Article
Reference
Search for the Higgs boson decays H→ee and H→eμ In Collisions at with the ATLAS detector
ATLAS Collaboration
ADORNI BRACCESI CHIASSI, Sofia (Collab.), et al.
Abstract
Searches for the Higgs boson decays H→ee and H→eμ are performed using data corresponding to an integrated luminosity of 139fb−1 collected with the ATLAS detector in pp collisions at √s = 13 TeV at the LHC. No significant signals are observed, in agreement with the Standard Model expectation. For a Higgs boson mass of 125 GeV, the observed (expected) upper limit at the 95% confidence level on the branching fraction B(H→ee) is 3.6×10−4 (3.5×10−4) and on B(H→eμ) is 6.2×10−5 (5.9×10−5). These results represent improvements by factors of about five and six on the previous best limits on B(H→ee) and B(H→eμ) respectively.
ATLAS Collaboration, ADORNI BRACCESI CHIASSI, Sofia (Collab.), et al . Search for the Higgs boson decays H→ee and H→eμ In Collisions at with the ATLAS detector. Physics Letters. B , 2020, vol. 801, p. 135148
DOI : 10.1016/j.physletb.2019.135148
Available at:
http://archive-ouverte.unige.ch/unige:131174
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Contents lists available atScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Search for the Higgs boson decays H → ee and H → e μ 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:
Received24September2019
Receivedinrevisedform20November2019 Accepted6December2019
Availableonline10December2019 Editor: M.Doser
SearchesfortheHiggsbosondecaysH→eeandH→eμareperformedusingdatacorrespondingtoan integratedluminosityof139 fb−1collectedwiththeATLASdetectorinppcollisionsat√
s=13 TeVat theLHC. Nosignificantsignalsareobserved,inagreementwiththeStandardModelexpectation.Fora Higgsbosonmassof125 GeV,theobserved(expected)upperlimitatthe95%confidencelevelonthe branchingfractionB(H→ee)is3.6×10−4(3.5×10−4)andonB(H→eμ)is6.2×10−5(5.9×10−5).
Theseresults representimprovements byfactors ofaboutfive andsix onthepreviousbestlimits on B(H→ee)andB(H→eμ)respectively.
©2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Thediscoveryofaheavy scalarparticlebyATLAS andCMS [1, 2] providedexperimentalconfirmationoftheEnglert–Brout–Higgs mechanism [3–8], which spontaneously breaks electroweak (EW) gaugesymmetryandgeneratesmasstermsfortheW andZ gauge bosons.Inthe StandardModel(SM) thefermionmassesaregen- erated via Yukawa interactions. The Yukawa couplings to third- generationfermions were determined by measurements ofHiggs boson production and decays [9–15], andfound to be in agree- mentwiththeexpectationsoftheSM.However,thereiscurrently noevidenceofHiggsbosondecaysintofirst- orsecond-generation quarksorleptons.
ThisLetter presentsthe first ATLAS searches for H→ee and forthelepton-flavour-violating decay H→eμ usingthe fullRun 2datasetofproton–proton(pp)collisions atacentre-of-mass en- ergyof √
s=13 TeV, withan integratedluminosity of 139 fb−1. TheCMSCollaborationhaspreviouslyperformedsearchesforH→ ee[16] andH→eμ[17] usingLHCRun1ppdataat√
s=8 TeV correspondingtoanintegratedluminosityof19.7 fb−1.
In the SM the H →ee branching fraction is given by GFmHm2e/(4√
2πH)5×10−9,wheremH andH aretheHiggs massandwidthrespectively.This branchingfractionis farbelow the sensitivity of the LHC experiments. Contributions from dia- gramsthat do not depend on the electron Yukawa coupling Yee and are non-resonant e.g. H→eeγ, are expected to be signifi- cantlylarger,althoughstillmuchsmallerthanpresentsensitivity.
E-mailaddress:atlas.publications@cern.ch.
The LHCoffers thebest constrainton Yee [18], whichmaybe larger than predicted by the SM. The SM forbids lepton-flavour- number-violating Higgs boson decays. There are strong indirect constraintsontheoff-diagonalYeμcoupling,thestrongestderived fromlimitsonthebranchingfractionof μ →eγ andtheelectric dipole momentoftheelectron [19].However, theseindirectcon- straintsassumeSMvaluesfortheasyetunmeasuredYeeandYμμ Yukawa couplings. Searchingfor H→eμ allows Yeμ to be con- straineddirectly.
BothanalysespresentedinthisLettercloselyfollowthesearch forthe SM Higgsboson decay H→μμ [20]. Thesignal is sepa- ratedfromthebackgroundprimarilybyidentifyinganarrowpeak in the distribution of theinvariant mass ofthe two leptons m corresponding to the mass of the Higgs boson of 125 GeV [21].
The backgroundinthe ee search isdominatedby Drell–Yan(DY) Z/γ∗ production,withsmallercontributions fromtop-quarkpair (tt)¯ anddibosonproduction(Z Z,W ZandW W).Intheeμsearch, a much smalleryield ofSM background events is expected. The DYbackgroundonlycontributesthroughdecaysof Z/γ∗→τ τ→
eντνeμντνμ.Thustheproductionoftopquarks,dibosons(mainly through W W →eνeμνμ), W+jetsand multijetevents,withjets misidentifiedasleptons,aremoreimportantthanintheeesearch.
2. ATLASdetector
The ATLAS experiment [22–24] at the LHC is a multipurpose particle detector with a forward–backward symmetric cylindrical geometryandanear4π coverageinsolidangle.1 Itconsistsofan
1 ATLASuses aright-handedcoordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam
https://doi.org/10.1016/j.physletb.2019.135148
0370-2693/©2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
innertrackingdetector(ID)surroundedbyathinsuperconducting solenoidprovidinga2 Taxialmagneticfield,electromagneticand hadroncalorimeters,andamuonspectrometer.
TheIDcoversthepseudorapidityrange|η|<2.5.Itconsistsof silicon pixel, silicon microstrip, and transition radiation tracking detectors. Lead/liquid-argon (LAr) sampling calorimeters provide electromagnetic (EM) energy measurements with high granular- ity.Asteel/scintillator-tilecalorimeterinthecentral pseudorapid- ityrange|η|<1.7 measurestheenergies ofhadrons.The endcap and forward regions are instrumented withLAr calorimeters for boththe EMandhadronicenergymeasurements up to |η|=4.9.
The muon spectrometer (MS) surrounds the calorimeters up to
|η|=2.7 and is based on three large air-core toroidal supercon- ducting magnets with eight coils each. The field integral of the toroidsrangesbetween2.0and6.0 T macrossmostofthedetec- tor.Themuonspectrometerincludesasystemofprecisiontracking chambersandfastdetectorsfortriggering.
Atwo-leveltriggersystemisusedtoselectevents [25].Itcon- sists of a first-level trigger implemented in hardware and using a subset of the detectorinformation to reduce theevent rateto 100 kHz. This is followed by a software-based high-level trigger thatemploysalgorithms similartothoseusedofflineandreduces therateofacceptedeventsto1 kHz.
3. Simulatedeventsamples
Samples of simulated signal events with a Higgs boson mass of mH =125 GeV were generated as described below and pro- cessed through the full ATLAS detectorsimulation [26] based on GEANT4 [27]. Higgs boson production via the gluon–gluon fu- sion (ggF) process was simulated using the POWHEG NNLOPS program [28–35] with the PDF4LHC15 set of parton distribution functions(PDFs) [36]. TheHiggsboson rapidity inthesimulation was reweighted to achieve next-to-next-to-leading-order (NNLO) accuracy in QCD [37]. Higgs boson production via vector-boson fusion(VBF)andwithanassociatedvectorboson(V H)weregen- erated atnext-to-leading-order (NLO) accuracy inQCD usingthe POWHEG-BOXprogram [38–40].The Z H sampleswere simulated forprocesseswithquark–quarkinitialstates,andthesmallcontri- butionfromgluon–gluoninitialstatesisaccountedforinthenor- malisationofthe Z H cross section.The parton-level eventswere processed with PYTHIA8 [41] for the decay of the Higgs bosons into theee oreμ final states andto simulateparton showering, hadronisation and the underlying event, using the AZNLO set of tuned parameters [42]. All samples were normalised to state-of- the-art predictions using higher-order QCD and electroweak cor- rections [43–66].Theeffectsarisingfrommultipleppcollisions in thesameorneighbouringbunchcrossings(pile-up)wereincluded inthesimulationbyoverlayinginelastic ppinteractions generated withPYTHIA8usingtheNNPDF2.3LOsetofPDFs [67] andtheA3 set of tuned parameters [68]. Events were reweighted such that thedistribution ofthe average numberofinteractions per bunch crossingmatchesthatobservedindata.Simulatedeventswerecor- rectedtoreflecttheleptonenergyscaleandresolution,andtrigger, reconstruction,identificationandisolationefficienciesmeasuredin data.
Toevaluatetheuncertaintyinthebackgroundmodellinginthe eechannel,adedicatedfastsimulationforthedominantDY back- groundwas used to produce a sample of 109 events, equivalent
pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms ofthepolarangleθasη= −ln tan(θ/2).Angulardistanceismeasuredinunitsof R≡
(η)2+(φ)2.
to 40 times the integratedluminosity ofthe data. For thissam- ple, Z/γ∗+(0,1)-jet events were generated inclusively at NLO accuracyusingPOWHEG-BOX [69] withtheCT10PDFset [70].Ad- ditional Z/γ∗+2-jeteventsweregeneratedwithALPGEN [71] at leading-orderaccuracywiththeCTEQ6L1PDFset [72].Theevents were interfaced to PHOTOS [73] to simulate QED final-state ra- diation. The effects ofpile-up and a fastparameterisation of the responseofthedetectortoelectronsandjets,usingsimplesmear- ingfunctions,wasthenappliedtothegeneratedevents.
4. Eventselection
Events are recorded using triggers that require either an iso- latedelectronoranisolatedmuonaboveatransversemomentum (pT)thresholdof26 GeV [25,74].Electronsarereconstructedinthe range|η|<2.47 from clustersofenergydepositsinthecalorime- ter matched to a track in the ID [75]. Muons are reconstructed in therange |η|<2.5 by combiningtracks inthe ID eitherwith tracksintheMSor,for|η|<0.1,withcalorimeterenergydeposits consistentwithamuon [76].Theelectronsandmuonsarerequired tobeassociatedwiththeprimaryppcollisionvertex,whichisde- fined asthecollisionvertexwithlargestsumofp2T oftracks,and to beisolated fromother tracks [75,76]. Eacheventmustcontain eitherexactlytwo electrons oran electronandamuon. Onelep- tonmusthave pT>27 GeV toensureahightriggerefficiencyand theothermustbeofoppositechargeandhavepT>15 GeV.
Requirementsonjetsareusedinthisanalysistosuppressback- groundanddefinea categorythathasa highsensitivitytosignal produced intheVBFproductionmode.Jetsintherange|η|<4.5 and pT>30 GeV are reconstructed from energy deposits in the calorimeter [77],usingtheanti-kt algorithm [78,79] witharadius parameterof0.4.Trackinginformationiscombinedusingamulti- variatelikelihoodtosuppressedjetsfrompile-upinteractions [80].
Backgrounds with top quarks are suppressed by identifying b-hadronsandneutrinosinthefinal state.Jetsintherange|η|<
2.5 containingb-hadronsareidentifiedasb-jetsusingamultivari- atealgorithmthatusescalorimeterandtrackinginformation [81].
Eventsarerejectedifthereisatleastoneidentifiedb-jet.Different workingpoints are usedforthe ee andeμchannels becausethe latterhasalargertop-quarkbackground.Forthe ee(eμ) channel theb-jet identificationefficiencyisabout60%(85%)witharejec- tionfactorofabout1200(25)forlight-flavourjets [82].Neutrinos produced insemileptonic top-quark decays escape detectionand lead tomissingtransversemomentum EmissT ,reconstructedasthe magnitudeofthevectorsumofthetransversemomentaofallcal- ibrated leptons andjetsand additionalID tracks associated with the primary vertex(soft term) [83].Backgrounds with significant EmissT aresuppressedby requiring EmissT /√
HT<3.5(1.75)GeV1/2 fortheee(eμ)channel,where HTisthescalarsumofthetrans- versemomentaofleptonsandjetsand√
HT isproportionaltothe EmissT resolution.
Backgroundfromtheprocess H→γ γ,wherethephotonsare misreconstuctedaselectrons,isstudiedwithsimulatedeventsand found to contribute about0.07% inthe ee channel fora H→ee branching fractionat theexpectedlimit. It isthereforeneglected intherestoftheanalysis.
Thesearchisperformedintherangeofdileptoninvariantmass 110<m<160 GeV, which allows the background to be de- termined withanalyticfunctionsconstrainedby thesidebands to eithersideofthepotentialsignal.
The eventsample passing thebasicleptonselection isdivided into seven (eight) categories for the ee (eμ) channel that differ intheirexpectedsignal-to-backgroundratios,toimprovetheover- all sensitivityof the search. Thesecategories are based on those
usedinRef. [20], andarefoundtoprovidegoodsensitivityinthe presentanalyses.
First,a low-pT lepton category ‘Low pT’ is definedin the eμ
channel with events in which the subleading lepton has pT<
27 GeV.Thisregionhasasignificantfractionofeventsinwhichei- therreconstructedleptonisofnon-promptoriginorisamisidenti- fiedphotonorhadron,hereaftercalledafakelepton.Theseevents arenotseparatedoutintheeechannelbecausetherelativecontri- butionfromfakeleptonsissmaller.Acategoryenriched inevents fromVBF productionisdefinedfromtheremainingeventsbyse- lectingthosecontainingtwojetswithpseudorapiditiesofopposite signs,apseudorapidityseparation|ηj j|>3 andadijetinvariant massmj j>500 GeV.
Events that fail to meet the criteria ofthe ‘Low pT’ and VBF categoriesareclassifiedas‘Central’ifthepseudorapiditiesofboth leptons are |η|<1 or as ‘Non-central’ otherwise. For each of thesetwo categories, threeranges inthedileptontransversemo- mentum pT are considered: ‘Low pT’ (pT ≤15 GeV),‘Mid pT’ (15<pT ≤50 GeV),and ‘High pT’ (pT >50 GeV). These cate- gories exploit differences in the dilepton mass resolution, which is better for more central leptons, as well as differences in the expected signal-to-background ratio between the signal and the backgroundprocesses asfunction ofdilepton transverse momen- tumandrapidity.
5. Signalandbackgroundparameterisation
Analyticfunctionsareusedtodescribethemdistributionsfor boththesignalandthebackground.The H→eeandH→eμsig- nals consideredare narrow resonances witha massanda width set to the SM values of mH =125 GeV and 4.1MeV respec- tively.Theobservedsignalshapesarethusdeterminedbydetector resolution effects and are parameterised as a sum of a Crystal Ballfunction (FCB) [84] and a Gaussian function (FGS) following Ref. [20]:
PS(m) = fCB×FCB(m|mCB,σCB,α,n) +(1− fCB)×FGS
m|mGS,σGSS.
Theparameters αandndefinethepower-lawtailoftheFCBdistri- bution,whilemCB,mGS, σCB,and σGSS denotethe FCB meanvalue, FGS meanvalue, FCB width,and FGS widthrespectively.Therela- tivenormalisationbetweenthetermsisgovernedbytheparame- ter fCB.Theseparameters aredeterminedbyfittingthesimulated signalm distributionineachcategory.Intheee(eμ)channelthe signalmassresolutionvariesbetweenabout2.0GeV (2.3GeV) for thecentraland2.9GeV (3.0GeV)forthenon-centralcategories.
The background parameterisation for the ee channel follows Ref. [20] asthebackgroundisverysimilar.Themeedistributionsin eachcategory aredescribed by asumofaBreit–Wigner function (FBW)convolvedwitha FGS,andan exponentialfunction divided byacubicfunction:
PB(mee) = f× [FBW(mee|mBW,BW)⊗FGS(mee|σGSB)]
+(1−f)×CeA·mee/m3ee,
where f representsthefractionofthe FBW componentwheneach individualcomponent isnormalisedto unityand C isa normali- sationcoefficient.The σGSB parameterineach category isfixed to thecorrespondingaveragemresolutionasdeterminedfromsim- ulated signal events. For all the categories, the FBW parameters are fixed tomBW=91.2 GeV and BW=2.49 GeV [85]. The pa-
rameters f and A andthe overall normalisation are left free to bedeterminedinthefitanduncorrelatedbetweendifferentcate- gories.
ABernstein polynomial ofdegreetwo isused toparameterise the meμ distributionof the backgroundineach of the eightcat- egories in the eμ channel, with parameters uncorrelated across categories. The choice of background function is validated by an F-testconsideringBernstein polynomialsoffirst,second andthird degree.
The signal yield, which is allowed to be positive ornegative, is constrained using separate binned maximum-likelihood fits to theobservedm distributionsintherange110<m<160 GeV inthetwo channels.Thefits areperformedusingthesumofthe signalandbackgroundmodels(‘S+B model’)andareperformed simultaneouslyinallthecategories.Inadditiontothebackground- modelparametersdescribedearlier,thebackgroundnormalisation ineach categoryandthebranchingfractionofthesignal arefree parametersinthefit.
6. Systematicuncertainties
Thesignalexpectationissubjecttoexperimental andtheoreti- caluncertainties,whicharecorrelatedacrossthecategories.
The uncertainty in the combined2015–2018 integratedlumi- nosity is 1.7% [86], obtained usingthe LUCID-2detector [87] for the primary luminosity measurements. Other sources of experi- mentaluncertaintyincludethe electronandmuontrigger, recon- struction,identificationandisolationefficiencies [75,76], theb-jet identification efficiency [81], the pile-up modelling [88], the de- termination ofthe EmissT soft term [83], andthe jet energy scale andresolution [89].Theuncertainties intheelectronenergyscale andresolution [75] andinthemuonmomentumscaleandresolu- tion [76] affecttheshape ofthesignal distributionaswell asthe signalacceptance.
Thetotalexperimentaluncertaintyinthepredictedsignalyield in each ggF category is between 2% and 3% for the ee channel and between4% and6% forthe eμ channel. It is dominated by the luminosity, EmissT soft term and pile-up effects, and the last twocontributions arelargerintheeμ analysisduetothetighter EmissT /√
HT requirement. TheexperimentaluncertaintyintheVBF category isbetween7% and15% fortheee channel andbetween 6% and22% fortheeμ channel,dueto largercontributions from thejetenergyscaleandresolution.
Thetheoreticaluncertainties intheproductioncrosssection of the Higgs boson are taken from Ref. [43]. In addition, theoreti- calmodellinguncertaintiesaffectingtheacceptanceforthesignals are calculated separately for the ggF and VBF Higgs boson pro- ductionprocessesineachanalysiscategory.Theuncertaintyinthe acceptanceforthe V Hprocessisneglected.Theeffectsofmissing higher-order termsin the perturbative QCD calculationsare esti- matedbyvaryingtherenormalisationandfactorisationscales.For theggF processtheuncertainties are approximatedastwocorre- latedsources that rangefromaround 1% to11% forthe different analysiscategoriesinboth channels. FortheVBF process theun- certaintiesintheacceptanceduetotheQCDscalesarefoundtobe small.Theeffectsofuncertainties inthepartondistributionfunc- tionsandthevalueof αSareestimatedusingthePDF4LHC15rec- ommendations [36] andfoundtobeverysmall.Theuncertaintyin themodellingofthepartonshower,underlyingevent,andhadro- nisation isassessedbycomparingthe acceptanceofsignal events showeredbyPYTHIAwiththatofeventsshoweredbyHERWIG [90, 91].Thetotalvariationsduetotheseuncertaintiesrangefromless than 1%to 11% fortheggF signalprocess andfrom1% to8% for theVBFsignalprocessdependingontheanalysiscategory.