Article
Reference
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
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al.
Abstract
A search for dark matter pair production in association with a Higgs boson decaying to a pair of bottom quarks is presented, using 3.2 fb −1 of pp collisions at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the LHC. The decay of the Higgs boson is reconstructed as a high-momentum bb¯ system with either a pair of small-radius jets, or a single large-radius jet with substructure. The observed data are found to be consistent with the expected backgrounds. Results are interpreted using a simplified model with a Z′ gauge boson mediating the interaction between dark matter and the Standard Model as well as a two-Higgs-doublet model containing an additional Z′ boson which decays to a Standard Model Higgs boson and a new pseudoscalar Higgs boson, the latter decaying into a pair of dark matter particles.
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . 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. Physics Letters. B , 2017, vol. 765, p. 11-31
DOI : 10.1016/j.physletb.2016.11.035
Available at:
http://archive-ouverte.unige.ch/unige:89831
Disclaimer: layout of this document may differ from the published version.
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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
Availableonlinexxxx 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:atlas.publications@cern.ch.
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.
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Fig. 1.Diagramsshowingthesimplifiedmodelswhere(a)aZdecaystoapairof DMcandidatesχχ¯afteremittingaHiggsbosonh,andwhere(b)aZdecaystoa HiggsbosonhandthepseudoscalarAofatwo-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], interfacedtoPythia8.186[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,ismodelledusingPythia8.186andtheNNPDF2.3PDF set.Thesamplesare normalisedusingtheSMtotalcross-sections calculatedatnext-to-leadingorder(NLO)[41]andnext-to-next-to- leadingorder(NNLO)[42]inQCDforW handZh,respectively,and include NLO electroweak corrections [43]. In all cases, the Higgs bosonmassissetto125 GeV.
Simulated samples of vector boson production in association with jets, W/Z +jets, where the W or Z bosons decay in all leptonic decay modes, are generated using SHERPA2.1.1 [44], in- cluding b- and c-quark mass effects,and the CT10PDF set [45].
Matrixelements are calculatedforup totwo partonsatNLOand four partons at LO using the Comix [46] and OpenLoops [47]
matrix element generators and merged withthe SHERPA parton shower[48]usingtheME+PS@NLOprescription[49].Thecross- sectionsaredeterminedatNNLO[50] inQCD.Furthermore,these backgrounds are split intodifferentcomponents accordingto the true flavour of the two jets that are used to identify the flavor of the reconstructed Higgsboson candidate,as described in Sec- tion 5:l denotesa light quark (u,d,s) ora gluon andtheheavy quarks are denoted by c and b.Thisdivision isperformedto al- low accurate modelling ofthe W/Z+heavy-flavour backgrounds inthecombinedfitdescribedinSection8.
Diboson productionmodes, including Z Z, W W,andW Z pro- cesses, with one boson decaying hadronically andthe other lep- tonically are simulatedusingtheSHERPA2.1.1 generatorwiththe CT10 PDF set. They are calculated for up to one (Z Z) or zero (W W/W Z) additionalpartonsatNLOandup tothreeadditional partons at LO using the Comix and OpenLoops matrix element generatorsandmergedwiththeSHERPApartonshowerusingthe
