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Searches for lepton-flavour-violating decays of the Higgs boson in √s = 13 TeV pp collisions with the ATLAS detector

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Physics Letters B 800 (2020) 135069

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Searches for lepton-flavour-violating decays of the Higgs boson in

s = 13 TeV pp collisions with the ATLAS detector

.TheATLASCollaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received13July2019

Receivedinrevisedform27August2019 Accepted13September2019

Availableonline4November2019 Editor:M.Doser

This Letter presents directsearches forlepton flavour violationin Higgs bosondecays, Heτ and Hμτ,performed with the ATLASdetector at the LHC. The searches are based on adata sample of proton–proton collisions ata centre-of-mass energy

s=13 TeV, corresponding to an integrated luminosityof36.1 fb1.NosignificantexcessisobservedabovetheexpectedbackgroundfromStandard Model processes. The observed (median expected) 95% confidence-level upper limits onthe lepton- flavour-violatingbranchingratiosare0.47% (0.34+00..1310%)and0.28% (0.37+00..1410%)forHeτandHμτ, respectively.

©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

1. Introduction

The search for processes beyond the Standard Model (SM) is one of the main goals of the Large Hadron Collider (LHC) pro- gramme at CERN. A possible sign of such processes is lepton flavour violation (LFV) in decays of the Higgs boson [1,2]. Many beyond-SMtheories predict LFV decaysof the Higgsboson, such assupersymmetry [3,4], other modelswithmore thanone Higgs doublet [5,6], composite Higgs models [7], models with flavour symmetries [8] or warped extra dimensions [9–11] models and others [12,13].

In this Letter, searches for LFV decays of the Higgs boson, Heτ andHμτ,attheLHCwiththeATLASexperimentare presented.Studiesare basedonproton–proton(pp)collisiondata recordedin 2015–2016 at a centre-of-mass energy

s=13 TeV.

Thedatasetcorrespondstoanintegratedluminosityof36.1 fb1. PreviousATLASsearches [14,15] placedanupperlimitof1.04%

(1.43%)onthe Heτ (Hμτ)branchingratio(B)witha95%

confidencelevel(CL)usingRun 1datacollectedat

s=8 TeV,cor- respondingtoanintegratedluminosityof20.3 fb1.TheCMSCol- laborationrecentlyprovided95%CLupperlimitsonthesebranch- ingratiosof0.61%and0.25%,respectively,usingdatacollectedat

s=13 TeV,withanintegratedluminosityof35.9 fb1 [16].

Thesearchespresentedhereinvolvebothleptonic(τ→ νν¯1) andhadronic(τhadrons+ν)decaysof τ-leptons,denoted τ and τhad respectively. The dilepton final state τ only consid- erspairsofdifferent-flavourleptons.Same-flavourleptonpairsare

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

1 Unlessexplicitly mentionedotherwise, leptons(denotedbyor )referto electronsormuons.

rejected due to the large lepton pair-production Drell-Yan back- ground.Twochannelsareconsideredforeachofthetwosearches:

eτμ and eτhad for the Heτ search, μτe and μτhad for the Hμτ search. The analysisisdesignedsuch that anypotential LFVsignal overlapbetweenthe Heτ and Hμτ searchesis negligible.Manymethodsarereusedfromthemeasurementofthe Higgsbosoncross-sectionintheHτ τ finalstate [17].

The ATLAS detector2 is described in Refs. [18–20]. It con- sists of an inner tracking detector covering the range |η|<2.5, surrounded by a superconducting solenoid providing a 2T ax- ialmagneticfield,high-granularityelectromagnetic(|η|<3.2)and hadroniccalorimeters (|η|<4.9), anda muon spectrometer(MS) which coversthe range|η|<2.7 and includes fasttriggercham- bers(|η|<2.4)andsuperconductingtoroidalmagnets.

2. Simulationsamples

SamplesofMonteCarlo(MC)simulatedeventsareusedtoop- timizetheeventselection,andtomodelthesignal andseveralof the background processes. The samples were produced with the ATLAS simulationinfrastructure [21] usingthefull detectorsimu- lationperformedbythe Geant4 [22] toolkit.TheHiggsbosonmass wassettomH=125 GeV [23].The fourleadingHiggsbosonpro- ductionmechanismsareconsidered:thegluon–gluonfusion(ggF), vector-boson fusion (VBF) and two associated production modes

2 ATLASuses aright-handedcoordinatesystemwith itsoriginat thenominal interactionpointinthecentreofthedetectorandthez-axisalongthebeampipe.

Theazimuthalangleφrunsaroundthebeampipe,thepseudorapidityisdefinedin termsofthepolarangleθasη≡ −ln tan(θ/2).Angulardistanceintheηφspace isdefinedas R

( η)2+ ( φ)2. https://doi.org/10.1016/j.physletb.2019.135069

0370-2693/©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.

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Table 1

Generatorsusedtodescribethesignalandbackgroundprocesses,partondistributionfunction(PDF)setsforthehardprocess,and modelsusedfor partonshowering, hadronizationandtheunderlyingevent(UEPS).Theordersofthetotalcross-sectionsusedtonormalizetheeventsarealsogiven.MoredetailsaregiveninRef. [17].

Process Generator PDF UEPS Cross-section order

ggF Powheg-Box v2 [26–30] NNLOPS [31] PDF4LHC15 [32] NNLO Pythia 8.212 [25] N3LO QCD + NLO EW [33–36]

VBF Powheg-Box v2 MiNLO [30] PDF4LHC15 NLO Pythia 8.212 NNLO QCD + NLO EW [37–39]

W H, Z H Powheg-Box v2 MiNLO PDF4LHC15 NLO Pythia 8.212 NNLO QCD + NLO EW [40–42]

W/Z+jets Sherpa 2.2.1 [43] NNPDF30NNLO [44] Sherpa 2.2.1 [45] NNLO [46,47]

V V/Vγ Sherpa 2.2.1 NNPDF30NNLO Sherpa 2.2.1 NNLO

tt¯ Powheg-Box v2 [26–28,48] CT10 [49] Pythia 6.428 [50] NNLO+NNLL [51]

Single t Powheg-Box v1 [52,53] CT10 Pythia 6.428 NLO [54–56]

(W H , Z H ),whiletheothersgivenegligiblecontributions andare ignored.Thecross-sectionsofallHiggsbosonproductionprocesses werenormalizedtotheSM predictions [24].TheLFVHiggsboson decaysaswell asthe Hτ τ andHW W background decays weremodelledwithPythia 8 [25].Otherbackgroundprocessesin- volve electroweak productionof W/Z bosons via VBF, Drell–Yan productionof W/Z inassociation withjet(s) as well asdiboson, single top-quarkandtop-quarkpair(t¯t)production.TheMC gen- eratorsusedfortheSM Hτ τ cross-sectionmeasurement [17]

werealsoemployedhereforallbackgroundcomponents.Thegen- eratorsandpartonshower modelsusedtosimulatedifferentpro- cessesaresummarizedinTable1.

3. Objectreconstruction

The correct identification of H→ τ events requires recon- struction of severaldifferent objects (electrons, muons, and jets, includingthoseinitiatedbyhadronicdecaysof τ-leptons)andthe missing transverse momentum pmissT , whose magnitude is called EmissT .

Electrons are reconstructed by matching tracks in the in- ner detector to clusteredenergy deposits in the electromagnetic calorimeter [57]. Loose likelihood-based identification [58], pT>

15 GeV and fiducial volume requirements (|η|<2.47, excluding thetransitionregionbetweenthebarrelandtheendcapcalorime- ters 1.37<|η|<1.52) are applied.Medium identification, corre- sponding to an efficiencyof 87% at pT=20 GeV, is imposed for thebaselineelectronselection.

Muonsare identified by tracks reconstructed inthe inner de- tectorandmatchedto tracksintheMS. Looseidentification [59], pT>10 GeV and |η|<2.5 requirements are applied. Medium identification(efficiencyof96.1%formuonswith pT>20 GeV) is imposedforthebaselinemuonselection.

Isolation criteriaexploiting calorimeter and track-based infor- mation are applied to both electrons and muons. The gradient working point is used, featuring an efficiency of 90% (99%) ob- tainedforleptonswithpT>25 GeV (60 GeV)originatingfromthe Z→ process [58,59].

Jetsare reconstructed using the anti-kt algorithm [60] as im- plemented by the FastJet [61] package.The algorithm is applied to topologicalclustersof calorimetercells [62] with aradius pa- rameter R=0.4. Only jets with pT>20 GeV and |η|<4.5 are considered.Jetsfromother pp interactionsinthesameandneigh- bouringbunch crossings(pile-up)are suppressedusingjet vertex tagger(JVT) algorithms [63,64]. Jetscontaining b-hadrons (b-jets) areidentified by theMV2c20algorithm [65,66] in thecentral re- gion (|η|<2.4). A working point corresponding to 85% average efficiencydetermined forb-jets int¯t simulatedeventsis chosen, rejectionfactorsare2.8and28againstc-jetsandlight-flavourjets respectively.

The reconstruction of the object formed by the visible prod- uctsofthe τhad decay(τhad-vis)beginsfromjetsreconstructedby

the anti-kt jet algorithm with a radius parameter R=0.4. Infor- mation fromtheinnerdetectortracksassociatedwiththeenergy deposits in the calorimeteris incorporatedin the reconstruction.

Only τhad-vis candidates with pT>20 GeV and|η|<2.5 arecon- sidered.3 One or three associated tracks with an absolute total charge |q|=1 are required. An identification algorithm [67,68]

based on boosted decisiontrees (BDT) [69–71] is used to reject

τhad-vis candidates arising from misidentification of jets or from decays of hadrons with b- or c-quark content. Unless otherwise indicated, atightidentification(ID)workingpoint isusedforthe

τhad-vis, corresponding to an efficiency of 60% (45%) for 1-prong (3-prong)candidates.Jetscorresponding toidentified τhad-vis can- didates are removed from the jet collection. The τhad-vis candi- dates withonetrackoverlappingwithanelectroncandidatewith high ID score,as determined by a multivariate (MVA) approach, are rejected. Leptonic τ-decays are reconstructed as electrons or muons.

Events considered in the analysis are triggered with single- electron or single-muon triggers. The pT thresholds depend on theisolationrequirementanddata-takingperiod [72,73].Thelow- est trigger thresholds correspond to 2527 GeV (electrons) and 2127 GeV (muons).

4. Eventselectionandcategorization

Events selected in the τ channel contain exactly one elec- tronandonemuonofopposite-sign(OS)charges.Similarlyinthe

τhad channel, alepton anda τhad-vis ofOScharges arerequired, andevents withmorethan one baselinelepton are rejected. The selection criteriaaresummarizedinTable2fortheanalysiscate- goriesaswell asthecontrolregions(CRs),whicharedescribedin Section5.

Intheτ channel,1 and2denotetheleadingandsublead- ing lepton in pT,respectively. Events wheretheleading lepton is anelectron(muon)areusedinthesearchforHeτμ (Hμτe).

A requirement on the dilepton invariant mass, equal to the in- variantmassoftheleptonandthevisible τ-decayproducts,mvis, reducesbackgroundswithtopquarks,andthecriterionappliedto the track-to-cluster pT ratioofthe electronreducesthe Zμμ

background wherea muon deposits a large amountof energyin theelectromagneticcalorimeterandismisidentifiedasanelectron in the μτe channel. The contribution fromthe Hτ τ decayis reducedbytheasymmetricpT selectionofthetwoleptons.

In the τhad channel, the criterion based on the azimuthal separations of lepton–EmissT and τhad-vis–EmissT ,



i=,τhad-viscos φ (i,EmissT ), reduces the W + jets background whereas therequirementon| η(, τhad-vis)|reducesbackgrounds withmisidentified τhad-viscandidates.

Forbothchannelsofeachsearch,ab-vetorequirementreduces the single-top-quarkandt¯t backgrounds.Eventsare furthercate-

3 Thetransitionregioninηisexcluded,similarlytoelectrons.

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The ATLAS Collaboration / Physics Letters B 800 (2020) 135069 3

Table 2

Baselineeventselectionandfurthercategorizationfortheτ andτhadchannels.Thesamecriteriaarealso usedforthecontrolregion(CR)definitionsintheτ channel(Section5),butonerequirementofthebaseline selectionisinvertedtoachieveorthogonaleventselection.ThereisnoCRintheτhadchannel.

Selection τ τhad

Baseline

exactly 1e and 1μ, OS exactly 1and 1τhad-vis, OS pT1>45 GeV pT>27.3 GeV pT2>15 GeV pτThad-vis>25 GeV,|ητhad-vis| <2.4 30 GeV<mvis<150 GeV 

i=,τhad-vis

cos φ(i,EmissT ) >0.35 peT(track)/peT(cluster) <1.2 (μτeonly) | η(,τhad-vis)| <2

b-veto (for jets with pT>25 GeV and|η| <2.4)

VBF

Baseline

2 jets, pjT1>40 GeV, pjT2>30 GeV

| η(j1,j2)| >3, m(j1,j2) >400 GeV

pτThad-vis>45 GeV

Non-VBF

Baseline plus fail VBF categorization

mT(1,EmissT ) >50 GeV

mT(2,EmissT ) <40 GeV

| φ(2,EmissT )| <1.0

pτT/pT1>0.5

Top-quark CR inverted b-veto:

VBF and non-VBF 1 b-tagged jet (pT>25 GeV and|η| <2.4) Zτ τCR inverted pT1requirement:

VBF and non-VBF 35 GeV<pT1<45 GeV

gorizedintoVBF(withafocusontheVBFproductionoftheHiggs boson)andnon-VBFcategories. TheVBFselectionisbasedonthe kinematics ofthe two jets withthe highest pT,where j1 andj2 denotetheleadingandsubleadingjetinpT,respectively.Thevari- ablesm(j1,j2) and η(j1,j2) stand forthe invariant mass and η

separationofthesetwojets.Thenon-VBFcategorycontainsevents failingtheVBFselection.Inthedileptonchannel,additionalselec- tioncriteriaareappliedtofurtherrejectbackgroundeventsinthis category.ThesecriteriaarealsolistedinTable2,wheremT stands forthe transversemass4 ofthetwo objectslisted inparentheses, and T represents the magnitude of the vector sum of pT2 and EmissT .Therequirementon T/pT1 reducesthebackgroundarising fromjetsmisidentifiedasleptons.TheVBFandnon-VBFcategories ineachoftheτ andτhad channelsgiveriseto foursignalre- gionsineachsearch.

TheanalysisexploitsBDTalgorithmstoenhancethesignalsep- arationfrom thebackground inthe individual searches,channels andcategories.Thecomponentsofthefour-momentaoftheanaly- sisobjectsaswellasderivedeventvariables(e.g.invariantmasses andangular separations) are the input variables ofthe BDT dis- criminant. Correlations between these input variables have been carefullychecked,highly correlatedvariables have beenremoved and the remaining ones are ranked according to their discrimi- nation power [74,75]. The list of variables isthen optimized, re- movingthelowest-rankedvariableswithmarginalcontributionto the sensitivity. The final list of variables is presented in Table 3 foreachchannelandcategory.TheinvariantmassoftheHiggsbo- sonreconstructedundertheH→ τ decayhypothesisexhibitsthe highestsignal-to-backgroundseparationpowerandithelpstodis- tinguishLFVsignalfromHτ τ andHW W backgrounds.For theτ channeltheinvariantmassisreconstructedwiththeMMC algorithm [76] and isdenoted bymMMC; forthe τhad channel it isreconstructedwiththe collinearapproximation [76] andis de- notedbymcoll.The analysisCRsare used tovalidate the levelof

4 ThetransversemassoftwoobjectsisdefinedasmT=

2pT 1pT 2(1cos φ), wherepT iaretheindividualtransversemomentaand φistheanglebetweenthe twoobjectsintheazimuthalplane.

agreement between data and simulated distributions of the BDT scoreandinputvariables,aswellastheircorrelations.

5. Backgroundmodelling

Themostsignificantbackgroundsinthesearcharefromevents with Zτ τ decaysorwith(singleorpair-produced)topquarks, especiallyintheτ channel,aswellasfromeventswithmisiden- tified objects, which are estimatedusing data-driven (d.d.) tech- niques.Therelativecontributionfrommisidentified objectstothe total background yield is 5–25% in the τ channel and 25–45%

in the τhad channel, depending on the search andthe analysis category. The shapes of distributions fromthe Zτ τ andtop- quark (single-top-quark and tt)¯ processes are modelled by sim- ulation in both the τ and τhad decay channels. In the τ

channel,the relativecontributionsof Zτ τ andtop-quarkpro- duction processes are20–35% and20–55%, respectively; thetop- quark background dominates in the VBF category. In the τhad

channel, the top-quark background fraction is 1–10%, while the Z τ τ process contributes to 45–55% of the total background.

The individual contributionsare listed inTables 4and 5. Smaller backgroundcomponentsare alsomodelledby simulationandare grouped together: Z μμ, diboson production, Hτ τ and HW W .

GoodmodellingofthebackgroundisdemonstratedinFig.1for a selectionof importantBDT input variables.Detailsof theback- groundestimationtechniquesaregivenbelow.

5.1. τchannel

TwosetsofCRs,asdefinedinTable2,areusedtoconstrainthe normalizationof Zτ τ andtop-quarkbackgroundcomponents.

These CRs inherit their definitions from the corresponding anal- ysiscategory but invertone requirementto ensure orthogonality with the nominal selection. The normalization factors are deter- mined during the statistical analysis by fitting the event yields in all signal andcontrol regions simultaneously. Foreach search, separate Z τ τ normalizationfactors are usedfor theVBF and non-VBF categories. In the case of the top-quark background, in

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