Searches
for
a
heavy
scalar
boson
H
decaying
to
a
pair
of
125 GeV
Higgs
bosons
hh
or
for
a
heavy
pseudoscalar
boson A
decaying
to Zh,
in
the
final
states
with
h
→
τ τ
.
CMS
Collaboration
CERN,Switzerland
a
r
t
i
c
l
e
i
n
f
o
a
b
s
t
r
a
c
t
Articlehistory: Received5October2015
Receivedinrevisedform10January2016 Accepted26January2016
Availableonline9February2016 Editor:M.Doser Keywords: CMS Physics MSSM τ Higgs
AsearchforaheavyscalarbosonHdecayingintoapairoflighterstandard-model-like125 GeVHiggs bosons hh andasearchforaheavypseudoscalarboson AdecayingintoaZandanhbosonarepresented. The searches are performedon adata set corresponding toan integrated luminosity of19.7 fb−1 of pp collisiondataatacentre-of-massenergyof8 TeV,collectedbyCMSin2012.A finalstateconsisting oftwo
τ
leptonsandtwobjetsisusedtosearchfortheH→hh decay.A finalstateconsistingoftwoτ
leptonsfromtheh bosondecay,andtwoadditionalleptonsfromtheZ bosondecay,isusedtosearch forthedecayA→Zh.Theresultsareinterpretedinthecontextoftwo-Higgs-doubletmodels.Noexcess isfoundabovethestandardmodelexpectationandupperlimitsaresetontheheavybosonproduction crosssectionsinthemassranges260<
mH<350 GeV and220<mA<350 GeV.©2016CERNforthebenefitoftheCMSCollaboration.PublishedbyElsevierB.V.Thisisanopenaccess articleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
ThediscoveryofadditionalHiggsbosonsattheLHCwould pro-vide directevidence ofphysics beyondthe standardmodel (SM). ThereareseveraltypesofmodelsthatrequiretwoHiggsdoublets
[1–3].Forexample theminimal supersymmetricextension ofthe SM(MSSM)requiresthe introductionofanadditionalHiggs dou-blet,whereoneHiggsdoublet couplestoup-typequarksandthe othertodown-typequarks
[4–11]
.Thisleads tothepredictionof fiveHiggsparticles:onelightandoneheavyCP-evenHiggsboson, handH,oneCP-oddHiggsbosonA,andtwochargedHiggsbosons H±[2,12].Themassesandcouplingsofthesebosonsare interre-latedand,attreelevel,canbedescribedbytwoparameters,which areoftenchosentobethemassofthepseudoscalarbosonm
A and theratioofthevacuumexpectationvaluesoftheneutral compo-nentsofthe twoHiggsdoublets tanβ
.However, radiative correc-tions[13–17]
introducedependenciesonotherparametersnamely themassofthetopquarkm
t,thescaleofthesoftsupersymmetry breakingmassesM
SUSY,thehiggsinomassparameterμ
,thewino mass parameter M2, the third-generation trilinear couplings, At,Ab,and Aτ , themass ofthe gluinomg˜,andthe third-generation sleptonmassparameter
M
˜3.
E-mailaddress:cms-publication-committee-chair@cern.ch.
Direct searchesfortheneutralMSSM Higgsbosons havebeen performed by the CMS and ATLAS Collaborations [18–20] using thebenchmarkscenarios proposedinRef.[21].Inthesescenarios theparameters involvedintheradiativecorrectionsfortheHiggs bosonmassesandcouplingshavebeenfixed,andonlythetwo pa-rameters
m
A andtanβ
remainfree.Thevalue ofM
SUSY wasfixed ataround 1 TeV, whichproduces a lightest CP-evenHiggs boson with a mass mh lower than the observed Higgs boson mass of 125.
09±
0.
21(
stat)
±
0.
11(
syst)
GeV[22],forvaluesoftanβ
6.If, however,MSUSY ismuch largerthan1 TeV, assuggestedby the non-observationsofSUSY partner particles attheLHC sofar, low valuesof tan
β
canproduce an h boson withmh125 GeV[23,24].TheinterpretationoftheHiggsbosonmeasurementsinthe framework oftherecentlydevelopedMSSM benchmarkscenarios
[24–27] suggests that the mass of the CP-odd Higgs boson, mA, can be smaller than 2mt. In the mass region below 2mt and at low valuesof tan
β
,the decaymode oftheheavy scalar H→
hh andthatofthepseudoscalarA→
Zh canhavesizeablebranching fractions.Thisencouragesaprogrammeofsearchesintheso-called“low tan
β
”channels[23,28]
:•
for220 GeV<
mA<
2mt:A→
Zh;•
for260 GeV<
mA<
2mt:H→
hh;•
form
A>
2mt:A/
H→
t¯
t. http://dx.doi.org/10.1016/j.physletb.2016.01.0560370-2693/©2016CERNforthebenefitoftheCMSCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
The decay modes H
→
hh and A→
Zh, studied in this pa-per, are alsopresentin other typesoftwo-Higgs-doublet models (2HDM)[2,3]
.Therearedifferenttypesof2HDMwiththosemost similar to the MSSM (i.e. whereup-type fermions coupleto one doublet and down-type fermions to the other) being “Type II” 2HDM.Thediscovery ofaHiggsbosonattheLHC[29–31]
witha massaround125 GeV pushesthe2HDMparameterspacetowards eitherthealignmentordecouplinglimits[24]
.Intheselimitsthe propertiesofh areSM-like.Inthealignment limitof2HDMwhen cos
(β
−
α
)
1 (whereα
is themixinganglebetweenthetwo neutralscalarfields),the Hhh and AZh couplings vanish at Born level [32]. However, in the MSSM, the Hhh and AZh couplings do not vanish, even in thealignmentlimit,becauseofthelargeradiativecorrectionsthat arise in the model. In the decoupling limit of 2HDM the scalar HiggsbosonH hasaverylargemassandthedecayH→
t¯
t domi-nates[32].Thispaperreportsthe resultsofsearches forthedecays H
→
hh→
bbτ τ
and A→
Zh→
τ τ
(wheredenotes
μμ
or ee). Thechoiceofτ
pairfinalstatewasdrivenbyitsquiteclean signa-tureandbythemostrecentresults,whichgavestrongerevidence ofthe125 Higgsbosoncouplingtothefermions[33].Thisanalysis exploitssimilartechniquesasusedforthesearchfortheSMHiggs boson at 125 GeV [34] and several differentτ τ
signatures are studied.Forthechannel H→
hh→
bbτ τ
,theμτ
h,eτ
h,andτ
hτ
h final states are used,whereτ
h denotes the visibleproducts ofa hadronicallydecayingτ
,whereasforthechannelA→
Zh→
τ τ
, theμτ
h,eτ
h,τ
hτ
h,andeμ
finalstatesareselected.SearchesforthedecaysH
→
hh,andA→
Zh havealreadybeen performedbytheATLAS[35–38]
andCMSCollaborations[39–41]
indi-photon,multileptonandbb finalstates.
This analysishas the powerto bring important results inthe low tan
β
region for the mA range, which has been previously discussed andwhere theseprocesses havean enhanced sensitiv-ity [23]. This region has not yet been excluded by the direct or indirectsearches fora heavy scalaror pseudoscalarHiggsboson, that have been mentioned above, therefore the described decay modeslooktobequitepromising.Forsimplicityofthepaper,weareneitherindicatingthecharge oftheleptonsnortheparticle–antiparticlenatureofquarks. 2. TheCMSdetector,simulationanddatasamples
A detailed description of the CMS detector can be found in Ref. [42]. The central feature of the CMS apparatus is a super-conductingsolenoidof6 m internal diameterproviding afield of 3.8 T.Withinthefieldvolumeareasiliconpixelandstriptracker, a crystal electromagnetic calorimeter (ECAL), and a brass/scintil-lator hadron calorimeter. Muons are measured in gas-ionisation detectorsembeddedinthesteelreturnyokeofthemagnet.
TheCMScoordinatesystemhastheorigincentred atthe nom-inalcollision pointandis orientedsuch thatthe x-axis points to thecentre oftheLHCring,the
y-axis
pointsverticallyupwardand the z-axis isinthedirectionofthebeam. Theazimuthal angleφ
ismeasuredfromthe
x-axis
inthexy plane
andtheradial coordi-nateinthisplaneisdenotedby r.Thepolarangleθ
isdefinedin therz plane
andthepseudorapidityisη
= −
ln[
tan(θ/
2)
]
[42].The momentumcomponenttransversetothebeamdirection,denoted by pT,iscomputedfromthex- and
y-components.Thefirstlevel(L1)oftheCMStriggersystem,composedof cus-tomhardware processors, usesinformationfromthecalorimeters andmuondetectorstoselectthemostinterestingeventsinafixed time interval of less than 4 μs. The high-level Trigger processor farm decreases the L1 accept rate from around 100 kHz to less than1 kHz beforedatastorage.
Thedatausedforthissearchwere recordedwiththeCMS de-tectorinproton–protoncollisionsattheCERNLHCandcorrespond to an integrated luminosity of19
.
7 fb−1 at acentre-of-mass en-ergy of√
s=
8 TeV. The H→
hh signals are modelled with the pythia6.4.26[43]
eventgeneratorwhilethe A→
Zh signalswere modelled with MadGraph 5.1 [44]. When modelling background processes,the MadGraph 5.1generatorisusedforZ+
jets,W+
jets, t¯
t,anddibosonproduction,and powheg 1.0[45–48]
forsingle top quark production.The powheg and MadGraph generators are in-terfacedwith pythia forpartonshoweringandfragmentationusing the Z2*tune [49].All generators are interfacedwith tauola[50]forthe simulationof the
τ
decays.All generatedeventsare pro-cessed through a detailed simulation of the CMS detector based on Geant4 [51] andare reconstructed with the samealgorithms as the data. Parton distribution functions (PDFs) CT10 [52] or CTEQ6L1[53]
fortheprotonareused,dependingonthegenerator inquestion, togetherwithMSTW2008[54]
accordingto PDF4LHC prescriptions[55].3. Eventreconstruction
Duringthe2012LHCrun therewere anaverageof21proton– proton interactions per bunch crossing. The collision vertex that maximises the sumofthe squares ofmomenta components per-pendiculartothebeamline(transverse momenta)ofalltracks as-sociated withit,
p2T, is taken to be the vertex of the primary hard interaction.The otherverticesare categorisedaspileup ver-tices.
A particle-flow algorithm [56,57] is used to reconstruct indi-vidual particles, i.e. muons, electrons, photons, charged hadrons andneutralhadrons,usinginformationfromallCMSsubdetectors. Composite objects such as jets, hadronically decaying
τ
leptons, andmissingtransverseenergyarethenconstructedusingthelists ofindividualparticles.Muonsare reconstructed byperforming a simultaneousglobal trackfit to hitsinthe silicontracker andthemuon system [58]. Electronsare reconstructedfromclustersofECALenergydeposits matched to hits in the silicontracker [59].Muons andelectrons assumed to originate from W or Z bosondecays are required to be spatially isolatedfromother particles [59,60].The presenceof charged and neutral particles from pileup vertices is taken into account in the isolation requirement of both muons and elec-trons. Muon andelectron identification and isolation efficiencies aremeasuredviathetag-and-probetechnique[61]usinginclusive samples of Z
→
events from data and simulation. Correction factors are applied to account for differences between data and simulation.Jets are reconstructed fromall particles using the anti-kT jet clustering algorithm implemented in fastjet [62,63] with a dis-tance parameter of 0.5. The contribution to the jet energy from particles originatingfrom pileup vertices is removed following a procedure based on the effective jet area described in Ref. [64]. Furthermore, jet energy corrections are applied as a function of jet pT and
η
correcting jet energies to the generator level re-sponse ofthe jet, on average. Jets originatingfrom pileup inter-actions areremoved by amultivariate pileupjetidentification al-gorithm[65].The missingtransverse momentum vector
pmissT is definedas the negative vector sumofthe transversemomenta ofall recon-structedparticlesinthevolumeofthedetector(electrons,muons, photons, andhadrons). Its magnitudeis referred toas Emiss
T . The
Emiss
T reconstructionisimprovedbytakingintoaccountthejet en-ergyscalecorrectionsandthe
φ
modulation,duetocollisions not beingatthenominalcentreofCMS[66]
.A multivariateregression correctionof Emisshadron-plus-stripsalgorithm
[68]
,whichconsiderscandidateswith one chargedpion andup to two neutralpions, or threecharged pions. The neutral pions are reconstructed as “strips” of elec-tromagnetic particles taking into account possible broadening of calorimeterenergydepositionsintheφ
directionfromphoton con-versions.Theτ
h candidatesthat are alsocompatiblewithmuons orelectronsarerejected.Jetsoriginatingfromthehadronisation of quarksandgluonsaresuppressedbyrequiringtheτ
hcandidateto beisolated.Thecontributionofchargedandneutralparticlesfrom pileupinteractionsisremovedwhencomputingtheisolation. 4. EventselectionTheeventsare selectedwitha combinationof electron,muon and
τ
trigger objects [34,59,60,69]. The identification criteria of these objects were progressively tightened and their transverse momentum thresholds raised asthe LHC instantaneous luminos-ityincreasedoverthedatatakingperiod.A tag-and-probemethod wasusedtomeasuretheefficienciesofthesetriggersindataand simulation,andcorrectionfactorsareappliedtothesimulation.Electrons,muons,and
τ
hareselectedusingthecriteriadefined intheCMSsearch fortheSM Higgsbosonat125 GeV[34]. Spe-cificrequirementsfortheselectionoftheH→
hh→
bbτ τ
andthe A→
Zh→
τ τ
channelsaredescribedbelow.4.1. Event selection of H
→
hh→
bbτ τ
IntheH
→
hh→
bbτ τ
channel,the threemostsensitive final statesare analysed, distinguished by the decaymode of the twoτ
leptonsoriginatingfromtheh boson(μτ
h,eτ
handτ
hτ
h). Intheμτ
handeτ
hfinalstates,eventsareselectedwithamuon with pT>
20 GeV and|
η
|
<
2.
1 or an electron of pT>
24 GeV and|
η
|
<
2.
1,and an oppositely chargedτ
h of pT>
20 GeV and|
η
|
<
2.
3. To reduce the Z→
μμ
,
ee contamination, events with twomuonsorelectronsof pT>
15 GeV,ofopposite charges,and passinglooseisolationcriteriaarerejected.In the
μτ
h and eτ
h final states, the transverse mass of the muonorelectronandpmissTmT
=
2pTEmissT
(1
−
cosφ),
(1)where pT isthelepton transversemomentumand
φ
isthe dif-ferenceintheazimuthalanglebetweentheleptonmomentumandpmissT ,isrequiredto be lessthan 30 GeV to rejecteventscoming fromW
+
jets andt¯
t backgrounds.Them
T distributionfortheμτ
h finalstateisshowninFig. 1
.In the
τ
hτ
h final state, events with two oppositely charged hadronically decayingτ
leptons with pT>
45 GeV and|
η
|
<
2.
1 areselected.Inadditiontothe
τ τ
selection,each selectedeventmust con-tain atleast two jetswith pT>
20 GeV and|
η
|
<
2.
4.These pT andη
requirementsare necessary to selectjetsthat have a well definedvalueoftheCSVdiscriminator(Section3),whichis impor-tant forcategorisingsignal-like events withtwo b jet candidates comingfromthe125 GeV Higgsbosondecayingtobb.Fig. 1. DistributionofmTforeventsinthe
μτ
h finalstate,containingatleasttwo additionaljets.TheW+jets backgroundisincludedinthe“electroweak”category. MultijeteventsareindicatedasQCD.TheH→hh→bbτ τselectionrequiresmT< 30 GeV fortheμτ
handeτhfinalstates.Simulationstudiesshowthatthemajorityofsignaleventswill have at least one jet passing the medium working point of the CSVdiscriminator.ThejetsareorderedbyCSVdiscriminatorvalue, suchthattheleadingandsubleadingjetsaredefinedasthosewith the two highest CSV values. Then the events are separated into categories,definedas:
•
2jet–0tagwhen neithertheleading norsubleadingjet passes themediumCSVworkingpoint.Onlyasmallamountofsignal iscollectedinthiscategory,whichisbackground-dominated.•
2jet–1tag when only the leading but not the subleading jet passesthemediumCSVworkingpoint.•
2jet–2tagwhenboththeleadingandsubleadingjetspassthe mediumCSVworkingpoint.Thesignalextractionisperformedusingthedistributionofthe reconstructedmassoftheHbosoncandidate.
4.2. Event selection of A
→
Zh→
τ τ
IntheA
→
Zh→
τ τ
channeleight finalstatesareanalysed. ThesearecategorisedaccordingtothedecaymodeoftheZboson andthe decaymode oftheτ
leptons originatingfromthe h bo-son.The Z bosonis reconstructedfrom twosame-flavour, isolated, andoppositelychargedelectronsormuons.IntheZ
→
μμ
(ee) fi-nalstatethemuons(electrons)arerequiredtohave|
η
|
<
2.
4 (2.5) with pT>
20 GeV for the leading lepton and pT>
10 GeV for the subleading lepton. The invariant mass of the two leptons is required to be between60 GeV and 120 GeV. When more than onepairofleptonssatisfythesecriteria,thepairwithaninvariant massclosesttotheZbosonmassisselected.AftertheZ candidatehasbeenchosen,theh
→
τ τ
decayis se-lectedbycombiningthedecayproductsofthetwoτ
leptonsinthe four final statesμτ
h,
eτ
h,
τ
hτ
h,
eμ
.The combination of thelarge contributionfromtheirreducibleZZ backgroundandofthesmall branching fractions ofleptonic taudecays makes theμμ
andee final stateslesssensitivetothe signal,andthereforethey arenot used in the analysis. Depending on the final state, a muonwithpT
>
10 GeV and|
η
|
<
2.
4, or an electron of pT>
10 GeV andFig. 2. DistributionofthevariableLh
Tforeventsintheτhτh finalstate.The re-duciblebackgroundisestimatedfromdata,insteadtheZZirreduciblebackground fromsimulation.
formanoppositely chargedpair.Eventswithadditionallight lep-tonssatisfyingtheserequirementsarerejected.
A requirement on LhT, which is the scalar sum of the visible transverse momenta of the two
τ
candidates originating from the h boson, is applied to lower the reducible background from misidentifiedleptonsaswellastheirreduciblebackgroundfrom ZZ production.The thresholdsofthisrequirementdepend onthe fi-nalstate andhavebeen choseninsuch awayastooptimisethe sensitivityoftheanalysistothepresenceofan A→
Zh signal for A masses between 220 and350 GeV. The distribution of LhT for eventsinthe
τ
hτ
hfinalstatecanbeseeninFig. 2
.In order to reduce the t
¯
t background, eventscontaining a jet with pT>
20 GeV,|
η
|
<
2.
4 and passing the medium working pointoftheCSVbtaggingdiscriminatorareremoved.Thefourfinalobjectsarefurtherrequiredtobeseparatedfrom eachother by
R
=
(
η
)
2+ (
ϕ
)
2 largerthan 0.5(whereϕ
is inradians),andtocomefromthesameprimaryvertex.Inthischannelthesignalextractionisperformedusingthe dis-tributionofthereconstructedmassoftheAbosoncandidate. 5. Backgroundestimation
5.1. Background estimation for H
→
hh→
bbτ τ
ThebackgroundstotheH
→
hh→
bbτ τ
finalstateconsist pre-dominantlyoft¯
t events,followedbyZ→
τ τ
+
jets events,W+
jets events, and QCD multijet events, with other small contributions from Z→
, diboson, and single top quark production.The es-timation of the shapes of the reconstructed H mass and of the yields of the major backgrounds isobtained fromdata wherever possible.TheZ
→
τ τ
process constitutesanirreduciblebackgrounddue to its final state involving twoτ
leptons, whichonly differ from theh→
τ τ
signalbyhavinganinvariantmassclosertothemass ofthe Z bosoninstead ofthe Higgsboson. Requiringtwo jetsin the eventgreatly reduces this backgroundand the btagging re-quirements reduce it even further. Nevertheless, it still remains an important source of background events, in particular in the 2jet–1tag and2jet–0tag categories. This background isestimated usingasampleofZ→
μμ
eventsfromdata,obtainedbyrequiring two oppositely charged isolated muons, where the reconstructedFig. 3. Distributionsofthereconstructedfour-bodymasswiththekinematicfitafter applyingmassselectionsonmτ τandmbbinthe
μτ
hchannel.Theplotsareshown foreventsinthe2jet–0tag(top),2jet–1tag(middle),and2jet–2tag(bottom) cate-gories.Theexpectedsignalscaledbyafactor10isshownsuperimposedasanopen dashedhistogramfortanβ=2 andmH=300 GeV inthelowtanβscenarioofthe MSSM.Expectedbackgroundcontributionsareshownforthevaluesofnuisance pa-rameters(systematicuncertainties)obtainedafterfittingthesignalplusbackground hypothesistothedata.Fig. 4. Distributionsofthereconstructedfour-bodymasswiththekinematicfitafter applyingmassselectionsonmτ τandmbbintheeτhchannel.Theplotsareshown foreventsinthe2jet–0tag(top),2jet–1tag(middle),and2jet–2tag(bottom) cate-gories.Theexpectedsignalscaledbyafactor10isshownsuperimposedasanopen dashedhistogramfortanβ=2 andmH=300 GeV inthelowtanβscenarioofthe MSSM.Expectedbackgroundcontributionsareshownforthevaluesofnuisance pa-rameters(systematicuncertainties)obtainedafterfittingthesignalplusbackground hypothesistothedata.
Fig. 5. Distributionsofthereconstructedfour-bodymasswiththekinematicfitafter applyingmassselectionsonmτ τandmbbinthe
τ
hτhchannel.Theplotsareshown foreventsinthe2jet–0tag(top),2jet–1tag(middle),and2jet–2tag(bottom) cate-gories.Theexpectedsignalscaledbyafactor10isshownsuperimposedasanopen dashedhistogramfortanβ=2 andmH=300 GeV inthelowtanβscenarioofthe MSSM.Expectedbackgroundcontributionsareshownforthevaluesofnuisance pa-rameters(systematicuncertainties)obtainedafterfittingthesignalplusbackground hypothesistothedata.Fig. 6. InvariantmassdistributionsfordifferentfinalstatesoftheA→Zh processwhereZ decaystoee.Theexpectedsignalscaledbyafactor5isshownsuperimposed asanopendashedhistogramfortanβ=2 andmA=300 GeV inthelowtanβscenarioofMSSM.Expectedbackgroundcontributionsareshownforthevaluesofnuisance parameters(systematicuncertainties)obtainedafterfittingthesignalplusbackgroundhypothesistothedata.(Forinterpretationofthereferencestocolourinthisfigure, thereaderisreferredtothewebversionofthisarticle.)
muons are replaced by the reconstructed particles from simu-lated
τ
decays. A correction for a contamination from t¯
t events is applied to the Z→
μμ
selection. Thistechnique substantially reduces the systematic uncertainties dueto the jet energy scale andthe missingtransverse energy, asthese quantities are mod-elledwithdata.Forthet
¯
t background,bothshapeandnormalisationaretaken from Monte Carlo simulation (MC), and the results are checked against data ina control region wherethe presence of t¯
t events isenhanced by requiringeμ
in thefinal state insteadof aditau, andatleastonebtaggedjet.Another significant source of background is from QCD multi-jetevents,whichcanmimicthesignalinvariousways, e.g.where oneormorejetsaremisidentifiedas
τ
h.Intheμτ
handeτ
h chan-nels,the shapeoftheQCD backgroundisestimatedusingan ob-servedsample ofsame-sign (SS)τ τ
events.Theyield isobtained by scaling theobserved numberof SSevents by theratio ofthe opposite-sign(OS)to SSeventyields obtainedina QCD-enriched regionwithrelaxedleptonisolation.Intheτ
hτ
hchannel,theshape isobtainedfromOS eventswithrelaxedτ
isolation. The yieldis obtained by scaling these events by the ratio of SS events with tighterandrelaxedτ
isolation.In the
μτ
h and eτ
h channels, W+
jets events in which there isa jetmisidentified asaτ
h are anothersizeablesourceof back-ground. The W+
jets shape is modelledusing MC simulationand the yieldisestimatedusinga controlregion ofeventswithlargemTclosetotheW mass.Inthe
τ
hτ
hchannelthisbackgroundhas been foundto be lessrelevantandits shape andyieldare taken fromMCsimulation.The contribution of Drell–Yan production of muon and elec-tron pairs is estimatedfrom simulationafter rescaling the simu-latedyieldtothatmeasuredfromobservedZ
→
μμ
events.Inthe eτ
hchannel, theZ→
ee simulationisfurthercorrectedusingthe e→
τ
h misidentification ratemeasured indata using a tag-and-probetechnique[61]onZ→
ee events.Finally the contributions of other minor backgrounds such as diboson and single top quark events are estimatedfrom simula-tion. Possible contributions fromSM Higgs boson production are estimatedandfoundtohaveanegligibleeffectonthefinalresult.
5.2. Background estimation for A
→
Zh→
τ τ
The backgrounds to the A
→
Zh channel can be divided into a reduciblecomponentandanirreduciblecomponentwhich con-tributeinequalparts.Fig. 7. InvariantmassdistributionsfordifferentfinalstatesoftheA→Zh processwhereZ decaysto
μμ
.Theexpectedsignalscaledbyafactor5isshownsuperimposed asanopendashedhistogramfortanβ=2 andmA=300 GeV inthelowtanβscenarioofMSSM.Expectedbackgroundcontributionsareshownforthevaluesofnuisance parameters(systematicuncertainties)obtainedafterfittingthesignalplusbackgroundhypothesistothedata.(Forinterpretationofthereferencestocolourinthisfigure, thereaderisreferredtothewebversionofthisarticle.)Thepredominant source ofirreducible backgroundis from ZZ production that yields exactly the same final states as the ex-pectedsignal. Other “rare” sources ofirreducible background are SMHiggsbosonassociatedproductionwithaZ boson,t
¯
tZ produc-tionwheretheZ bosondecaysintoamuonoranelectronpairand both top quarksdecay leptonically (to e,μ
,orτ
h), andtriboson events(WWZ,WZZ,ZZZ). The contributions ofall theirreducible backgrounds after the final selection are estimated from simula-tion.Thereduciblebackgroundshaveatleastoneleptoninthefinal statethatisduetoamisidentifiedjetthatpassesthelepton iden-tification.In
τ
hτ
hfinalstates,thereduciblebackgroundis essen-tiallycomposed ofZ+
jetsevents withatleast twojets, whereas inμτ
h ande
τ
hfinal states,the maincontributionto the re-duciblebackgroundcomesfromWZ+
jetswiththreelightleptons. Thecontributionfromtheseprocessestothefinalselectedevents isestimatedusingcontrolsamplesindata.Theprobabilities fora jet that passesrelaxedlepton selection criteriatopassthefinalidentificationandisolationcriteriaof elec-trons,muons, and
τ
leptonsare measuredin a signal-freeregionasafunctionofthetransversemomentumoftheobjectclosestto thecandidate, f
(
pfakeT)
.Inthisregion,eventsarerequiredtopass all thefinal stateselections, exceptthatthe reconstructedτ
can-didates are required to have the same sign and to pass relaxed identificationandisolationcriteria. Thiseffectivelyeliminates any possiblesignal,whilemaintainingroughly thesameproportionof reduciblebackgroundevents.Inordertousethemisidentificationprobabilities f
(
pfake T)
, side-bands are defined for each channel, where, unlike the relaxed criterion, the final identification or isolation criterion is not sat-isfied for one or more of the final state lepton candidates. The number of reducible background events due to a lepton being misidentified in the final selection is estimated by applying the weight f(
pfakeT
)/(
1−
f(
pfakeT))
to the observed events with lep-toncandidatesinthesidebandthatsatisfytherelaxedbutnotthe finalidentificationorisolationcriterion.Finally,thereducible back-groundshape ofthe reconstructedA mass isobtainedfroma SS signal-free region wheretheτ
candidates havethe same charge andrelaxedisolationcriteria.PossiblecontributionsfromSMHiggs bosonproductionareestimatedandfoundtohaveanegligible ef-fectonthefinalresult.Fig. 8. Upperlimitsat95%CLontheH→hh→bbτ τ crosssectiontimesbranchingfractionforthe
μτ
h(topleft),eτh(topright),τ
hτh(bottomleft),andforfinalstates combined(bottomright).6. Systematicuncertainties
The shape of the reconstructed mass of the A and H boson candidates, used for signal extraction, and the normalisation are sensitivetovarioussystematicuncertainties.
The main contributions to the normalisation uncertainty that affectthe signal andthe simulated backgrounds include the un-certaintyinthetotalintegratedluminosity,whichamountsto2.6%
[70], andthe identificationandtriggerefficiencies ofmuons(2%) andelectrons (2%). The
τ
h identificationefficiency has a 6% un-certainty(8%intheτ
hτ
hchannel),whichismeasuredinZ/
γ
∗→
τ τ
→
μτ
h eventsusinga tag-and-probetechnique. Thereis a3% uncertaintyintheefficiencyonthe hadronicpartoftheμτ
h and eτ
h triggers,anda4.5%uncertaintyoneachofthetwoτ
h candi-datesrequiredbytheτ
hτ
htrigger.Thebtaggingefficiencyhasan uncertaintyof2–7%,andthemistag rateforlight-flavourpartons is accurate to 10–20% depending onη
and pT [67]. The back-groundnormalisation uncertainties fromthe estimation methods discussedinSection5arealsoconsidered.IntheH→
hh→
bbτ τ
channel this uncertainties amount to 2–40% depending on the eventcategoryandonthefinalstate.Theuncertaintiesofreducible backgroundstotheA
→
Zh channelareestimatedbyevaluatinganindividual uncertainty for each lepton misidentification rate and applyingittothebackgroundcalculation.Thisamountsto15–50% depending on the final
τ τ
state considered. The main uncer-tainty in the estimation of the ZZ background arises from the theoreticaluncertaintyintheZZ productioncrosssection.Uncertaintiesthat contribute to variations inthe shape ofthe mass spectrum include the jet energy scale, which varies with jet pT andjet
η
[71],andtheτ
lepton(3%)energyscale[34].Theoretical uncertainties onthe crosssection forsignal derive fromPDFandQCDscaleuncertaintiesanddependonthechoiceof signalhypothesis.Formodelindependentresultsnochoiceofcross sectionismadeandhencenotheoreticaluncertaintiesare consid-ered.FortheMSSMinterpretationtheuncertaintiesdependon
m
A andtanβ
andamountto2–3%forPDFuncertaintiesand5–9%for scale uncertainties, evaluated as described in [27] andusing the PDF4LHC recommendations [55]. No theoretical uncertainties are consideredinthe2HDMinterpretation.7. Resultsandinterpretation
The ditau(mττ ) massisreconstructedusinga dedicated algo-rithmcalled SVFit[72],whichcombinesthevisiblefour-vectorsof
Fig. 9. Upperlimitsat95%CLoncrosssectiontimesbranchingfractiononA→Zh→LLτ τ foreμ(topleft),μτh (topright),eτh(bottomleft),andτhτh(bottom right)finalstates.
the
τ
leptoncandidatesaswellasthe EmissT andits experimental resolutioninamaximumlikelihoodestimator.Forthe H
→
hh→
bbτ τ
process, the chosen distribution for signalextractionisthefour-bodymass.Thedecayproducts ofthe twoh bosonsneedtofulfillstringentkinematicconstraints,dueto thesmallnaturalwidthoftheh.Theseconstraintscanbeusedin akinematic fit inorder toimprove theevent reconstruction and tobetterseparatesignaleventsfrombackground.Thecollinear ap-proximationfor the decay products of theτ
leptons isassumed inthefit,sincetheτ
leptons arehighlyboostedasthey originate froman objectthat isheavy when comparedto their ownmass. Furthermore,it is assumed that the reconstruction of the direc-tionsofallfinalstateobjectsisaccurateandtheuncertaintiescan beneglected comparedto theuncertainties ontheenergy recon-struction.Inthedecayofthetwoτ
leptons,atleasttwoneutrinos are involved and there is no precise measurement of the origi-nalτ
lepton energies. Forthisreason, theτ
lepton energies are constrainedfromthebalanceofthefittedH bosontransverse mo-mentumandthereconstructedtransversalrecoildeterminedfromEmissT reconstructionalgorithms,asdescribedinSec.3.The recon-structedmassobtainedwiththekinematicfitisdenotedby
m
kinfitH (seetheSupplementarymaterialforadetaileddescription).
The signal-to-background ratio is greatly improved by select-ing events that are consistent with a mass of 125 GeV for both thedijet(mbb)massandtheditaumass(mττ )reconstructedwith SVFit.The mass windows of theselections are optimised to col-lect as much signal as possible while rejecting a large part of the background. They correspond to 70
<
mbb<
150 GeV and 90<
mττ<
150 GeV.TheinvariantmassdistributionsoftheH bo-sonindifferentfinalstatesareshowninFigs. 3,
4 and 5.For the A
→
Zh→
τ τ
process, the A boson mass is recon-structed from the four-vector information of the Z boson can-didate and the four-vector information of the h boson candi-date as obtainedfrom SVFit. The invariant mass distributions of the A boson in the different final states are shown in Figs. 6 and 7. Theτ
hτ
h final states have a comparable contribution from reducible and irreducible backgrounds, while thee
μ
fi-nal states are dominated by the irreducible ZZ production. The backgroundlabelledas“rare”collects togetherthesmaller contri-butions fromthe triboson processesasdiscussed inthe previous section.In neithersearch do the invariant massspectra show any ev-idence ofa signal.Model independentupper limitsat95% confi-dencelevel(CL)onthecrosssectiontimesbranching fractionare
Fig. 10. Upperlimitsat95%CLoncrosssectiontimesbranchingfractiononA→ Zh→LLτ τforallτ τ finalstatescombined(top)andcomparisonofthedifferent finalstates(bottom).
setusingabinned maximumlikelihoodfitforthe
signal plus
back-ground and backback-ground-only hypotheses. Thelimitsaredetermined using the CLs method [73,74] and the procedure is described in Refs.[75,76].Systematicuncertaintiesaretakenintoaccountasnuisance pa-rameters in the fit procedure: normalisation uncertainties affect the signal and backgroundyields. Uncertaintieson the
τ
energy scaleandjetenergyscalearepropagatedasshapeuncertainties.The model independent expected and observed cross section times branching fraction limits for the H
→
hh→
bbτ τ
process areshowninFig. 8
andfortheA→
Zh→
LLτ τ
processinFigs. 9
and10whereL=
e,
μ
orτ
inordertoreflectthe smallZ→
τ τ
contributiontothesignalacceptance.
We interpret the observed limits on the cross section times branchingfractionintheMSSMand2HDMframeworks,discussed inSection1.
IntheMSSMweinterprettheminthe“lowtan
β
”scenario[27, 78]inwhichthevalueofM
SUSY isincreaseduntilthemassofthe lightestHiggsbosonisconsistentwith125 GeV overarangeoflow tanβ
andm
A values.The exclusionregionin them
A–tanβ
plane forthecombinationofthe H→
hh→
bbτ τ
andA→
Zh→
τ τ
Fig. 11. The95%CLexclusionregioninthemA–tanβplaneforthelowtanβ sce-narioasdiscussedintheintroduction,combiningtheresultsoftheH→hh→bbτ τ
and theA→Zh→ τ τ analysis.Theareahighlighted inblue belowthe black curvemarkstheobservedexclusion.Thedashedcurveandthegreybandsshow theexpectedexclusionlimitwiththerelativeuncertainty.Theredareawiththe back-slashlinesatthelower-leftcorneroftheplotindicatestheregionexcludedby themassoftheSM-likescalarbosonbeing125 GeV.Thelimitfallsoffrapidlyas mAapproaches350 GeV becausedecaysoftheA totwotopquarksarebecoming kinematicallyallowed.(Forinterpretationofthereferencestocolourinthisfigure legend,thereaderisreferredtothewebversionofthisarticle.)
analyses,insuchascenario,isshownin
Fig. 11
.Thelimitfallsoff rapidlyasm
Aapproaches350 GeV becausedecaysoftheA totwo topquarksarebecomingkinematicallyallowed.The interpretationoftheobserved limitsinaType II2HDMis performedinthe“physicsbasis”.The inputstothisinterpretation are the physicalHiggs boson masses(mh,
m
H, mA,m
H±),the ra-tioofthevacuumexpectationenergies(tanβ
),theCP-evenHiggs mixingangle(α
) andm
212
=
m2A[
tanβ/(
1+
tanβ
2)
]
.Forsimplicity weassumethatm
H=
mA=
mH±.The cross sections andbranching fractionsin the2HDMwere calculated as described by the LHC Higgs Cross Section Working Group [77,78].The exclusionregions,calculatedusingthe combi-nation ofthe H
→
hh→
bbτ τ
andA→
Zh→
τ τ
analyses, in the cos(β
−
α
)
vs. tanβ
plane forsuch a Type II2HDMscenario withaheavy Higgsbosonmassof300 GeV areshowninFig. 12
. ThiscanbecomparedtoFig. 5inRef.[41].8. Summary
A search for a heavy scalar Higgs boson (H) decaying into a pairofSM-likeHiggsbosons(hh)andasearchforaheavyneutral pseudoscalarHiggsboson (A)decayingintoa Z bosonanda SM-like Higgsboson(h), havebeenperformedusingeventsrecorded by theCMS experiment atthe LHC. The data set corresponds to anintegratedluminosityof19
.
7 fb−1,recordedat8 TeV centre-of-massenergyin2012.Noevidenceforasignalhasbeenfoundand exclusion limitson the production cross section times branching fraction forthe processes H→
hh→
bbτ τ
andA→
Zh→
LLτ τ
are presented. The results are also interpreted in the context of theMSSMand2HDMmodels.
Fig. 12. The95%CLexclusionregionsinthecos(β−α)vs.tanβplaneof2HDM Type IImodelformA=mH=300 GeV,combiningtheresultsofthe H→hh→ bbτ τ andA→Zh→ τ τ analysis.Theareashighlightedinblueboundedbythe blackcurvesmarktheobservedexclusion.Thedashedcurvesandthegreybands showtheexpectedexclusionlimitwiththerelativeuncertainty.(Forinterpretation ofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheweb versionofthisarticle.)
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,andNSFC(China);COLCIENCIAS (Colombia);MSESandCSF(Croatia);RPF(Cyprus);MoER,ERCIUT andERDF(Estonia); AcademyofFinland,MEC, andHIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic ofKorea); LAS (Lithuania);MOE andUM (Malaysia); CINVESTAV, CONACYT,SEP,andUASLP-FAI(Mexico);MBIE(NewZealand);PAEC (Pakistan);MSHEandNSC(Poland);FCT(Portugal);JINR(Dubna); MON,RosAtom,RASandRFBR(Russia);MESTD(Serbia);SEIDIand CPAN(Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEPCenter,IPST,STARandNSTDA(Thailand);TUBITAKandTAEK (Turkey);NASUandSFFR(Ukraine);STFC (UnitedKingdom);DOE andNSF(USA).
Individuals have received support from the Marie-Curie pro-gramme and the European Research Council and EPLANET (Eu-ropean Union); the Leventis Foundation;the A. P. Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherchedansl’Industrieetdansl’Agriculture(FRIA-Belgium);the
doctoral Fellowship,Chulalongkorn University(Thailand); andthe WelchFoundation,contractC-1845.
Appendix A. Supplementarymaterial
Supplementarymaterialrelatedtothisarticlecanbefound on-lineat
http://dx.doi.org/10.1016/j.physletb.2016.01.056
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YerevanPhysicsInstitute,Yerevan,Armenia
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NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus
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UniversiteitAntwerpen,Antwerpen,Belgium
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N. Daci,
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VrijeUniversiteitBrussel,Brussel,Belgium
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GhentUniversity,Ghent,Belgium
S. Basegmez,
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UniversitédeMons,Mons,Belgium
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M. Vutova
InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria
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I. Glushkov,
L. Litov,
B. Pavlov,
P. Petkov
UniversityofSofia,Sofia,Bulgaria
M. Ahmad,
J.G. Bian,
G.M. Chen,
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T. Cheng,
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UniversidaddeLosAndes,Bogota,Colombia
N. Godinovic,
D. Lelas,
I. Puljak,
P.M. Ribeiro Cipriano
UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia
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M. Kovac
UniversityofSplit,FacultyofScience,Split,Croatia
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K. Kadija,
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H. Rykaczewski
UniversityofCyprus,Nicosia,Cyprus
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M. Finger
10,
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V. Karimäki,
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J. Tuominiemi,
E. Tuovinen,
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HelsinkiInstituteofPhysics,Helsinki,Finland
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T. Tuuva
LappeenrantaUniversityofTechnology,Lappeenranta,Finland
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F. Couderc,
M. Dejardin,
D. Denegri,
B. Fabbro,
J.L. Faure,
C. Favaro,
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A. Rosowsky,
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A. Zghiche
DSM/IRFU,CEA/Saclay,Gif-sur-Yvette,France
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P. Verdier,
S. Viret
UniversitédeLyon,UniversitéClaudeBernardLyon1,CNRS–IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France
T. Toriashvili
17GeorgianTechnicalUniversity,Tbilisi,Georgia
Z. Tsamalaidze
10TbilisiStateUniversity,Tbilisi,Georgia
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S. Thüer
RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany
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RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany
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A. Vanhoefer,
B. Vormwald
UniversityofHamburg,Hamburg,Germany
M. Akbiyik,
C. Barth,
C. Baus,
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E. Butz,
T. Chwalek,
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InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece
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N. Saoulidou,
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UniversityofAthens,Athens,Greece
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G. Flouris,
C. Foudas,
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J. Strologas
UniversityofIoánnina,Ioánnina,Greece