Search for light bosons in decays of the 125 GeV Higgs boson in proton-proton collisions at sqrt(s) = 8 TeV

0.87 MB·

by  CMS Collaboration

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-EP/2016-292
2017/01/10
CMS-HIG-16-015
Search for light bosons in decays of the 125GeV Higgs
√
boson in proton-proton collisions at s = 8TeV
7
∗
1 The CMS Collaboration
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2

n
a
J

8 Abstract

]
x
e A search is presented for decays beyond the standard model of the 125GeV Higgs
-
p bosonstoapairoflightbosons,basedonmodelswithextendedscalarsectors. Light
e bosonmassesbetween5and62.5GeVareprobedinfinalstatescontainingfourτ lep-
h
tons,twomuonsandtwobquarks,ortwomuonsandtwoτ leptons. Theresultsare
[
from data in proton-proton collisions corresponding to an integrated luminosity of
1
19.7fb−1,accumulatedbytheCMSexperimentattheLHCatacenter-of-massenergy
v
2 of8TeV. Noevidenceforsuchexoticdecaysisfoundinthedata. Upperlimitsareset
3
ontheproductofthecrosssectionandbranchingfractionforseveralsignalprocesses.
0
2 Theresultsarealsocomparedtopredictionsoftwo-Higgs-doubletmodels,including
0 thosewithanadditionalscalarsinglet.
.
1
0
7
SubmittedtotheJournalofHighEnergyPhysics
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:
v
i
X
r
a
(cid:13)c 2017CERNforthebenefitoftheCMSCollaboration.CC-BY-3.0license
∗SeeAppendixAforthelistofcollaborationmembers

1
1 Introduction
Studies of the recently discovered spin-0 particle h [1–3], with a mass of 125GeV and with
properties consistent with the standard model (SM) Higgs boson [4], severely constrain SM
extensions that incorporate scalar sectors [5–7]. There are many well-motivated models that
predict the existence of decays of the Higgs boson to non-SM particles [8]. Without making
assumptionsabouttheh(125)couplingstoquarks,leptons,andvectorbosons,otherthanthat
thescalarsectoriscomposedonlyofdoubletsandsinglets,theATLASandCMScollaborations
attheCERNLHCexcludeata95%confidencelevel(CL)branchingfractionsoftheHiggsbo-
sontobeyondSM(BSM)particles,B(h → BSM),greaterthan49%and52%,respectively[5,6].
Branching fractions as low as 34% can be excluded at 95% CL by combining the results ob-
tainedbythetwoexperiments[4,9]. TheLHCexperimentsareexpectedtobeabletoconstrain
branchingfractionstonewparticlesbeyondthe5-10%levelusingindirectmeasurements[10–
12]. In this context, it is interesting to explore the possibility of decays of the SM-like Higgs
particle to lighter scalars or pseudoscalars [8, 13–15]. The SM Higgs boson has an extremely
narrowwidthrelativetoitsmass,becauseofitsexceedinglysmallYukawacouplingstotheSM
fermions,whichsuggeststhatanynon-SMfinalstateislikelytohavealargepartialwidth,and
thereforeanon-negligiblebranchingfractioncomparedtodecaystoSMparticles[8]. Examples
ofBSMmodelsthatprovidesuchadditionaldecaymodesincludethoseinwhichtheHiggsbo-
sonservesasaportaltohidden-sectorparticles(e.g. darkmatter)thatcancoupletoSMgauge
bosonsandfermions[16]. Othermodelshaveextendedscalarsectors,suchasthoseproposed
in two-Higgs-doubletmodels (2HDM) [17–21], in thenext-to-minimal supersymmetric model
(NMSSM)[22,23],orinothermodelsinwhichasingletHiggsfieldisaddedtotheSMdoublet
sector. The NMSSM is particularly well motivated as it provides a solution to the µ prob-
lem associated with supersymmetry breaking, and can provide a contribution to electroweak
baryogenesis[24,25]. Both2HDMandNMSSMmaycontainalightenoughpseudoscalarstate
(a),whichcanyieldalargeh → aabranchingfraction. In2HDM,themassofthepseudoscalar
boson a is a free parameter, but, if m < m /2, fine-tuning of the 2HDM potential is required
a h
to keep the branching fraction B(h → aa) consistent with LHC data [26]. In NMSSM, there
aretwopseudoscalarHiggsbosons,a anda . ConstraintsfromthePeccei–Quinn[27,28]and
1 2
R[23,29]symmetriesimplythatthelightera islikelytohaveamasssmallerthanthatoftheh
1
boson[25],and,sinceitistypicallyasinglet,suppressionofB(h → a a )toalevelcompatible
1 1
with observations is a natural possibility. The minimal supersymmetric model (MSSM) con-
tains a single pseudoscalar (A), but the structure of the MSSM Higgs potential is such that its
masscannotbebelowabout95GeVwhenthescalar(tobeidentifiedwithh)hasmasscloseto
125GeVandisSM-likeasimpliedbytheLHCdata[30]. Thephenomenologyofdecaysofthe
observed SM-like Higgs boson to a pair of lighter Higgs bosons is detailed in Refs. [8, 31–38]
for2HDM,inRefs.[8,39–42]inthecontextofNMSSMorNMSSM-like,andinRefs.[8,43,44]
in the general case of adding a singlet field to the SM or to a 2HDM prescription. The 2HDM
Φ Φ
contains two Higgs doublet fields, and , which, after symmetry breaking, lead to five
1 2
physicalstates. Oneofthefreeparametersinthe2HDMistanβ,theratiobetweenthevacuum
expectation valuesfor the two doublets, expressed astanβ = v /v . The lightest scalar ofthe
2 1
2HDM is compatible with the SM-like properties of the discovered boson in the limit where
the other scalars all have large masses (decoupling limit), and also in the alignment limit [45],
inwhichtheneutral Higgsbosonmasseigenstateisapproximatelyaligned withthedirection
of the vacuum expectation values for the scalar field. Approximate alignment, which is suf-
ficient for consistency with LHC data, is possible for a large portion of parameter space [45],
particularly when the pseudoscalar boson has sufficiently small mass to make h → aa decays
possible. At lowest order, there are four types of 2HDM without flavor-changing neutral cur-
rents(FCNC),whichcanbecharacterizedthroughthecouplingofeachfermiontothedoublet
2 1 Introduction
structure, as shown in Table 1. The ratios of the Yukawa couplings of the pseudoscalar boson
of the 2HDM relative to those of the Higgs boson of the SM are functions of tanβ and of the
type of 2HDM, and are given in Table 2. Type-1 and type-2 models are the ones commonly
considered, andthelatterarerequiredinsupersymmetricmodels. Inthesetwocases,thelep-
tons have the same couplings as the down-type quarks. In type-3 2HDM, all quarks couple
Φ Φ
to and all leptons couple to , with the result that all leptonic or quark couplings of the
2 1
pseudoscalar a are proportional to tanβ or cotβ, so that for large tanβ the leptonic decays of
a dominate. As implied previously, a complex SU(2) singlet field S can be added to 2HDM;
L
such models are called 2HDM+S, and include the NMSSM as a special case. If S mixes only
weakly with the doublets, one of the CP-even scalars can again have SM-like properties. The
addition of the singlet S leads to two additional singlet states, a second CP-odd scalar and a
thirdCP-evenscalar,whichinheritamixtureofthefermioninteractionsoftheHiggsdoublets.
Aftermixingamongthespin-0states,theresultistwoCP-oddscalars,a anda ,andthreeCP-
1 2
evenscalars,h ,h ,andh . Ofthelatter,onecanbeidentifiedwiththeobservedSM-likestate,
1 2 3
h. ThebranchingfractionofthehbosontoapairofCP-evenorCP-oddbosonscanbesizeable,
leadingtoawidevarietyofpossibleexotichdecays. Inthe2HDManditsextensions,theratio
Table 1: Doublets to which the different types of fermions couple in the four types of 2HDM
withoutFCNCatlowestorder.
Type-1 Type-2 Type-3(lepton-specific) Type-4(flipped)
Φ Φ Φ Φ
Up-typequarks
2 2 2 2
Φ Φ Φ Φ
Down-typequarks
2 1 2 1
Φ Φ Φ Φ
Chargedleptons
2 1 1 2
Table 2: Ratio of the Yukawa couplings of the pseudoscalar boson a of the 2HDM relative to
thoseoftheHiggsbosonoftheSM,inthefourtypesof2HDMwithoutFCNCatlowestorder.
Type-1 Type-2 Type-3(lepton-specific) Type-4(flipped)
Up-typequarks cotβ cotβ cotβ cotβ
Down-typequarks −cotβ tanβ −cotβ tanβ
Chargedleptons −cotβ tanβ tanβ −cotβ
ofthedecaywidthsofapseudoscalarbosontodifferenttypesofleptonsdependsonlyonthe
massesoftheseleptons. Inparticular,fordecaysintomuonsandτ leptons,andapseudoscalar
bosonofmassm ,wecanwrite[8,46]:
a
(cid:113)
Γ(a → µ+µ−) m2µ 1−(2mµ/ma)2
= . (1)
Γ(a → τ+τ−) (cid:113)
m2 1−(2m /m )2
τ τ a
This kind of relation can also be written for electrons and muons. In models where the pseu-
doscalarbosonadecaysonlytoleptons,itsbranchingfractiontoτ leptonsisgreaterthan99%
for pseudoscalar boson masses above 5GeV. This is a good approximation for pseudoscalar
masses below twice the bottom quark mass, or for type-3 2HDM, assuming loop-induced de-
cays such as a → gg are ignored. In type-1 and -2, and their extensions, a similar relation
existsbetweenthepartialdecaywidthsofthepseudoscalarbosontoleptonsandtodown-type
3
quarks,forexample,formuonsandbquarks,wecanwrite[8,46]:
(cid:113)
Γ(a → µ+µ−) m2µ 1−(2mµ/ma)2
= . (2)
Γ(a → bb) 3m2(cid:113)1−(2m /m )2(1+QCDcorrections)
b b a
Thefactorofthreeinthedenominatorreflectsthenumberofb quarkcolors, andperturbative
quantum chromodynamic (QCD) corrections are typically ≈20% [8]. In models of type-3 or
-4, however, the ratio of the partial decay widths depends on tanβ. Three searches for decays
of the 125GeV Higgs boson to pairs of lighter scalars or pseudoscalars are described in this
paper, where, for notational simplicity, the symbol a refers to both the light scalar and light
pseudoscalar:
• h → aa → 4τ,
• h → aa → 2µ2b,
• h → aa → 2µ2τ.
Thefirstanalysisfocusesonlightbosonmassesabovetwicetheτ mass,usingdedicatedtech-
niques to reconstruct the Lorentz-boosted τ lepton pairs. The two other analyses focus on
masses large enough that the decay products are well separated from each other, and below
half of the Higgs boson mass. The results of these searches are interpreted in the 2HDM and
2HDM+S contexts, together with the two other analyses described in greater detail in the ref-
erencesgivenbelow:
• h → aa → 4µ[47];
• h → aa → 4τ, using a different boosted τ lepton reconstruction technique than the
analysiswiththesamefinalstatelistedabove[48].
Theseanalysesarebasedonproton-protoncollisiondatacorrespondingtoanintegratedlumi-
nosityof19.7fb−1, recordedbytheCMSexperimentattheLHCatacenter-of-massenergyof
8TeV. TheD0CollaborationattheFermilabTevatronpublishedresultsforh → aa → 2µ2τand
h → aa → 4µ searchesforpseudoscalarmassesm between3.5and19GeV[49],whileATLAS
a
reported a search for h → aa → 2µ2τ decays with m between 3.7 and 50GeV, using special
a
techniques to reconstruct Lorentz-boosted τ lepton pairs [50]. Additionally, CMS performed
searches for direct production of light pseudoscalars with mass between 5.5 and 14GeV that
decay to pairs of muons [51], and with mass between 25 and 80GeV that decay to pairs of τ
leptons[52].
2 The CMS detector, event simulation, and reconstruction
The central feature of the CMS apparatus is a superconducting solenoid of 6m internal diam-
eter, providing an axial magnetic field of 3.8T. Within the solenoid volume are a silicon pixel
andstriptracker,aleadtungstatecrystalelectromagneticcalorimeter(ECAL),andabrassand
scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections.
Extensive forward calorimetry complements the coverage provided by the barrel and endcap
detectors. Muons are detected in gas-ionization chambers embedded in the steel flux-return
yoke outside the solenoid. The first level of the CMS trigger system, composed of specialized
hardwareprocessors,usesinformationfromthecalorimetersandmuondetectorstoselectthe
mostinterestingeventsinafixedtimeintervaloflessthan4µs. Thehigh-leveltriggerproces-
sor farm further decreases the event rate from around 100kHz to less than 1kHz, before data
storage. AdetaileddescriptionoftheCMSdetector,togetherwithadefinitionofthecoordinate
4 2 TheCMSdetector,eventsimulation,andreconstruction
systemusedandtherelevantkinematicvariables, canbefoundinRef.[53]. Samplesofsimu-
latedeventsareusedtomodelsignalandbackgroundprocesses. Drell-Yan,W+jets,tt,anddi-
bosoneventsaresimulatedwithMADGRAPH5.1.3.30[54]usingthematrixelementcalculation
at leading-order (LO) precision in QCD; PYTHIA 6.426 [55] is used for parton showering, had-
ronization,andmostparticledecays;andTAUOLA27.121.5[56]isusedspecificallyforτ lepton
decays. Single top quark events produced in association with a W boson are generated using
POWHEG1.0r1380[57–60],interfacedtoPYTHIAforpartonshowering. Signalsamplesaregen-
eratedwithPYTHIAusingitsbuilt-in2HDMandNMSSMgeneratorroutines. Backgroundand
signalsamplesusethe CTEQ6L [61]partondistributionfunctions(PDFs). Minimum-biascol-
lisioneventsgeneratedwith PYTHIA areaddedtoallMonteCarlo(MC)samplestoreproduce
the observed concurrent pp collisions in each bunch crossing (pileup). The average number
of pileup interactions in 2012 data was 20. All generated events are passed through the full
GEANT4 [62, 63] based simulation of the CMSapparatus and are reconstructed with the same
CMSsoftwarethatisusedtoreconstructthedata. Eventreconstructionreliesonaparticle-flow
(PF) algorithm, which combines information from different subdetectors to reconstruct indi-
vidual particles [64, 65]: neutral and charged hadrons, photons, electrons, and muons. More
complexobjectsarereconstructedbycombiningthePFcandidates. Adeterministicannealing
algorithm [66, 67] is used to reconstruct the collision vertices. The vertex with the maximum
sum in the squared transverse momenta (p2) of all associated charged particles is defined as
T
the primary vertex. The longitudinal and radial distances of the vertex from the center of the
detectormustbesmallerthan24and2cm,respectively. Muonsarereconstructedbymatching
hits in the silicon tracker and in the muon system [68]. Global muon tracks are fitted from
hits in both detectors. A preselection is applied to the global muon tracks, with requirements
on their impact parameters, to suppress non-prompt muons produced from the pp collision
or muons from cosmic rays. Electrons are reconstructed from groups of one or more associ-
atedclustersofenergydepositedintheECAL.Electronsareidentifiedthroughamultivariate
(MVA)method[69]trainedtodiscriminateelectronsfromquarkandgluonjets[70]. Themuon
andelectronrelativeisolationisdefinedas:
(cid:34) (cid:32) (cid:33)(cid:35)
∑ ∑ ∑ 1 ∑
I = p +max 0, p + p − p /p , (3)
rel T T T T T
2
charged neutral γ charged,PU
whereΣ p isthesumofthemagnitudesofthetransversemomentaofchargedhadrons,
charged T
electronsandmuonsoriginatingfromtheprimaryvertex, Σ p isthecorrespondingsum
neutral T
for neutral hadrons and Σ for photons, and Σ p is the sum of the transverse mo-
γ charged,PU T
mentum of charged hadrons, electrons, and muons originating from other reconstructed ver-
tices. The particles considered in the isolation calculation are inside a cone with a radius
√
∆R = (∆η)2+(∆φ)2 =0.4aroundtheleptondirection,where∆η and∆φarethedifferences
of pseudorapidity and azimuthal angle in radians between the particles and the lepton direc-
tion,respectively. Thefactor 1 originatesfromtheapproximateratiooftheneutraltocharged
2
candidates in a jet. In the search for h → aa → 4τ, the isolation criteria are extended to veto
thepresenceofreconstructedleptonswithinthe∆R=0.4cone,asdetailedinSection3. Jetsare
reconstructedbyclusteringchargedandneutralparticlesusingananti-k algorithm[71]witha
T
distanceparameterof0.5. Thereconstructedjetenergyiscorrectedforeffectsfromthedetector
response as a function of the jet p and η. Furthermore, contamination from pileup, under-
T
lying events, and electronic noise is subtracted on a statistical basis [72]. An eta-dependent
tuning of the jet energy resolution in the simulation is performed to match the resolution ob-
served in data [72]. The combined secondary vertex (CSV) algorithm is used to identify jets
that are likely to originate from a b quark (”b jets”). The algorithm exploits the track-based
lifetime information together with the secondary vertices associated with the jet to provide a
5
likelihoodratiodiscriminatorfortheb jetidentification[73]. Asetof p -dependentcorrection
T
factors are applied to simulated events to account for differences in the b tagging efficiency
betweendataandsimulation[73]. Tauleptonsthatdecayintoajetofhadronsandaneutrino,
denoted τ , are identified with a hadron-plus-strips (HPS) algorithm, which matches tracks
h
and ECAL energy deposits to reconstruct τ candidates in one of the one-prong, one-prong +
π0(s),andthree-prongdecaymodes[74]. Reconstructedτ candidatesareseededfromanti-k
h T
jets with a distance parameter of 0.5. For each jet, τ candidates are constructed from the jet
constituents according to criteria that include consistency with the vertex of the hard interac-
tionandconsistencywiththeπ0masshypothesis. Twomethodsforrejectingquarkandgluon
jets are employed, depending on the analysis. The first is a straightforward selection based
on the isolation variable, while the second uses a multivariate analysis (MVA) discriminator
thattakesintoaccountvariablesrelatedtotheisolation,tothetransverseimpactparameterof
the leading track of the τ candidate, and to the distance between the τ production point and
h
the decay vertex in the case of three-prong decay modes [74]. MVA-based discriminators are
implemented rates at which electrons or muons are misidentified as τ candidates. Muons or
h
electrons from leptonic decays of τ leptons are indistinguishable from prompt leptonic decay
productsofWandZbosonsandarereconstructedasmentionedpreviously. Themissingtrans-
verseenergy, Emiss,isdefinedasthemagnitudeof(cid:126)pmiss,whichisthenegativesumof(cid:126)p ofall
T T T
PFcandidates. ThejetenergycalibrationintroducescorrectionstotheEmiss measurement. The
T
Emiss significance variable, which estimates the compatibility of the reconstructed Emiss with
T T
zero,iscalculatedviaalikelihoodfunctiononanevent-by-eventbasis[75]. Aspartofthequal-
ity requirements, events in which an abnormally high level of noise is detected in the HCAL
barrelorendcapdetectorsarerejected[76].
3 Search for h → aa → 4τ decays
This analysis considers 4τ final states arising from h → aa → 4τ decay, where the Higgs bo-
son is produced via gluon fusion (ggh), in association with a W or Z boson (Wh or Zh), or
via vector boson fusion (VBF). Light boson masses are probed in the range 5−15GeV, where
the branching fraction of the light boson to τ leptons is expected to be large in certain 2HDM
models. Toillustratetheperformanceoftheanalysis,amassof9GeVischosenasabenchmark
modelthroughoutthissection;itrepresentsatype-22HDMvariantinwhichthepseudoscalar
branching fraction to τ leptons is dominant. The large Lorentz boost of the a boson at such
lightmassescausesitsdecayproductstooverlap. Tomaximizethesensitivitytooverlappingτ
leptons, a special boosted ττ pair reconstruction technique is employed, based on the specific
final state in which one τ lepton decays to a muon. This analysis is performed in two search
regions based on the transverse mass (m ) formed from a high-p muon and the pmiss. These
T T T
two regions are designed to distinguish between the Wh production mode and other modes
(primarily ggh) without significant pmiss. Events considered in this search must have an iso-
T
lated muon with p > 24GeV and |η| < 2.1 reconstructed in the CMS trigger system. Two
T
further sets of muon identification criteria [68] are used in events passing the trigger. We de-
fine the “trigger muon”, µ , as a muon located within ∆R < 0.1 of the object reconstructed
trg
in the trigger system. It is required to have p > 25GeV, |η| < 2.1, be well reconstructed in
T
boththemuondetectorsandthesilicontracker,haveahigh-qualitytrackfit,andbeconsistent
with originating from the primary pp interaction in the event. In addition, it is required to be
isolatedfromotherphotons, hadrons, andleptonsinthedetector. Isolationfromphotonsand
hadrons is enforced by requiring that the muon relative isolation, as defined in Eq. (3), is less
than0.12. Tobeisolatedfromotherleptons,thetriggermuonisrequiredtohavenoidentified
electrons (p > 7GeV, |η| < 2.5), muons (p > 5GeV, |η| < 2.4, passing ”τ ” criteria below),
T T µ
6 3 Searchforh→aa→4τ decays
or τ leptons (p > 10GeV, |η| < 2.3, passing modified HPS criteria, as described below) re-
T
constructed within ∆R = 0.4 of the trigger muon direction. The requirement of isolation from
nearbyleptons,inadditiontotheisolationrequirementofEq.(3),ensuresthatatriggermuon
originating from a τ lepton decay, where the τ lepton originates from a pseudoscalar decay,
is well isolated from the other τ lepton in the pseudoscalar decay pair. In this way, the high
level trigger and “trigger muon” identification criteria are efficient for low-p τ decay muons
T
expected to pass the trigger in the ggh and VBF production modes, provided that τ leptons
from the pseudoscalar decay are well separated or one of the τ leptons has p low enough
T
not to affect the isolation of the other τ lepton. The isolation requirements are also efficient
for high-p isolated muons from W boson decays expected in the Wh associated production
T
mode. Themuonfromtheτ leptondecayingviathemuonchannel(τ )isrequiredtohave p
µ T
> 5GeV and |η| < 2.4, be well reconstructed in the silicon tracker, have a high-quality track
fit, be consistent with originating from the primary vertex in the event, and be separated by
at least ∆R = 0.5 from the trigger muon. Because no isolation requirement is placed on the τ
µ
candidate, it can be identified with high efficiency in the presence of a nearby τ lepton. Over-
all, the trigger and τ quality criteria are similar, but the τ criteria are optimized for low-p
µ µ T
non-isolatedmuons,whilethetriggermuoncriteriaareoptimizedforhigh-p isolatedmuons.
T
Sincethefinalstateinthisanalysisincludesapairofboostedτ leptonsfrompseudoscalarde-
cay, the HPS algorithm is modified to maintain high efficiency for overlapping τ leptons. All
jet constituents are checked for the presence of τ candidates as defined above. Only jets that
µ
have at least one muon candidate passing the τ criteria among their constituents are used to
µ
seed the HPS reconstruction. Within these selected jets, the muon is excluded from the set of
jetconstituentsbeforerunningtheHPSreconstructionalgorithm. TheHPSreconstructionthen
proceeds as described in Section 2, and the resulting τ lepton is required to have p > 20GeV
T
and |η| < 2.3. The combination of the τ and isolated HPS τ candidates resulting from this
µ
selection are collectively referred to as a τ τ object, as it is designed to reconstruct boosted
µ X
a → τ τ decays. The HPS τ candidate is referred to as τ because no anti-electron or anti-
µ X X
muondiscriminatorsareappliedtoit;althoughτ leptonsdecayingtoelectronsandmuonscan
thus pass the HPS selection, the vast majority (∼97%) of selected τ candidates in simulated h
→ aasamplesarehadronicallydecayingτ leptons. ThemodifiedHPSτ leptonreconstruction
andisolationrequirementshaveasimilarefficiencyforh → aadecaysasthestandardHPSand
isolationrequirementshaveforZ → ττ decays. Thisanalysisrequiresatleastoneτ τ object,
µ X
which reconstructs a single a → ττ decay, per event. The τ τ object consists of a muon, one
µ X
or three other charged particle tracks, and zero or more neutral hadrons, and could therefore
arisefrommisidentifyingthedecayproductsofabottomquarkjet. Tofurtherdistinguishτ τ
µ X
objectsfrombackground,theseedjetoftheHPSreconstructedτ (excludinganyidentifiedτ
X µ
candidate) is required not to be identified as a b jet. The main background contributions to
this search arise from Drell-Yan dimuon pairs produced in association with jets, (W → µν) +
jets,tt withmuonsinthefinalstate,andQCDmultijetevents. InordertoreducetheDrell-Yan
background, the trigger muon and τ candidates are required to have the same sign (SS) of
X
electric charge. To minimize backgrounds with jets misidentified as τ candidates, the τ and
µ
τ objectsarerequiredtohaveoppositesign. Thesignalregionisdefinedbyeventspassingall
X
therequirementsdescribedabove,aswellasmµ+X ≥4GeV,wheremµ+X istheinvariantmass
calculatedfromthefour-vectorsofthetwocomponentsoftheτ τ object. Thechoiceof4GeV
µ X
reduces the expected background contribution by about 95%, while keeping approximately
75%oftheexpectedeventsinthecaseofthegghbenchmark9GeVpseudoscalarmasssample.
Signal acceptance is calculated from the simulated samples for masses between 5 and 15GeV.
The expected signal acceptance is corrected using p - and |η|-dependent scale factors to ac-
T
countforknown differencesintheb vetoefficiencybetween data andsimulation[73]. Events
7
Table3: Expectedsignalyieldsfortheh → aa → 4τ processforarepresentativepseudoscalar
massof 9GeV, in both m bins, assumingSMcross sectionsand B(h → aa)B2(a → τ+τ−) =
T
0.1,inthecontextoftheh → aa → 4τ search. Expectedbackgroundyieldsaswellasobserved
numbersofeventsarealsoquoted. Onlythestatisticaluncertaintyisgivenforsignalyields.
m ≤ 50GeV m > 50GeV
T T
ggh 4.6±0.3 0.8±0.1
Wh 0.27±0.02 0.70±0.03
Zh 0.068±0.005 0.19±0.01
VBF 0.51±0.03 0.09±0.01
SMbackground 5.4±1.0(stat)+4.2(syst) 6.1±1.6(stat)+3.7(syst)
−4.6 −3.6
Observed 7 14
areclassifiedintotwoanalysisbinsdependingonthevalueofthetransversemassbetweenthe
triggermuonmomentumandthe(cid:126)pmiss,definedas
T
(cid:113)
m = 2pµtrgEmiss[1−cos∆φ(µ ,(cid:126)pmiss)], (4)
T T T trg T
where ∆φ(µ ,(cid:126)pmiss) is the azimuthal angle between the trigger muon position vector and
trg T
(cid:126)pmiss vector. The contribution of signal events for the different production modes in the low-
T
m and high-m bins for a representative pseudoscalar mass of 9GeV, and assuming B(h →
T T
aa)B2(a → τ+τ−) = 0.1, is given in Table 3. For m ≤ 50GeV, ggh fusion production ac-
T
counts for about 85% of the expected signal, VBF accounts for another 10%, and associated
production accounts for the rest. For m > 50GeV, ggh and Wh productions each account
T
for about 40% of the expected signal and Zh and VBF productions account for the rest. Di-
viding selected events in two m categories increases the sensitivity to models (for example
T
Ref.[77])wherethegghproductionratewouldbemodifiedwithrespecttotheSMexpectation
becauseofdifferentYukawacouplingsofthefermionsappearingintheloop,whereastheWh
and Zh production rates would be similar as in the SM in the case of the alignment limit of
2HDM. There are several mechanisms that result in τ τ misidentification, for example jets
µ X
with semileptonic decays, jets with double semileptonic decays, or resonances in b or light-
flavorjetfragmentation. Itisimpracticaltosimulateallbackgroundstotherequiredstatistical
precision. Therefore,thenumberofbackgroundeventsinthelow-m (high-m )signalregion,
T T
denoted Nblokwg-mT(high-mT)(mµ+X ≥ 4GeV), is estimated independently from three event sam-
ples. In each background estimation sample, the isolation energy around the τ candidate is
X
required to be between 1 and 5GeV, as opposed to the signal sample requirement of isolation
energylessthan1GeV. Thethreesamplesare:
1. Observedeventspassingallothersignalselections;
2. SimulatedDrell-Yan,W+jets,tt,anddibosoneventspassingallothersignalselections;
3. Observed events passing all other signal selections, but with inverted µ relative isola-
trg
tion.
The background estimate from each sample is normalized to match the observed data yield
in the signal-free region with mµ+X < 2GeV. The final background prediction in the low-
m (high-m ) bin is taken as the arithmetic mean of the estimates from the three background
T T
8 4 Searchforh→aa→2µ2bdecays
estimation samples with m ≤ 50GeV(m > 50GeV). The positive (negative) systematic un-
T T
certainty is taken as the difference between the largest (smallest) of the three plus (minus)
its statistical uncertainty and the average. In the low-m bin, the background yield is es-
T
timated to be 5.4±1.0(stat)+4.2(syst) events, while in the high-m bin it is estimated to be
−4.6 T
6.1±1.6(stat)+3.7(syst) events. Seven and fourteen events are observed in the low- and high-
−3.6
m bins, respectively. The relaxed τ isolation requirement common to each sample implies
T X
that these background estimation samples should be enriched in events with jets. Simulated
samples of W+jets and tt events, in which the τ τ candidate arises from misidentified jets,
µ X
have been used to check that events with nonisolated τ candidates have the same kinematic
X
properties as those of the signal sample. Figure 1 shows the resulting misidentified jet back-
groundestimate,thesearchregiondata,andsimulationsofthefoursignalproductionmodels
forbothm bins.
T
19.7 fb-1 (8 TeV) 19.7 fb-1 (8 TeV)
n104 n104
bi CMS Data ggh ma = 9 GeV bi CMS Data ggh ma = 9 GeV
Events / 103 Low mT MBkisgi.d s. yjestt .b ukngc.. WVZhBh Fm m ama = a= =9 9 9G G GeeVeVV Events / 103 High mT MBkisgi.d s. yjestt .b ukngc.. WVZhBh Fm m ama = a= =9 9 9G G GeeVeVV
102 Counting experiment signal window mm+X ‡ 4 GeV 102 Counting experiment signal window mm+X ‡ 4 GeV
10 10
1 1
10-1 10-1
0 2 4 6 8 10 0 2 4 6 8 10
m (GeV) m (GeV)
m +X m +X
Figure1: Comparison,fortheh → aa → 4τ search,ofmµ+X distributionsfordata(blackmark-
ers) and the misidentified jet background estimate (solid histogram) in the low-m (left) and
T
high-m (right)bins. Predictedsignaldistributions(dottedlines)foreachofthefourHiggsbo-
T
sonproductionmechanismsarealsoshown; thedistributionsarenormalizedtoanintegrated
luminosity of the data sample of 19.7fb−1, assuming SM Higgs boson production cross sec-
tions and B(h → aa)B2(a → τ+τ−) = 0.1. The last bin on the right contains all the events
withmµ+X ≥ 4GeV,whichcorrespondtothenumbersreportedinTable3.
4 Search for h → aa → 2µ2b decays
In the search for h → aa → 2µ2b decays, events are triggered based on the presence of two
muonswithp > 17GeVandp > 8GeV. Fortheofflineselection,theleadingmuonp thresh-
T T T
old is increased to 24GeV, while the subleading muon p must exceed 9GeV. The two muon
T
candidates are required to have opposite electric charges and to be isolated. If more than one
muon is found for a given sign, the one with the highest p is selected. At least two jets with
T
p > 15GeVand|η| < 2.4arerequiredtosatisfyb-tagrequirementsthatallowonlyO(1%)of
T
the light quark jets to survive, for an efficiency of ∼65% for genuine b jets. The Emiss signifi-
T
canceoftheeventhastobelessthan6. Eventsoutsidethe|m −125GeV| < 25GeVwindow
µµbb
arediscarded. Thesearchforanewscalarisrestrictedtomassesbetween25and62.5GeV. The
upper bound is imposed by the kinematic constraint of m = 125GeV, while there is a sensi-
h
tivity loss for this search below the lower bound due to overlap between the two b jets or the
two muons arising from an increased boost of the pseudoscalars [78]. A slightly wider pseu-
doscalar mass range is however used for the selection, the optimization aiming at maximum