# Presentation: 536308

## Search for the Standard Model Higgs Boson at High Mass at the Tevatron

- Lidija Živković,
- Columbia University
- on behalf of the
- CDF            and              DØ
- collaborations

*XLIIId Rencontres de MORIOND *
*La Thuile, March 1-8, 2008*

## Outline

- Motivation
- Theoretical overview
- Analysis overview
- CDF
- DØ
- Combined Limits
- Future perspectives

- up to 2.4 fb-1

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## Motivation - The Higgs Mechanism

- Essential ingredient of the Standard Model
- Complex scalar field with potential
- Used to break the el. weak symmetry......
- ..... and to generate fermion masses:
- Search for the Higgs boson is a key issue for experiments at current and  future colliders
- Experimental challenges:
- Higgs boson discovery
- Measurement of Higgs boson parameters (couplings to bosons and fermions) and the Higgs self coupling
- Mass limits: lower 114.4 GeV/c2 and upper 182 GeV/c2 from combined direct and indirect searches

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## Production and Decay

- Dominant production process is gluon fusion
- Associated with vector boson is also significant

- Dominant decay for high mass Higgs boson is WW
- For mH = 160 GeV, BR = 390 fb;several hundreds HWW so far at either DØ or CDF (2 fb-1 per exp.)few tens to two leptons and neutrinos

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## Analysis overview

- To suppress hadron backgrounds we look into final states where both W decay to leptons, i.e. ee,  and e
- Major backgrounds: Diboson (mainly WW), W+jets, Drell-Yan,  tt, Multijets
- Signature:
- Two energetic isolated leptons with opposite charge
- Large missing transverse energy

- signal

- WW background

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## Analysis overview

- Characteristics:
- In signal WW pair is coming from spin 0 Higgs boson
- Leptons prefer to point in same direction
- Di-lepton opening angle ll discriminates against dominant WW background.
- Dilepton mass is small and broad
- Discriminates against Drell-Yan

- DØ Run II Preliminary

- signal

- signal

- em [rad]

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## Multivariate techniques

- In order to better separate signal from backgrounds we use different multivariate techniques: CDF combines Leading Order (LO) Matrix Elements and Neural Networks (NN), DØ uses Neural Networks
- LO Matrix Elements are used to calculate event probabilitiesand calculate likelihood ratio:
- Neural network:
- CDF uses ME and kinematic variables as inputs
- DØ uses kinematic variables as inputs
- Single output from NN is used as discriminant variable

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## CDF - selection

- Basic Selection:
- Lepton trigger selection
- Several categories of lepton(track) pairs with opposite charge divided into two groups – high signal to background and low signal to background
- Lepton and missing ET cuts applied to reduce backgrounds:pT(l1) > 20 GeV, pT(l2) > 10 GeV, ET·sin(min(/2,(ET,l or jet))) > 25 GeV,njets < 2 (pT(jet) > 15 GeV, || < 2.5), mll > 16 GeV, trilepton veto
- Data are well described

- /

- /

- MET

- signal

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## CDF – event yields 

- CDF – event yields

- All five channels are combined leading to 626 expected events from known SM processes and 661 observed  events
- 9.5 signal events are expected for Higgs mass of 160 GeV

- signal

- ee     e         etrk    trk

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## CDF – final discriminant

- ME calculated from lepton 4-vectors and missing transverse energy is used as an input to NN together with several kinematic distributions

- signal

- NN output

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## CDF - results

- Binned maximum likelihood fit of NN
- discriminant used to determine limit

- BR < 0.8 pb @ 95% CL
- for mH=160 GeV

- Observed Limit/SM (NNLL) ~ 1.6
- Expected Limit/SM (NNLL) ~ 2.4

- Various sources of systematic uncertainties affect the background estimation and the signal efficiency:
- Theoretical uncertainty of background production cross sections (10-15%), lepton id (2%), trigger efficiency (~5%)

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## DØ - selection

- Basic Selection:
- Combination of several lepton triggers ensures trigger efficiency ~95%
- Two isolated leptons with opposite charge
- Lepton and missing ET cuts applied to reduce backgrounds
- Final selection cuts optimized for each Higgs mass separately
- Data are well described

- signal

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## DØ – event yields

- Combining three analyzed channels there are 50.6 expected events from known SM processes and 45 observed  events
- 3.71 signal events expected for Higgs mass of 160 GeV

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## DØ – final discriminant

- Kinematic distributions are used as inputs for the NN
- NN trained on WW background samples, run on all backgrounds; separate optimization for each channel and Higgs mass

- Final result determined from fit to NN output

- signal

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## DØ – result

- Various sources of systematic uncertainties affect the background estimation and the signal efficiency:
- Dominated by background normalization (6-20%), others include theoretical uncertainty of background production cross sections (~4%), Jet Energy Scale – JES (5-10%), electron and muon reconstruction efficiencies and resolutions (2-11%), trigger efficiency (~5%)

- Observed Limit/SM (NNLL) ~ 2.4
- Expected Limit/SM (NNLL) ~ 2.8

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## WH->WWW(*)->l±l±+X

- Helps to cover intermediate mass region
- Basic selection requires two same charge leptons with pT > 15 GeV
- Two main types of backgrounds:
- With two real same charge leptons like WZlll
- Instrumental – measured from data:
- “QCD” with misidentified lepton
- “flip charge” when charge of the lepton is mismeasured
- Main source of systematic uncertainty is coming from instrumental background (~30%)
- Limit: 0.9 pb at 95% CL for mH=160 GeV

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## Combined limits – CDF and DØ

- Combined limits from December 2007
- New results shown today not yet included!
- Expect both experiments to show improved limits next week

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## Combined limits – Tevatron

- _[http://arxiv.org/abs/0712.2383](http://arxiv.org/abs/0712.2383)_

- New results shown today not yet included!
- CDF limits improved more than 20% (10% from improved analysis)
- mH = 160 GeV: (exp) 3.1 2.4
- We expect significant improvement of DØ limits (next week)
- Expect new improved combination from Tevatron (next week)

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## Summary and Future Perspectives

- Still no evidence for Higgs boson but the Tevatron is closing in on the SM at large values of Higgs mass
- Current limit on the cross-section is only 1.4 times bigger then what we expect from Standard model for mH = 160 GeV
- We expect further improvements from
- More data (luminosity)
- Further improvement of lepton identification
- Optimization of multivariate techniques
- Including new channels
- Look for better limits already next week and expect exciting summer

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## Backup

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## CDF and DØ experiments in Run II

- Both detectors are upgraded in Run II
- New silicon micro-vertex trackers
- New tracking systems
- Upgraded muon chambers

- CDF: new Plug Calorimeters, new TOF

- DØ: new solenoid, new pre-showers,
-  LØ for SMT in RunIIb, new L1Cal trigger

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## Tevatron projections

- Including data taking efficiency projected full data set will be:
- 5.5 fb-1 by the end of 2009
- 6.8 fb-1 by the end of 2010
- Assumption: projected sensitivity for mH = 115 GeV 2 times higher than current for full data set
- Improvment from 2005-2007 was ~1.7
- Possibilities:
- Better b-tagging
- Better dijet mass resolution
- Better multivariate techniques

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## CDF WW limits

- CDF WW

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##  DØ, Tevatron limits

- Tevatron

- DØ

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## DØ, Tevatron LLR plots

- Distributions can be interpreted as follows:
- The separation between LLRb and LLRs+b provides a measure of the discriminating power of the search. This is the ability of the analysis to separate the s + b and b-only hypotheses.
- The width of the LLRb distribution  provides an estimate of how sensitive the analysis is to a signal-like fluctuation in data, taking account of the presence of systematic uncertainties. For example, when a 1 background,  fluctuation is large compared to the signal expectation, the analysis sensitivity is thereby limited.
- The value of LLRobs relative to LLRs+b and LLRb indicates whether the data distribution appears to be more signal-like or background-like. As noted above, the signicance of any departures of LLRobs from LLRb can be evaluated by the width of the LLRb distribution.

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