Standard Model Higgs Combination from CMS with up to 1.7 fb-1 of data

0.24 MB·English

by  Michal Bluj for the CMS Collaboration

Standard Model Higgs Combination from CMS with up to 1.7 fb 1 of data
−
MichałBluja,bfortheCMSCollaboration
CNRS-IN2P3/LLR-E´colePolytechnique,Palaiseau,France
Abstract. The combination is presented of searches for a standard model (SM) Higgs boson in eight de-
2 cay modes: H γγ, H ττ, H bb, H WW 2ℓ2ν, H ZZ 4ℓ, H ZZ 2ℓ2τ, H ZZ 2ℓ2ν, and
∗ ∗
1 H ZZ 2ℓ2q→.Thesea→rchesw→ereperfo→rmedby→theCMSC→ollabo→rationus→ing1.1→–1.7fb 1o→finte→gratedlumi-
−
0 → →
nosity,dependingontheanalysis.NoexcesscompatiblewithaSMHiggssignalhasbeenobserved;thelargest
2
excursionoftheobserveddatafromtheexpectedbackgroundhasaprobabilityof0.4aftertakingintoaccount

n thelook-elsewhereeffect.TheSMHiggsbosonisexcludedat95%C.L.inthreemassranges145–216,226–288,
a and310–400GeV/c2,whiletheexpectedexclusionrangeis130–440GeV/c2.
J

7
2 1 Introduction Table 1. Summary information on the analyses included in the

combination.
]
Understandingof the mechanism of electroweak symme-
x
e trybreakingisoneofthemaingoalsoftheCMSphysics channel massrange lumi. no.of typeof
- program.Inthestandardmodel(SM)theelectroweaksym- [GeV/c2] [fb−1] sub-chan. analysis
p H γγ 110–150 1.7 8 massshape(unbinned)
e metrybreakingisdescribedbyso-calledHiggsmechanism H→ττ 110–140 1.1 6 massshape(binned)
[h wbthohissiocpnhar(lteHica)lde[s1ht,ao2v,pe3r,g4ei,dv5ie,c6nt]ino.enSgooaftfiavoren,reeexsspcuealtrlsaimralpelonawrtatiilcnslgeeta-orcaehxHecsliugfdgoesr HHH→→WHZZW→Z→Z∗∗b→→b2ℓ242ℓℓτ2ν 111181110000––––616603000500 1111....1175 8535 mmaassssccsshhuuaattpp&&eecc((uuoonnuubbnniittnnnneedd))
1 itsmassbelowm <114.4GeV/c2(LEPexperiments[7]) H→ZZ→2ℓ2ν 250–600 1.6 2 cut&count
8v andformH ∈100H–108GeV/c2andmH ∈156–177GeV/c2 H→→ZZ→→2ℓ2q 226–600 1.6 6 massshape(unbinned)
(Tevatronexperiments[8]). Indirectsearchesusing fits to
5
8 precisemeasurementspredictrelativelylightHiggsboson
5 withmH <158GeV/c2[9]. ground.Itisthemostsensitivechannelatlowmassesde-
. In this report combination is presented of searches for spiteasmallbranchingfraction(Br(H γγ) 1 2 10 3).
1 −
the Higgs boson in eight decay modes: H γγ, H ττ, → ∼ − ×
0 Alleventsaredividedintoeightcategoriesbasedonwhether
→ →
2 H bb,H WW∗ 2ℓ2ν,H ZZ∗ 4ℓ,H ZZ 2ℓ2τ, the transverse momentum of the di-photon system
→ → → → → → →
1 H ZZ 2ℓ2ν,andH ZZ 2ℓ2q. pγγ > 40 GeV/c, whether both photons are in the cen-
: T→hecro→sssectionsfor→Higg→sbosonproduction,itsdecay trTal part of the CMS detector, and whether both photons
v
branchingfractions,andtheiruncertaintiesaretakenfrom
i areunconverted(havecompactelectromagneticshowers).
X thereportpreparedbyLHCHiggsCrossSectiongroup[10]. The categorisation is motivated by different resolution in
r In Section 2 an overview of all eight analysis used in eachcategory(1-3%).
a the combination is provided, then in Section 3 a statisti-
The background under the expected signal peak is de-
calmethodologyusedinthisworkisbrieflydescribed,and
rivedfromsidebandswithoutuseofMonteCarlosimula-
finallyinSection4thecombinedresultispresented.
tion.
2 Channels used in the combined search
2.2 H ττchannel[12]
→
The combination presented in this report bases on eight
analysescorrespondingtomaindecaymodesoftheHiggs In this analysis a broad excess in the visible di-tau mass
boson as summarised in Table 1. In the following sub- mvis distributionislookedfor(resolutionofmvis 20%).
sectionsabriefdescriptionis providedof eachindividual Tττhreedi-taufinalstatesareused:eµ,eτ ,µτττ(∼τ stands
h h h
analysis. foraτdecayinghadronically).Eachofofthesethreecat-
egoriesisfurtherdividedintotwomutuallyexclusivesub-
categories: events with the Vector Boson Fusion (VBF)
2.1 H γγchannel[11]
→ signature(twojetsseparatedinpseudorapiditywithnoad-
ditionaljetsinbetween),andeventswithlessthantwojets
TheH γγanalysisisasearchforanarrowpeakinthedi-
→ orwithexactlytwojetsthatfailVBFrequirements.
photonmassm distributionontopofalargefallingback-
γγ
ThemainirreduciblebackgroundisZ ττwithnormal-
→
a OnleavefromNCBJ,Warsaw,Poland isation taken from the Z ℓℓ cross section measurement
b e-mail:[email protected] and shape of mvis model→led using simulated events. The
ττ
EPJWebofConferences
reduciblebackgrounds(W+jets,QCD,t¯t,Z ℓℓ)areeval- 2.6 H ZZ 2ℓ2τchannel[16]
→ → →
uatedbasingondata.
In the H ZZ 2ℓ2τ search, the presence is required of
→ →
onedi-leptonpair(eeorµµ)forminganon-shellZboson.
2.3 H bbchannel[13] ThenasecondZbosonisrequiredtodecaytoτ-pair,with
→
oneofthefollowingfourdecaymodes:eµ,eτ ,µτ ,τ τ
h h h h
The H bb search exploits the Higgs boson production (τh stands for a τ decaying hadronically). It makes eight
in assoc→iation with W or Z bosons(V). The followingW exclusive sub-channels. In the analysis, the mass of two
and Z boson decay modes are considered: W ℓν and leptonsandvisibleproductsoftwotaudecays(withoutac-
Z ℓℓ,νν(ℓ=e,µ).Itisrequiredthatthesyste→moftwo countingformissingneutrinos)isafinalobservable.
b-t→aggedjets (a Higgs boson decay candidate) is boosted ThedominantbackgroundistheelectroweakZZproduc-
inthetransverseplane,whichreducesbackgroundandim- tionwhichistakenfromsimulationandnormalisedusing
provesthedi-jetmassresolution. the measured yield of Z ℓℓ events scaled by the ratio
→
The result of the analysis bases on event count in sig- ofZZandZcrosssections.Thesub-leadingbackgrounds
nalregionsdefinedbytheoutputofamultivariateanalysis withjetsfakingtaucomefromZ+jets(includingZW)and
classifier (MVA). The classifier was trained for a number t¯t,areevaluatedfromdatausingfake-ratemethod.
ofHiggsbosonmasses.
TheratesofthemainbackgroundsV+jets,Vbb¯,andt¯tare
estimated from control samples and then applied to sim- 2.7 H ZZ 2ℓ2νchannel[17]
ulation. The WZ and ZZ backgrounds with Z bb¯, and → →
→
thesingle-topbackgroundaremodelledwithMonteCarlo In the H ZZ 2ℓ2ν analysis, events with one di-lepton
simulation. pair (ee o→r µµ)→consistent with an on-shell Z boson, and
significantEmissareselected.Thenthetransversemassm
T T
from the di-leptonpair momentaand Emiss is constructed
T
2.4 H WW∗ 2ℓ2νchannel[14] assumingthatEmissarisesfromtheZ ννdecay1.Finally,
→ → T →
events are counted in a m dependent window in the m
H T
The signatureof the H WW∗ 2ℓ2νsignal is the pres- distribution.
→ →
enceofexactlytwooppositesign,isolatedleptonsandsig- ThemainZZandWZbackgroundsaretakenfromsimu-
nificantEmiss.Thereisnosignalmasspickduetoescaping lation,whileallotherbackgroundsareevaluatedfromcon-
T
energy due to presence of two neutrinos from W decays. trolsamples.
Thesearchisbasedoneventcounting.
Eventsaresplitintothreecategoriesbasingonajetmul-
tiplicityintheevent(0,1or2jets)withdifferentsignal-to-
2.8 H ZZ 2ℓ2qchannel[18]
background ratios. For the 0-jet category the main back- → →
ground is the electroweak WW production; for the 1-jet
The H ZZ 2ℓ2q analysis is a search fora peak in the
category the WW and t¯t processes. Both the 0- and 1-jet → →
massofthedi-leptonplusdi-jetsystem(m ).Thereare
categoriesarefurthersplitintosame-flavourandopposite- 2ℓ2j
sixexclusivefinalstatesusedinthesearchwiththelepton
flavourdi-leptonsub-channels,sincedifferentcontribution
pair inoneof twopossible flavours(eeor µµ) andthe jet
of the Drell-Yan background. The 2-jet category is opti-
pairwith0,1or2b-tags.Bothleptonandjetpairsarere-
mised to take advantageof the VBF productionsignature
quiredtobeconsistentwiththeZbosonmass.Background
(jets separation in pseudorapidity).The main background
isfurthersuppressedbyemployingamultivariateangular
forthiscategoryist¯t.
likelihoodconstructedfromthekinematicvariablesofthe
To separate the H WW signal from the electroweak
→ twoleptonsandthetwojets.
WW background the scalar nature of the Higgs boson is
Thebackgroundm distributionisobtainedusingcon-
explored. 2ℓ2j
trolregionsindata.
Contributions from all main backgroundsare estimated
basingondata.
3 Statisticalmethodology
2.5 H ZZ 4ℓchannel[15]
∗
→ → ThemodifiedfrequentistconstructionCLs[19,20]isused
for calculations of exclusion limits presented in this re-
The H ZZ 4ℓ analysis is a search for a four-lepton
∗
→ → port.Tocompletelydefinethemethod,oneneedstomake
masspeakoverthecontinuumbackground.Threefinalstates
achoiceofteststatisticandoftreatmentofthesystematic
4e,4µ,2e2µareconsideredseparately,aseachofthemhas
uncertaintiesintheconstructionoftheteststatisticandin
adifferentresolutionofm andadifferentcompositionof
4ℓ
generatingpseudo-data.HeretheLHCHiggsCombination
backgroundwithjetsfakingleptons.
Groupprescription[21]isfollowed.
Thedominantirreduciblebackgroundistheelectroweak
The likelihood (data µ,θ) used to construct the test
ZZproduction.Itscontributionismodelledusingsimula-
L |
statisticisdefinedasfollows:
tion and normalised using the measured yield of Z ℓℓ
→
eventsscaledbytheratioofZZandZcrosssections.The
reduciblebackgroundswithjetsfakingleptons(Z+jets,Zbb¯, L(data|µ,θ)=Poisson(data|µ·s(θ)+b(θ))·p(θ˜|θ), (1)
t¯t) are evaluated from data using control regions. Their
2 miss 2
contributionwasfoundtobesmall. 1 m2T=(cid:18)qp2T,Z+MZ2+ q(ETmiss)2+MZ2(cid:19) −(cid:18)→−pT,Z+→−ET (cid:19)
The2011HadronColliderPhysicssymposium(HCP-2011),Paris,France,November14-182011
where Poisson(dataµ s(θ)+b(θ)) is the Poisson proba- 4 Result of combined search
| ·
bility to observe“data”, assumingsignal andbackground
models, s(θ) and b(θ), that depend on some nuisance pa- ThecombinedresultofthesearchfortheSMbosonispre-
rametersθ.Thefreeparameterµisacommonsignalstrength sented in Figure 1 which shows the combined 95% C.L.
modifieraffectingsignaleventyieldsinallproductionmodes upperlimitsonthe signalstrengthmodifier(σ/σ ) asa
SM
(σ/σSM).Nuisanceparametersθcorrespondwithindepen- functionoftheHiggsbosonmass.Thedashedlinestands
dentsourcesofsystematicuncertainties.Theprobabilityof for the median expected results for the background-only
“measuring”θ˜ whichisbestknownestimateoftruevalue hypothesis, while the green (yellow) bands indicate the
θisp(θ˜θ)anddescribesthescaleofthesystematicuncer- range of 68% (95%) deviation from the median. The ob-
|
tainty. servedlimitisindicatedbythesolidlinewithpoints.The
Then,theteststatisticsisdefiniedas: SM Higgs boson is excluded at 95% C.L. in three mass
ranges145–216,226–288,and310–400GeV/c2,whilethe
(dataµ,θˆ )
q = 2lnL | µ , where0 µˆ µ, (2) expectedexclusionrangeis130–440GeV/c2.
µ − (dataµˆ,θˆ) ≤ ≤
L |
where“data”standseitherfortherealobservationorpseudo-
data.Boththedenominatorandnumeratoraremaximized.
Inthenumerator,µisfixedandonlythenuisanceparame-
tersθcanfloat.Theirvaluesatwhich reachesthemax-
imumaredenotedasθˆ .InthedenomLinator,bothµandθ
µ
are allowed to float in the fit, and µˆ and θˆ are parameters
at which reaches its global maximum. The lower con-
L
straint on µˆ (0 µˆ) is imposed as the signal rate cannot
≤
be negative; the upper constraint (µˆ µ) forces the limit
≤
to be one-sided.Thevalueof the test statistic forthe real
observationisdenotedasq .
obs
Inthenextstep,thevaluesofthenuisanceparametersθˆobs
0
andθˆobs bestdescribingtheobserveddata(maximizing )
µ L
areobtainedforthebackground-onlyandsignal+background
hypotheses,respectively.Usingthesebest-fitvaluesofthe
nuisanceparameters,toyMonteCarlopseudo-dataisgen- Fig. 1. The 95% C.L. upper limit on the signal cross section
eratedtoconstructthetest statistic samplingdistributions σ/σSM fortheSMHiggsbosonhypothesis asafunctionofthe
for the both signal+background hypothesis (with signal SM Higgs boson mass. The observed values are shown by the
solidline.Thedashed blacklineindicatesthemedianexpected
strengthµ)andforthebackground-onlyhypothesis(µ=0).
The“measurements”θ˜arealsorandomizedineachpseudo- resultsforthebackground-onlyhypothesis,whilethegreen(yel-
low)bandsindicatetherangeof68%(95%)deviationfromthe
data.
median.
Withsignal+backgroundandbackground-onlysampling
distributionsfortheteststatisticq onecanfindtheprob-
µ
ability to obtaina test statistic valueashighas, orhigher A contribution of each individual Higgs decay mode to
than,theoneobservedindata,underthesignal+background thecombinedresultisillustratedinFigure2.Forhighmasses
hypothesis,andobtainCLs(µ)fromtheratio (m >200GeV/c2)theexclusionisdrivenbytheH ZZ
H
decaychannels,foranintermediaterange130–200Ge→V/c2
P q qobs µ s(θˆobs)+b(θˆobs)
CLs(µ)= (cid:16) µ ≥P qµ | q·obs µb(θˆobs) µ (cid:17). (3) wthheielexcfloursiloonwismdoasmseinsa(tmedHb<y1th3e0HG→eVW/cW2)∗H→2ℓγ2γνmisotdhee,
µ ≥ µ | 0 mostsignificantcontributor. →
(cid:16) (cid:17)
TheCLs αforagivenµmeansthatthesignalwithstrenthµ Thedifferencesbetweentheobservedandexpectedlimits
isexclude≤datthe(1 α)confidencelevel(C.L.).Toquote areconsistentwithstatisticalfluctuations,astheobserved
the95%C.L.upper−limitonµ,weadjustµuntilwereach limitsliewithinthe68%and95%bands.ForthelowHiggs
CLs=1 0.95. boson mass range,we observe an excess of eventswhich
Toquan−tifyanexcessofeventsthealternativeteststatis- leads to weaker observed limit than expected in the ab-
ticq0isused: senceoftheSMHiggsboson.Theobservedlocalp-valuep0
whichquantifiestheconsistencyoftheobservedexcesses
q = 2lnL(data|0,θˆ0) andµˆ 0. (4) withthebackground-onlyhypothesis,isshowninFigure3
0 − (data µˆ,θˆ) ≥ (top panel) for the combined search, and split into indi-
L | vidual decay modes in Figure 4. A broad offset for low
This test statistic allows to evaluate significances(Z) and
masses, of about1σ, correspondsto the excesses seen in
p-values(p )fromtheasymptoticformula:
0 theH WW 2ℓ2νchannelcharacterisedbypoormass
∗
→ →
resolution.TheexcessesobservedintheH ZZ 4ℓand
Z = qqo0bs, (5) the H→γγ channels result two narrow mi→nima→of the p-
value. The minimal p-value is p 0.01, but after ac-
min
p =P(q qobs)= ∞ e−x2/2 dx= 1 1 erf Z/√2 , countingforthe look-elsewhereeffec∼twhichis important
0 0 ≥ 0 ZZ √2π 2h − (cid:16) (cid:17)i forthisstudy,itisreducedtoaglobalprobabilitypglobal∼0.4.
(6) Thelook-elsewhereeffectwasesimetedforthewholeex-
whereqobsistheobservedteststatisticforµ=0. ploredmassrange110–600GeV/c2.
0
EPJWebofConferences
Fig. 2. The observed 95% C.L. upper limit on the signal cross Fig.4.Theobservedlocalp-valueasafunctionoftheSMHiggs
sectionσ/σ fortheSMHiggsbosonhypothesisasafunction bosonmassforeightdecaymodesandtheircombination.
SM
of the SM Higgs boson mass for eight decay modes and their
combination.
hasaprobabilityof0.4aftertakingintoaccountthelook-
elsewhereeffectforthewholeexploredmassrange(110–
Thebest-fitvalueofσ/σSM isalsopresentedinFigure3 600GeV/c2).
(bottompanel).Thebest-fitvalueisafactorbywhichthe
SMHiggsbosoncrosssectionhastoberescaledtobestde-
scribe observeddata. In the mass range between 115 and
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−
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is130–440GeV/c2.Theobserveddataallowedtoexclude CMSNOTE-2011/005(2011)
theSMHiggsbosonat95%C.L.inthreemassranges145–
216, 226–288, and 310–400 GeV/c2. The largest excur-
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