Search for lepton flavour violating decays of the Higgs boson to eτ and eμ in proton-proton collisions at √s=8 TeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Search

for

lepton

flavour

violating

decays

of

the

Higgs

boson

to

e

τ

and

e

μ

in

proton–proton

collisions

at

√

s

=

8 TeV

.

The

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received13July2016

Receivedinrevisedform21August2016 Accepted14September2016

Availableonline6October2016 Editor:M.Doser Keywords: CMS Physics Higgs Lepton-flavour-violation

A direct search for lepton flavour violating decays of the Higgs boson (H) in the H →e

τ

and H →e

μ

channels is described. The data sample used in the search was collected in proton–proton collisions at √s=8 TeV with the CMS detector at the LHC and corresponds to an integrated luminosity of 19.7 fb−1_{. No evidence is found for lepton flavour violating decays in either final state. Upper limits}

on the branching fractions, B(H →e

τ

)<0.69% and B(H →e

μ

)<0.035%, are set at the 95% confidence level. The constraint set on B(H →e

τ

)is an order of magnitude more stringent than the existing indirect limits. The limits are used to constrain the corresponding flavour violating Yukawa couplings, absent in the standard model.

©2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3_.

1. Introduction

The discovery of the Higgs boson [1–3] has generated great interest in exploring its properties. In the standard model (SM), lepton flavour violating (LFV)decays of the Higgsboson are for-bidden. Such decays can occur naturally in models with more than one Higgs boson doublet [4]. They also arise in supersym-metric models [5–11], composite Higgs models [12,13], models with flavour symmetries [14], Randall–Sundrum models [15–17], and others [18–26]. The CMS Collaboration has recently pub-lished a search in the H

→

μτ

channel [27] showing an excess of data with respect to the SM background-only hypothesis at

mH

=

125 GeV witha significanceof2

.

4 standarddeviations (

σ

). Aconstraintisseton thebranchingfraction

B(

H

→

μτ

)

<

1

.

51% at95%confidencelevel(CL),whilethebestfitbranchingfractionis

B(

H

→

μτ

)

= (

0

.

84+₋0_0..39₃₇

)

%. TheATLASCollaborationfinds a devi-ationfromthebackgroundexpectationof1

.

3

σ

significanceinthe H

→

μτ

channelandsets an upperlimit of

B(

H

→

μτ

)

<

1

.

85% at 95% CL with a best fit branching fraction of

B(

H

→

μτ

)

=

(

0

.

77

±

0

.

62

)

% [28]. To date, no dedicated searches have been published forthe H

→

e

μ

channel. The ATLAS Collaboration re-cently reported searches for H

→

e

τ

and H

→

μτ

, finding no significant excessofeventsover thebackgroundexpectation. The searches in channels with leptonic taudecays are sensitive only to a difference between

B(

H

→

e

τ

)

and

B(

H

→

μτ

)

. These are

 _{E-mailaddress:cms-publication-committee-chair@cern.ch.}

combined with the searches in channels with hadronic tau de-cays to set limitsof

B(

H

→

e

τ

)

<

1

.

04%,

B(

H

→

μτ

)

<

1

.

43% at 95% CL [29]. There are alsoindirect constraints. The presence of LFV Higgs boson couplings allows,

μ

→

e,

τ

→

μ

, and

τ

→

e to proceed via a virtual Higgs boson [30,31]. The experimental limits on these decays have been translated into constraints on

B(

H

→

e

μ

)

,

B(

H

→

μτ

)

and

B(

H

→

e

τ

)

[32,33].The nullresult for

μ

→

e

γ

[34]stronglyconstrains

B(

H

→

e

μ

)

<

O(

10−8

)

. How-ever, the constraint

B(

H

→

e

τ

)

<

O(

10%

)

ismuch lessstringent. Thiscomesfromsearchesforrare

τ

decays[35]such as

τ

→

e

γ

, andthemeasurementoftheelectronmagneticmoment.Exclusion limitsontheelectricdipolemomentoftheelectron[36]also pro-videcomplementaryconstraints.

This letterdescribesa search for LFVdecays ofthe Higgs bo-son with mH

=

125 GeV, based on proton–proton collision data recorded at

√

s

=

8 TeV withthe CMSdetectorattheCERN LHC, correspondingtoanintegratedluminosityof19.7 fb−1_.Thesearch is performed in three decay channels, H

→

e

τ

μ, H

→

e

τ

h, and H

→

e

μ

, where

τ

μ and

τ

h correspond to muonic and hadronic decay channels of tau leptons, respectively. The decay channel, H

→

e

τ

e, is not considered due to the large background contri-butionfromZ

→

ee decays.Theexpectedfinalstatesignaturesare verysimilartotheSMH

→

τ

e

τ

handH

→

τ

e

τ

μ decays,studiedby

CMS[37,38] andATLAS [39],butwithsome significantkinematic differences. The electron in the LFV H

→

e

τ

decay is produced promptly, andtends to havea largermomentum than intheSM H

→

τ

e

τ

hdecay.IntheH

→

e

μ

channel,mHcanbemeasuredwith goodresolutionduetotheabsenceofneutrinos.

http://dx.doi.org/10.1016/j.physletb.2016.09.062

0370-2693/©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

This letter is organized as follows. After a description of the CMSdetector(Section2) andof thecollision dataandsimulated samplesused intheanalysis(Section3),theeventreconstruction isdescribed in Section 4. The eventselection andtheestimation ofthebackgroundanditscomponentsaredescribedseparatelyfor thetwoHiggsdecaymodesH

→

e

τ

andH

→

e

μ

inSections5and

6.TheresultsarethenpresentedinSection7.

2. TheCMSdetector

AdetaileddescriptionoftheCMSdetector,togetherwitha def-inition ofthecoordinate systemusedandthe relevantkinematic variables,canbe foundinRef.[40].Themomentaofcharged par-ticlesaremeasuredwithasiliconpixelandstriptrackerthat cov-ersthe pseudorapidity range

|

η

|

<

2

.

5, in a 3.8 Taxial magnetic field. Alead tungstate crystalelectromagnetic calorimeter (ECAL) and a brass and scintillator hadron calorimeter, both consisting of a barrel section and two endcaps, cover the pseudorapidity range

|

η

|

<

3

.

0.Asteelandquartz-fibre Cherenkovforward detec-torextendsthe calorimetriccoverage to

|

η

|

<

5

.

0. Theoutermost componentoftheCMSdetectoristhemuonsystem,consistingof gas-ionizationdetectorsplacedinthesteelflux-returnyokeofthe magnet to identify the muons traversing the detector. The two-levelCMStriggersystemselects eventsofinterestforpermanent storage.Thefirsttriggerlevel,composedofcustomhardware pro-cessors,uses informationfromthe calorimetersandmuon detec-torsto selectevents inlessthan 3.2 μs. Thesoftware algorithms ofthe high-level trigger, executed ona farm of commercial pro-cessors,reducetheeventratetolessthan1 kHzusinginformation fromalldetectorsubsystems.

3. Collisiondataandsimulatedevents

ThetriggersfortheH

→

e

τ

μ and H

→

e

μ

analysesrequirean

electronanda muoncandidate.The trigger forH

→

e

τ

h requires asingleelectron.Moredetailsonthetriggerselectionaregivenin Sections5.1and

6.1

,fortheH

→

e

τ

andH

→

e

μ

channels respec-tively.Simulatedsamplesofsignalandbackgroundeventsare pro-ducedwith severalevent generators. The CMSdetector response ismodelledusing Geant4 [41].TheHiggsbosonsareproducedin proton–protoncollisions predominantlybygluonfusion(GF) [42], butalsobyvectorbosonfusion(VBF)[43]andinassociationwith a W or Z boson [44]. The H

→

e

τ

decay sample is produced with pythia 8.176[45]usingtheCTEQ6Lpartondistribution func-tions (PDF). The H

→

e

μ

decaysample is produced with pythia 6.426 [46] usingtheCT10 partondistributionfunctions[47].The SMHiggsbosonsamplesaregeneratedusing powheg 1.0[48–52], withCT10partondistributionfunctions,interfacedto pythia 6.426. The MadGraph 5.1.3.30[53]generatorisusedforZ

+

jets,W

+

jets, topanti-topquarkpairproductiontt,anddibosonproduction,and powheg forsingle top quark production.The powheg and Mad-Graph_{generatorsareinterfacedto pythia 6.426forpartonshower} andhadronization.The pythia parametersfortheunderlyingevent descriptionare settothe Z2*tune.The Z2*tuneis derivedfrom theZ1tune [54],which usesthe CTEQ5L partondistributionset, whereasZ2*adoptsCTEQ6L.Duetothehighluminositiesattained during data-taking,manyevents havemultipleproton–proton in-teractionsper bunch crossing (pileup).All simulatedsamples are reweightedtomatchthepileupdistributionobservedindata.

4. Eventreconstruction

Datawerecollectedatanaveragepileupof21interactionsper bunch crossing. The tracking system is able to separate collision

vertices as close as 0.5 mm to each other along the beam di-rection [55]. The primary vertex, assumed to correspond to the hard-scattering process, is the vertex for which the sum of the squared transverse momentum p2_T of all the associated tracks is thelargest.Thepileupinteractionsalsoaffecttheidentificationof mostofthephysicsobjects,suchasjets,andvariablessuchas lep-tonisolation.

A particle-flow (PF)algorithm [56–58] combines the informa-tion from all CMS subdetectors to identify and reconstruct the individual particles emerging from all interactions in the event: chargedandneutralhadrons,photons,muons,andelectrons.These particles are then required to be consistent with the primary vertex and used to reconstruct jets, hadronic

τ

decays, quantify the isolation of leptons and photons and reconstruct E



miss_T . The missing transverse energy vector,



Emiss_T , is defined as the nega-tive of the vector sum of the pT of all identified PF objects in theevent[59].Its magnitudeis referredto as Emiss

T .The variable



R

=



(

η

)

2

_{+ (φ)}

2_,where

_φ

_{is theazimuthal co-ordinate, is} usedto measurethe separationbetweenreconstructedobjects in thedetector.

Electron reconstruction requires the matching of an energy clusterintheECALwithatrackinthesilicontracker[60].Electron candidates are accepted in the range

|

η

|

<

2

.

5, with the excep-tion of the region 1

.

44

<

|

η

|

<

1

.

56 where service infrastructure for the detector is located. Electron identification uses a multi-variate discriminant that combines observables sensitive to the amount ofbremsstrahlungalong the electron trajectory, the geo-metricalandmomentummatchingbetweentheelectrontrajectory andassociatedclusters, andshower-shapeobservables.Additional requirementsare imposed toremove electrons produced by pho-tonconversions.The electronenergyiscorrectedforimperfection ofthereconstruction usinga regressionbasedonaboosted deci-siontree[61].

Muon candidatesare obtainedfromcombinedfitsoftracks in the tracker andmuon detectorseeded by tracksegments in the muon detector alone, including compatibility with small energy depositions in the calorimeters. Identification is based on track qualityandisolation.Themuonmomentumisestimatedwiththe combined fit. Any possible bias in the measured muon momen-tumisdeterminedfromthepositionoftheZ

→

μμ

masspeakas a functionofmuonkinematic variables,andasmallcorrection is obtainedusingtheproceduredescribedinRef.[62].

Hadronically decaying

τ

leptons arereconstructed and identi-fied using an algorithm [63] that selects the decay modes with onechargedhadronanduptotwoneutralpions,orthreecharged hadrons. A photon froma neutral–pion decaycan convertin the tracker material into an electron–positron pair, which can then radiate photons. These particles give rise to severalECAL energy deposits at the same

η

value but separated in

φ

. They are re-constructed asseveral photons by the PF algorithm. To increase theacceptancefortheseconvertedphotons, theneutralpionsare identifiedbyclusteringthereconstructedphotonsinnarrowstrips alongthe

φ

direction.Thechargeof

τ

hcandidatesisreconstructed by summingthechargesofallparticlesincludedinthe construc-tionofthecandidate,exceptfortheelectrons containedinstrips. Dedicateddiscriminatorsvetoagainstelectronsandmuons.

Jetsmisidentifiedaselectrons,muonsortausaresuppressedby imposingisolationrequirements,summingtheneutralandcharged particle contributions in cones of



R about the lepton. The en-ergydepositedwithintheisolationconeiscontaminatedbyenergy frompileup andthe underlyingevent. The effectofpileup is re-ducedby requiring thetracks considered inthe isolation sum to be compatiblewithoriginatingfromtheproductionvertexofthe lepton.The contributionto theisolation frompileupandthe un-derlyingeventissubtractedonanevent-by-eventbasis.Inthecase

ofelectrons,thiscontributionisestimatedfromtheproductofthe measured energy density

ρ

for the event, determined using the

ρ

medianestimatorimplementedin FastJet[64],andan effective areacorrespondingtotheisolationcone.Inthecaseofmuonsand hadronicallydecaying

τ

leptons,itisestimatedonastatistical ba-sisthroughthemodified

β

correctiondescribedinRef.[63].

Jetsarereconstructedfromalltheparticlesusingtheanti-kTjet clusteringalgorithm [65]implementedin FastJet, witha distance parameterof



R

=

0

.

5.Thejetenergiesarecorrectedby subtract-ingthecontributionofparticlescreatedinpileupinteractionsand in the underlying event[66]. Particlesfrom different pileup ver-tices can be clusteredintoa pileup jet,or significantly overlapa jet fromthe primary vertex belowthe selected jet pT threshold. Thesejetsareidentifiedandremoved[67].

5. H

→

e

τ

analysis 5.1. Eventselection

The H

→

e

τ

h selection begins by requiring an eventrecorded withasingle electrontrigger(pe_T

>

27 GeV,

|

η

e

_|

_<

₂

_.

_{5).The H}

_→

e

τ

μ channelrequiresamuon–electrontrigger(peT

>

17 GeV,

|

η

e

|

<

2

.

5, pμ_T

>

8 GeV,

|

η

μ

|

<

2

.

4). Thetriggersalsoapplyloose identi-ficationandisolationrequirementstotheleptons.

A loose selection is then made for both channels. Electron, muon andhadronic tauleptoncandidates arerequired tobe iso-latedand tolie inthe pseudorapidity ranges where they can be well reconstructed;

|

η

e

_|

_<

₁

_.

_{44 or 1}

_.

₅₇

_<

_|

_η

e

_|

_<

₂

_.

_30,

_|

_η

μ

_|

_<

₂

_.

₁

and

|

η

τh

|

_<

₂

_.

_{3,respectively.Leptonsarealsorequiredtobe} com-patiblewiththeprimaryvertexandto beseparatedby



R

>

0

.

4 fromanyjetin theeventwith pT

>

30 GeV.The H

→

e

τ

μ

chan-nel then requires an electron (pe_T

>

40 GeV) and an oppositely charged muon (pμ_T

>

10 GeV) separated by



R

>

0

.

1. Events in this channel with additional muons (pT

>

7 GeV) or electrons (pT

>

7 GeV) are also rejected. The H

→

e

τ

h channel requires an electron (pe_T

>

30 GeV) and an oppositely charged hadronic taulepton(pτh

T

>

30 GeV). Events inthischannel withadditional muons(pT

>

5 GeV),electrons(pT

>

10 GeV),orhadronictau lep-tons(pT

>

20 GeV)arerejected.

Theeventsarethendividedintocategorieswithineachchannel accordingtothenumberofjetsintheevent. Jetsarerequiredto passidentificationcriteria,havepT

>

30 GeV,andlieintheregion

|

η

|

<

4

.

7. The 0-jet and1-jet categoriescontain events primarily producedbyGF.The2-jetcategoryisdefinedtoenrichthe contri-butionfromeventsproducedviatheVBFprocess.

The main observable used to discriminate betweenthe signal and the background is the collinear mass, Mcol, which provides an estimate of MH usingthe observed decayproducts. It is con-structedusing thecollinearapproximation basedonthe observa-tionthat,sincemH



Mτ ,the

τ

decayproductsarehighlyLorentz boostedinthedirectionofthe

τ

[68].Theneutrinomomentacan be approximated to havethe same directionasthe other visible decayproductsofthe

τ

(

τ

vis_{) andthecomponentofthe}



_Emiss

T in the directionofthe visible

τ

decay products isused to estimate the transverse component of the neutrino momentum (pν_T,est). The collinear mass can then be derived from the visible mass ofthe

τ

–e system (Mvis) as Mcol

=

Mvis

/



xvisτ , where xvisτ isthe

fractionof energycarriedby the visible decayproducts ofthe

τ

(x_τvis

=

pτ_Tvis

/(

pτ_Tvis

+

pν_T,est

)

).

Fig. 1showstheobservedMcoldistributionandestimated back-groundsfor each category andchannel, after the looseselection. Thesimulatedsignalfor

B(

H

→

e

τ

)

=

100% isshown. The princi-palbackgroundsareestimatedwithcollisiondatausingtechniques described in Section 5.2. There is good agreement between the

observed distributionsandthecorresponding background estima-tions. The agreement is similar in all of the kinematic variables that are subsequently used to suppress backgrounds. The analy-sis is subsequentlyperformed blinded by using a fixed selection andcheckingtheagreement betweenrelevant observedand sim-ulateddistributionsoutsidethesensitiveregion100 GeV

<

Mcol

<

150 GeV.

Next, a set of kinematic variables is defined, and the event selection criteria are set to maximise the significance S

/

√

S

+

B, whereSandBaretheexpectedsignalandbackgroundeventyields inthemasswindow100 GeV

<

Mcol

<

150 GeV.Thesignalevent yield corresponds to the SM Higgs boson production cross sec-tionatmH

=

125 GeV with

B(

H

→

e

τ

)

=

1%.Theselectioncriteria foreach categoryandchannel are giveninTable 1.The variables usedare:theleptontransversemomentap_Twith



=

e

,

μ

,

τ

h; az-imuthal angles between the leptons

φ

_

p1_T−p2_T ; azimuthal angle

betweentheleptonandtheEmiss

T vector

φ

p

T−EmissT ;thetransverse massM T

=



2p TETmiss

(

1

−

cos

φ

p T−EmissT

)

.

Events inwhichatleastone ofthejetsisidentified asarising from a b quark decayare vetoed using the combined secondary vertex (CSV) b-tagging algorithm [69]. To enhance the VBF con-tribution in the 2-jet category further requirements are applied. In the H

→

e

τ

h channel, events in this category are additionally required to have two jets separated by

|

η

|

>

2

.

3 and a dijet invariant mass Mj j

>

400 GeV. In the H

→

e

τ

μ channel, the

re-quirementsare

|

η

|

>

3 andMj j

>

200 GeV.

After the full selection, a binned likelihood is used to fit the distributions of Mcol forthe signalandthe background contribu-tions. The modified-frequentistCLs method[70,71] isusedto set upperboundsonthesignal strength

μ

,ordetermineasignal sig-nificance.

5.2. Backgroundprocesses

The contributions from the dominant background processes are estimatedusingcollision datawhilethe lesssignificant back-grounds are estimated usingsimulation. The largest backgrounds arefromZ

→

τ τ

decaysandfromW

+

jets andQCDmultijet pro-duction.Inthelatter,PFobjects(predominantlyjets),are misiden-tifiedasleptons.

5.2.1. Z

→

τ τ

background

TheZ

→

τ τ

backgroundcontributionisestimatedusingan em-bedding technique [38,72]. First, a sample of Z

→

μμ

events is selected from collision data using the loosemuon selection. The muons are then replaced with simulated

τ

decays reconstructed withthe PFalgorithm.Thus, thekey features oftheevent topol-ogysuchasjetmultiplicity,instrumentalsourcesof Emiss_T ,andthe underlying event are takendirectly fromcollision data.Only the

τ

leptondecaysaresimulated.Thenormalizationofthesampleis obtainedfromsimulation.Thetechniqueisvalidatedbycomparing thecollinearmassdistributionsobtainedfromtheZ

→

τ τ

simula-tion andtheembedding techniqueapplied toasimulatedsample ofZ

→

μμ

events.Ashiftof2%inthemasspeakoftheembedded sample relativetosimulationisobserved.Thisshiftreflectsabias in theembedding technique,which doesnot take thedifferences between muonsand taus in final-state radiationof photons into account, andis corrected for. Identification and isolation correc-tions obtainedfromthecomparisonareappliedtotheembedded sample.

5.2.2. Misidentifiedleptonbackground

Themisidentifiedleptonbackgroundisestimatedfromcollision data by defining a sample with the same selection as the

sig-Fig. 1. Comparisonoftheobservedcollinearmassdistributionswiththebackgroundexpectationsafterthelooseselectionrequirements.Theshadedgreybandsindicatethe totalbackgrounduncertainty.TheopenhistogramscorrespondtotheexpectedsignaldistributionsforB(H→eτ)=100%.TheleftcolumnisH

→

eτμandtherightcolumn isH→eτh;theupper,middleandlowerrowsarethe0-jet,1-jetand2-jetcategories,respectively.

Table 1

Eventselectioncriteriaforthe kinematicvariables afterapplyinglooseselection requirements.

Variable [GeV]

H→eτμ H→eτh

0-jet 1-jet 2-jet 0-jet 1-jet 2-jet

pe T >50 >40 >40 >45 >35 >35 pμ_T >15 >15 >15 – – – pτh T – – – >30 >40 >30 MTμ – <30 <40 – – – Mτh T – – – <70 – <50 [radians] φpT,e−pT,τ_h – – – >2.3 – – φ_pT,μ−ETmiss <0.8 <0.8 – – – – φpT,e−pT,μ – >0.5 – – – – Table 2

Definitionofthesamplesusedtoestimatethe misiden-tifiedlepton ()background.They are definedby the chargeofthetwoleptonsandbytheisolation require-mentsoneach.Thedefinitionofnot-isolateddiffers be-tweenthetwochannels.

Region I Region II

±1 (isolated) ±1 (isolated)

∓2 (isolated) ±2 (isolated) Region III Region IV

±1 (isolated) ±1 (isolated)

∓2 (not-isolated) ±2 (not-isolated)

nalsample,butinvertingtheisolationrequirementsononeofthe leptons,toenrichthecontributionfromW

+

jets andQCD multi-jets.TheprobabilityforPFobjectstobemisidentifiedasleptonsis measuredusing anindependent collisiondataset,definedbelow, andthisprobabilityisappliedtothebackgroundenrichedsample tocomputethemisidentifiedleptonbackgroundinthesignal sam-ple.Thetechniqueisshownschematicallyin

Table 2

inwhichfour regionsaredefinedincludingthesignal (I)andbackground (III) en-richedregionsandtwocontrolRegions (II & IV), definedwiththe sameselectionsasRegions I & IIIrespectively,exceptwithleptons ofthesamecharge.

ThemisidentifiedelectronbackgroundisnegligibleintheH

→

e

τ

μ channel duetothehigh pT electronthreshold.The misiden-tifiedmuonbackgroundisestimatedwithRegion Idefinedasthe signalselectionwithanisolatedelectronandanisolatedmuonof oppositecharge.Region IIIisdefinedasthesignalselectionexcept themuonisrequirednottobeisolated.Smallbackgroundsources ofpromptleptonsare subtractedusingsimulation.The misidenti-fiedmuonbackgroundinRegion Iisthenestimatedbymultiplying theeventyieldinRegion IIIbyafactor fμ,where fμ istheratioof isolated tononisolatedmuons.It iscomputedonan independent collisiondatasampleofZ

→

μμ

+

X events,whereXisanobject identifiedasamuon,inbinsofmuon pT and

η

.Intheestimation of fμ,backgroundsourcesofthreepromptleptons,predominantly WZ and ZZ, are subtracted from the Z

→

μμ

+

X sample using simulation. The technique is validated usinglike-sign lepton col-lision datainRegions II andIV.In Fig. 2(left) the eventyieldin Region II iscompared tothe estimatefromscaling theRegion IV samplebythemeasuredmisidentificationrate.TheRegion II sam-ple isdominatedby misidentifiedleptons butalsoincludessmall contributionsoftrueleptonsarisingfromvectorbosondecays, es-timatedwithsimulatedsamples.

IntheH

→

e

τ

hchanneleitherleptoncandidatecanarisefroma misidentified PFobject,predominantlyinW

+

jets and QCD mul-tijet events,but also from Z

→

ee

+

jets and tt production. The misidentification rates fτ and fe are defined as the fraction of looselyisolated

τ

horelectroncandidatesthatalsopassatight iso-lationrequirement. Thisismeasured inZ

→

ee

+

X collision data events,whereXisanobjectidentifiedasa

τ

h ore. The misiden-tified

τ

h contribution is estimated with Region I defined as the signal selection. Region IIIisthe signal selectionexceptthe

τ

h is requiredtohavelooseandnottightisolation.Themisidentified

τ

h leptonbackgroundinRegion Iisthenestimatedbymultiplyingthe eventyieldinRegion IIIbyafactor fτ

/(

1

−

fτ

)

.The same proce-dureisusedtoestimatethemisidentifiedelectronbackgroundby defining Region Iasthesignal selectionandRegion IIIasthe sig-nalselectionbutwitha looseandnottightisolatedelectron,and scaling by fe

/(

1

−

fe

)

.To avoiddoublecounting, the eventyield in Region III,multiplied bya factor fe

/(

1

−

fe

)

×

fτ

/(

1

−

fτ

)

,is subtracted fromthe sumofmisidentifiedelectrons andtaus. The procedureisvalidatedwiththelike-signe

τ

samples.

Fig. 2

(right) shows the collision data in Region II compared to the estimate

Table 3

Thesystematicuncertaintiesintheexpectedeventyieldsinpercentagefortheeτhandeτμchannels.Alluncertaintiesaretreatedas corre-latedbetweenthecategories,exceptwhentwovaluesarequoted,inwhichcasethenumberdenotedbyanasteriskistreatedasuncorrelated betweencategories.

Systematic uncertainty H→eτμ H→eτh

0-jet 1-jet 2-jet 0-jet 1-jet 2-jet

Muon trigger/ID/isolation 2 2 2 – – –

Electron trigger/ID/isolation 3 3 3 1 1 2

Efficiency ofτh – – – 6.7 6.7 6.7

Z→τ τ background 3⊕5∗ 3⊕5∗ 3⊕10∗ 3⊕5∗ 3⊕5∗ 3⊕10∗

Z→μμ,ee background 30 30 30 30 30 30

Misidentified leptons background 40 40 40 30 30 30

Pileup 2 2 10 4 4 2

WW,WZ,ZZ+jets background 15 15 15 15 15 15

tt background 10 10 10⊕10∗ 10 10 10⊕33∗

Single top quark background 25 25 25 25 25 25

b-tagging veto 3 3 3 – – –

Luminosity 2.6 2.6 2.6 2.6 2.6 2.6

Table 4

TheoreticaluncertaintiesinpercentagefortheHiggsbosonproductioncrosssectionforeach productionprocessandcategory.Alluncertaintiesaretreatedasfullycorrelatedbetween cat-egoriesexceptthosedenotedbyanegativesuperscriptwhicharefullyanticorrelateddueto themigrationofevents.

Systematic uncertainty Gluon fusion Vector boson fusion 0-jet 1-jet 2-jet 0-jet 1-jet 2-jet Parton distribution function 9.7 9.7 9.7 3.6 3.6 3.6 Renormalization/factorization scale 8 10 30− 4 1.5 2 Underlying event/parton shower 4 5− 10− 10 <1 1−

derived from Region IV. The method assumes that the misiden-tificationrateinZ

→

ee

+

X eventsisthesameasintheW

+

jets andQCDprocesses.Tocheckthisassumption,themisidentification ratesare alsomeasured ina collisiondatacontrol sample ofjets comingfromQCDprocessesandfoundtobeconsistent.This sam-pleisthesameZ

→

ee

+

X sampleasabovebutwithoneofthe electroncandidatesrequiredtobenotisolatedandthe pT thresh-oldlowered.

5.2.3. Otherbackgrounds

Theleptonic decayof Wbosons fromtt pairs produces oppo-sitesigndileptons and Emiss_T . Thisbackground isestimatedusing simulatedtt events tocompute the Mcol distributionanda colli-siondatacontrolregionfornormalization.Thecontrolregionisthe 2-jetselectiondescribed inSection5.1,includingtheVBF require-ments,withtheadditionalrequirementthatatleastoneofthejets isb-taggedinordertoenhancethett contribution.Othersmaller backgrounds enter from SM Higgs boson production (H

→

τ τ

), WW,WZ, ZZ

+

jets, W

γ

(∗)

₊

_{jets processes,andsingle top quark}

production.Eachoftheseisestimatedusingsimulation[38].

5.3.Systematicuncertainties

Systematicuncertainties are implemented asnuisance param-eters in the signal and background fit to determine the scale of their effect. Some of these nuisance parameters affect only the backgroundandsignalnormalizations,whileothersalsoaffectthe shapeofthe Mcol distributions.

5.3.1. Normalizationuncertainties

Thevaluesofthesystematicuncertaintiesimplementedas nui-sance parameters in the signal and background fit are summa-rizedin Tables 3 and 4. The uncertainties inthe muon, electron and

τ

h selection efficiencies (trigger, identification, andisolation) are estimated using collision data samples of Z

→

μμ

,

ee

,

τ

μ

τ

h events[63,72]. The uncertainty in the Z

→

τ τ

background yield

comes fromthecross sectionuncertainty measurement(3% [73]) andfrom the uncertaintyin the

τ

identification efficiency when applyingtotheembeddedtechnique(5–10%uncorrelatedbetween categories).Theuncertaintiesintheestimationofthemisidentified leptonratecomefromthedifferenceinratesmeasuredindifferent collisiondatasamples(QCDmultijetandW

+

jets).Thesystematic uncertaintyinthepileupmodellingisevaluatedbyvaryingthe to-tal inelastic cross section by

±

5% [74]. The uncertainties in the production cross sectionsestimated from simulation are also in-cluded[38].

Uncertaintiesondibosonandsingletopproductioncorrespond to the uncertainties of the respective cross section measure-ments[75,76].A10%uncertaintyfromthecrosssection measure-ment

[77]

isappliedtotheyieldofthett background.Inthe2-jet categoriesanadditionaluncertainty(10%forH

→

e

τ

μ and33%for

H

→

e

τ

h)isconsideredcorrespondingtothestatisticaluncertainty ofthett backgroundyield.

There areseveral theoreticaluncertainties on theHiggs boson production cross section that depend on the production mech-anism and the analysis category, as reported in Table 4. These uncertainties affect both the LFV Higgs boson andthe SM Higgs bosonbackgroundandarefullycorrelated.Theuncertaintyinthe partondistribution functionis evaluatedby comparing theyields in each category, that span the parameter range of three differ-ent PDF sets, CT10 [47], MSTW [78], NNPDF [79] following the PDF4LHC[80] recommendation.Theuncertaintyduetothe renor-malization and factorization scales,

μ

R and

μ

F, is estimated by

scaling up and down by a factor of two relative to their nomi-nal values(

μR

=

μF

=

MH

/

2). The uncertaintyin the simulation of the underlying eventand partonshowers is estimated by us-ing two different pythia tunes, AUET2 andZ2*. All uncertainties aretreatedasfullycorrelatedbetweencategoriesexceptthose de-notedby anegativesuperscriptwhicharefullyanticorrelateddue tothemigrationofevents.

Table 5

Systematicuncertaintiesintheshapeofthesignalandbackground dis-tributions,expressedinpercentage.Thesystematicuncertaintyandits implementationaredescribedinthetext.

Systematic uncertainty H→eτμ H→eτh

Z→τ τbias 2 –

Z→ee bias – 5

Jet energy scale 3–7 3–7

Jet energy resolution 1–10 1–10 Unclustered energy scale 10 10

τhenergy scale – 3

5.3.2. Mcolshapeuncertainties

Thesystematicuncertaintiesthatleadtoachangeintheshape ofthe Mcol distributionare summarizedin

Table 5

. A2% shiftin theMcol distributionoftheembeddedZ

→

τ τ

sampleusedto es-timatethebackgroundisobservedrelativetosimulation.Itoccurs onlyintheH

→

e

τ

μ channelastheeffectsofbremsstrahlungfrom

themuonareneglectedinthesimulation.The Mcol distributionis corrected by 2

±

2% for thiseffect. There is a systematic uncer-taintyof5%inZ

→

ee backgroundintheH

→

e

τ

hchannel,dueto themismeasured energyof theelectron reconstructedasa

τ

h.It causesashiftintheMcol distribution,estimatedbycomparing col-lisiondatawithsimulationinacontrolregionofZ

→

ee eventsin whichoneofthetwoelectronsthatformtheZpeakisalso iden-tifiedasa

τ

h [63].Correctionsareappliedforthejetenergyscale andresolution[66].Theyaredeterminedwithdijetand

γ

/

Z

+

jets collisiondataandthemostsignificantuncertaintyarisesfromthe photonenergyscale.Otheruncertaintiessuchasjetfragmentation modelling, single pion response, and uncertainties in the pileup corrections are also included. The jet energy scale uncertainties (3–7%) are applied asa function of pT and

η

, including all cor-relations,toalljetsintheevent,propagatedtothe Emiss_T ,andthe resultant Mcol distribution isusedin thefit.There isalsoan ad-ditional uncertainty to account for the unclustered energy scale uncertainty.Theunclusteredenergycomesfromjetsbelow10 GeV andPF candidatesnot within jets. It isalso propagated to Emiss

T . TheseeffectscauseashiftoftheMcoldistribution.Theuncertainty inthejetenergyresolutionisusedtosmearthejetsasafunction of pT and

η

andtherecomputed Mcol distribution isusedinthe fit.A3% uncertaintyinthe

τ

h energyscaleisestimatedby com-paring Z

→

τ τ

events in collision data and simulation.Potential uncertaintiesintheshapeofthemisidentifiedleptonbackgrounds arealsoconsidered.IntheH

→

e

τ

μ channelthemisidentified

lep-tonratesareappliedinbinsofpT and

η

.IntheH

→

e

τ

hchannel, the

τ

h misidentification rate is found to be approximately inde-pendentof pT but todepend on

η

.These ratesare all varied by onestandarddeviationandthedifferencesintheshapesareused

asnuisanceparametersinthefit.Finally,thedistributionsusedin thefithavestatisticaluncertaintiesineachmassbinwhichis in-cludedasanuncertaintythatisuncorrelatedbetweenthebins.

6. H

→

e

μ

analysis 6.1. Eventselection

To select H

→

e

μ

events,the trigger requirementis an elec-tronandamuonwithpTgreater than17and8 GeVrespectively. Toenhancethesignal sensitivitythe eventsample isdividedinto nine different categories according to the region of detection of the leptons andthenumberofjets, anda furthertwo categories enriched invector bosonfusionproduction.The resolutionofthe reconstructed mass of the electron muon system, Meμ, depends

on whether the leptons are detected in the barrel (

|

η

e

|

<

1

.

48,

|

η

μ

|

<

0

.

80)orendcap(1

.

57

<

|

η

e

|

<

2

.

50,0

.

8

<

|

η

μ

|

<

2

.

4),while

the composition and rate of backgrounds varies with the num-ber ofjets. The definitionof the categories is shownin Table 6. Thetwoleptonsarerequiredtobeisolatedinallcategories. Cate-gories0–8,whichareselectedaccordingtotheregionofdetection oftheleptonandnumberofjets,aremutuallyexclusivewithjets requiredto havepT

>

20 GeV.Tosuppressbackgroundswith sig-nificant Emiss_T ,suchasWW

+

jets,Emiss_T isrequiredtobelessthan 20, 25 or 30 GeV, depending on the category. Jets arising from b quark decays are vetoed using the CSV discriminant to signif-icantly reduce the tt background.In the VBF categories, the two highest pT jets are required to have

|

η

|

<

4

.

7 and to be sepa-rated by

|

ηj

1

−

ηj

2

|

>

3

.

0. In addition the jets are required to have

|

η

∗

|

= |

η

12

−

η_j1+η_j2

2

|

<

2

.

5, where



=

e or

μ

,

η

12 de-notes the pseudorapidity of the dilepton system and j1

,

j2 are the two jets. The

φ

betweenthe dijetsystemandthe dilepton system isrequiredto be greater than2.6 rad.The VBF tight cat-egory selection further requires that both jets have pT

>

30 GeV and the dijetinvariant mass be Mj1j2

>

500 GeV, while the VBF loosecategoryrelaxesthesecond jetrequirementto pT

>

20 GeV with Mj1j2

>

250 GeV andis exclusiveto theVBF tightcategory. The leptons inboth VBFcategoriescan bein eitherthebarrelor endcap. To avoidan eventappearing in more than one category theVBFassignmentismadefirst.Eventswithmorethantwojets arenotconsidered.Theselectionefficiency,summedoverall cate-gories,is24%(22%)fortheGF(VBF)productionmechanism.

6.2. Signalandbackgroundmodelling

The signal model is the sum of two Gaussian functions, de-termined from simulation for each category. The reconstructed

Table 6

TheH→eμeventselectioncriteriaandbackgroundmodelforeacheventcategory.Thecategoriesareprimarilydefined accordingtowhethertheleptonsaredetectedinthebarrel(B)orendcap(EC),andthenumberofjets(N-jets). Require-mentsarealsomadeon p

T, ETmissandavetoonjetsarisingfromab-quarkdecay.Thebackgroundmodelfunctionand orderofthatfunctionarealsogiven.

Category Description N-jets p

T [GeV] EmissT [GeV] Background model Function Order 0 eBμB 0 >25 <30 polynomial 4 1 eBμB 1 >22 <30 polynomial 4 2 eBμB 2 >25 <25 power law 1 3 eBμEC 0 >20 <30 polynomial 4 4 eBμEC 1 >22 <20 exponential 1 5 eBμEC 2 >20 <30 exponential 1 6 eECμB or EC 0 >20 <30 polynomial 4 7 eECμB or EC 1 >22 <20 power law 1 8 eECμB or EC 2 >20 <30 polynomial 4 9 VBF Tight 2 >22 <30 exponential 1 10 VBF Loose 2 >22 <25 exponential 1

massresolutions depend onwhether the leptons are in the bar-rel(B) orendcap(EC)calorimeterandare:2.0–2.1 GeV foreB

μ

B, 2.4–2.5 GeVforeB

μ

EC,3.2–3.6 GeVforeECμB or EC categoriesand 2.4(4.0) GeVfortheVBFtight(loose)categories. Thebackground, modelled as either a polynomial function, a sum of exponential functions,orasumofpowerlawfunctionsisgivenin

Table 6

for each category. The procedureto determine the background func-tionfollowsthemethoddescribed in[3].Itisdesignedtochoose amodelwithsufficientparameterstoaccuratelydescribethe back-groundwhile ensuringthat thesignal shapeis notabsorbedinto thebackgroundfunction.Thebackgroundmodelforeachcategory ischosenindependentlyusingthisprocedure.

In a first step, referencefunctions are selected for each type offunction(polynomial,sumofexponentials,sumofpowerlaws). Theorderofthefunctionischosensuchthatthenexthigherorder doesnot give a significantly better fit resultwhen fit to the ob-servedMeμ distributionintherange110 GeV

<

Meμ

<

160 GeV.

Ina second step, an ensemble ofdistributions isdrawn from each ofthethree referencebackground modelscombined witha signalcontributioncorrespondingto

B(

H

→

e

μ

)

=

0

.

1%,andfitted forsignal andbackgroundwitheach ofthethreeclassesof func-tionsofdifferentorders.

On average, the signal yield extracted from the distributions usingasignalplusbackgroundfitwilldifferfromtheinjected sig-nal due to the imperfect modelling of the background. The bias is defined as the median deviation of the fit signal event yield fromthegeneratednumberofsignalevents.Thepossible combina-tionsofgenerateddistributionswiththefitsignalplusbackground modelsare then reduced by requiringthe bias to be lessthan a thresholdwhichresultsinlessthan 1%uncertaintyinthe fit sig-nalevent yield.The combinationin which thefit modelhas the leastparameters is then selected andthe fit function isused as thebackgroundmodelforthecollisiondata.Ifthereismorethan onemodelwiththesameminimalnumberofparametersthenthe onewiththeleastbiasisselected.

6.3.Systematicuncertainties

The systematic uncertainties are summarized in Table 7. The backgroundisfittotheobservedmassdistributionwitha negligi-blesystematicuncertaintyof

<

1%inthesignalyieldarisingfrom thechoiceofbackgroundmodelasdescribedabove.Correction fac-torsare applied tothe leptontrigger, isolation,andidentification efficienciesforeachsimulatedsignalsample toadjustfor discrep-ancieswiththecollision data.The uncertaintyinthesignal yield fromtheleptonisolationandidentificationcorrectionsis2.0%and is estimated withthe “tag-and-probe”method [72] applied to a collisiondatasampleofZ bosonsdecayingtoleptonpairs[60,62]. Theuncertaintiesintheleptonenergyscaleandthedileptonmass resolutionaretakenfromtheH

→

ZZ analysis[61].Thesystematic uncertaintyinthepileupmodellingisevaluatedbyvaryingthe to-talinelastic crosssection by

±

5% [74].It variesaccordingto the productionprocessandcategorybetween0.7%and2.3%.Thereare systematicuncertainties in the efficiency ofthe b quark jet veto thatalsovarywithproductionprocessandcategoryfrom0.05%to 0.7%.Theuncertaintyontheintegratedluminosityis2.6%[81].The effectsofsystematicuncertaintiesinthejetenergyscaleand res-olution,andthe uncertainties inPDF’sonthe selectionefficiency areestimatedasdescribedinSection5.3.2fortheH

→

e

τ

channel. Thelargest valuesof thesesystematicuncertainties occur dueto themigrationofeventsto,orfrom,acategorywithlowstatistics.

The theoretical uncertainties on the Higgs boson production crosssectionarealsodescribedinSection5.3.2.

Table 7

SystematicuncertaintiesinpercentageontheexpectedyieldforH

→

eμ.Ranges aregivenwheretheuncertaintyvarieswithproductionprocessandcategory.All uncertaintiesaretreatedascorrelatedbetweencategories.

Experimental uncertainties

Background model <1

Trigger efficiency 1.0

Lepton identification 2.0

Lepton energy scale 1.0

Dilepton mass resolution 5.0

Pileup 0.7–2.3

b quark jet veto efficiency 0.05–0.70

Luminosity 2.6

Jet energy scale (inclusive categories) 0.6–22 Jet energy scale (VBF categories) 0.1–78 Jet energy resolution (inclusive categories) 2.8–12 Jet energy resolution (VBF categories) 0.0–49 Acceptance (PDF variations) 0.8–5.1 Theoretical uncertainties

GF normalization/factorization scale +₋77..28 GF parton distribution function +7.5 −6.9 VBF normalization/factorization scale ±0.2 VBF parton distribution function +₋22..68

7. Results 7.1. H

→

e

τ

The distributions of the fitted signal and background contri-butions, afterthe full selection, areshown inFig. 3andthe cor-responding event yields in the mass range 100 GeV

<

Mcol

<

150 GeV aregivenin

Table 8

.Thereisnoevidenceofasignal.

Ta-ble 9showstheexpectedandobserved95%CLmeanupperlimits on

B(

H

→

e

τ

)

whicharesummarizedin Fig. 4fortheindividual categoriesin thee

τ

μ ande

τ

h channelsandfor thecombination. The combined observed (expected)upperlimit on

B(

H

→

e

τ

)

is 0.69 (0.75)%at95%CL

[70,71,82]

.

7.2. H

→

e

μ

The Meμ distributionofthecollisiondatasample, afterall

se-lectioncriteria,forallcategoriescombinedisshownin

Fig. 5

.Also shownarethecombinationsoftheinclusivejet-taggedcategories (0–8)andtheVBFcategories(9–10).Theexpectedyieldsofsignal (

B(

H

→

e

μ

)

=

0

.

1%)andbackgroundeventsfor124 GeV

<

Meμ

<

126 GeV,afteralltheselection criteria,are givenin

Table 10

and comparedtothecollisiondataeventyield.Thecontributionstothe background are takenfrom simulationand givenfor information only,they arenotusedintheanalysis. Thedominantbackground contributions are from Drell–Yan production of

τ

lepton pairs andelectroweakdibosonproduction.Thereis nosignal observed. An exclusion limit on the branching fraction

B(

H

→

e

μ

)

with

MH

=

125 GeV isderived usingthe CLs asymptoticmodel[83].It isshownin

Fig. 4

fortheinclusivecategoriesgroupedbynumber of jets, the VBF categories, and all categories combined. The ex-pected limitis

B(

H

→

e

μ

)

<

0

.

048% at 95% CLandthe observed limitis

B(

H

→

e

μ

)

<

0

.

035% at95%CL.

7.3. Limitsonleptonflavourviolatingcouplings

The constraints on

B(

H

→

e

τ

)

and

B(

H

→

e

μ

)

can be inter-preted in terms of the LFV Yukawa couplings

|

Yeτ

|

,

|

Yτe

|

and

|

Yeμ

|

,

|

Yμe

|

respectively[33].TheLFVdecaysH

→

e

τ

,e

μ

ariseat treelevelintheLagrangian, LV,fromtheflavour-violatingYukawa

Fig. 3. Comparisonoftheobservedcollinearmassdistributionswiththebackgroundexpectationsafterthefit.Thesimulateddistributionsforthesignalareshownforthe branchingfractionB(H→eτ)

=

0.69%.TheleftcolumnisH

→

eτμ andtherightcolumnisH→eτh;theupper,middleandlowerrowsarethe0-jet,1-jetand2-jet categories,respectively.

Table 8

Eventyieldsinthesignalregion,100 GeV<Mcol<150 GeV,afterfittingforsignalandbackgroundfortheH

→

eτ channel,normalizedtoanintegratedluminosityof 19.7 fb−1_{.TheLFVHiggsbosonsignalistheexpectedyieldforB(H→eτ)=0.69% assumingtheSMHiggsbosonproductioncrosssection.}

Jet category H→eτμ H→eτh

0-jet 1-jet 2-jet 0-jet 1-jet 2-jet

Misidentified leptons 85.2±5.9 38.1±3.9 2.1±0.7 3366±25 223±11 8.7±2.2 Z→ee,μμ 2.3±0.6 5.4±0.5 – 714±30 85±4 3.2±0.2 Z→τ τ 84.7±2.1 113.3±4.2 8.5±0.6 270±10 32±3 1.6±0.3 tt,t,t 13.8±0.3 69.4±2.3 12.7±0.8 10±2 13±2 0.5±0.2 ZZ,WZ,WW 83.0±2.7 51.7±2.0 3.6±0.4 53±2 6±1 0.3±0.1 Wγ(∗) 2.2±1.0 1.2±0.6 – – – – SM H background 2.3±0.3 3.6±0.4 1.1±0.2 12±1 3±1 1.0±0.1 Sum of background 273.5±6.1 282.0±6.0 28.1±1.3 4425±28 363±11 15.3±2.3 Observed 286 268 33 4438 375 13 LFV H signal 23.1±1.6 16.0±1.2 5.9±1.0 61±4 15±1 2.8±0.5

Fig. 4. 95%CLupperlimitsbycategoryfortheLFVdecaysforMH=125 GeV.Left:H

→

eτ.Right:H→eμforcategoriescombinedbynumberofjets,theVBFcategories

combined,andallcategoriescombined.

Table 9

Theexpectedand observedupper limits at 95%CL,and best fit valuesfor the branchingfractions B(H

→

eτ)fordifferentjetcategoriesand analysischannels. Theasymmetriconestandard-deviationuncertaintiesaroundtheexpectedlimits areshowninparentheses.

0-jet 1-jet 2-jet

Expected limits at 95% CL (%) eτμ <1.63  +0.66 −0.44  <1.54+0.71 −0.47  <1.59+0.93 −0.55  eτh <2.71  +1.05 −0.75  <2.76+1.07 −0.77  <3.55+1.38 −0.99  eτ <0.75+0.32 −0.22  Observed limits at 95% CL (%) eτμ <1.83 <0.94 <1.49 eτh <3.92 <3.00 <2.88 eτ <0.69



α

= 

β_{.ThesubscriptsL andR refertotheleftandrighthanded}

componentsoftheleptons,respectively. LV

≡ −

Yeμe

¯

L

μ

RH

−

Yμe

μ

¯

LeRH

−

Yeτe

¯

L

τ

RH

−

Yτe

τ

¯

LeRH

−

Yμτ

μ

¯

L

τ

RH

−

Yτ μ

τ

¯

L

μ

RH

Thedecaywidth

(

H

→ 

α



β

)

intermsof theYukawacouplings isgivenby:

Table 10

Eventyieldsinthemasswindow124 GeV<Meμ<126 GeV fortheH→eμ chan-nel.Theexpectedcontributions,estimatedfromsimulation,arenormalizedtoan integratedluminosityof19.7 fb−1_{.TheLFVHiggsbosonsignalistheexpectation} forB(H→eμ)=0.1% assumingtheSMproductioncrosssection.Valuesfor back-groundprocessesaregivenforinformationonlyandarenotusedfortheanalysis. TheexpectednumberofbackgroundeventsintheVBFcategoriesobtainedfrom simulation areassociated with largeuncertaintiesand aretherefore notquoted here;weexpect1.5±1.2 eventsfromsignalandobserve2events.

Jet category 0-jet 1-jet 2-jet

Drell–Yan 17.8±4.2 6.1±2.5 1.9±1.4 tt 1.4±1.2 3.1±1.8 14.1±3.8 t,t <1.0 <1.0 2.7±1.6 WW, WZ, ZZ 21.6±4.7 5.3±2.3 1.9±1.4 SM H background <0.07 0.1±0.2 <0.07 Sum of backgrounds 40.8±6.4 14.6±3.8 20.7±4.5 Observed 49 6 17 LFV H signal 21.2±4.6 9.1±3.0 2.6±1.6

(

H

→ 

α



β

)

=

MH 8

π

(

|

Yβα

|

2

_{+ |}

_Y αβ

|

2

),

andthebranchingfractionby:

B

(

H

→ 

α



β

)

=

(

H

→ 

α



β

₎

(

H

→ 

α



β

₎

_{+ }

SM

Fig. 5. Observedeμmass spectra(points),backgroundfit(solidline)and signal model(bluedashedline)forB(H→eμ)=0.1%.Top:inclusivejetcategories com-bined(0–8).Middle:VBFjettaggedcategoriescombined(9–10).Bottom:all cate-goriescombined.(Forinterpretationofthereferencestocolourinthisfigurelegend, thereaderisreferredtothewebversionofthisarticle.)

TheSMHiggsbosondecaywidthis



SM

=

4

.

1 MeV fora125 GeV Higgsboson[84].The95%CLconstraintsontheYukawacouplings, derivedfrom

B(

H

→

e

τ

)

<

0

.

69% and

B(

H

→

e

μ

)

<

0

.

035% using theexpressionforthebranchingfractionaboveare:



|

Yeτ

|

2

+ |

Yτe

|

2

<

2

.

4

×

10−3

,



|

Yeμ

|

2

+ |

Yμe

|

2

<

5

.

4

×

10−4

.

Figs. 6comparetheseresultstotheconstraintsfromprevious in-direct measurements. The absence of

μ

→

e

γ

decays implies a limitof



|

Yeμ

|

2

+ |

Yμe

|

2

<

3

.

6

×

10−6 [33]assuming thatflavour

Fig. 6. ConstraintsontheflavourviolatingYukawacouplings

|

Yeτ|, |Yτe|(top)and

|Yeμ|, |Yμe|(bottom).Theexpected(redsolidline)andobserved(blacksolidline) limitsarederivedfromthelimitsonB(H

→

eτ)andB(H→eμ)fromthepresent analysis.TheflavourdiagonalYukawacouplingsareapproximatedbytheirSM val-ues.Thegreen(yellow)bandindicatestherangethatisexpectedtocontain68% (95%)ofallobservedlimitexcursions.Theshadedregionsintheleftplotare de-rivedconstraintsfromnullsearchesforτ→3e (grey),τ→eγ (darkgreen)and thepresentanalysis(lightblue).Theshadedregionsintherightplotarederived constraintsfromnullsearchesforμ

→

eγ (darkgreen),μ

→

3e (lightblue)and

μ

→

e conversions(grey).Thepurplediagonallineisthetheoreticalnaturalness limitYi jYji≤mimj/v2[33].(Forinterpretationofthereferencestocolourinthis figurelegend,thereaderisreferredtothewebversionofthisarticle.)

changingneutralcurrentsaredominatedby theHiggsboson con-tributions.However,thislimitcanbedegradedbythecancellation ofleptonflavourviolating effectsfromothernewphysics.The di-rectsearchforH

→

e

μ

decayspresentedhereistherefore comple-mentaryto indirectlimitsobtainedfromsearchesforraredecays atlowerenergies.

8. Summary

A search for lepton flavour violating decays of the Higgs bo-son to e

τ

ore

μ

,based onthe full

√

s

=

8 TeV collision data set collected by the CMS experiment in 2012, is presented. No evi-denceis foundforsuch decays.Observedupperlimitsof

B(

H

→

e

τ

)

<

0

.

69% and

B(

H

→

e

μ

)

<

0

.

035% at 95% CL are set for

MH

=

125 GeV.TheselimitsareusedtoconstraintheYeτ andYeμ

Yukawacouplings asfollows:



|

Yeτ

|

2

+ |

Yτe

|

2

<

2

.

4

×

10−3 and



|

Yeμ

|

2

+ |

Yμe

|

2

<

5

.

4

×

10−4 at95%CL.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technicalandadministrativestaffs atCERN andatother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentresand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythecomputinginfrastructure essential toour analyses. Finally, we acknowledge the enduring support for the construc-tionandoperationofthe LHCandtheCMSdetectorprovided by thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China); COLCIEN-CIAS(Colombia);MSESandCSF(Croatia);RPF(Cyprus);SENESCYT (Ecuador); MoER, ERC IUT and ERDF (Estonia); Academy of Fin-land,MEC,andHIP(Finland);CEAandCNRS/IN2P3(France);BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hun-gary);DAEandDST (India);IPM(Iran);SFI(Ireland);INFN (Italy); MSIPandNRF (RepublicofKorea);LAS(Lithuania);MOE andUM (Malaysia); BUAP,CINVESTAV, CONACYT, LNS, SEP, and UASLP-FAI (Mexico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland);FCT (Portugal); JINR(Dubna); MON, RosAtom, RAS and RFBR (Russia); MESTD (Serbia); SEIDI and CPAN (Spain); Swiss Funding Agencies (Switzerland); MST (Taipei); ThEPCenter, IPST, STAR and NSTDA (Thailand); TUBITAK and TAEK (Turkey); NASU andSFFR(Ukraine);STFC(UnitedKingdom);DOEandNSF(USA).

Individuals have received support from the Marie-Curie pro-grammeandthe European ResearchCouncil andEPLANET (Euro-pean Union); the Leventis Foundation; the Alfred P. Sloan Foun-dation; the Alexander von Humboldt Foundation; the Belgian Federal Science Policy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technolo-gie (IWT-Belgium); the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Science and In-dustrial Research, India; the HOMING PLUS programme of the Foundation for Polish Science, cofinanced from European Union, Regional Development Fund, the Mobility Plus programme of the Ministry of Science and Higher Education, the OPUS pro-gramme contract 2014/13/B/ST2/02543 and contract Sonata-bis DEC-2012/07/E/ST2/01406oftheNationalScienceCenter(Poland); theThalisandAristeiaprogrammescofinancedbyEU-ESFandthe Greek NSRF; the National Priorities Research Program by Qatar National Research Fund; the Programa Clarín-COFUND del Prin-cipado de Asturias; the Rachadapisek Sompot Fund for Postdoc-toralFellowship,ChulalongkornUniversity andtheChulalongkorn AcademicintoIts2ndCenturyProjectAdvancementProject (Thai-land);andtheWelchFoundation,contractC-1845.

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