Vector Boson Fusion Production of the Standard Model Higgs at the LHC

1.2 MB·English

by  Monica Vazquez Acosta

Vector Boson Fusion Production of the Standard
Model Higgs at the LHC
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Mónica Luisa Vázquez Acosta (on behalf of the CMS Collaboration)∗
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ImperialCollegeLondon
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J E-mail: [email protected]

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ThecrosssectionmeasurementsoftheHiggsbosonproductioninthevectorbosonfusion(VBF)

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x process at the LHC followed by a Higgs boson decay into ττ, WW and γγ will significantly
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extend the possibility of Higgs boson coupling measurements. Prospective analyses with the
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p CMS experiment are discussed for the H →γγ, WW and ττ decay channels for an integrated
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h LHCluminosityof30fb−1. ForaHiggsbosonmassintherange115to140GeV,anobservation
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withasignificanceabove2standarddeviationsisexpectedintheHtoγγ channel, andabove3

1 standarddeviationsintheHtoττ channel. TheHtoWWchanneloffersadiscoveryreachabove
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5sigmainthemassrange140to200GeV.Anewcompletestrategyispresentedforthecontrol
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9 ofsystematicsandearlysearchesatverylowluminositiesoftheorderof1fb−1.
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2008PhysicsatLHC
September29-4October2008
Split,Croatia
∗Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
VBFHiggsProductionattheLHC MónicaLuisaVázquezAcosta(onbehalfoftheCMSCollaboration)
1. Introduction
Vector Boson Fusion (VBF) Higgs boson production is the second largest production mech-
anism at the LHC. The cross section measurements of the VBF process, VV →H (qq→qqH),
followed by Higgs boson decays into ττ, WW and γγ will significantly extend the possibility of
Higgsbosoncouplingmeasurements[1,2].
2. VectorBosonFusionSignature
EventsproducedbyVBFarecharacterizedbyadisticttopologyofthefinalstate: twoforward
jets with little extra hadronic activity and the decay products of the Higgs boson. The rapidity
distribution of the 3rd jet with respect to the two forward jets, η∗, is shown in Fig. 1 (left) which
j3
shows a double-peak structure for the electroweak processes, including the VBF signal, and is
more central for the QCD background samples. Applying a central jet veto (CJV) is a poweful
rejection method against the QCD background. To avoid considering jets from pile-up events in
theCJV,jetsareassociatedtothesignalvertexusingtracks. Foreveryextrajetonecandefinethe
quantityα =Σp /E ,wherep isthep oftracksfromthesignalvertexwithinthejetcone
j3 Ttrk Tj3 Ttrk T
and E is the jet measured raw E . Figure 1 (right) shows α tends to peak at low values for
Tj3 T 3j
non-signaljets. Theefficiencyofthevetoforthebackgroundsamplesversusthesignalefficiency
is shown in Figure 2 (left) for events containg a 3rd jet with E larger than different threshold
T
values. Anoptimalthresholdwherethesignalprocesshas∼80%efficiencywhilethebackgrounds
are suppressed below 50% is used [3]. An alternative approach is to consider a track counting
veto(TCV)[4],wherethenumberoftracksbetweenthetwoleadingjetsiscountedwithdifferent
p thresholds. Figure 2 (right) shows the performance of the TCV algorithm, i.e the efficiency of
T
selectingthesignalversusthebackgroundforeventswithanincreasingcutonthetrackmultiplicity
and p . The black star indicates the performance of the CJV based on calorimeter jets. The TCV
T
algorithmcanreachsimilardiscriminationpowerthanthecentraljetveto.
s 90
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ntri 80
e 70
of
er 60 Signal, qqH
mb 50 QCD 2t+2/3jet
Nu 40 EW 2t+2jet
tt fi WbWb
30
W+3/4jet
20
10
0
-4 -2 0 2 4
h* = h - (h + h )/2
j3 j3 j1 j2
Figure1: Theη distributionofthe3rdjetwithrespecttothetwoforwardjets(left). Thedistributionofα
3j
whichisusedtomatchjetstothesignalvertex(right).
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VBFHiggsProductionattheLHC MónicaLuisaVázquezAcosta(onbehalfoftheCMSCollaboration)
Figure 2: Efficiency of the CJV for background versus signal (M =135 GeV), for increasing 3rd jet E
H Tth
threshold (left). TCV performance for different ptrack and track multiplicity thresholds compared to the
t
performanceoftheCJV.
3. VectorBosonFusionHiggsDiscoveryPotential
The observability of the VBF Higgs boson production has been studied with the full CMS
detector simulation in the H → ττ, γγ and WW decay channels [5]. VBF H → ττ production
has been studied in the Higgs mass range of 115 to 145 GeV in the lepton plus τ final state.
jet
Figure3(left)showstheexpecteddi-τ massdistributionusingthecollinearapproximation[3]for
a luminosity of 30 fb−1. Figure 3 (right) shows the significance of the expected number of signal
eventsfordifferentHiggsmasses. Astatisticalsignalsignificanceof3.9σ isexpectedforaHiggs
massof135GeV.
2 GeV/c 5 SEttbiWcpppcpppg01201222 // a nn dd ff n /11r ..1144 112233Q 44..661188a 00 ..22––00..77 66 ––00 W22–– ..33 66 //44.. 226611l..44338866C (+D1j3e 25tstG+ejeVts/c2) MNSH(3[G0efVb−]1) 1101.457 71.2759 71.3954 31.4653
-1) / 5vts (30fb 43 FFFSiiiuttt mtttooo oSZttfb/i ggfai*ntrs a (Wfil+ 2jet)ts NSSccBPP(aa3tt033f00bff−bb1−−)11((nσoBu=nc7e.8rt%ai)nty) 343...709047 233...276117 133...899484 122...411298
Ne 2 S at60fb−1(σ =5.9%) 5.67 5.26 5.64 3.19
cP B
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Mt t [GeV/c2]
Figure 3: Di-τ invariant mass expected for a luminosity of 30 fb−1 (left). Significance of the expected
numberofsignaleventsfordifferentHiggsbosonmasses(right).
VBFH →WW productionintheleptonplustwojetfinalstatehasbeenstudiedintheHiggs
massrangebetween120and250GeV.Figure4(left)showsthesignalsignificanceexpectedwith
30fb−1 fordifferentcentraljetvetoselections[6]. Inthemassrangebetween140-200GeVa5σ
significance can be achieved. VBF H →γγ production has also been studied in the Higgs mass
rangebetween115and150GeV[7]. Figure4(right)showsthesignalsignificanceexpectedwith
30 and 60 fb−1. With 60 fb−1 of collected data a 3σ significance can be achieved for a low mass
Higgsintherange115to130GeV.
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VBFHiggsProductionattheLHC MónicaLuisaVázquezAcosta(onbehalfoftheCMSCollaboration)
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e 4
9 anc 3.5
8 c
nce 67 Signifi 2.35 CMS, 60 fb-1
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nific 5 2
g 4
Si 1.5
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1 0.5
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120 140 160 180 200 220 240 260 111100 111155 112200 112255 113300 113355 114400 114455 115500 115555 116600
M, GeV/c2 HHiiggggss mmaassss,, GGeeVV
H
Figure 4: Signal significance of VBF H →WW for 30 fb−1. The high (low) curves correspond to full
(loose)extrajetveto(left). SignalsignificanceofVBFH→γγ for30and60fb−1(right).
4. SearchofHiggs→ττ →lepton+τ with1fb−1
jet
A selection strategy for the search of VBF Higgs →ττ →lepton+τ with 1 fb−1 has been
jet
developed and is described in detail in [8]. The di-τ invariant mass will be analyzed to search
for the presence of a Higgs boson in the region above the Z → ττ mass peak. It is important
to know well the shape of the Z → ττ background. The dominant uncertainty comes from the
modeling of the missing transverse momentum related to the effects of pile-up, underlying event
andthecalorimeternoiseandresponse. Amethodtomodelthedi-τ masshasbeendeveloped[4].
Z→µµ dataeventsareselectedandthemuonsareremovedfromtherealevent. Di-τ MonteCarlo
eventsaregeneratedwiththesamekinematicsastherealmuonsandtheirdetectorresponseisfully
simulated. FinallytherealZ →µµ eventswiththemuonsremovedandthesimulateddi-τ events
aresuper-imposedtoformoneevent,Z→τ τ ,andthedi-τ massiscalculated. Thereconstructed
µ µ
di-τ mass for real and fake Z →ττ events for inclusive Drell-Yan and Z+jets events are shown in
Fig.5. Agoodagreementbetweenthedi-τ massshapesisobtained.
The expected di-τ mass distribution for the background and the Higgs signal for 1 fb−1 is
shown in Fig. 6 (left). A profile likelihood method is used to evaluate the upper limit on the
number of signal events. Figure 6 (right) shows the expected 95%CL limit on the cross section
timesbranchingratioasafunctionoftheHiggsbosonmass.
events / 10 GeV111122246802000000000000 (cid:242) L ZZdt fifi= 1 3tt0m tp bt-m1
1000
800
600
400
200
) / ratio (ttttmm011...920110 50 100 150 200 250
0.8
0.7 CMS preliminary
mt t [GeV/c2]
Figure5:Reconstructeddi-τmassforrealandfakeZ→ττeventsforthefinalstates(left)ττ→µνν+µνν
frominclusiveDrell-Yaneventsand(right)ττ →lνν+τ ν fromZ+jetsevents.
jet
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VBFHiggsProductionattheLHC MónicaLuisaVázquezAcosta(onbehalfoftheCMSCollaboration)
-1-1N events at 1 fbN events at 1 fb67896789 ∫C LM dSt P=r e1l ifmb-1inary ZZγtWQZ Ht++++C,+ jWjjj→eeeeDjettttW s sstsPτs,,,τ YZ Z,→ ZW( T1 →→→ H 3a→ 5Ilτe Ale τleGνe,e ,j Veet / →c→2 )τ τ x 10 SMSM 95% CL / 95% CL / σσσσ22220505 C∫ ML Sd tP =re 1li mfbi-n1ary
55 1155
44
33 1100
22 Without systematics
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0000 5500 110000 115500 MM22((00ττττ00)) [[GGeeVV22//cc552200]] 00 H11 →1155 ττ →11 22l00 νν + 1τ1h22a55d ν 113300 113355 11W4400ith smmysHHt e[[mGG11a44eeti55cVVs//cc22]]
Figure6: Di-τ massdistributionofexpectedbackgroundswith1fb−1 afterallselection. Backgroundsare
showncumulative. Thesignalmassdistributionscaledbyafactor10isalsoshownforM =135GeV.
H
5. Conclusion
A selection strategy for the Standard Model Higgs boson produced in vector boson fusion
decayingtoapairofτ leptonswith1fb−1 ofearlyCMSdataattheLHChasbeenpresented. No
signalevidenceisexpectedandupperlimitonthecrosssectiontimesbranchingratioisevaluated.
ProspectiveanalysesfortheH →γγ,WW andττ decaychannelsforaluminosityof30fb−1 have
also been discussed. For a Higgs boson mass in the range 115 to 140 GeV, an observation with a
significance above 2 standard deviations is expected in the H to γγ channel, and above 3 standard
deviations in the H to ττ channel. The H to WW channel offers a discovery reach above 5 sigma
inthemassrangeof140to200GeV.
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H
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