Contents lists available atScienceDirect
Physics
Letters
B
www.elsevier.com/locate/physletb
Search
for
an
exotic
decay
of
the
Higgs
boson
to
a
pair
of
light
pseudoscalars
in
the
final
state
with
two
b quarks
and
two
τ
leptons
in
proton–proton
collisions
at
√
s
=
13
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:
Received25May2018
Receivedinrevisedform16July2018 Accepted7August2018
Availableonline31August2018 Editor:M.Doser Keywords: CMS Physics Higgsboson Exoticdecays NMSSM 2HDM+S
A search for an exotic decay of the Higgs boson to a pair of light pseudoscalar bosons is performed for the first time in the final state with two b quarks and two
τ
leptons. The search is motivated in the context of models of physics beyond the standard model (SM), such as two Higgs doublet models extended with a complex scalar singlet (2HDM+S), which include the next-to-minimal supersymmetric SM (NMSSM). The results are based on a data set of proton–proton collisions corresponding to an integrated luminosity of 35.9 fb−1, accumulated by the CMS experiment at the LHC in 2016 at a center-of-mass energy of 13 TeV. Masses of the pseudoscalar boson between 15 and 60 GeV are probed, and no excess of events above the SM expectation is observed. Upper limits between 3 and 12% are set on the branching fractionB(h →aa→2
τ
2b)assuming the SM production of the Higgs boson. Upper limits are also set on the branching fraction of the Higgs boson to two light pseudoscalar bosons in different 2HDM+S scenarios. Assuming the SM production cross section for the Higgs boson, the upper limit on this quantity is as low as 20% for a mass of the pseudoscalar of 40 GeV in the NMSSM.©2018 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
Within the standard model (SM), the Brout–Englert–Higgs mechanism[1–6] isresponsible forelectroweaksymmetry break-ingandpredictstheexistenceofascalarparticle—theHiggsboson. AparticlecompatiblewiththeHiggsbosonwasdiscoveredbythe ATLAS and CMScollaborations at the CERN LHC [7–9]. Measure-mentsofthe couplingsandpropertiesofthisparticleleave room forexoticdecaysto beyond-the-SMparticles,witha limit of34% onthisbranchingfractionat95%confidencelevel(CL),usingdata collectedatcenter-of-massenergiesof7and8TeV [10].
The possible existence of exotic decays of the Higgsboson is well motivated [11–16]. The decay width of the Higgs boson in the SM isso narrow that a smallcoupling to a light state could lead tobranching fractionsof theHiggs bosonto that light state oftheorderofseveralpercent.Additionally,the scalarsectorcan theoretically serve as a portal that allows matter froma hidden sectortointeractwithSM particles [17].Ingeneral, exoticdecays oftheHiggsbosonareallowedinmanymodelsthatareconsistent withallLHCmeasurementspublishedsofar.
E-mailaddress:cms-publication-committee-chair@cern.ch.
An interesting class of such processes consists ofdecays to a pair oflight pseudoscalar particles,which then decayto pairs of SM particles. This type of process is allowed in various models, includingtwoHiggsdoubletmodelsaugmentedbyascalarsinglet (2HDM
+
S). Sevenscalarandpseudoscalarparticlesarepredicted in 2HDM+
S. Oneofthem, h, isa scalarthat canbe compatible withthediscoveredparticlewithamassof125GeV,andanother, thepseudoscalara,canbelightenoughsothath→
aa decaysare allowed.Fourtypesof2HDM,andbyextension2HDM
+
S,forbid flavor-changing neutralcurrentsattreelevel [18].Intype I,all SM par-ticlescoupletothefirstdoublet.Intype II,up-typequarkscouple to the firstdoublet, whereas leptons anddown-type quarks cou-ple to the second doublet. The next-to-minimal supersymmetric SM (NMSSM) is a particular case of 2HDM+
S of type II that brings asolution to theμ
problem [19]. In type III, quarks cou-ple tothefirstdoublet,andleptons tothesecond one.Finally,in type IV, leptons and up-type quarks couple to the first doublet, while down-typequarks couple to the second doublet [15]. The branching fractionsofthe lightpseudoscalars topairsofSM par-ticlesdependonthetypeof2HDM+
S,onthepseudoscalarmass ma,andon tanβ
,definedastheratioofthevacuumexpectationvalues ofthesecond andfirst doublets. Thevalue ofthe
branch-https://doi.org/10.1016/j.physletb.2018.08.057
0370-2693/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Fig. 1. Predicted B(aa
→
bbτ τ) for ma=40GeV in the different models of 2HDM+S,asafunctionoftanβ.Thepictureisessentiallythesameforallma hy-pothesesconsideredinthisLetter.Thebranchingfractionsarecomputedfollowing theformulasofRef. [15].ingfraction
B(
aa→
bbτ τ)
isslightlyabove10%intheNMSSM—or moregenerallyin2HDM+
S type II—fortanβ >
1,andcanreach uptoabout50%in2HDM+
Stype IIIwithtanβ
∼
2,asshownin Fig.1.SeveralsearchesforexoticdecaysoftheHiggsbosontoapair oflight short-lived pseudoscalar bosons havebeen performedby the CMS Collaboration with data collected at a center-of-mass energy of 8TeV in different final states: 2μ2b for 25
.
0<
ma<
62
.
5GeV [20], 2μ2τ for 15.
0<
ma<
62.
5GeV [20], 4τ for 4<
ma<
8GeV [21] and 5<
ma<
15GeV [20], and 4μ for 0.
25<
ma<
3.
50GeV [22].TheCMSCollaborationalsostudiedthe2μ2τfinalstatefor15
.
0<
ma<
62.
5GeV at acenter-of-massenergyof13TeV [23]. The ATLASCollaboration reportedresultsforthe fol-lowingfinalstatesatacenter-of-massenergyof8TeV:4μ,4e,and 2e2μ for15
<
ma<
60GeV [24]; 4γ for 10<
ma<
62GeV [25];and2μ2τ for 3
.
7<
ma<
50.
0GeV [26]. At a center-of-massen-ergyof 13TeV, theATLAS Collaboration published results forthe 4b decaychannelfor20
<
ma<
60GeV [27],and4μ,4e,and2e2μfor 1
<
ma<
60GeV [28]. The 2b2τ final state has never beenprobedso far. Thisfinal state benefits fromlarge branching frac-tionsinmostmodelsbecauseofthelargemassesof
τ
leptonsand b quarkswithrespect toother leptons andquarks.The presence oflightleptons originatingfromtheτ
decaysallowseventstobe triggeredinthedominantgluonfusionproductionmode.ThisLetterreportsonthefirstsearchwiththeCMSexperiment forexoticdecaysoftheHiggsbosontoapairoflightpseudoscalar bosons,inthefinalstatewithtwo
τ
leptonsandtwob quarks.The searchfocusesonthemassrangebetween15and60GeV.Forlow mavalues,betweenthebb thresholdand15GeV,thedecayprod-uctsofeachofthepseudoscalarbosonsbecomecollimated,which wouldnecessitatetheuseofspecialreconstructiontechniques.
The search is based on proton–proton(pp) collision data col-lectedata center-of-massenergyof13TeV andcorresponding to an integratedluminosity of35
.
9fb−1. ThroughoutthisLetter, the termτ
h denotesτ
leptons decaying hadronically. Theτ τ
finalstatesstudiedinthissearchareeμ,eτh,and
μτ
h.Despiteitslargebranchingfraction, the
τ
hτ
h final state isnot considered becausethesignalacceptanceisnegligiblewiththetransverse momentum (pT) thresholdsavailableforthe
τ
hτ
h triggers.The ee andμμ
fi-nalstatesforthe
τ τ
pairare notconsidered either,becausethey havealowbranchingfractionandsufferfromalargecontribution ofDrell–Yanbackgroundevents.2. TheCMSdetector
The central feature of the CMS apparatus is a superconduct-ingsolenoidof6m internaldiameter,providingamagneticfieldof 3
.
8T. Within the solenoid volume, there are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorime-ter (ECAL), and a brass and scintillator hadron calorimeter, each composedofabarrelandtwoendcapsections.Forward calorime-ters extend the pseudorapidity coverage provided by the barrel andendcapdetectors.Muonsaredetectedingas-ionization cham-bersembeddedinthesteelflux-returnyokeoutsidethesolenoid. Events of interest are selected using a two-tiered trigger sys-tem [29].A moredetaileddescriptionoftheCMSdetector,together witha definitionofthe coordinatesystemused andtherelevant kinematicvariables,canbefoundinRef. [30].3. Simulatedsamplesandeventreconstruction
Thesignalandsomeofthebackgroundprocessesaremodeled withsamplesofsimulatedevents.The MadGraph5_amc@nlo [31] 2.3.2 generator isused fortheh
→
aa→
2τ2b signalprocess, in gluonfusion(ggh),vectorbosonfusion(VBF),orassociatedvector boson(Wh,Zh)processes.Thesesamplesaresimulatedatleading order (LO) inperturbative quantumchromodynamics (QCD) with theMLMjetmatchingandmerging [32].TheZ+
jets andW+
jets processesarealsogeneratedwiththe MadGraph5_amc@nlo gen-eratoratLOwiththeMLMjetmatchingandmerging.TheZ+
jets simulation contains non-resonant Drell–Yan production, with a minimum dilepton mass threshold of 10GeV. The FxFx merging scheme [33] is used to generate diboson background with the MadGraph5_amc@nlo generator at next-to-LO (NLO). The tt and singletopquarkprocessesaregeneratedatNLOwiththe powheg 2.0 and 1.0 generator [34–39]. Backgrounds from SM Higgs bo-son production are generatedat NLO withthe powheg 2.0 gen-erator [40], and the minlohvj [41] extension of powheg 2.0 is used for the Wh and Zh simulated samples. The generators are interfaced with pythia 8.212 [42] to model the parton shower-ingandfragmentation,aswell asthedecayoftheτ
leptons.The CUETP8M1tune [43] is chosenforthe pythia parametersthat af-fectthedescriptionoftheunderlyingevent.Thesetofparton dis-tributionfunctions(PDFs)isNLO NNPDF3.0forNLOsamples,and LO NNPDF3.0 for LO samples [44]. The full next-to-NLO (NNLO) plus next-to-next-to-leading logarithmic (NNLL) order calculation [45–50], performed withthe Top++ 2.0 program [51], is used to compute a tt production cross section equal to 832+−2029(scale)±
35(
PDF+
α
S)
pb setting the top quark mass to 172.
5GeV. Thiscross section is used to normalize the tt background simulated with powheg.
All simulated samples includeadditional proton–proton inter-actions per bunch crossing, referred to as “pileup”, obtained by generatingconcurrentminimumbiascollisioneventsusing pythia. The simulated events are reweighted insuch a way to have the same pileupdistribution asdata.Generated eventsare processed throughasimulationoftheCMSdetectorbasedon Geant4 [52].
The reconstruction of events relies on the particle-flow (PF) algorithm [53], which combines information from the CMS sub-detectors toidentify andreconstructtheparticles emerging from pp collisions: chargedand neutralhadrons, photons, muons, and electrons.CombinationsofthesePFobjectsareusedtoreconstruct higher-levelobjectssuchasjets,
τ
hcandidates,andmissingtrans-versemomentum.
ElectronsarereconstructedbymatchingECALclusterstotracks inthetracker.Theyarethen identifiedwithamultivariate analy-sis (MVA)discriminantthatmakes useofvariablesrelatedto the
energydepositsintheECAL,thequalityofthetrack,andthe com-patibilitybetweenthepropertiesoftheECALclustersandthetrack that havebeen matched [54]. The MVAworkingpoint chosen in thissearch has an efficiency ofabout 80%.The reconstruction of muon candidates is performedcombiningthe information ofthe trackerandthemuon systems.Muons arethen identified onthe basis of the track reconstruction quality and on the number of measurementsinthetrackerandthemuonsystems [55].The rel-ativeisolationofelectronsandmuonsisdefinedas:
I
≡
chargedpT+
max 0,
neutralpT−
12 charged,PUpT p T.
(1)In this formula,
chargedpT is the scalar sum of the transversemomentaofthe chargedparticles,excluding thelepton itself, as-sociatedwiththeprimaryvertexandinaconearoundthelepton direction,withsize
R
=
√
(
η
)
2+ (φ)
2=
0.
3 forelectrons, or0.4formuons. The sum
neutralpT representsa similar quantityforneutralparticles.ThelasttermcorrespondstothepTofneutral
particlesfrompileupvertices,which,asestimatedfromsimulation, is equal to approximately half of that of charged hadrons asso-ciated withpileup vertices,denoted by
charged,PUpT. The pT ofthelepton isdenoted p
T.Theazimuthal angle,
φ
,ismeasured inradians.
JetsarereconstructedfromPFobjectswiththeanti-kT
cluster-ing algorithmimplementedin the FastJet library[56,57],usinga distanceparameterof0.4.Correctionstothejetenergyareapplied asafunctionofthepTand
η
ofthejet [58].Thejetsinthissearchare requiredto be separatedfrom theselected electrons, muons, or
τ
h, byR
≥
0.
5. Jets that originate from b quarks, called bjets, are identified with the combined secondary vertex (CSVv2) algorithm [59]. The CSVv2 algorithm builds a discriminant from variables related to secondary vertices associated with the jet if any,andfromtrack-basedlifetimeinformation.Theworkingpoint choseninthissearchprovidesanefficiencyforb quarkjetsof ap-proximately70%,andamisidentificationrateforlight-flavorandc quarkjetsofapproximately1and10%,respectively.
Hadronically decaying
τ
leptons are reconstructed with the hadrons-plus-strips (HPS) algorithm [60,61] as a combination of tracks and energy deposits in strips of the ECAL.The tracks are the signature of the charged hadrons, andthe strips that of the neutral pions, which decay to a pair of photons with potential electron–positron conversion. The reconstructedτ
h decay modesare one track, one trackplusat leastone strip, andthreetracks. Therateforjetstobe misidentifiedas
τ
h isreducedby applyingan MVAdiscriminatorthat usesisolation aswell aslifetime vari-ables.Its workingpoint hasbeenchosen tohave anefficiencyof approximately45% fora misidentificationrate oflight-flavor jets of the orderof 0.1%. Additionally, discriminatorsthat reduce the rateswithwhichelectronsandmuonsare misidentifiedas
τ
hareapplied. Loose working points with an efficiency above 90% are chosen because the Z
→
ee/
μμ
background does not contribute muchintheregionwherethesignalisexpected.Toaccountforthecontributionofundetectedparticles,suchas theneutrinos,themissingtransversemomentum,
pmissT ,isdefined asthenegativevectorialsumofthetransversemomentaofallPF objects reconstructed in the event. The magnitude ofthis vector isdenoted pmissT . The reconstructedvertex withthe largestvalue ofsummedphysics-object p2T istakentobetheprimary pp inter-action vertex. The physics objects are the objects constructed by ajet findingalgorithm [56,62] applied toall chargedtracks asso-ciated withthevertex, and thecorresponding associated missing transversemomentum.Table 1
Baseline selectioncriteriaforobjectsrequiredinvarious fi-nalstates.Thenumbersgivenforthe pT thresholdsofthe electronand muonintheeμfinalstatecorrespondtothe leadingandsubleadingparticles.ThepTthresholdfortheτh candidates istheresultofanoptimization oftheexpected exclusionlimitsofthesignal.
eμ eτh μτh
pT(e) >24/13 GeV >26 GeV —
pT(μ) >24/13 GeV — >20 GeV
pT(τh) — >25 GeV >25 GeV
pT(b) >20 GeV >20 GeV >20 GeV
|η(e)| <2.4 <2.1 — |η(μ)| <2.4 — <2.1 |η(τh)| — <2.3 <2.3 |η(b)| <2.4 <2.4 <2.4 Isolation (e) <0.10 <0.10 — Isolation (μ) <0.15 — <0.15 Ident. (τh) — MVA MVA
4. Eventselection
Events are selectedinthree different
τ τ
final states:eμ,eτh,and
μτ
h. They are additionally required to contain at least oneb-taggedjet.Thedominantbackgroundswiththeseobjectsinthe finalstatearett andZ
→
τ τ
production.Anotherlargebackground consistsof eventswithjetsmisidentified asτ
h,such asW+
jetsevents,thebackgroundfromSMeventscomposeduniquelyofjets producedthroughthestronginteraction,referredtoasQCD multi-jetevents,orsemileptonictt events.
All eventsare requiredto haveat leastone b-taggedjet with pT
>
20GeV and|
η
|
<
2.
4.About 90% of simulatedsignal eventspassing thiscondition haveonly one such jet, asa resultof the typically soft b jet pT spectrum and of the limited efficiency of
the b taggingalgorithm. Eventsinthe eμ final stateare selected withatriggerthatreliesonthepresenceofbothanelectronand a muon, wherethe leading leptonhas pT
>
23GeV andthesub-leading one pT
>
12GeV if it is an electron or 8GeV if it is amuon. Intheeτh finalstate, thetriggerisbasedon thepresence
of an isolatedelectron with pT
>
25GeV,whereas in theμτ
hfi-nal state events are selected witha combination oftriggers that requireeitheranisolatedmuonwithpT
>
22GeV,oramuonwith pT>
19GeV andaτ
hcandidatewithpT>
21GeV.Duringthe2016datatakingperiod,noneoftheavailabletriggersthatrequiredthe presenceofbothanelectronanda
τ
hcandidatecouldincreasethesignalacceptancesignificantlywithrespecttothetriggerbasedon thepresenceofan electrononly.Tighter selectioncriteriaare ap-pliedatthereconstructionlevel.Theelectrons,muons,and
τ
hcan-didatesarerequiredtobewellidentifiedandisolated [54,55,61],to haveoppositecharge, andtobeseparatedbyatleast
R
=
0.
4 if thereisaτ
h,or0.3otherwise.Table1detailsthe pT,η
,isolation,andidentification criteriaforthe various objects,inthe different finalstates.
Toincrease the sensitivityoftheanalysis, eventsin eachfinal state are separated into four categories with different signal-to-background ratios.The categoriesare definedonthe basis ofthe invariant massofthevisibledecayproductsofthe
τ
leptonsand the b-taggedjet withthe highestpT,denotedbymvisbτ τ .This vari-able is typically low for signal events because the three objects originate froma 125GeV Higgs boson,butit isonaverage much largerforbackgroundevents,wherethethreeobjectsdonot orig-inatefromadecayofaresonance,asshowninFig.2fortheμτ
hfinalstate.Thethresholdsthatdefinethecategoriesdependonthe
τ τ
final state:they are lowerinthe eμfinal state becausethere aremoreneutrinosnotincludedinthemasscalculation,andthey are higher in the eτh final state to keep enough events despiteFig. 2. Visibleinvariantmassoftheleptonsandtheleadingb jet,mvis
bτ τ,afterthe baselineselection,inthe μτh finalstate, forthe signalwithdifferentmass hy-potheses(top).Distributionofmvis
bτ τintheμτhfinalstate(bottom).The“jet→τh” contributionincludesalleventswithajetmisidentifiedasaτhcandidate,whereas therestofbackgroundcontributionsonlyincludeeventswherethereconstructed τhcorrespondstoaτh,amuon,oranelectron,atthegeneratorlevel.The“Other” contributionincludeseventsfromsingletopquark,diboson,andSMHiggsboson processes.Thesignalhistogramcorrespondsto10timestheSMproductioncross sectionforggh,VBF,andVh processes,andassumesB(h→aa→2τ2b)=100%. (Forinterpretationofthe colorsinthefigures,thereaderisreferredtotheweb versionofthisarticle.)
Signaleventswithma
25GeV contributemostlytothefirsttwocategories, whereas those with ma
25GeV are concentrated inthesecond andthirdcategories.Thiscanbeexplainedbythefact thatthemissing b jetin themasscalculation wouldbe closerto thereconstructedb jet fora signalwithlowermabecauseofthe
boostofthepseudoscalarbosons,leadingtoalargerreconstructed mass.Thelastcategoryhaslargebackgroundyields;itisusefulto constrain the various backgrounds and provides some additional sensitivityforlow-masignalsamples.Theresultsofthesearchare
extractedfroma fitofthevisible
τ τ
mass(mvisτ τ )distributionsin eachof thecategories, because thisisa resonant distributionfor signalevents.
Selectioncriteriaareappliedtooptimizetheexpectedlimitson theproductofthesignalcrosssectionandbranchingfraction.The samethresholdswouldbeobtainedwithanoptimizationbasedon thediscovery potential.One suchcriterion isbased onthe trans-versemassof
pmissT andeach oftheleptons.The transversemass mTbetweenaleptonandpmissT isdefinedas
mT
(,
pmissT)
≡
2pTpmissT
[
1−
cos(φ)
],
(2)where pTisthetransversemomentumofthelepton
,and
φ
is theazimuthalanglebetweentheleptonmomentumandpmissT . Se-lectingeventswithlowmTstronglyreducesthebackgroundsfromW
+
jets andtt events,which arecharacterizedby alarger pmissT . Inaddition,forW+
jets eventsinwhichtheselectedleptoncomes fromtheW bosondecay,mThasaJacobianpeakneartheW bosonmass. The distributions of mT
(
μ
,
pmissT)
and mT(
τ
h,
pmissT)
in theμτ
hfinalstatebeforethemvisbτ τ -basedcategorizationareshownin Fig.3(topandcenter).Anotherselectioncriterion isbasedonthe variable Dζ,which
isdefinedas
Dζ
≡
pζ−
0.
85 pvisζ,
(3)where pζ is the component of
pmissT along the bisector of thetransversemomenta ofthe two
τ
candidatesand pvisζ isthesum ofthecomponentsofthelepton pT alongthesamedirection [63].AsshowninFig.3(bottom),theZ
→
τ τ
backgroundtypicallyhas Dζ valuesclose to zero because pmissT is approximately collinearto the
τ τ
system, whereas the tt background is concentrated at lower Dζ valuesbecause oftypically large pmissT notalignedwiththe
τ τ
system. The signal liesinan intermediate region because pmissT is approximatelyaligned with the
τ τ
system, but pmissT isusuallysmall.Theprecisecriteriaforeachfinalstateandcategory areindicatedinTable2.
5. Backgroundestimation
TheZ
→
backgroundisestimatedfromsimulation.The dis-tributionsofthepTofthedileptonsystemandthevisibleinvariantmass between the leptons and the leading b jet are reweighted with corrections derived using data from a region enriched in Z
→
μμ
+ ≥
1 b events. The simulation is separated between Z→
τ τ
, where the reconstructedτ
candidates correspond toτ
leptons atgeneratorlevel,andZ
→
ee/
μμ
decays,whereatleast oneelectronormuonismisidentifiedasaτ
hcandidate.Backgroundswithajetmisidentifiedasa
τ
hcandidateareesti-matedfromdata.TheyconsistmostlyofW
+
jets andQCDmultijet events,aswellasthefractionoftt,diboson,andsingletopquark productionwherethereconstructedτ
hcandidatecomesfromajet.Theprobabilitiesforjetsto bemisidentifiedas
τ
hcandidates,de-noted f , areestimated fromZ
→
μμ
+
jets eventsin data.They are parameterized withLandaudistributions asa function ofthe pT oftheτ
h candidate,separatelyfor everyreconstructedτ
hde-cay mode. Events that pass all the selection criteria, except that the
τ
h candidatefailstheisolation condition,arereweightedwithaweight f
/(
1−
f)
toestimatethecontributionofeventswithjets inthesignalregion.Thecontributionofeventswithgenuine elec-trons, muons, orτ
h candidates inthecontrol region isestimatedfromsimulationandsubtractedfromdata.
Intheeμfinalstate,thesmallW
+
jets backgroundisestimated fromsimulation [64].Sucheventstypicallyhaveagenuinelepton coming from the W boson decay and a jet misidentified asthe other lepton. The QCD multijet background, which also contains jets misidentified asleptons, isestimated from data.Its normal-ization corresponds to the difference between the data and the sumofall theother backgroundsina so-calledsame-signregion where theτ
candidates have the same sign. A smooth distribu-tionisobtainedbyadditionallyrelaxingtheisolationconditionsof bothleptons.Acorrectionthatisextractedfromdataisappliedto extrapolatethenormalizationobtainedinthesame-sign regionto thesignalregion.Fig. 3. DistributionsofmT(μ,pmissT )(top),mT(τh,
pmissT )(center),andDζ (bottom) intheμτhfinalstatebeforethembτ τvis-basedcategorization.The“jet→τh” contri-butionincludesalleventswithajetmisidentifiedasaτhcandidate,whereasthe restofbackgroundcontributionsonlyincludeeventswherethereconstructedτh correspondstoaτh, amuon,oranelectron,atthegenerator level.The“Other” contributionincludeseventsfromsingletopquark,diboson,andSMHiggsboson processes.Thesignalhistogramcorrespondsto10timestheSMproductioncross sectionforggh,VBF,andVh processes,andassumesB(h→aa→2τ2b)=100%.
Table 2
Optimizedselectionandcategorizationinthevariousfinalstates.Theselection cri-terion Dζ >−30GeV in the eμ finalstatereduces the largett background.In the otherfinalstatesthe tt backgroundisless important,and onlyeventswith
Dζ>0GeV arediscardedinoneofthecategoriesoftheμτhfinalstatetoreduce theZ→τ τbackground.Thisselectioncriteriondoesnotimprovethesensitivityin theeτhfinalstatebecauseofthelowerexpectedsignalandbackgroundyields,and isthereforenotapplied.
Variable Category 1 Category 2 Category 3 Category 4 eμ
mvisbτ τ <65 GeV ∈[65,80]GeV ∈[80,95]GeV >95 GeV
mT(e,pmissT ) <40 GeV <40 GeV <40 GeV <40 GeV
mT(μ,pmissT ) <40 GeV <40 GeV <40 GeV <40 GeV
Dζ >−30 GeV >−30 GeV >−30 GeV >−30 GeV eτh
mvis
bτ τ <80 GeV ∈[80,100]GeV ∈[100,120]GeV >120 GeV
mT(e,pmissT ) <40 GeV <50 GeV <50 GeV <40 GeV
mT(τh,pTmiss) <60 GeV <60 GeV <60 GeV <60 GeV μτh
mvis
bτ τ <75 GeV ∈[75,95]GeV ∈[95,115]GeV >115 GeV
mT(μ,pmissT ) <40 GeV <50 GeV <50 GeV <40 GeV
mT(τh,pTmiss) <60 GeV <60 GeV <60 GeV <60 GeV
Dζ — <0 GeV — —
Otherprocesses,includingdiboson,tt,andsingletopquark pro-duction without jet misidentified as a
τ
h candidate, as well asSMHiggsbosonprocessesinvariousproductionanddecaymodes, areestimatedfromsimulation.Thett productionisamajor back-ground, especiallyinthe eμfinal state. Thett simulation models thevariablesusedinthisanalysiswell,asithasbeenverifiedina controlregionintheeμfinalstatewherenoselectioncriterionis applied onmT
(
e,
pmissT)
ormT(
μ
,
pmissT)
,andwhere the Dζselec-tioncriterionisinverted.
Inthe eτhand
μτ
h final states,whereallbackgrounds withajet misidentified asa
τ
h candidateare estimatedfromdata,sim-ulated eventswith a reconstructed
τ
h that is not matched toanelectron, a muon,or a
τ
h atthe generatorlevel are discarded toavoid doublecounting. Approximately 30% ofsimulatedtt events aftertheselectionhaveareconstructed
τ
h thatisnotmatchedtoanelectron,amuon,ora
τ
hatthegeneratorlevel.6. Fitmethodandsystematicuncertainties
The search for an excess of signal events over the expected backgroundinvolvesaglobalbinnedmaximumlikelihoodfitbased on themvisτ τ distributionsinthedifferentchannelsandcategories. Thestatisticaluncertaintylargelydominatesoversystematic uncer-taintiesinthissearch.Thesystematicuncertaintiesarerepresented by nuisance parameters that are varied in the fit according to their probability densityfunctions. Alog-normal probability den-sity function is assumedfor the nuisance parameters that affect the event yields of the various background andsignal contribu-tions,whereassystematicuncertaintiesthataffectthedistributions arerepresentedbynuisanceparameterswhosevariationresultsin acontinuousperturbationofthespectrum [65] andwhichare as-sumedtohaveaGaussianprobabilitydensityfunction.
Totakeintoaccountthelimitedsizeofsimulatedsamplesand ofdatainthecontrolregions usedtoestimate someofthe back-groundprocesses,statisticaluncertainties inindividualbinsofthe mvisτ τ distributions are considered asPoissonian nuisance parame-ters.The uncertaintycanbe aslarge as40%forsome binsinthe low-mvisbτ τ categories.Thecombinedeffectofalltheseuncertainties isthedominantsystematicuncertaintyinthissearch.
The uncertainties inthe jet energy scale [58] affect the over-all yieldsofprocessesestimatedfromsimulation,aswell astheir
Fig. 4. Distributionsofmvis
τ τinthefourcategoriesoftheeμchannel.The“Other”contributionincludeseventsfromsingletopquark,diboson,SMHiggsboson,andW+jets productions.ThesignalhistogramcorrespondstotheSMproductioncrosssectionforggh,VBF,andVh processes,andassumesB(h→aa→2τ2b)=10%.Thenormalizations ofthepredictedbackgrounddistributionscorrespondtotheresultoftheglobalfit.
relativecontribution tothe differentcategories becausethe cate-gorizationisbasedon thevalueofmvisbτ τ foreach event.Theyare functionsof the jet pT and
η
.The pmissT is recomputedfor eachvariationofthe jet energyscale.The uncertaintyin
pmiss T relatedtothemeasurementoftheenergythatisnotclusteredinjets [66] isevaluatedevent-by-event,andisalsoconsideredasashape un-certainty.
Correctionsfortheefficiencyofthe identificationofelectrons, muons, and
τ
h candidates are derived from data usingtag-and-probe methods [67], and are applied to simulated events as a functionofthelepton pTand
η
.Uncertaintiesrelatedtothesecor-rectionsamountto2% forelectrons, 2%formuons,and5% for
τ
hcandidates.Additionally,eventswithanelectronormuon misiden-tifiedasa
τ
hcandidatehaveayielduncertaintyof5%.Triggerscalefactorsare also estimatedwithtag-and-probe methods and their corresponding uncertainties in the yields of simulated processes are1%.
Theenergyscale of
τ
h candidatesiscorrected foreachrecon-structeddecay mode,andthe uncertaintyof 1.2%for each single decay mode is considered as a shape uncertainty. Uncertainties intheenergyscales ofelectrons andmuonsare alsoincludedas shapeuncertainties.
Correctionstotheefficiencyforidentifyingab quarkjetasab jet,aswellasformistaggingajetoriginatingfromadifferent
fla-vor,areappliedtosimulatedeventsonthebasisofthegenerated flavorofthejets.Theuncertaintiesinthescalefactorsdependon the pT ofthejetandarethereforeconsideredasshape
uncertain-ties. Theyamountto1.5%forajetoriginatingfroma b quark,5% fromac quark,and10%fromalight-flavorparton [59].
The uncertainty in the yield of the backgrounds with jets misidentified as
τ
h candidates accounts for possibly differentmisidentification rates in Z
+
jets events (where the misidenti-fication rates are measured), and in W+
jets and QCD multijet events (which dominate the constitution of the reducible back-groundinthesignalregion),andfordifferencesbetweendataand predictedbackgroundsobservedinaregion enrichedinreducible backgroundevents byinverting thecharge requirementon theτ
candidates and removing the mT and Dζ selection criteria. This
uncertaintyamounts to20%,andisconstrainedtoabout7% after themaximumlikelihoodfitbecauseofthelargenumberofevents contributingtothelastmvisbτ τ category.Uncertaintiesinthe param-eterizationofthemisidentificationprobabilityofjetsasafunction of pT resultin shapeuncertainties for thebackgrounds withjets
misidentifiedas
τ
hcandidates.TheuncertaintyintheyieldoftheQCDmultijetbackgroundin theeμfinalstateis20%;thevaluecomesfromtheuncertaintyin theextrapolationfactorfromthesame-signregiontothe opposite-sign region. The uncertainty in the W
+
jets background in this channelalsoamountsto20%,andaccountsforapotentialmismod-Fig. 5. Distributionsofmvis
τ τ inthefourcategoriesoftheeτhchannel.The“jet→τh”contributionincludesalleventswithajetmisidentifiedasaτhcandidate,whereasthe restofbackgroundcontributionsonlyincludeeventswherethereconstructedτhcorrespondstoaτh,amuon,oranelectron,atthegeneratorlevel.The“Other”contribution includeseventsfromsingletopquark,diboson,andSMHiggsbosonprocesses.ThesignalhistogramcorrespondstotheSMproductioncrosssectionforggh,VBF,andVh processes,andassumesB(h→aa→2τ2b)=10%.Thenormalizationsofthepredictedbackgrounddistributionscorrespondtotheresultoftheglobalfit.
elinginsimulationofthemisidentificationrateofjetsaselectrons ormuons.
The theoretical yield uncertainty of the tt background is re-latedtothePDF uncertaintyandtothe uncertaintyassociatedto the strong couplingconstant
α
S in thefull NNLO plus NNLLor-der calculation ofthecross section;it amountsto about4%. The yield uncertainties for other backgrounds estimated from simu-lationare taken fromrecentCMS measurements:6% for diboson processes [68],13%forsingletopquarkprocesses [69],and7%for Z
+
jets eventswithatleastoneb-taggedjetinthefinalstate [70]. TheuncertaintyinthecorrectionofthedileptonpTdistributionforDrell–Yaneventsisequalto10%ofthesizeofthecorrectionitself. Theuncertaintyinthecorrectionofthemvisbτ τ distributionisequal tothecorrectionitself,andconsideredasashapeuncertainty. Un-certaintiesintheproductioncrosssectionsandbranchingfractions forSMHiggsbosonprocessesaretakenfromRef. [71].The uncer-taintyintheintegratedluminosityamountsto2.5% [72].
7. Results
Themvisτ τ distributionsinthe differentchannelsandcategories areshowninFigs.4–6.Thebinningcorresponds tothebinsused inthelikelihoodfit.
No excess is observed relatively to the SM background pre-diction. Upper limits at 95% CL are set on
(
σ
(
h)/
σ
SM)B(
h→
aa
→
2τ2b)
using the modified frequentist construction CLs inthe asymptotic approximation [73–77], for pseudoscalar masses between 15 and60GeV.In this expression,
σ
SM denotes theSMproductioncross sectionoftheHiggsboson, whereas
σ
(
h)
isthe h productioncrosssection.Thelimitsperchannelandforthe com-binationofthethreechannelsare showninFig.7.The most sen-sitivefinalstateisμτ
h.Thesensitivityoftheeτhandeμchannelsis approximatelyequivalent; thefirst channel suffersfromhigher trigger thresholds and lower object identification efficiency than
μτ
h,andthe second onesuffers fromalower branching fractionthan
μτ
h. At low ma, the eμ final state hasa highersignalac-ceptancethanthe otherfinal states,especially eτh.Thelimitsare
more stringent in the intermediate mass range. The low-ma
sig-nalshavealoweracceptancebecauseoftheoverlapoftheleptons related to the boost ofthe pseudoscalar bosons, andof the typ-ically softer lepton andb jet pT spectra. The highma signalslie
in a region where more backgrounds contribute, leading also to lower sensitivitythan inthe intermediate massregion. The cate-gories are complementary over the probed mass range,with the low-mvisbτ τ signalregionsmoresensitiveforheavyresonances,and thehigh-mvis
Fig. 6. Distributionsofmvis
τ τinthefourcategoriesoftheμτhchannel.The“jet→τh”contributionincludesalleventswithajetmisidentifiedasaτhcandidate,whereasthe restofbackgroundcontributionsonlyincludeeventswherethereconstructedτhcorrespondstoaτh,amuon,oranelectron,atthegeneratorlevel.The“Other”contribution includeseventsfromsingletopquark,diboson,andSMHiggsbosonprocesses.ThesignalhistogramcorrespondstotheSMproductioncrosssectionforggh,VBF,andVh processes,andassumesB(h
→
aa→2τ2b)=
10%.Thenormalizationsofthepredictedbackgrounddistributionscorrespondtotheresultoftheglobalfit.The combined limit at intermediate mass is aslow as3% on
B(
h→
aa→
2τ2b)
,assumingtheSMproductioncrosssectionand mechanisms forthe Higgs boson, and isup to 12% for the low-est mass point ma=
15GeV. Computing the branching fractionsof the light pseudoscalar to SM particles [15,78], this translates to limits on
(
σ
(
h)/
σ
SM)
B(
h→
aa)
of about 20% in 2HDM+
Stype II—including the NMSSM—with tan
β >
1 for ma=
40GeV.Thisimprovesbymorethanoneorderofmagnitudeprevious lim-itson
B(
h→
aa)
obtainedinthe2μ2τ finalstatebyCMSfor15<
ma
<
25GeV [20,23], andby upto afactorfivethoseobtainedinthe2μ2b finalstatebyCMSfor25
<
ma<
60GeV [20].Inthesce-nariowiththehighestbranchingfraction,2HDM
+
Stype III with tanβ
=
2,theexpected limitis aslow as6%at intermediatema.Fig.8showstheobservedlimitsat95% CL on
(
σ
(
h)/
σ
SM)B(
h→
aa
)
asafunctionofmaandtanβ
fortype IIIandtype IV2HDM+
S,forwhichthereisastrongdependencewithtan
β
.Fig.9showsthe observedlimitsat95%CL on(
σ
(
h)/
σ
SM)B(
h→
aa)
forafewsce-nariosof2HDM
+
S,assumingthebranchingfractionsofthelight pseudoscalar to SM particles computed using Refs. [15,78]. The limit shownfortype II2HDM+
S is approximatelyvalidfor any valueoftanβ >
1,andthatfortype I2HDM+
Sdoesnotdepend ontanβ
. Inthema rangeconsideredin theanalysis, thebranch-ingfraction
B(
aa→
bbτ τ)
rangesbetween0.10and0.11intype I 2HDM+
S,between0.11and0.13fortanβ
=
2 intype II2HDM+
S,between0.44and0.46fortan
β
=
2 intype III2HDM+
S,and be-tween0.16and0.21fortanβ
=
0.
5 intype IV2HDM+
S.8. Summary
ThefirstsearchforexoticdecaysoftheHiggsbosontopairsof light bosons with two b quark jets andtwo
τ
leptons inthe fi-nal state hasbeen performedwith 35.
9fb−1 ofdata collected at 13TeV center-of-mass energy in 2016. This decay channel has a large branching fraction in manymodels where the couplings to fermions are proportional to the fermion mass,and can be trig-gered withhighefficiency inthe dominantgluon fusion produc-tion modebecauseof thepresenceof lightleptons fromleptonicτ
decays. No excess of events is found on top of the expected standardmodelbackgroundformassesofthelightboson,ma,be-tween 15 and 60GeV. Upper limits between 3 and 12% are set on the branching fraction
B(
h→
aa→
2τ2b)
assuming the SM productionofthe Higgsboson.This translatestoupperlimitsonB(
h→
aa)
as low as20% forma=
40GeV in the NMSSM.Theseresultsimprove by morethan one orderofmagnitudethe sensi-tivitytoexoticHiggsbosondecaysto pairsoflightpseudoscalars intheNMSSMfrompreviousCMSresultsinother finalstatesfor 15
<
ma<
25GeV,andbyafactoruptofivefor25<
ma<
60GeVFig. 7. Expectedandobserved95%CL limitson(σ(h)/σSM)B(h→aa→2τ2b)in%.Theeμresultsareshowninthetopleftpanel,eτhinthetopright,μτhinthebottom left,andthecombinationinthebottomright.Theinner(green)bandandtheouter(yellow)bandindicatetheregionscontaining68and95%,respectively,ofthedistribution oflimitsexpectedunderthebackground-onlyhypothesis.
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,wegratefullyacknowledgethecomputingcentersand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythe computinginfrastructureessential to ouranalyses. Finally, we acknowledge the enduring support for the construc-tionandoperation oftheLHCandthe CMSdetectorprovidedby 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, andHGF (Germany); GSRT (Greece);NKFIA (Hungary); DAE andDST (India);IPM (Iran);SFI (Ireland);INFN (Italy); MSIP and NRF(RepublicofKorea);LAS(Lithuania);MOEandUM(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, CPAN, PCTI and FEDER (Spain); Swiss Funding Agencies (Switzerland); MST (Taipei); ThEPCenter, IPST, STAR, and NSTDA (Thailand); TÜBITAK and TAEK (Turkey);
NASU andSFFR (Ukraine); STFC (United Kingdom);DOEand NSF (USA).
Individuals have received support from the Marie-Curie pro-gramandtheEuropeanResearchCouncilandHorizon2020Grant, contract No. 675440 (European Union); the Leventis Founda-tion; the Alfred P. Sloan Foundation; the Alexander von Hum-boldt 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 Technologie (IWT-Belgium); the F.R.S.-FNRS and FWO (Belgium) under the “Excellence of Science – EOS” – be.h projectNo. 30820817;theMinistryofEducation,YouthandSports (MEYS) of the Czech Republic; the Lendület (“Momentum”) Pro-gramme and the János Bolyai Research Scholarship of the Hun-garian Academy of Sciences, the New National Excellence Pro-gram ÚNKP, the NKFIA research grants 123842, 123959, 124845, 124850 and 125105 (Hungary); the Council of Science and In-dustrial Research,India;theHOMING PLUSprogramofthe Foun-dation for Polish Science, cofinanced from European Union, Re-gional Development Fund, the Mobility Plus program of the Ministry of Science and Higher Education, the National Science Centre (Poland), contracts Harmonia 2014/14/M/ST2/00428, Opus 2014/13/B/ST2/02543, 2014/15/B/ST2/03998, and 2015/19/B/ST2/ 02861,Sonata-bis2012/07/E/ST2/01406;theNationalPriorities Re-search Program by Qatar National Research Fund; the Programa Estatal de Fomento de la Investigación Científica y Técnica de
Fig. 8. Observed95%CL limitson(σ(h)/σSM)B(h→aa)in2HDM+Softype III (top),andtype IV(bottom).Thecontourscorrespondingtoa95%CL exclusionof
(σ(h)/σSM)B(h
→
aa)=
1.00 and 0.34aredrawnwith dashed lines.The num-ber34%correspondstothelimitonthebranchingfractionoftheHiggsbosonto beyond-the-SMparticlesatthe95%CL obtainedwithdatacollectedat center-of-massenergiesof7and8TeV bytheATLASandCMSexperiments [10].Fig. 9. Observed95%CL limitson(σ(h)/σSM)B(h
→
aa)forvarious2HDM+Stypes. Thelimitintype I2HDM+Sdoesnotdependontanβ.ExcelenciaMaríade Maeztu,grant MDM-2015-0509andthe Pro-grama Severo Ochoa del Principado de Asturias; the Thalis and Aristeiaprograms cofinancedby EU-ESF andthe GreekNSRF; the Rachadapisek Sompot Fund for Postdoctoral Fellowship, Chula-longkornUniversityandtheChulalongkornAcademic intoIts2nd CenturyProjectAdvancement Project(Thailand);theWelch Foun-dation,contractC-1845;andtheWestonHavensFoundation(USA).
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TheCMSCollaboration
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YerevanPhysicsInstitute,Yerevan,Armenia
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M. Spanring,
D. Spitzbart,
A. Taurok,
W. Waltenberger,
J. Wittmann,
C.-E. Wulz
1,
M. Zarucki
InstitutfürHochenergiephysik,Wien,Austria
V. Chekhovsky,
V. Mossolov,
J. Suarez Gonzalez
InstituteforNuclearProblems,Minsk,Belarus
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D. Di Croce,
X. Janssen,
J. Lauwers,
M. Pieters,
M. Van De Klundert,
H. Van Haevermaet,
P. Van Mechelen,
N. Van Remortel
UniversiteitAntwerpen,Antwerpen,Belgium
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F. Blekman,
J. D’Hondt,
I. De Bruyn,
J. De Clercq,
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VrijeUniversiteitBrussel,Brussel,Belgium
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B. Clerbaux,
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N. Postiau,
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UniversitéLibredeBruxelles,Bruxelles,Belgium
T. Cornelis,
D. Dobur,
A. Fagot,
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C. Roskas,
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M. Vidal Marono,
S. Wertz,
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F.L. Alves,
G.A. Alves,
L. Brito,
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UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil
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a,
C.A. Bernardes
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D. Romero Abad
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UniversidadeFederaldoABC,SãoPaulo,Brazil
A. Aleksandrov,
R. Hadjiiska,
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A. Marinov,
M. Misheva,
M. Rodozov,
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A. Dimitrov,
L. Litov,
B. Pavlov,
P. Petkov
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W. Fang
5,
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L. Yuan
BeihangUniversity,Beijing,China
M. Ahmad,
J.G. Bian,
G.M. Chen,
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TsinghuaUniversity,Beijing,China
C. Avila,
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B. Courbon,
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I. Puljak,
T. Sculac
UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia
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M. Kovac
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V. Brigljevic,
D. Ferencek,
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M. Finger
7,
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7E. Ayala
EscuelaPolitecnicaNacional,Quito,Ecuador
E. Carrera Jarrin
UniversidadSanFranciscodeQuito,Quito,Ecuador
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10AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt
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NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia
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DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland
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HelsinkiInstituteofPhysics,Helsinki,Finland
T. Tuuva
LappeenrantaUniversityofTechnology,Lappeenranta,Finland
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F. Couderc,
M. Dejardin,
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