Search for the SM Higgs Boson in H->ZZ* and H->WW* at the LHC

Search for the SM Higgs Boson in H->ZZ* and H->WW* at the

LHC

Sinjini Sengupta University of Minnesota

CMS Collaboration

The 16th International conference on Supersymmetry and the Unification of

Fundamental Interactions.

S.Sengupta, UMN SUSY08, Seoul, June 17th 2008

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Outline

o SM Higgs limits, production and decay modes

o The CMS detectoro About the H->WW* analysiso About the H->ZZ* analysiso SM Higgs Discovery Potential


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SM Limits on the Higgs Mass

Theoretical limits: finite and positive Higgs couplingExperimental limits:Direct (from LEP): mH > 114.4 GeV @ 95% CL

Indirect (from EW data): mH < 144 GeVRef: Phys.Lett.B 565(2003)


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SM Higgs Production

Gluon-gluon fusion(dominant mode) Vector Boson Fusion

(forward jets in final state)

Associated production(additional leptons or jets in the final state)


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SM Higgs Decay

For mH < 140 GeV/c2

o bb dominant decay mode but is hard to see because of large QCD background.

o γγ main discovery channel due to ECAL resolution

For 160 < mH < 180 GeV/c2

o H->WW* dominatesFor 140 < mH ~ 600 GeV/c2

o H->ZZ*->4l is important at higher mass


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CMS experiment

Muon Detectors

Superconducting Solenoid

Forward HCAL Hadronic

Calorimeter

Electromagnetic Calorimeter

Tracker


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The Muon Detector

o Drift Tubes (DT) in the barrel region

o Cathode Strip Chambers (CSC) in the endcap region

o Resistive Plate Chambers (RPC) as dedicated trigger detectors in both barrel and endcap.

4 layers of muon chambers, |η|=2.4

5 wheels inthe Barrel

30° (Φ)sectors


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Higgs―›WW*―›lνlν (where l=e,μ)


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Signal

o Main discovery channel for the Higgs Boson in the mass range 2MW < mH < 2MZ

o 3 Signal topologies: e+e-, μ+μ-, e±μ±o Geared for ∫L=1032cm-2s-1, 100 pb-1 of datao Pythia is re-weighted to follow NLO PT distribution.


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Background

o Primary backgrounds Continuum WW : uncertainty 17% at 5 fb-1

tt->2μ : uncertainty 16% at 5 fb-1

γ*/Z->2l

o Other backgrounds: ZW->3l, tWb->2l, ZZ->2lo Most samples generated with Pythia


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Analysis features

o Analysis uses full detector simulationo Single and di-lepton (e,μ) triggers at L1 and HLTo 2 charged leptons (e or μ). Leptons are required to

be well identified, well reconstructed and isolated30 GeV < PT

max < 55 GeV

PTmin > 25 GeV

o 2 neutrinos in the final state. Require ETmiss > 50 GeV

} mH = 160 GeV/c2


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Analysis features

o Due to presence of 2 neutrinos, there is no mass peak. Counting experiments Accurate background estimates from data are needed. Good reconstruction tools needed.

o Well identified lepton reduces W+jet bkgo To reduce tt and tWb bkg all central jets (|η| < 2.5) with PT > 15 GeV are

rejected. (this gets rid of 90% tt but keeps 50% signal)o Backgrounds are the dominant source of systematics (20% for 1 fb-1)


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Jet veto and Acoplanarity

Invariant mass before and aftercentral jet veto cut.

Azimuthal angular separation betweenThe di-lepton pair after all other cuts.(effective in reducing WW, ZZ and WZ backgrounds)

∆Фll < 45°12 < mll < 40 GeV


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Looking ahead…

o Currently cut vs multi-variate analysis is being studied. First results are very promising Shows an excess in signal

events.

5σ discovery potential can be achieved very rapidly.Need to understand detectorand backgrounds well.


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Higgs―›ZZ*―›4l (4l = 4e, 4μ or 2e2μ)


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The Golden Channel

Cleanest discovery channel for mH > 140 GeV/c2

H->ZZ->2e2μ

H->ZZ->4μ


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Signal and Background

o Signal:Potential for discovery over a wide mass rangePossible topologies: 4e, 4μ, 2e2μ

2e2μ has twice the rate as 4e an 4μ channels

o Backgrounds:qq -> ZZ*/γ*

Irreducible, dominant background

gg -> tt -> WWbb Zbb

Reducible backgroundsnon-isolated leptons from b decays


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Event Selection 2e2μ

o Require 2 pairs of leptons Flavor must be conserved Charge must be conserved Leptons must be isolated Originate at primary vertex Reconstruct to dilepton

invariant mass ~ mZ

o Analysis requires lepton identification with high efficiency and resolution down to low PT (~ 5 GeV/c)

2e2μ before selection

after selection

mH = 200 GeV

Selection cuts were optimized as a function of mH but mass independent cuts are being developed.

mH = 140 GeV


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Event Selection 4μo High muon efficiencyo Underlying event is calibrated

with data.

Largest systematic uncertaintiesare from background. Vary with mH


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mZ Resolution

o For mH < 180 GeV/c2, one Z is on shell, other is off shell

o For the on shell Z Z -> 2μ has 1.14% mass resolution Z -> 2e has 1.8% mass resolution (worse resolution due to Bremsstrahlung)

CMS note 2006/136mH = 130 GeV/c2

Ratio of the measured to the trueinvariant mass for μ+μ- pairs


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Isolation and Impact Parameter

o Isolation is important for reducing contamination from b-decay leptons Isolation is in the tracker only A cut is applied on the sum of the PT of

the tracks in a cone of radius ∆R around the signal lepton

Signal efficiency ~ 90%

o Impact Parameter Leptons from b quarks dont come from the primary vertex Apply cuts on the transverse and the 3D distance of fitted tracks from the vertex as well as on the impact parameter significance.


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Significance

o Expected events for signal and background for an integrated luminosity corresponding to a discovery significance of 5σ.

o Systematic uncertainties (5-30% from backgrounds) have little impact on the discovery potential.

mH = 140 GeV

mH = 200 GeV


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Discovery Potential for the Higgs

o Luminosity requirements: ~ 100 pb-1 will exclude certain regions with a 95% CL ~ 1-10 fb-1 has a discovery potential for mH ~ mWW at 5σ

~ 30 fb-1 has a discovery potential of upto mH ~ 600 GeV


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Conclusions

o The H -> WW* and H ->ZZ* analyses are already well developed at CMS Studies are being updated and new results are

expected shortly Multivariate approach over the counting strategy in

the case of H -> WW* shows significant early promise

o 5σ discovery potential for the Higgs Boson over a large mass range at the LHC with 5 fb-1 of data.

o H -> ZZ* has the cleanest signal and has the best discovery potential for a heavier Higgs Boson.


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Backup Slides


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Cut Flow Table for H-> ZZ* -> 2e2μ

mH = 140 GeV/c2. Errors are statistical only.All values (except number of events) are in fb


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Cut Flow Table for H -> ZZ* -> 2e2μ

mH = 200 GeV/c2. Errors are statistical only.All values (except number of events) are in fb


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Results for H -> ZZ* -> 2e2μ

Expected number of events from signal and backgroundprocesses for an integrated luminosity corresponding to a 5σ significance.


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Results for H-> WW* -> 2e

Expected number of events for a luminosity of 10 fb-1

Selection cuts optimized for mH = 150 GeV/c2.

Documents

Search for the SM Higgs Boson in H->ZZ* and H->WW* at the LHC