CMS search for Standard Model Higgs: H$\to$W$^+$W$^-$ and H$\to$ZZ

0.26 MB·English

by  Giuseppe Codispoti on behalf of the CMS Collaboration

CMS search for Standard Model Higgs: H→W+W− and H→ZZ
GiuseppeCodispoti1,aonbehalfoftheCMSCollaboration
UniversidadAuto´nomadeMadrid,28049Madrid,Spain.
Abstract. AsearchfortheStandardModelHiggsdecayingtoW+W− andZZinppcollisionsfromLHCat
√
s=7TeVusingupto1.7fb−1 ofdatarecordedbytheCMSdetectorispresented.Thissearchcoversamass
rangefrom110GeV/c2to600GeV/c2.NosignificantexcessaboveStandardModelbackgroundexpectationsis
2 observed.Upperlimitsontheproductioncrosssectionarederived.
1
0
2
1 Introduction

n
a The Standard Model (SM) is a very successful theory in 2c
J V/ 80 data Z+jets CMS preliminary
4 dseersvcerdibiinngpaasltmexopsterailmlepnhtesn.Oomneenoaftihnepkaerytircelmeapihnyinsgicqsuoebs-- Ge mH=160 top L = 1.55 fb-1
2 tionsistheoriginofthemassesofelementaryparticlesat- 5 WW WZ/ZZ
] tributed to the spontaneous breaking of electroweak sym- es / 60 W+jets
x metry. The existence of the associated field quantum, the ri
e Higgsboson(H),hasstilltobeexperimentallyestablished. nt
- e
p ThediscoveryorexclusionoftheSMHiggsbosonisone
40
e of the central goals of the CERN Large Hadron Collider
h (LHC)physicsprogram.
[ WereportonthesearchfortheSMHiggsdecayingto
√
1 W+W− andZZinppcollisionsfromLHCat s=7TeV 20
v using up to 1.7 fb−1 of data recorded by Compact Muon
1 Solenoid(CMS).Afulldescriptionofthedetectoraswell
3 as of the physics objects and technical challenges to trig-
9 ger interesting events and extract physics signal from the 0
4 highluminosityscenariocanbefoundin[1].Theanalysis 0 50 100 150 200
.
1 covers a wide mass range hypothesis for the Higgs mass, m [GeV/c2]
0 from 110 GeV/c2 to 600 GeV/c2. The main challenge of ll
2 the search analysis is to distinguish the candidate signal
1 from a wide set of backgrounds showing the very same Fig.1.Di-leptonmassm intheH→WW→2(cid:96)2νsearchinthe
ll
v: final states but much higher cross sections. Beside to re- H+0jetbinforanintegratedluminosityof1.55fb−1.
i duciblebackgroundssuchasZ/W+jets,t¯t,singletop,QCD
X
hardscatteringwherejetsaremisidentifiedasleptons,we
H → WW → 2l2ν + 0/1/2 jets (CLs)
ar hanavdeZtZopdreoadluaclstioonw.ithirreduciblebackgroundsfromWW σσ/n SM109 9955%% CCLL eexxcclluussiioonn:: 6m8e%di abnand
mit o 8 9ob5s%e rCveLd exclusion: 95% band
2 Higgs decaying in W+W− L Li 7
C
% 6
5
TheCMSsearchforH→W+W−[2]selectsdi-bosonscan- 9 5
didateswherebothbosonsdecayleptonically,yieldingan 4
experimentalsignatureoftwoisolated,hightransversemo-
3
mentum(p )oppositely-chargedleptons(electronsormuons)
T 2
and large missing transverse energy (Emiss) due to unde-
T 1
tected neutrinos. Given the large Emiss, a mass peak can-
T 0
not be reconstructed and therefore the analysis has a re- 100 120 140 160 180 200 220 240 260 280 300
Higgs mass, m [GeV/c2]
duced Higgs mass resolution. The zero spin of the Higgs H
boson gives angular constraints to the final state: the two Fig. 2. The expected and observed upper limits at 95% C.L.
leptons are expected with moderately small opening an- on σ(pp→H+X)×B(H→WW→2(cid:96)2ν) relative to the SM
gle while the Emiss with large angle with respect to the valueforanintegratedluminosityof1.55fb−1usingtheCLsap-
T
proach.Theresultsareobtainedusingacut-basedeventselection.
a e-mail:[email protected]
EPJWebofConferences
di-lepton system. Due to different signal sensitivities, the CMS Preliminary 2011 s = 7 TeV L = 1.66 fb-1
eventsareseparatedintothreeexclusivecategoriesaccord- 2c 6
ing to the jet multiplicity: H+0 jets, dominated by WW V/ DATA
e
andW+jetsbackgrounds;H+1jet,dominatedbyWWan G Z+jets
t¯t background; and H+2 jets where the main background 0 5
1 ZZ
is t¯t and the main production mechanism is Vector Bo- s/
son Fusion. The W+W− non resonant contribution is re- ent 4 mH=350 GeV/c2
duced requiring a small opening angle between the lep- Ev mH=200 GeV/c2
tons. The remaining background is estimated using side-
bandsextrapolationforthedi-leptonmassdistribution(fig. 3 mH=140 GeV/c2
1) for Higgs masses lower than 200 GeV/c2 and Monte
Carlo (MC) simulation for the high masses, where less
2
statisticsisavailable.TheW+jetsandQCDhardscattering
backgroundiscontrolledusingacontrolsampleofloosely
identified leptons extrapolated to the signal region. Back- 1
groundsinducedbyZbosonsarereducedrequiringthedi-
leptonmasstobeoutsidetheZmasswindow.Theremain-
0
ing contribution is estimated using the events inside the 100 200 300 400 500 600
Z mass window, rescaled using the ratio in/out estimated M [GeV/c2]
4l
using simulation. The t¯t background is reduced rejecting
events where a jet, considering jets with pT > 10 GeV/c, Fig.3.Distributionofthefour-leptonreconstructedmassforthe
can be identified as b-quarks. The tagged sample, domi- baselineselectioninthesumofthe4(cid:96)channels.Pointsrepresent
natedbyt¯tandtW,isusedtoextrapolatetheresidualcon- thedata,shadedhistogramsrepresentthesignalandbackground
tribution.Resultsfor1.55fb−1areshowninfigure2:aSM expectations.Thesamplescorrespondtoanintegratedluminosity
Higgs is excluded in the mass range from 147 GeV/c2 to of1.66fb−1.
194 GeV/c2, a wide sensitivity on the full mass range is
exploitedintheexclusionplot,evenatlowmasses. CMS preliminary 2011 1.66 fb-1 at s = 7 TeV
M
S 15
/n (cid:109)(cid:109) CCLLSS OExbpseecrvteedd LLiimmiitt
3 Higgs decaying in ZZ o CLS Expected ± 1(cid:109)
mit CLS Expected ± 2(cid:109)
The H → ZZ search involves the study of several final L li 10
states.ThegeneralstrategyidentifiesafirstZthatdecays C
into a pair (cid:96)+(cid:96)− and distinguish the case when the other %
5
decaysinto(cid:96)+(cid:96)− (4(cid:96)),intwoneutrinos(2(cid:96)2ν)orhadroni- 9
5
cally(2(cid:96)2q).Allthefinalstatesprovideastrongcontribu-
tionatthehighmassessearches.Someofthemmaycon-
tribute also to the low mass search, where one of the two
bosons can be off-shell. In particular the 4(cid:96) channel, de-
0
spite the low yield, provides very clean final states which 100 200 300 400 500 600
M [GeV/c2]
canbethencarefullyinspectedfortheHiggssignalsearch. H
In general a loss of sensitivity is present around mH = Fig.4. Theexpectedandobservedupperlimitsat95%C.L.on
180GeV/c2 due to the rapidly decreasing branching ratio σ(pp→H+X)×B(H→ZZ→4(cid:96))relativetotheSMvaluefor
ofH→ZZintheSM. anintegratedluminosityof1.66fb−1usingtheCLsapproach.The
resultsareobtainedusingashapeanalysismethod.
3.1 Thefourleptonsfinalstate
to be originated from the same primary vertex further re-
A clean signature of four well identified leptons belongs duce the background. A baseline selection, for the anal-
tothedecaychannelH→ZZ(∗)→(cid:96)±(cid:96)∓(cid:96)(cid:48)±(cid:96)(cid:48)∓ with(cid:96),(cid:96)(cid:48) = ysis in the full mass range imposes 20 GeV/c2 < m <
e,µ [3]. The search relies solely on the measurement of 120 GeV/c2. An high-mass selection, which is usedZ2for
leptons,andtheanalysisachieveshighleptonreconstruc- m > 2× m and the ZZ cross section measurement, re-
H Z
tion,identificationandisolationefficienciesforaZZ(∗)→4(cid:96) quires 60 GeV/c2 < m < 120 GeV/c2. The ZZ back-
Z2
system composed of two pairs of same flavour and oppo- ground estimation is performed using the MC prediction
site charge isolated leptons, e+e− or µ+µ−. On the other atNexttoLeadingOrder(NLO)thatgivesagooddescrip-
hand the small branching ratio associated to this decay tion of the mass distribution shape. The MC prediction is
channel leads to a low events yield and requires a very thennormalizedusingZ→(cid:96)+(cid:96)− eventsindataandtheex-
carefulstudyofthebackground.Thefirststepoftheanal- pected ratio σ /σ and acceptance obtained from MC.
ZZ Z
ysisrequirestheidentificationofthebestZcandidate(Z ) ThefakeleptonsfromZ+jetsinstrumentalbackgroundare
1
throughthetwooppositechargematchingflavourleptons estimated using a control sample containing the Z plus
1
withclosestinvariantmasstothenominalZmassandcon- looselyidentifiedleptons.TheZbb¯ andZt¯tbackgroundare
strainedto60GeV/c2 <m <120GeV/c2.Thenatleast estimated removing flavour, charge and isolation request
Z1
twootherleptonsarerequiredforthesecondZ(Z ).Isola- forZ andreversingtheimpactparametercut.Theinvari-
2 2
tion and impact parameter requests, imposing the leptons antmassdistributionoftheeventssurvivingtheselection
HadronColliderPhysicsSymposium2011
CMS Preliminary, s=7 TeV, 1.1 fb-1
70
M
S
H fi ZZ fi 2l2t
s/ 60
s
L
C Expected CLs Limit
%
5 50 – 1 s
9
– 2 s
s
40 Observed CLs Limit
30
20
10
200 250 300 350 400 450 500 550 600
m (GeV/c2)
H
Fig.5. Theexpectedandobservedupperlimitsat95%C.L.on Fig. 6. Missing transverse energy distribution in the
σ(pp→H+X)×B(H→ZZ→2(cid:96)2τ)relativetotheSMvalue H→ZZ→2µ2ν channel at pre-selection level for 1.6 fb−1
foranintegratedluminosityof1.1fb−1usingtheCLsapproach. ofdata.
is shown in figure 3. Results of the analysis are summa-
rizedintheexclusionplotinfigure4.
3.1.1 Thetwoleptonstwotausfinalstate
Aspecificanalysiscompletesthefourleptonsscenariowith
thespecificcasewheretheseconZdecaysintwoτleptons
[4].Theanalysisisperformedonadatasamplewhere4(cid:96)
events are already discarded. Additional requirements are
addedinordertobeabletoreconstructthe2τ.Resulsare
showninfigure5.
3.2 Thetwoleptonstwoneutrinosfinalstate
Fig.7. Theexpectedandobservedupperlimitsat95%C.L.on
Highbranchingratioandgoodsensibilitytothehighmass
range (250-600 GeV/c2) characterize the decay channel σ(pp→H+X)×B(H→ZZ→2(cid:96)2ν)relativetotheSMvalue
H→ZZ(∗) →(cid:96)+(cid:96)−νν¯[5]wherethesecondZdecaysintwo foranintegratedluminosityof1.6fb−1 usingtheCLsapproach.
Theresultsareobtainedusingacut-basedeventselection.
neutrinos.Twowellisolated,closeangle,chargedleptons
come from the decay of the first boosted Z. The second
Z decaying in two neutrinos brings to large Emiss, which
T tonsandtwojets[6].Moreover,thebranchingfractionof
characterizes the event and provides a good background
thedecaychannelH→ZZ(∗)→(cid:96)+(cid:96)−qq¯ isabout20times
suppressionpowerasshowninfigure6.Thet¯tbackground
higherthanthe4(cid:96) channel.Thismayleadtobettersensi-
is suppressed rejecting b-tagged events. The WZ and ZZ
tivitytoSMHiggsbosonproductionathighmasses,where
backgroundsareestimatedfromMCsimulation.Residual
backgrounds are estimated from data: Z+jets is modeled kinematicrequirementscaneffectivelysuppressbackground.
using photon + jets events and t¯t and WW are estimated For the analysis we select events containing at least two
eventswithdifferentflavourleptons(eµevents).Cutsop- leptons and two jets compatible with the Z mass hypoth-
esis. Since the final states are fully reconstructed we can
timizationisperformedonthebasisoftheHiggsmasshy-
performangularanalysis:theinitialzerospinoftheHiggs
pothesis. The dilepton pair is constrained to the Z mass,
bosons constraint the relative angles between the decays
theEmissisrequestedtobenotalignedwithjetsinorderto
T product.Adedicatedlikelihoodisusedtodiscriminatesig-
cope with jet energy mis-measurements. Transverse mass
nal compatible events. Since Z+jets events involve gluon
isalsousedtocharacterizethesample.Resultsofthisanal-
radiationwecanusegluon-jetscharacteristicstorejectthem.
ysisareshowninfigure7.
Inordertodothiswebuildalikelihooddiscriminantbased
on number of neutral and charged tracks and their trans-
3.3 Thetwoleptonstwojetsfinalstate versemomentumdistribution.Theeffectivenessofthegluon-
quarklikelihooddiscriminatoristestedonphoton+jetssam-
TheadvantageofthechannelwhereoneZdecayshadron- plewherejetsmainlycomefromquarks.Thet¯tbackground
ically is the fully reconstructable final state with two lep- is suppressed requesting small Emiss and controlled using
T
EPJWebofConferences
CMS Preliminary 2011, 1.6 fb-1 s = 7 TeV
V240 SM 102 CMS Preliminary, s = 7 TeV CHCHo o fifimm bbbbiibbnn ee dd ((11..11 ffbb--11))
20 Ge220200 S1 ibd-etabga n2dl2 Eqx dtraatpaolated Fit ss/mit on HHHHHHHHHHHHHH fifififififififififififififi tgtgWZZZZWZZZZttggZZZZZZZZ WW fifififififififi 42224222 llllllll 222 222 tntn q q ((((((((((((((11111111111111..............11117776765566 ffffffffffffffbbbbbbbbbbbbbb--------------11111111111111))))))))))))))
s / 180 Z + Jets L li 10
C
nt160 tt/tW %
e
v140 ZZ/WZ/WW 95
E
120 400 GeV SM Higgs×3 1
100
80
100 200 300 400 500 600
60 Higgs boson mass (GeV/c2)
40
Fig. 10. The observed 95% C.L. upper limits on the signal
20 strength modifier µ = σ/σ as a function of the SM Higgs
SM
boson mass for the eight CMS major analyses and their com-
200 300 400 500 600 700 bination,includedthefivedescribedinthispaper.Thelimitsare
m [GeV]
obtainedwiththeCLsmethod.
ZZ
Fig.8.Them invariantmassdistributionafterfinalselectionin
ZZ
the 1 b-tag category of the H → ZZ(∗) → 2(cid:96)2q channel. Points thantheMCsimulationbasedascanbeeasilyseeninfig-
witherrorbarsshowdistributionsofdata,solidhistogramsshow ure8.Resultsofthisanalysisareshowninfigure9.
expectationfromsimulatedevents,solidcurvedlineshowspre-
diction of background from sideband extrapolation procedure.
Thesamplescorrespondtoanintegratedluminosityof1.6fb−1. 4 Conclusions
CMS preliminary 2011 1.6 fb-1 at s = 7 TeV A search for the SM Higgs boson decaying into two Z or
16
M W bosons has been presented. The analisys is performed
S
σ CL Observed with the first 1.6 fb−1 of data recorded by CMS in 2011
/% 14 S overatotalofalmost5fb−1 andcoveramassrangefrom
5 CL Expected
σ9 S 130GeV/c2 to 440GeV/c2. No significant excess is ob-
12 CLS Expected ± 1σ servedthusnoevidenceofaHiggsbosonisfound.Com-
CL Expected ± 2σ bining the results from the different channels [7] we set
10 S
limitsontheStandardModelHiggsproductionintheranges
SM 145-216GeV/c2,226-288GeV/c2and310-400GeV/c2.The
8
limitsandthecontributionofthedecaychannelsillustrated
aresummarizedinfigure10.
6
4
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