Search for Higgs boson decays into a pair of light bosons in the bb μμ

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for Higgs boson decays into a pair of light bosons in the bb μμ

ﬁnal state in pp collision 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:

Received3July2018

Receivedinrevisedform12October2018 Accepted19October2018

Availableonline21December2018 Editor:W.-D.Schlatter

AsearchfordecaysoftheHiggsbosonintoapairofnewspin-zeroparticles,H→^aa,^where^the^a-bosons decayintoab-quarkpairand amuon pair,ispresented.The searchuses36.1 fb⁻¹ ofproton–proton collision data at√

s=13 TeV recorded by the ATLAS experiment atthe LHC in 2015and 2016. No significantdeviationfromtheStandardModelpredictionisobserved.Upperlimitsat95%confidencelevel areplacedonthebranchingratio(σH/σSM)×B(^H→âa→^bbμμ),rangingfrom1.2×¹⁰⁻⁴^to⁸.4×¹⁰⁻⁴ inthea-boson massrangeof20–60 GeV.Model-independentlimits are setonthe visibleproduction cross-sectiontimesthebranchingratiotothebbμμfinalstatefornewphysics,σvis(X)×B(X→^bbμμ), rangingfrom0.1 fbto0.73 fbform_μμbetween18and62 GeV.

©²⁰¹⁸^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

The discovery of the Standard Model (SM) Higgs boson [1,2]

hasopenedup newavenuestosearch forphysics beyondtheSM (BSM)withperspectivesto search for non-SMor “exotic”decays ofthe Higgsboson.Suchsearchescould provideuniqueaccessto hidden-sectorparticlesthataresingletsundertheSMgaugetrans- formations [3]. ExoticdecaysoftheHiggsboson arepredictedby manynew-physicsmodels [3,4],includingthosewithanextended Higgssector [5–9],darkmatter(DM)models [10–14],modelswith aﬁrst-orderelectroweakphasetransition [15,16] andtheorieswith neutralnaturalness [17,18]. Thesemodelshavealsobeen usedto explainthe observationsofa γ-rayexcess fromthe galacticcen- tre (GC) by theFermi LargeArea Telescope [19,20]. For example, amodelforthe GC γ-rayexcess was proposed inwhich30 GeV DMparticles pair-annihilate dominantly through a CP-oddscalar mediator that subsequently decays into SM fermions [13]. If the mediatorissuﬃcientlylighter thantheSMHiggsboson(H) then H decayintothemediatorpaircanbeobservedattheLHC.

ExistingmeasurementsconstraintheBSMor“exotic”branching ratio(B⁾ôf^the125 GeV Higgsbosondecaystolessthanapproxi- mately34% at95% confidencelevel [21].Duetothenarrowwidth (∼^{4 MeV)} ôf^the ^Higgs^boson, êven â^small ^non-SM^couplingôf O(¹⁰⁻²)canleadtoO(^10%)branchingratiointoBSMstates.This potentiallylargeB(H→^{BSM states})motivatesdirectsearchesfor exoticHdecays.

E-mailaddress:atlas.publications@cern.ch.

TheanalysispresentedinthisLetterperformsthesearchinthe bbμμ ﬁnal state. The a-boson can be eithera scalar ora pseu- doscalarunderparitytransformations,sincethedecaymodecon- sideredinthissearchisnotsensitivetothedifferenceincoupling.

Assumingthatthea-bosonmixeswiththeSMHiggsbosonandin- heritsitsYukawacouplingstofermions,thelargestbranchingratio isexpectedtobetotheheaviestfermionsaccessiblebykinematics (2ma<mH), where ma and mH are the a-boson and Higgs bo- sonmasses.Forma10 GeV thismeansthea-bosonwoulddecay preferentially into bb.However, in models withenhanced lepton couplings such as the Type-III 2HDM [22], the a→μμ branching ratio can also be relatively large. Additionally, the sensitivity of a givenchannel doesnot depend only on the expectedsignal ratein a particular model, butalso on theeﬃciency fortrigger- ing andreconstructingeventsofinterest. Thepresenceofaclean dimuonresonanceprovidesadistinctivesignaturethatcanbeused for triggering and precision mass reconstruction, which helps to suppressbackground.

Searches forthe Higgsboson witha mass of125 GeV decay- ingintotwo spin-zeroparticles, H→^aa,^have^been^performedⁱⁿ various ﬁnal states in ATLAS and CMS [23–29]. The CMS search with√

s=^{8 TeV data}ⁱⁿ^the^bbμμfinalstateset95%CLlimitson (σH/σSM)×B(H→âa→^bbμμ)between2×¹⁰⁻⁴ând⁸×¹⁰⁻⁴ⁱⁿ thea-bosonmassrangeof25–62.5 GeV [25].InType-III2HDM+^S scenariowithtanβ=^{2 [4],}^where^tanβ denotestheratioofthe vacuum expectation values of the two Higgs fields, these limits translateintoupperlimitson(σH/σSM)×B(H→âa)rangingbe- tween13%and50%.Someofthemoststringentlimitsup todate forType-III2HDM+^S ^with^tanβ=^{2 come} ^from ^the^CMS^search with √

s=13 TeV data in the bbτ τ ﬁnal state, setting the up- https://doi.org/10.1016/j.physletb.2018.10.073

0370-2693/©²⁰¹⁸^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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perlimitson(σH/σSM)×B(^H→^aa)between4%and26%inthe a-bosonmassrangeof15–60 GeV [28].

2. Dataandsimulation

The search presented inthis Letter is basedon the 36.1 fb⁻¹ datasetof proton–protoncollisions ata centre-of-mass energyof

√s=13 TeV recorded by the ATLAS experimentat theLHC dur- ing2015and2016. TheATLASexperiment [30] is amultipurpose particle detector witha forward–backward symmetric cylindrical geometry anda near 4π coverage in solid angle.¹ It consists of an inner tracking detector surrounded by a thin superconduct- ingsolenoidprovidinga 2 Taxialmagneticﬁeld, electromagnetic andhadronic calorimeters, anda muon spectrometer. Events are collected withsingle-muontriggers requiringthe muon pT tobe above 24 or 26 GeV, depending on the data-taking period. The trigger eﬃciencyforthe signal events withthe muon p_T onthe triggerplateauisabout 80%.

Simulatedevents are used to modelthe signal andSM backgrounds processes. Higgs boson production through the gluon–

gluon fusion (ggF) and vector-boson fusion (VBF) processes was modelled at next-to-leading order (NLO) using Powheg-Box v2 [31–33] interfacedwithPythia 8.186 [34] using theAZNLOsetof tunedparameters [35] forthesimulationofthebbμμdecayofthe Higgs boson,as well as forparton showering andhadronisation.

The ggF Higgs boson production rate is normalised to the total cross-sectionpredictedby anext-to-next-to-next-to-leading-order QCDcalculationwithNLOelectroweakcorrectionsapplied [36–40].

The VBF production rate is normalised to an approximate next- to-next-to-leading-order(NNLO)QCDcross-sectionwithNLOelec- troweakcorrectionsapplied [41–44].Fivemasspointsweresimu- latedintherangem_a=20–60 GeV instepsof10 GeV forbothggF andVBFproduction.

Sherpa2.2.1 [45] withthe NNPDF3.0[46] setofpartondistri- bution functions(PDF) was used forthe generationof Drell–Yan, W+^{jets and}^diboson⁽^{W W}^,^{W Z}^,^{Z Z}⁾backgrounds.Cross-sections were calculated at NNLO QCD accuracy for Z^(∗)/γ^∗₊jets and W +jets production [47] andat NLO including LOcontributions withtwoadditional partonsforthedibosonprocesses [45,48,49].

Thett¯andsingle-top-quarksamplesweregeneratedwithPowheg- Box v2 [32] using the CT10 PDF set [50] interfaced with Pythia v6.428 [51] and the Perugia 2012 set of tuned parameters [52]

forthepartonshower.Themassofthetopquark(m_t) wassetto 172.5 GeV.The parameterh_damp inPowheg,usedtoregulate the high-pTradiation,was settomt forimprovedagreementbetween dataandsimulationinthehighpTregion [53].Thecross-sectionof tt¯wascalculatedatNNLOinQCDincludingresummationofnext- to-next-to-leadinglogarithmic(NNLL)softgluonterms [54,55].The cross-section forsingle-top-quarkproductionwas calculated with theprescriptionsinRefs. [56,57].Theproductionoft¯tpairsinas- sociationwith W/Z bosons (denoted by tt V¯ ) was modelledwith samplesgeneratedatLOusingMadGraph5_aMC@NLOv2.2.2 [58]

andshoweredwithPythiav8.186.Thesamplesarenormalisedto NLOcross-sections [59,60].

Additional pp collisions generated with Pythia v8.186 were overlaid to model the effects of additional interactions in the sameandneighbouringbunchcrossings(pile-up)forallsimulated

1 TheATLASCollaborationusesaright-handedcoordinatesystemwithitsorigin atthenominalinteractionpoint(IP)inthecentreofthedetectorandthe z-axis alongthebeampipe.Thex-axispointsfromtheIPtothecentreoftheLHCring, andthey-axispointsupwards.Cylindricalcoordinates(r,φ)areusedinthetrans- verseplane,φbeingtheazimuthalanglearoundthebeampipe.Thepseudorapidity isdeﬁnedintermsofthepolarangleθasη= −^{ln tan}(θ/2).Angulardistanceis measuredinunitsofR≡

(η)²+(φ)².

events.The pile-upsimulation usedthe A2setoftuned parameters [61] andtheMSTW2008LOPDFset [62].Allthesampleswere processed through the full ATLAS detector simulation [63] based onGEANT4[64] andprocessedwiththesamereconstructionalgo- rithmasusedfordata.

3. Selectioncriteria

Interaction vertices from proton–proton collisions are recon- structedfromatleasttwotrackswithtransversemomentum(pT) larger than 0.4 GeV, andare required to be consistent with the beamspot envelope.The primary vertex (PV) is identiﬁed as the onewiththelargest

p²_T ofassociatedtracks [65].

Muoncandidatesarereconstructedusingtheinformationfrom the innerdetector andthemuon spectrometer [66].Theyare re- quiredtosatisfy“medium”identificationcriteria [66],bematched to the PV and have p_T>7 GeV and |η|<2.7. Additionally, the muons must satisfy the following criteria: theprojected longitu- dinal impact parameter |^z0sinθ| ^must ^be ^less^than ^{0.5 mm} ând the ratio of the transverse impact parameter d0 to its estimated uncertainty σd₀, |^d0/σd₀|^, ^must ^be ^less ^than ^3. ^Finally, ^the ^se- lected muons must fulfil requirements on the scalar sum of pT of additional inner detector tracks andon the sum of the E_T of calorimeter topological clusters [67] ina cone of size R=⁰.2 around the muon to ensure they satisfy “tight” isolation criteria [66].Theserequirementsselectsignalmuonswithanidentifi- cationefficiencyof∼^94%ândîsolationêfficiency^ranging^between

∼^91%^for^ma=^{20 GeV and}∼^95%^for^ma=^{60 GeV.}

Jets are reconstructed using the anti-k_t algorithm [68] imple- mented inthe FastJet package [69] with a radiusparameter R= 0.4 appliedtotopologicalclustersofenergydepositsincalorime- tercells.Jetsfrompile-uparesuppressedwiththeuseoftracking information asdetailedinRef. [70].All selectedjetsare required to have pT>20 GeV, |η|<2.5 and must pass quality requirements deﬁned to minimise the impact of detector effects, beam backgroundsandcosmicrays.

Jetsconsistentwiththehadronisationofab-quark(b-jets)are identified using a multivariate discriminant [71,72]. This analysis usesthe77%b-jetidentificationefficiencyworkingpointforwhich thepurityoftheb-taggedsampleisapproximately95%,whilethe probability ofmisidentifying ajetinitiatedby acharmquarkasa b-jetisapproximately16%,asdeterminedfromasampleofsimu- latedt¯t events.

Inordertorejectnon-promptmuonsfromthedecayofhadrons within a jet, an overlap removal algorithm is applied. If a jet is found within R=⁰.4 of themuon candidate,the overlapisre- solved in the following way: if there are more than two tracks with pT>500 MeV associatedwiththejet then themuon isre- movedfromtheevent,otherwisethemuonisretainedandthejet isremoved.

Themissingtransversemomentum(E^miss_T )usedintheanalysis iscalculatedasthemagnitudeofthenegativevectorsum(−→_pmiss

T )

ofthetransversemomentaofallselectedandcalibratedobjectsin the event andthe additional “soft” term that takes into account tracksnotassociatedwithanyofthetheseobjects [73].The“soft”

term is calculatedfrom inner detector tracksmatched to the PV andincludedtoachieveabetterE^miss_T resolution.

Eventsarerequiredtohaveexactlytwob-taggedjetswithp_T>

20 GeV andexactly tworeconstructed muonsofopposite charge, withtheleadingmuonhavingpT>27 GeV tobeinthemaximum- eﬃciency regime of thetrigger andthe subleading muon having pT>7 GeV. The dimuon invariant mass (mμμ) isrequired to be between16 GeV and64 GeV.Theupperboundonmμμ isdeﬁned by theassumptionthatthe125 GeV Higgsbosondecaysintotwo

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Fig. 1.The(a)mμμ,(b)mbbbeforetheKLfit,(c)mbbμμbeforeand(d)m^KL_bb_μμaftertheKLfitforeventsafterthepreselectionstage,butremovingtheupperboundonmμμ. Thet¯tcontributionismodelledwiththesimulatedsamplenormalisedtothetheoreticalcross-section.TheDrell–Yancontributionistakenfromdatatemplates(describedin thetext)andnormalisedtothetotalyieldpredictedbytheDrell–Yansimulation.ThesignaldistributionsforallfivesimulatedmaarealsoshownassumingtheSMHiggs bosoncross-section(includingggF,VBFandV H production)andB(^H→âa→^bbμμ)=^10%.^The^branching^ratioⁱⁿ^thisândâll^subsequent^figuresîs^chosen^soâs^to^give goodvisibilityontheplot.

on-shellparticlesofequalmasses,whilethelowerboundismoti- vatedbythekinematicsofthea-bosondecays.Forlowervaluesof ma,mostofthesignaljetsfallbelowthereconstructionthreshold andthe jetstend tooverlapgeometricallyinthedetectorso that thesensitivityoftheanalysistothe H→^aa^signal^decreases.^This setofselectioncriteriaisreferredtoasthe“preselection”.

Signal events are characterised by the invariant mass of the twob-jets(mbb)beingequal,withinthedetectorresolution,tothe dimuoninvariantmassandthefour-objectmass(mbbμμ)beingap- proximately125 GeV.Oneside ofthe H→âa^decay^(a→μμ)is measuredwithapproximatelytentimesbetterresolutionthanthe othersideofthedecay(a→^bb),âs^shownⁱⁿ^Figs.^1(a)ând^1(b).

Akinematic-likelihood(KL)fit [74] exploitingthesymmetryof H→âa^decaysîs^performed^to^test ^thecompatibilityofan event withthe m_bb^mμμ hypothesis and improve the m_bbμμ resolu- tioninsignal events.The KLfit findsthe energies oftheleading (Eˆ_b₁)andsubleading(Eˆ_b₂) b-jets thatmaximisethelikelihoodfor aneventwithmeasuredleadingandsubleadingb-jetenergies E_b₁ and Eb2 andwithdimuon invariant massmμμ. Thelikelihood is definedasfollows,

L=^W(Eˆ_b₁,E_b₁)·^W(Eˆ_b₂,E_b₂)·^FBW(m^KL_bb,m_μμ),

where m^KL_bb is the dijet invariant mass computed from the b-jet four-momenta corresponding to Eˆb1 and Eˆb2, W is the transfer functionoftheb-jets,andF_BWisaBreit–Wignerfunctioncentred

onmμμwithawidththatissmallcomparedtothembbresolution.

Thetransferfunction W(Eˆ_b₁,E_b₁)isadoubleGaussianprobability density function derived from simulated events as a function of jet p_T and ηusingthedifferencebetweentrueandreconstructed energies.Thefitdeterminesamaximum-likelihoodvalueofL(de- notedby ln(L^max)),whichquantifieshowwellaneventfitstothe constraints.Theb-jet momentadeterminedby thefitareused to recomputethefour-bodymassdenotedm_bb^KLμμ.AsseeninFig.1(d), theresolutionofthem^KL_bbμμ distributionforthesignalisimproved by up to a factor of two compared to the pre-fit m_bbμμ shown inFig.1(c), whilethe backgroundshapewithin them_bbμμ signal peak remains almost unchangedwiththe yields risingby ∼^20%.

This allows the analysis to place tighter constraints on the dif- ference between the reconstructed invariant mass of the bbμμ

systemandmH,rejecting morebackground eventsandobtaining highersignalsigniﬁcance.

Twocriteriabased on the kinematiclikelihood fitare applied to selectsignal-like events andreject background eventsthat do not fit the mbb=^mμμ constraint well: |^m^KL_bb_μμ−^mH|<15 GeV andln(L^max)>−^8.^Finally,^the Ê^miss_T <60 GeV requirementrejects a largeportion oftt¯ pairswhere bothtop quarksdecaysemilep- tonically,while retainingmostofthe signal events.Adding these threerequirementsafterthepreselection stagedefinesthesignal- enhanced region (SR). A search for a localised excess above the expectedbackgroundisperformedinmultiplemμμ binsoftheSR