Search for a low-mass pseudoscalar Higgs boson produced in association with a bb̅ pair in pp collisions at √s=8 TeV

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for a low-mass pseudoscalar Higgs boson produced in association with a bb pair in pp collisions at √

s = ^{8 TeV}

.CMS Collaboration

^

CERN,Switzerland

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

Articlehistory:

Received11November2015 Receivedinrevisedform21April2016 Accepted2May2016

Availableonline6May2016 Editor:M.Doser

Keywords:

CMS Physics Higgs

AsearchisreportedforalightpseudoscalarHiggsbosondecayingtoapairofτ leptons,producedin association with abb pair,in thecontext oftwo-Higgs-doublet models.The results are basedonpp collision data atacentre-of-mass energyof 8 TeVcollected bythe CMS experiment atthe LHCand corresponding to an integratedluminosity of19.7 fb⁻¹.Pseudoscalar boson massesbetween 25 and 80 GeVareprobed.Noevidenceforapseudoscalarbosonisfoundandupperlimitsaresetontheproduct ofcrosssectionandbranchingfractiontoτpairsbetween7and39 pbatthe95%confidencelevel.This excludespseudoscalarAbosonswithmassesbetween25and80 GeV,withSM-likeHiggsbosonnegative couplingstodown-typefermions,producedinassociationwithbb pairs, inType II,two-Higgs-doublet models.

©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

The discovery of a newboson with a mass closeto 125 GeV [1–3], consistentwiththe standard model(SM) Higgsboson,has shedlightononeofthemostimportantquestionsofphysics:the originofthemassofelementaryparticles. Althoughallthe mea- surementsmadeuptonowareinimpressiveagreementwiththe predictionsoftheSM[4,5],theSMcannot addressseveralcrucial issues such as the hierarchy problem, the origin of the matter- antimatterasymmetry andthenatureofdarkmatter[6–9].Theo- riespredictingnewphysicsbeyondthestandardmodelhavebeen proposed toaddress theseopen questions.Manyof thempredict theexistenceofmorethanoneHiggsboson.

Two-Higgs-doublet models (2HDM) [10–14] are a particularly simpleextension ofthe SM. Starting withthe two doubletfields

1 and2 andassumingan absenceofCPviolationintheHiggs sector, after SU(2)L symmetry breaking five physical states are left: two CP-even (h and H), one CP-odd (A), and two charged (H^±)bosons.Toavoidtree-levelflavourchangingneutralcurrents, one imposes a Z2 symmetry according to which the Lagrangian is requiredto be invariant under 1→ 1, 2→ −2.The re- sultis four distinct classes ofmodels, corresponding to different patternsofquarkandleptoncouplings.Themostcommonlycon- sideredare Type I and Type II.In Type I, all quarks andleptons

 E-mailaddress:[email protected].

obtain masses from 1^{. In Type II, up-type quarks masses are} derived from1≡^v1 anddown-typequarks andcharged lep- tonsmassesare derived from2≡^v2.In thelimit ofan exact Z2symmetry[15],theHiggssectorofa2HDMcanbedescribedby sixparameters:fourHiggsboson masses(mh,mH,mA,andm_H±), the ratio of the vacuum expectation values of the two doublets (tanβ≡^v2/v1)andthemixingangle

α

ofthetwoneutralCP-even Higgs states.Allowing a softbreaking of theZ2 symmetry intro- ducesanewHiggsmixingparameterm²₁₂[11].Inthe“decoupling limit”of2HDMs[16,17],themassesmH,mA,andm_H± arealllarge, cos(β−

α

)^{1, andh is the observed bosonat} 125 GeV andis SM-like.AnSM-likeh orH at125 GeV canalsobeobtainedinthe

“alignment limit” [16,17] without the other bosons being heavy.

ThisisaninterestingcaseandcanbecompatiblewiththeSM-like Higgs boson total width measurements and branching fractions evenifoneormoreofthelightHiggsbosonshave amassbelow halfof125 GeV providedoneadjuststhemodelparameterssothat thebranchingfractionoftheSMHiggsbosontopairsoflightHiggs bosonsisverysmall.ThisscenariocanbetestedattheCERNLHC bysearchingforsinglyproducedlightbosonsdecayingtoapairof

τ

leptonswithlargecrosssections.InType II2HDMs,iftheHiggs couplingtothethirdgenerationofquarksisenhanced,ashappens atlargetanβ,alargeproductioncrosssectionisexpectedforthe production ofthe low-mass A boson in association withbb. The crosssection isoftheorderof1 pbforregionsofthe2HDMpa- rameterspacewithsin(β−

α

)≈^1,cos(β−

α

)>0 andsmallm²₁₂. The cross section can be much larger, between 10 and 100 pb, http://dx.doi.org/10.1016/j.physletb.2016.05.003

0370-2693/©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

forsomeotherregionsoftheparameterspace,i.e. sin(β±

α

)≈^1, cos(β−

α

)<0 and tanβ >5 [18,19], wherethe coupling ofthe SM-likehbosontodown-typefermionsisnegative(“wrongsign”

Yukawacoupling).Consequently, giventhelarge productioncross sectionoftheA bosoninsuchscenarios,theLHCdataaresensitive toitspresenceforsomecombinationsofmodelparameters.

Previoussearches fordi-

τ

resonances [20,21]have mainly fo- cusedonmassesgreaterthanthemassoftheZ boson,forexam- pleinthecontextoftheminimalsupersymmetricstandardmodel (MSSM)[22–24],whichisahighlyconstrained2HDMofType II.In fact,alightpseudoscalarHiggsbosonisexcludedintheMSSM,but anA bosoncanstillhavequitealowmassingeneral2HDMs,even givenalltheconstraintsfromLEP,TevatronandLHCdata[18,19].

Thisletterpresentsasearchforalow-masspseudoscalarHiggs boson produced in association with a bb pair and decaying to a pair of

τ

leptons. Associated production of the A boson with a bb pair has the advantage that there is a higher signal over backgroundratio relative togluon–gluon fusionproduction. Such a signature is also relevant in the context of light pseudoscalar mediatorsandcoydarksectors[25].The analysisisbasedon pp collisiondataatacentre-of-massenergyof8 TeV recordedbythe CMS experiment at the LHC in 2012. The integrated luminosity amounts to 19.7 fb⁻¹.The

τ

leptons are reconstructed via their muon, electron andhadronic decays.In the following, the terms leptonsrefer toelectronsandmuons, whereas

τ

s thatdecayinto hadrons+

ν

τ aredenoted by

τ

h.The invariant massdistributions ofthe

τ

pairsinallthreechannelsareusedtosearchforpseudo- scalarbosonswithmassesbetween25and80 GeV.

2. TheCMSdetectorandeventsamples

The central feature of the CMS apparatus is a superconduct- ingsolenoidof 6 minternal diameter,providing amagnetic field of3.8 T.Withinthesolenoidvolumeare asilicon pixelandstrip tracker,aleadtungstatecrystalelectromagneticcalorimeter(ECAL), andabrassandscintillatorhadroncalorimeter(HCAL),each com- posed of a barrel and two endcap sections. Muons are detected ingas-ionisation detectorsembeddedinthesteelflux-returnyoke outsidethesolenoid. Extensive forward calorimetrycomplements thecoverage provided by the barrelandendcap detectors.A de- taileddescription ofthe CMSdetector,together withadefinition ofthecoordinatesystemusedandtherelevantkinematicvariables, canbefoundinRef.[26].

The first level of the CMS triggering system (Level-1), com- posedofcustom hardware processors, uses informationfromthe calorimetersandthemuonsdetectorstoselectthemostinterest- ingeventsinafixedtimeintervaloflessthan4 μs.Thehigh-level trigger(HLT)processorfarmfurtherdecreasestheeventratefrom around100 kHztolessthan1 kHz,beforedatastorage.

Asetof MonteCarlo(MC) simulatedevents isused tomodel thesignalandbackgrounds.Drell–Yan, Wbosonproductionasso- ciated to additional jets, production of top quark pairs (tt), and diboson(WW, WZ and ZZ) backgrounds are generatedusing the leading order (LO) MadGraph 5.1 package [27]. Single top quark samplesareproduced usingthenext-to-leading-order (NLO)gen- erator powheg (v1.0) [28]. Simulatedsamplesof gluon–gluon fu- sion to bbA signal events are generated with pythia 6.426 [29]

formassesbetween25and80 GeV in5 GeV steps.As noloopis involvedatleadingorderinthebbA productionprocess,theprod- uct of acceptance and efficiency for signal only depends on the A boson mass, withno dependence on other model parameters.

The simulated samples are produced using the CTEQ6L1 parton distributionfunction (PDF) set [30]. All thegenerated signal and backgroundsamplesareprocessedwiththesimulationoftheCMS detectorbasedon geant 4[31].

Additional eventsareadded tothe MC-simulatedevents,with weights corresponding totheluminosity profile indata,tosimu- lateLHCconditionsandthepresenceofother softppinteractions (pileup)inthesameorneighbouringbunchcrossingsofthemain interaction. Finally, identical algorithms and procedures are used toreconstructbothsimulatedeventsandthecollecteddata.

3. Eventreconstruction

Event reconstruction is based on the particle-flow (PF) algo- rithm[32,33],whichaimstoexploittheinformationfromallsub- detectors to identifyindividual particles (PF candidates): charged andneutralhadrons, muons,electrons,andphotons. Complexob- jects, such as

τ

leptons that decayinto hadronsand a neutrino, jets,andtheimbalanceinthetransversemomentumintheevent arereconstructedfromPFcandidates.

Thedeterministicannealingalgorithm[34,35]isusedtorecon- structthecollisionvertices.Thevertexwiththemaximumsumof squaredtransversemomenta(p²_T)ofallassociatedtracksisconsid- eredastheprimaryvertex.Muons,electrons,and

τ

hsarerequired tooriginatefromtheprimarycollisionvertex.

Muon reconstruction starts by matching tracks in the silicon trackerwithtracksintheoutermuonspectrometer[36].A global muontrackisfittedtothehitsfrombothtracks.A preselectionis appliedto thesemuon tracksthat includesrequirementsontheir impact parameters, to distinguish genuine prompt muons from spurious muonsor muons fromcosmic rays. In addition, muons are required to pass isolation criteriato separate prompt muons from those associated witha jet, usually from the semi-leptonic decaysofheavy quarks.The muonrelative isolation isdefinedas thefollowing[26]:

I_rel

=

⎡

⎣ 

charged

p_T

+

max

⎛

⎝

0

, 

neutral

pT

+ 

γ pT

−

¹

2



charged,PU

pT

⎞

⎠

⎤

⎦ /

p^μ_T

,

(1)

where all sums are over the scalar p_T ofparticles inside a cone with size of R=

(

η

)²+ ( φ)²=⁰.4 relative to the muon direction, where

η

is the pseudorapidity and φ is the azimuthal angle (in radians) in theplane transverse to the beamaxis, and

“charged” corresponds to charged hadrons, muons, andelectrons originating from the primary vertex, “neutral” refers to neutral hadronsand“charged, PU”refers to chargedhadrons,muons, and electronsoriginatingfromotherreconstructedvertices.Thelastof thesesumsisusedtosubtracttheneutralpileupcomponentinthe computation, andthefactorof1/2 reflectsthe approximateratio ofneutraltochargedparticlesinjets[37].

Electron reconstruction starts from ECAL superclusters, which aregroupsofoneormoreassociatedclustersofenergydeposited intheECAL.Superclustersarematchedtotrackseedsintheinner tracker(the closest layers ofthe trackerto theinteraction point) andelectrontracksare formedfromthose.Trajectoriesarerecon- structed based on the modelling of electron energy loss due to bremsstrahlung,andarefittedusingtheGaussian sumfilteralgo- rithm[38].Electronidentificationisbasedonamultivariate(MVA) boosteddecisiontreetechnique[39] todiscriminategenuineelec- tronsfromjetsmisidentifiedaselectrons[40].The mostpowerful variables for thediscrimination of

τ

h candidates are the ratio of energydepositions inthe ECALandHCAL,the angulardifference betweenthetrackandsupercluster,andthedistributionofenergy depositionsintheelectronshower.Relativeisolationisdefinedin

an analogous way to that of Eq. (1) and is used to distinguish promptelectronsfromelectronswithinajet.

JetsarereconstructedfromPFcandidatesusingtheanti-k_T[41]

algorithm witha distanceparameter of 0.5, in the FastJet pack- age [42]. Several corrections are applied to the jet energies to reducetheeffectofpileupandcorrectforthenonlinearresponse of the calorimeters [37]. To identify and reject jets from pileup, an MVA discriminator is defined based on information fromthe vertexand the jet distribution [43].Jets identified as originating fromabquark,calledb-taggedjets,areidentifiedusingthecom- binedsecondaryvertex(CSV)algorithm[44],whichisbasedona likelihoodtechnique, andexploitsinformationsuchastheimpact parametersofcharged-particle tracksandthepropertiesofrecon- structeddecayvertices.

The hadron-plus-strips (HPS) algorithm [45,46] is used to re- constructthe

τ

h candidates.It starts froma jet,andsearches for candidatesproduced bythemain hadronicdecaymodesofthe

τ

lepton:eitherdirectlytoone chargedhadron,orviaintermediate

ρ

anda1(1280) mesons to one chargedhadron plus one ortwo neutral pions, or three charged hadrons with up to one neutral pion.Thechargedhadronsare usuallylong-livedpions,whilethe neutralpions decayrapidlyintotwo photons. The HPSalgorithm takesinto account the possible conversion ofphotons into e⁺e⁻ pairs in material in front of the ECAL, and their corresponding bremsstrahlunginthemagneticfieldwithconsequent broadening ofthe distribution of theshower. Strips are formed from energy depositionsin the ECAL arising from electrons and photons. The stripsizes inECALare0.05×⁰.20 in

η

× φ^.The

τ

hdecaymodes are reconstructed by combining the charged hadrons with ECAL strips.Neutrinos producedin

τ

hdecaysarenot reconstructedbut contributetoE^miss_T .IsolationrequirementsbasedonanMVAtech- nique take into account the p_T of PF candidates around the

τ

leptondirectionandinformationrelatedtoitslifetime,suchasthe transverse impact parameter ofthe leading track of the

τ

h can- didate and its significance for decays to one charged hadron or thedistancebetweenthe

τ

hproductionanddecayverticesandits significancefordecaystothreechargedhadrons.Electronscanbe misidentified as

τ

h candidates withonetrackandECAL strip.An MVAdiscriminatorbasedon propertiesofthereconstructed elec- tron, such asthe distributionof theshower and theratio ofthe ECALandHCALdepositedenergies, isusedto improvepion/elec- tronseparation.Finally,anotherMVAdiscriminatorisusedtosup- pressmuonsreconstructedas

τ

hcandidateswithonetrack.Itex- ploitsinformationabouttheenergydepositedinthecalorimeters with

τ

hcandidates,aswell ashitsandsegmentsreconstructedin themuon spectrometersthatcan bematched tothecomponents ofthe

τ

h.

The missing transverse momentum vector p^miss_T is defined as theprojectionontheplaneperpendiculartothebeamsoftheneg- ativevectorsumofthemomentaofall reconstructedparticles in anevent.ItsmagnitudeisreferredtoasE_T^miss.Toimprovetheres- olution,andreduce theeffectofpileup,a p^miss_T basedonanMVA regressiontechnique[47]isused,whichtakesintoaccountseveral collectionsofparticlesfromdifferentvertices.

Theinvariantmassofthe

τ

pair(mττ )isusedastheobservable for the statistical interpretation of results in all channels andis reconstructedusingthe SVFit algorithm[48].The SVFit algorithm usesa maximumlikelihood techniquewhere thelikelihoodtakes asinputthefour-momentaofthevisibledecayproducts ofthe

τ

, the projection of p^miss_T along the x- and y-axes, as well as the covariancematrixofthecomponentsof p^miss_T .

The relative mττ resolution obtained through the SVFit algo- rithmisabout15%overthewholemassrange.Itisslightlyhigher forthee

μ

channelbecauseofthepresenceofoneadditionalneu- trino.

4. Eventselection

Three di-

τ

final states are considered:

μτ

h, e

τ

h,and e

μ

. The

μμ

andee finalstatesarediscardedbecauseoftheirsmallbranch- ing fractionsandlargebackgrounds,while

τ

h

τ

h isnot considered becauseofinefficienciesduetothetriggerthreshold.

The selection of events in the

μτ

h or e

τ

h final state starts from a trigger that requires a combination of a muon or elec- tronwithp_T>17 or22 GeV,respectively,andanisolated

τ

hwith pT>20 GeV.Thiscombinedtriggerisseededbyasinglemuonor electron, withpT>16 or20 GeV atLevel-1. Theoffline selection requires a muon or electron with pT>18 or 24 GeV, respec- tively,and|

η

|<2.1,andanoppositelycharged

τ

h candidatewith pT>22 GeV and |

η

_|<2.3.Leptons are requiredto pass a tight identification[36,40] andhavea relativeisolation, Irel, <0.1. The

τ

hcandidateshavetopassatightworkingpointoftheMVAdis- criminantthatcombinesisolationandlifetimeinformation(result- ingina

τ

hreconstructionandisolationefficiencyofabout30%and ajetto

τ

hmisidentificationratebetween0.5and1.0 per mille),as well as the requirements to suppress electron and muon candi- dates misidentified as

τ

h, described inSection 3. Leptonsand

τ

h candidatesarerequiredtobeseparatedby R>0.5.Eventswith additionalidentifiedandisolatedelectronsormuonsarediscarded.

TosuppressW+jets andtt backgrounds,thetransversemassbe- tween thelepton transversemomentum ^p _T ^{and pmiss}_T ^,definedin Eq.(2),isrequiredtobesmallerthan30 GeV,

MT

( ,

p

^miss_T

) = 

2p _TE^miss_T

(

1

−

^cos

φ),

(2) where φ is the azimuthal angle betweenthe lepton transverse momentumandthe^{p miss}_T ^vectors.

Events selected in the e

μ

channel must pass a trigger that requires a combination of an electron and a muon, with pT>

17(8) GeV forthe leading (subleading)lepton. Dependingon the flavour of the leading lepton that passes the trigger selection, events are required to have either a muon with pT>18 GeV andan electronwith pT>10 GeV,ora muonwith pT>10 GeV andanelectronwithpT>20 GeV.Thefiducialregionsformuons (electrons)aredefinedby|

η

|<2.1(2.3).Additionally,leptonswith oppositechargeareselectedandrequiredtobespatiallyseparated by R>0.5.

The muonsandelectronsarerequiredtobe isolated,withrel- ative isolation lessthan 0.15 in thebarrel(|

η

|<1.479) andless than0.1 intheendcaps(|

η

_|>1.479).Inaddition,bothmuonsand electrons are required to pass the tight identification criteria as describedinSection3.Eventshavingadditionalidentifiedandiso- latedleptonsarevetoed,similarlytothe

μτ

hande

τ

hchannels.To reducethelargett backgroundinthee

μ

finalstate,alinearcom- binationofthe Pζ andP^vis_ζ variables[49]isused.Pζ and P_ζ^visare definedasfollows:

P_ζ

=

p^μ_T

+

^pe_T

+

^pmiss_T



· ˆζ

^and P^vis_ζ

= 

p^μ_T

+

^peT

 · ˆζ,

⁽³⁾

where ˆζ is the unit vector of the axis bisecting the angle be- tween ^pμ_T ^{and pe}_T ^{of the muon and electron} candidates, respec- tively.Thesevariablestakeintoaccountthefactthattheneutrinos producedin

τ

decaysaremostlycollinearwiththevisible

τ

decay products,butthisisnottrueforneutrinosfromtheothersources, nor for misidentified

τ

h candidates from background. The linear combination P_ζ −

α

P^vis_ζ isrequiredto be greater than −^{40 GeV,} withanoptimalvalueof

α

of1.85,determinedintheCMSsearch foraMSSM Higgsboson inthe

τ τ

final state [21].Tofurtherre- ducett andelectroweakbackgroundsinthee

μ

finalstate,theM_T

betweenthedileptontransversemomentumand ^pmiss_T ^,definedas inEq.(2),isrequiredtobelessthan25 GeV.

Inaddition to theabove selections, events inall channels are alsorequiredtohaveatleastone b-taggedjet withp_T>20 GeV and|

η

_|<2.4,whichpassestheworkingpointoftheCSVb-tagging discriminant (corresponding to b-tagging efficiencyof about 65%

and light-jet misidentification rate of about 1%) and the pileup MVAdiscriminant forjets,andis separatedby atleast R=⁰.5 fromthesignalleptons.

5. Backgroundestimation

Oneofthemainbackgrounds inall threechannelsis Z/

γ

^∗→

τ τ

.Drell–Yaneventswithinvariant masslarger than50 GeV are modelled using “embedded” eventsamples, as follows: Z→

μμ

events are selected in data with an invariant mass larger than 50 GeVtoremovethemassrangebiasedbyatriggerrequirement.

Thereconstructedmuonsarereplacedbysimulated

τ

leptonsthat aresubsequently decayedvia tauola [50].To modelthe detector responsetothe

τ

decayproducts theGEANTbaseddetectorsim- ulation is used.Jets, p^miss_T , and

τ

h are then reconstructed, while lepton isolations are recomputed [51]. This substantially reduces theuncertaintiesrelatedtothemodellingofthe E^miss_T ,thejeten- ergyscale,andtheb jetefficiency.Low-mass Z/

γ

^∗_→

τ τ

events, whichcannot becoveredbytheembedded samples,aretakendi- rectlyfromasimulatedsample.

Multijeteventsoriginatedby QCD processescompriseanother majorbackground,especiallyatlowdi-

τ

mass.Thecontributionof theQCDmultijetbackgroundarisesfromjet→

τ

hmisidentification and to a lesser extent from jet→

μ

and jet→e misidentifica- tion,depending onthefinal state.Other contributions aredueto the presence of muons or electrons from the semi-leptonic de- cays of heavy flavour quarks.This background is estimatedfrom data.

Multijet background normalisation in the

μτ

h and e

τ

h final statesis determined from a sample definedin the same wayas the signal selection described in Section 4, except that the lep- ton andthe

τ

h candidateare required to have electriccharge of samesign(SS).TheeventswiththeSSselectionaredominatedby multijets,andthelimitedcontributionfromtheother processesis subtracted using predictions from simulatedevents. To take into accountthe difference inthe multijetnormalisation betweenthe SS andopposite-sign (OS) regions, an OS/SS extrapolation factor isused to multiply themultijet yield in the SS region. Thisfac- torismeasuredinsignal-freeeventsselectedwithinvertedlepton isolations(0.2<I_rel<0.5) and arelaxed

τ

h isolation. TheOS/SS extrapolation factor is parameterised as a function of mττ , and fittedwithan exponentiallydecreasing function.Thisratiois ap- proximatelyequalto1.2fordi-

τ

massesof20 GeV,anddecreases toabout1.1formassesabove50 GeV.

The mττ distribution for the QCD multijet background is ob- tainedfroma control region in databy inverting the leptoniso- lationandrelaxing the

τ

h isolation. These two selectionsare re- quired to attain a control region populated with QCD multijet eventsandobtainasufficientlysmoothmττ distribution.A correc- tionhasbeenapplied toaccount forthedifferencesbetweenthe nominalselectionandtheselectionusedtoestimatetheQCDmul- tijetmττ distribution.The correctiondependsonthe

τ

h misiden- tification rate (the probability fora

τ

h,that passes a looseriso- lationrequirement, topassthetight isolationselection). Thisrate isparameterised asa function of the pT of the

τ

h in three bins ofpseudorapidity. It was checked that the mττ distributionsob- tainedwhen thelepton isolation is invertedandthe

τ

h isolation isrelaxed, are consistent within statistical uncertainties withthe normalsearchprocedure.

Inthee

μ

finalstate,theQCDmultijetbackgroundismeasured simultaneouslywithotherbackgroundsusingmisidentifiedleptons indata,througha“misidentified-lepton” method[51],andrequir- ing atleast one jet misidentified asa lepton. The probability for loosely preselected leptons, mainly dominated by leptons within jets, tobe identified asgoodleptons is measuredin samplesde- pletedofisolated leptons asa functionofthe p_T and

η

.Weights obtainedfromthismeasurementareappliedtoeventsindatawith electrons and muons passing the loose preselection but not the nominalselectioncriteria,toextracttheQCDmultijetbackground contribution.

Inthe

μτ

hande

τ

hfinalstates,theW+jets backgroundarises from events with a genuine isolated and identified lepton from the leptonic decayofa W boson andajet misidentified asa

τ

h. Its contributionis highly suppressed by requiring the MT of the lepton and ^pmiss_T ^{of Eq.(2)to be} <30 GeV (low-MT region). The W+jets normalisationisdeterminedfromcollisiondatausingthe yield in the high-M_T (>70 GeV) sideband, multiplied by an ex- trapolation factor that is the ratio of the W+jets events in the high- andlow-MT regions in simulatedevents. The smallcontri- bution from other backgrounds in events selected with high-MT selection issubtractedusingthepredictionfromsimulations.The distribution of mττ for the W+jets background is taken from simulation. A correction to the distribution, measured in a sam- pleenrichedinW+^{jets andasafunctionofthep}Tofthelepton originatingfromtheW boson,isappliedtocorrectthedifferences betweenobservedandsimulatedevents.Inthee

μ

final state,the W+jets background isestimated together withthebackgrounds thatcontainatleastonejetmisidentifiedasalepton,suchasQCD multijets,aspreviouslydescribed.

The Z/

γ

^∗_→

μμ

andZ/

γ

^∗_→ee processes contribute,respec- tively,tothe

μτ

h ande

τ

h finalstates,becauseofthemisidentifi- cationofalepton asa

τ

h.The normalisationandthe distribution ofmττ forthesebackgroundsareobtainedfromsimulation.

The presence of genuine b jetsfrom top quark decays makes the tt background contributionimportant.The tt background has true

τ

h≈^{70% of the times and} misidentified

τ

h in ≈^{30% of} the times. The distribution of mττ for tt events is taken from simulation, but normalised to the measurement of the tt cross section [52]. A reweighting is applied to generated tt events to improve the modelling of the top quark pT spectrum. This reweighting only depends on the simulated p_T of top and anti- top quarks [52], andhasa negligible impacton the final results.

Inaddition,themττ distributions observedindataandpredicted byMCsimulationsarecomparedinaregionwithhighpurityoftt events,anddepletedinsignal,obtainedbyraisingthepTthreshold oftheleptonsand

τ

h,andrequiringatleasttwob-taggedjetswith ahigherp_T thresholdthanthatusedineventselectionsdescribed in Section 4. Good agreement is found between distributions in dataandMCsimulation.

Single top quark,diboson (WW, WZ, ZZ),andSM Higgsback- grounds represent a small fraction of the total background, and are takenfromsimulations andnormalisedtotheNLO crosssec- tions[51,53,54].

Scalefactorstocorrectforresidualdiscrepanciesbetweendata andMC simulationrelatedtothelepton triggering,identification, and isolation are applied to the signal and the backgrounds es- timated fromMC simulations. These correction factors are deter- minedusingthe“tag-and-probe”technique[45,46,55],whichrelies onthepresenceoftwoleptonsfromZ bosondecays.Nocorrection factoris appliedto the

τ

hcandidate nortothe selectedb jet,as thecorrections arefound tobe consistent withunity.The uncer- taintiesrelatedtothesescalefactorsaredescribedinSection6.

Table 1

Systematicuncertaintiesthataffectthenormalisation.

Systematic source Systematic uncertainty

μτh eτh eμ

Normalisation

Integrated luminosity 2.6% 2.6% 2.6%

Muon ID/trigger 2% — 2%

Electron ID/trigger — 2% 2%

τhID/trigger 8% 8% —

Muon toτhmisidentification rate 30% — —

Electron toτhmisidentification rate — 30% —

b tagging efficiency 1–4% 1–4% 1–4%

b mistag rate 1–9% 1–9% 1–9%

E^miss_T scale 1–2% 1–2% 1–2%

Z/γ^∗→τ τnormalisation 3% 3% 3%

Z/γ^∗→τ τlow-mass normalisation 10% 10% 10%

QCD multijet normalisation 20% 20% —

Reducible background normalisation — — 30%

W+jets normalisation 30% 30% —

tt cross section 10% 10% 10%

Diboson cross section 15% 15% 15%

H→τ τsignal strength 30% 30% 30%

Theory Underlying event and parton shower 1–5% 1–5% 1–5%

Scales for A boson production 10% 10% 10%

PDF for generating signal 10% 10% 10%

NLO vs. LO 20% 20% 20%

6. Systematicuncertainties

The results of the analysis are extracted from a fit based on themττ distributionsineachfinalstate,asdiscussedinSection7.

Systematicuncertainties inthefit affectthenormalisationorthe shapeofthemττ distributionforthesignalandbackgrounds.The normalisationuncertaintiesaresummarisedinTable 1.

The uncertainty in normalisation that affects the signal and mostofthesimulatedbackgroundsisrelatedtotheintegratedlu- minosityat8 TeV,whichismeasuredwithaprecisionof2.6%[56].

Uncertaintiesinmuon andelectronidentificationandtriggereffi- ciency,aswellasinthe

τ

hidentificationefficiency,aredetermined using the “tag-and-probe” technique [45,46,55]. These uncertain- tiesareabout2%formuonandelectronand8%for

τ

h.Changesin acceptanceduetotheuncertaintyinthe btaggingefficiencyand the bmistag raterange from1to 9% depending onthe process.

ToestimatetheuncertaintyintheW+jets normalisation,theun- certaintyintheextrapolationfactorfromthehigh-M_Tsidebandto thesignalregionisobtainedbyvaryingE^miss_T anditsresolutionby their uncertainties, leading to a 30% uncertainty. The uncertainty in the normalisation of QCD multijet background is obtained by addingthestatisticaluncertaintyrelatedtothesamplesizeofthe QCDmultijet-dominatedcontrolregioninquadraturewiththeun- certaintyintheextrapolationfactorfromthecontrolregiontothe signalregion; thisamountsto20%.The normalisationuncertainty for the tt background amounts to 10%; it is determined from a controlregionwherebothWbosons originatingfromthetopand antitopquarksdecayto

τ

leptons[51].Uncertaintiesrelatedtothe dibosonbackgroundcrosssectionamountto15%[57].

A30% uncertainty inthe signal strength(ratio ofobserved to expectedcross sections) for the SM Higgs boson is applied [51].

Theoretical uncertainties arising from the underlying event and parton showering matching scale, PDF [58] and the dependence onfactorisationandnormalisationscalesareconsideredforsignal.

ThePDFuncertaintyistakenasthedifferenceinthesignalaccep- tanceforthesignalsimulationwithCTEQ6L1,MSTW2008NLO[59], and NNPDF2.3NLO [60] PDF sets, leading to a 10% uncertainty.

A 20% uncertainty in the signal normalisation is applied to take intoaccount the possibledifference inthe productofacceptance andefficiencybetween theLOsample generatedwith PYTHIA6.4 andtheNLOsamplegeneratedbythe MadGraph5_aMC@NLO gen- erator[61].

The

τ

h and electron energy scales are among the systematic uncertainties affecting the mττ distributions.To estimate the ef- fects ofthese uncertainties, the electron energyscale is changed by1% orby2.5% forelectronsreconstructedinthebarrelorinthe endcapregionsoftheECAL[40],respectively,whilethe

τ

henergy scaleisvariedby3%[46].ThetopquarkpTreweightingcorrection, used forsimulatedtt events to matchtheobserved pT spectrum ina dedicatedcontrolregion,ischangedbetweenzeroandtwice thenominalvalue[52,62].Theuncertaintyinthe

τ

hmisidentifica- tion ratecorrection ofthe QCDmultijetandW+jets background distributions hasbeentakenintoaccount.Toestimate thisuncer- tainty, the

τ

h misidentificationrate correction has been changed betweenzeroandtwiceitsvalue.Anadditionaltriggeruncertainty isappliedtothe

μτ

h ande

τ

hfinal statestocoverpossiblediffer- ences betweencollisiondata andsimulatedeventsinthe low-pT lepton region,wherethetriggerefficiencyhasnotyetreachedits plateau. These low-p_T leptons are attributed an uncertainty that corresponds to halfof the difference betweenthe measured and the plateau efficiencies. Finally, uncertainties due to the limited numberofsimulatedevents,orthenumberofeventsinthecon- trolregionsindata,aretakenintoaccount.Theseuncertaintiesare uncorrelatedacrossthebinsineachbackgrounddistribution[63].

Among all systematic uncertainties, the ones that have the largest impact on theresults are the

τ

h energy scale, theuncer- taintiesrelatedtothejettomuon,electronor

τ

hmisidentification rates,andtheuncertaintiesfromthelimitednumberofsimulated events (or the observed events indata control regions).The im- pact of theseindividual uncertainties on the combined expected limitrangesbetween5and10%dependingonmττ .

7. Results

The mass distributions for the

μτ

h,e

τ

h and e

μ

channels are shown inFig. 1. No significant excessof dataisobserved on top oftheSMbackgrounds.A binnedmaximumlikelihoodfithasbeen applied simultaneously to all three distributions, taking into ac- count thesystematicuncertaintiesasnuisanceparameters.A log- normal probability distribution function is assumed for the nui- sanceparameters thataffecttheeventyields ofthevariousback- ground contributions. Systematic uncertainties affecting the mττ distributions areassumedtohaveaGaussian probabilitydistribu- tionfunction.

Fig. 1. Observedandpredictedmτ τ distributionsintheμτh (top),eτh (middle), andeμ(bottom)channels.Theplotsontheleftarethezoomed-inversions formτ τ distributionsbelow50 GeV.A signalforamassofmA=^{35 GeV isshownforacrosssectionof40 pb.In}μτhandeτhfinalstates,theelectroweakbackgroundiscomposed ofZ→^{ee, Z}→μμ, W+^{jets,diboson,andsingletopquark}contributions.Inthe eμfinalstate, theelectroweak backgroundiscomposedofdibosonandsingletop backgrounds,whilethemisidentifiede/μbackgroundisduetoQCDmultijetandW+jets events.ThecontributionfromtheSMHiggsbosonisnegligibleandtherefore notshown.Expectedbackgroundcontributionsareshownforthevaluesofnuisanceparameters(systematicuncertainties)obtainedafterfittingthesignal+background hypothesistothedata.

Fig. 2. Observedandexpectedupperlimitsat95%CLontheproductofcrosssectionandbranchingfractionforalightpseudoscalarHiggsbosonproducedinassociation withtwobquarks,thatdecaystotwoτ leptons,intheμτh(left),eτh(middle),andeμ(right)channels.The1σ and2σ bandsrepresentthe1and2standarddeviation uncertaintiesontheexpectedlimits.

Upper limits on the product of cross section and branching fraction of the pseudoscalar Higgs boson to

τ τ

are set at 95%

confidence level (CL) usingthe modified frequentist construction CL_s [64,65] and the procedure is described in Refs. [66,67]. The observedandexpectedlimitsonthebbA→^bb

τ τ

processandthe oneandtwostandarddeviationuncertaintiesontheexpectedlim- its are shown in Fig. 2. Among the three channels,

μτ

h is the mostsensitiveonefortheentiremassrangebecauseofthehigher branchingfractionrelativetothee

μ

channel,lowertriggerandof- flinethresholds onthelepton pT relativetothee

τ

h channel,and highermuonthanelectronidentificationefficiency.Althoughback- groundyields increase sharply with the mass, the acceptance of thesignalgrowsfaster,providingtherebymorestringentlimitson the cross section at highermasses. The product of signal accep- tanceandefficiencyinthe

μτ

hchannel changesfrom1.5×¹⁰⁻⁵ atan A boson mass of25 GeV to 6×^{10−4 atm}A=^{80 GeV. In} thee

τ

hchannelitrangesfrom3×^{10−6 at25 GeV to2}×^{10−4 at} 80 GeV,andfinallyinthee

μ

channel,itrangesfrom1.3×^{10−5 at} 25 GeV to3.5×^{10−4 at80 GeV.Thetrigger}requirementsandthe p_Tthresholdoftheleptonsand

τ

hsarethemainfactorsindriving thesignalacceptanceandefficiency,especiallyatlowmasses.

The upper limits from the combination of all final states are presentedinFig. 3,withexactvaluesquotedinTable 2.Theyrange from7to39 pbforAbosonmassesbetween25and80 GeV.Inad- dition,superimposedinFig. 3areseveraltypicalproductioncross sectionsforthepseudoscalarHiggsbosonproducedinassociation witha pairofbquarksinType II2HDM,form_A lessthanhalfof the125 GeV Higgsboson(h), andforB(^h→^AA)<0.3 [19].The points are obtainedfrom a series of scans inthe 2HDM param- eterspace. Points withSM-like Yukawacoupling andsmalltanβ have sin(β−

α

)≈^{1, cos}(β−

α

)>0,and low m²₁₂, while points with “wrong sign” Yukawa coupling have sin(β±

α

)≈^{1, small} cos(β−

α

)<0,andtanβ >5.Whilethecombinedresultsofthe currentanalysisarenotsensitivetotheSM-likeYukawacoupling, they exclude the “wrong sign” Yukawa coupling for almost the entiremass range, and moregenerally for tanβ >5. For masses greater than mh/2, where the constraint on B(^h→^AA)<0.3 is automaticallysatisfied,theproductioncrosssectionofthepseudo- scalarHiggsbosoninassociationwitha pairofbquarksismuch larger[18];consequently,theexclusionlimitextendstomassesup to80 GeV.

8. Summary

AsearchbytheCMSexperimentforalightpseudoscalarHiggs boson produced in association witha bb pair anddecaying to a

Fig. 3. ExpectedcrosssectionsforType II2HDM,superimposedontheexpectedand observedcombinedlimitsfromthissearch.Cyanandgreenpoints,indicatingsmall valuesoftanβasshowninthecolourscale,havesin(β−α)≈^1,cos(β−α)>0, andlowm²₁₂,andcorrespondtomodelswithSM-likeYukawacoupling,whilered and orangepoints,withlargetanβ,havesin(β+α)≈1,smallcos(β−α)<0, andtanβ >5,andcorrespondtothemodelswitha“wrongsign”Yukawacoupling.

TheoreticallyviablepointsareshownonlyuptomA=mh/2[19].(Forinterpretation ofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheweb versionofthisarticle.)

pairof

τ

leptons isreported.Threefinalstates:

μτ

h,e

τ

h,ande

μ

, are usedwhere

τ

h representsahadronic

τ

decay.The resultsare basedonproton–protoncollisiondataaccumulatedatacentre-of- massenergyof8 TeV,correspondingtoanintegratedluminosityof 19.7 fb⁻¹.Pseudoscalarbosonmassesbetween25and80 GeV are probed.No evidenceforapseudoscalarbosonisfound andupper limitsare setontheproductofcross sectionandbranchingfrac- tionto

τ

pairsbetween7and39 pbatthe95%confidencelevel.

ThisexcludespseudoscalarAbosonswithmassesbetween25and 80 GeV, with SM-like Higgs boson negative couplings to down- type fermion, produced in association with bb pairs, in Type II, two-Higgs-doubletmodels.

Acknowledgements

WecongratulateourcolleaguesintheCERNacceleratordepart- ments for the excellent performance of the LHC and thank the technical andadministrativestaffs atCERNand atother CMSin- stitutes for their contributions to the success of the CMS effort.

Inaddition,wegratefullyacknowledgethecomputingcentresand

Table 2

Expectedand observedcombinedupper limitsat95% CLinpb,along withtheir1and2standard deviationuncertainties,intheproductofcrosssectionandbranchingfractionforpseudoscalarHiggs bosonsproducedinassociationwithbb pairs.

mA(GeV) Expected limit (pb) Observed limit (pb)

−²σ −¹σ Median +¹σ +²σ

25 20.4 28.1 41.3 63.1 95.5 35.8

30 14.6 20.0 29.1 44.3 66.3 38.7

35 12.2 16.6 24.3 36.7 55.1 37.4

40 10.3 14.1 20.6 31.1 46.5 31.3

45 8.4 11.6 16.8 25.3 37.9 20.3

50 7.0 9.5 13.7 20.7 30.8 13.2

55 6.7 9.2 13.3 20.1 29.9 10.5

60 6.1 8.2 12.0 18.0 26.7 10.6

65 5.6 7.7 11.2 17.0 25.4 8.3

70 5.1 7.0 10.2 15.6 23.3 7.1

75 5.3 7.2 10.5 15.9 23.8 7.9

80 5.5 7.5 10.9 16.6 25.0 8.0

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 AgentschapvoorInnovatiedoorWetenschapenTechnologie(IWT- Belgium);the Ministryof Education,Youth andSports(MEYS) of theCzechRepublic;theCouncilofScienceandIndustrialResearch, India; the HOMING PLUS programmeof the Foundation for Pol- ish Science, cofinanced from European Union, Regional Develop- ment Fund; the OPUS programme of the National Science Cen- ter(Poland);the Compagnia diSan Paolo (Torino); MIURproject 20108T4XTM (Italy); the Thalis and Aristeia programmes cofi- nancedbyEU-ESFandtheGreekNSRF;theNationalPrioritiesRe- searchProgrambyQatarNationalResearchFund;theRachadapisek SompotFundforPostdoctoralFellowship,ChulalongkornUniversity (Thailand);andtheWelchFoundation,contractC-1845.

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