsorbersprovidecoverageup to|η|=4.9.Intheoutermost part,air-coretoroidsprovidethemagneticfieldforthemuonspec-
1 ATLAS uses a right-handed coordinate system with itsorigin at the nomi- nalinteractionpoint(IP)inthe centreofthedetector andthe z-axisalongthe beampipe.Thex-axispointstowardsthecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φis the azimuthalanglearound the beampipe.The pseudorapidity ηis definedas η= −ln[tan(θ/2)],whereθ isthe polarangle.Finally,theangulardistance R isdefinedas
(φ)2+ (η)2.
trometer. The latterconsistsof threelayers of gaseous detectors:
monitoreddrifttubesandcathodestripchambersformuonidenti- ficationandmomentummeasurementsfor|η|<2.7,andresistive- plateandthin-gapchambersfortriggeringupto|η|=2.4.A two- level trigger system, custom hardware followed by a software- based level, is used to reduce the eventrate to about 1 kHz for offlinestorage.
3. Dataandsimulationsamples
The data sample used in this search, collected during nor- mal operation of the detector, corresponds to an integrated lu- minosity of 3.2fb−1. The primary data sample is selected using acalorimeter-based EmissT triggerwithathresholdof70 GeV.The trigger efficiencyforsignaleventsselected bythe offlineanalysis is about90%foreventswith EmissT of 150 GeVandreaches100%
foreventswithEmissT largerthan200 GeV.
Signal samples are generated at tree level with MadGraph5_aMC@NLO2.2.3 [37], interfacedto Pythia 8.186[38]
using the NNPDF2.3 parton distribution function (PDF) set [39]
andtheA14parametertune[40]forpartonshowering,hadronisa- tion,underlying-eventsimulation,andforsimulationoftheHiggs boson decay toa pair ofbottom quarks. Forthe vector-mediator simplified models, signals are generatedwith mediator massbe- tween 10 and 2000 GeV and DM particle mass between 1 and 1000 GeV. The event kinematics are largely independent of the other parametersofthemodel,andthusthesamevaluesofthese parametersarechosenfollowingtherecommendationsinRef.[26]:
gχ=1.0,gq=1/3, gZ=mZ,sinθ=0.3.FortheZ-2HDMmodel, pp→Z→Ah→χχ¯h samples are producedwith Z mass val- uesbetween600and1000 GeV, A massvaluesbetween300and 800 GeV (wherekinematically allowed),anda DMmassvalue of 100 GeV.Theotherparameterschosenforthismodelaretakento betanβ=1.0 andgZ=0.8.
Higgs boson production in association with a W or Z vector boson, V h,ismodelledusing Pythia 8.186andtheNNPDF2.3PDF set.Thesamplesare normalisedusingtheSMtotalcross-sections calculatedatnext-to-leadingorder(NLO)[41]andnext-to-next-to- leadingorder(NNLO)[42]inQCDforW h andZh,respectively,and include NLO electroweak corrections [43]. In all cases, the Higgs bosonmassissetto125 GeV.
Simulated samples of vector boson production in association withjets,W/Z+jets,wheretheW or Z bosonsdecayinalllep- tonicdecaymodes,aregeneratedusingSherpa2.1.1[44],including b- andc-quarkmasseffects,andtheCT10PDFset[45].Matrixele- mentsarecalculatedforuptotwopartonsatNLOandfourpartons at LOusing the Comix [46] andOpenLoops [47] matrix element generatorsandmergedwiththeSherpapartonshower[48]using theME+ PS@NLOprescription[49].Thecross-sectionsaredeter- mined atNNLO [50] inQCD.Furthermore,thesebackgrounds are splitintodifferentcomponentsaccordingtothetrueflavourofthe two jetsthat are used to identifythe flavor ofthe reconstructed Higgsbosoncandidate,asdescribedinSection5:l denotesalight quark (u,d,s) or a gluon and the heavy quarks are denoted by c and b. This division is performedto allow accurate modelling ofthe W/Z+heavy-flavour backgrounds inthecombinedfit de- scribedinSection8.
Diboson productionmodes, including Z Z , W W ,andW Z pro- cesses,withonebosondecayinghadronicallyandtheotherlepton- icallyaresimulatedusingtheSherpa2.1.1generatorwiththeCT10 PDFset.Theyarecalculatedforuptoone( Z Z )orzero(W W/W Z ) additionalpartonsatNLOanduptothreeadditionalpartonsatLO using the Comix and OpenLoops matrix element generators and merged withtheSherpapartonshower usingthe ME+PS@NLO
prescription.Theircross-sectionsare determinedbythegenerator atNLO.
The tt and¯ single-top-quark backgrounds are generated with PowhegBox [51] using the CT10 PDF set. It is interfaced with Pythia 6.428 [52] to simulate parton showering, fragmentation, and the underlying event, for which the CTEQ6L1 PDF set [53]
andthePerugia 2012parametertune [54]areused.Thett cross-¯ sectionisdeterminedatNNLOinQCDandnext-to-next-to-leading logarithms (NNLL)for softgluonradiation [55], whilethe single- top-quarkcross-sectionsarefixedtothoseinRefs.[56–58].Atop- quarkmassof172.5 GeVisusedthroughout.
The simulatedevent samples are processed with the detailed ATLAS detector simulation [59] based on Geant4 [60]. Effects of multiple proton–protoninteractions (pile-up) as a function of the instantaneous luminosity are taken into account by overlay- ing simulated minimum-bias events generated with Pythia8.186 withthe A2 tune [61] and MSTW2008LO PDF set [62] onto the hard-scatteringprocess, such that the distribution ofthe average numberofinteractionsper bunchcrossinginthesimulatedevent samplesmatchesthatinthedata.
4. Objectreconstruction
Proton–proton collision vertices are reconstructed using ID tracks with pT>0.4GeV. The primary vertex is defined as the vertexwiththehighest (ptrackT )2.Eacheventisrequiredtohave atleastonevertexreconstructedfromatleasttwotracks.
Muon candidates are identified by matching tracks found in theIDtoeitherfull tracksortracksegmentsreconstructedinthe muon spectrometer, and are required to satisfy the loose muon identification quality criteria [63]. Electron candidates are iden- tified as ID tracks that are matched to a cluster of energy in theelectromagneticcalorimeter.Electroncandidatesmustsatisfya likelihood-basedidentificationrequirement [64] basedon shower shapeandtrackselectioncriteria,andareselectedusingtheloose workingpoint.Boththemuonsandelectronsarerequiredtoorigi- natefromtheprimaryvertex,tohavepT>7GeV,andtoliewithin
|η|<2.5 formuonsand|η|<2.47 forelectrons.Theyarefurther requiredtobeisolatedusingrequirementsonthesumofpTofthe trackswithinaconearoundtheleptondirection.Theconesizeand therequirementsarevaried asa functionofthelepton pT to ob- tainanefficiencythatisfixedasafunctionof pT suchthata99%
efficiencyforpromptleptons isretainedacrossa broadkinematic range.
Jetsarereconstructedintwocategories,small-radius(small-R) andlarge-radius (large-R) jets. In both cases, the jetsare recon- structed from topological clusters of calorimeter cells using the anti-kt jet clustering algorithm [65]. In the case of small-R jets, a radius parameterof R=0.4 is usedand the effects ofpile-up arecorrectedforbyatechniquebasedonjetarea[66].Inthecase oflarge-R jets,aradiusparameter of R=1.0 isused andthejet trimming algorithm[67,68] is applied tominimise theimpact of energydepositionsduetopile-up andtheunderlying event.This algorithmreconstructssubjetswithin thelarge-R jetusingthe kt algorithm[69] withradiusparameter Rsub=0.2 andremovesany subjet with pT less than 5% of the large-R jet pT. The jet en- ergyscale,andalsointhecaseoflarge-R jetsthejetmassscale, iscalibratedusing pT- and η-dependentfactors determinedfrom simulation,with small-R jets receiving further calibrations using insitu measurements[70].Small-R jets within theID acceptance,
|η|<2.5, are called central in the following and are required to satisfy pT>20GeV.Those with2.5<|η|<4.5 are calledforward andare required tosatisfy pT>30GeV.To reduce the effectsof pile-upinsmall-R jetswith pT<50GeV and |η|<2.5,a signifi- cantfractionofthetracksassociated witheachjet musthavean
origincompatible withthe primary vertex, asdefinedby thejet vertextagger[71]. Furthermore,small-R jetsareremoved ifthey arewithinaR=0.2 conearoundanelectroncandidate.Large-R jetsarerequiredtosatisfy pT>250GeV and|η|<2.0.
Track jets are built from tracks using the anti-kt algorithm with R=0.2. Track jets with pT>10GeV and |η|<2.5 are se- lected andare matched by ghost-association [72] tolarge-R jets.
Small-R jets andtrack jetscontaining b-hadrons are identified –
“b-tagged”–usingaboosteddecisiontreethatcombinesinforma- tionabouttheimpactparameterandreconstructedsecondaryver- ticesofthetracksassociatedwiththesejets[35,36,73].Aworking pointisusedwhichachievesanaverageefficiencyof70%inidenti- fyingsmall-R calorimeterjet(trackjet)containingab-hadronwith misidentification probabilities of ∼12 (18)% for charm-quark jets and ∼0.2 (0.6)% for light-flavour jets, as determined in a simu- latedsampleoft¯t events.Trackjetshavehighermisidentification probabilitiesduetothesmallerradiusparameterused.
The missing transverse momentum, EmissT , is defined as the negative vector sumofthe transversemomenta ofthe calibrated physics objects (electrons, muons, small-R jets), with unassoci- atedenergydepositions,referredtoasthesoft-term,accountedfor using ID trackswith pT>0.5 GeV [74,75].Furthermore, a track- basedmissingtransversemomentumvector,pTmiss,iscalculatedas thenegativevectorsumofthetransversemomentaoftrackswith
|η|<2.5,consistentwithoriginatingfromtheprimaryvertex.2 5. Eventselection
For an eventto be considered inthe search, it is required to have EmissT >150GeV, pmissT >30GeV, andno identified, isolated muonsorelectrons.Thisisreferredtoasthezero-leptonregion.
Events with EmissT lessthan 500GeV are considered inthere- solvedregion. First, thisset of eventsis required to haveat least twocentralsmall-R jets.Followingthisselection,thereconstructed small-R jets are ranked asfollows. First, the central jets are di- videdintotwo categories, thosethat areb-taggedandthosethat arenot.EachofthesesamplesofjetsareorderedindecreasingpT. Theorderedsetofb-taggedjetsisconsideredwiththehighestpri- ority,whilethosethatarecentralbutnotb-taggedareconsidered withsecond priority, andfinally anyforwardjets, ordered inde- creasing pT,are consideredlast. Thetwo mosthighlyranked jets are usedtoreconstructtheHiggsboson candidate,hr,andthere- forecannot containforwardjets. Furthermore,atleastone ofthe jets constituting hr must satisfy pT>45GeV. Finally, events are dividedinto threecategoriesbasedonthe numberofcentraljets that are b-taggedbeingeitherzero, one,ortwo b-taggedcentral jets.Toachieve ahighEmissT triggerefficiency,eventsareretained ifthescalarsumofthepT ofthethreeleadingjetsisgreaterthan 150 GeV.Thisrequirementisloweredto120 GeVifonlytwocen- tralsmall-R jetsarepresent.
Additional selectionsare applied tofurther suppressthe mul- tijet background. Specifically, to reject events with EmissT due to mismeasured jets a requirement is placed on the minimum az- imuthal angle between the direction of the ETmiss and each of thejets,min
φ
EmissT ,jets
>20◦,forthethreehighest-ranked jets. Furthermore,theazimuthal anglebetweenthe EmissT andthe
pTmiss, φ
EmissT ,pTmiss
,is requiredto be lessthan 90◦,to sup- press events with misreconstructed missing transverse momen- tum.The Higgsboson candidateis requiredto be well separated
2 Throughoutthissearch,themagnitudeofEmissT isreferredtoasEmissT andthe magnitudeofpTmissisreferredtoaspmissT .Onlywhenthedirectionalityisnecessary doesthenotationusethevectorsymbol.
