# Presentation: 815510

## 

Searches for Higgs Particles with DØ

Avto Kharchilava
University of Notre Dame

For the DØ Collaboration

**Searches for Higgs Particles with DØ**
**Avto Kharchilava**
**University of Notre Dame**
**For the DØ Collaboration**

- Wine and Cheese seminar                                                             Fermilab, 15 April 2005

## ElectroWeak symmetry breaking in the SM

- Fundamental forces exhibit local gauge symmetry
- Gauge fields should have infinite range
- Gauge quanta, which mediate the field, should be massless
- However, the W and Z bosons are massive. Why ?
- Postulate presence of a Higgs field that “breaks” the gauge symmetry
- As a result of the EW symmetry breaking
- W and Z acquire masses, photon remains massless
- Fermion masses are generated if Higgs field couples to them
- Prediction: an excitation of this field will be a neutral, massive scalar particle
**the Higgs boson**
- Need to find it !

## How it all began (for the Tevatron)

- First proposal to search for the SM Higgs boson came in 1993 by A. Stange, W.J. Marciano and S. Willebrock (**Phys. Rev. D 49, 1354 (1994)**) when it was realized that at the Tevatron one can **efficiently tag b-jets** thus the associated production of Higgs boson with vector bosons can be explored
- “Conspicuously absent from this discussion [LHC+SSC]”
- Several analyses followed by CDF and DØ experimental groups
- TeV-2000 Study Group Collaboration, **D. Amidei et al., FERMILAB-PUB-96-082**
**A side note:** projected accuracy for the top mass ~ **13 GeV** for 70 pb-1 of data
- In about 2 years CDF+DØ precision reached **5.1 GeV**  **“data make us smarter”**
- Higgs Working Group Collaboration, **M. Carena et al., hep-ph/0010338**
- Tevatron Higgs Sensitivity Study Group, **L. Babukhadia et al., FERMILAB-PUB-03/320-E**
- Main results
- Demonstrated feasibility for a light Higgs boson discovery
- Evaluated experimental requirements

## A brief look into the future (of the Tevatron)

- Main conclusions
- There is no single channel which guarantees success
- Improved understanding of signal and background cross sections, kinematics, along with the detector performance figures, is vital
- To maximize sensitivity advanced analysis techniques have to be employed and results from two experiments combined

- The integrated luminosity required per experiment, to either exclude a SM Higgs at 95% C.L. or discover it at the 3 or 5 level; no systematics

- Where do we stand now ?
- After ~ 2 years of running
- With ~ 10% of data on tape

## 

Outline

Introduction
Current limits
DØ experiment

SM Higgs searches
W(e)H(bb)
Z()H(bb)
H  WW*  l+l- 

SUSY Higgs searches
bh/bbh(bb)

Summary

**Outline**
- Introduction
- Current limits
- DØ experiment
- SM Higgs searches
- W(e)H(bb)
- Z()H(bb)
- H  WW*  l+l-** **
- SUSY Higgs searches
- bh/bbh(bb)
- Summary

## Current status: direct and indirect limits

- Over the last decade, the focus has been on experiments at LEP
- Direct searches for Higgs production yield 		**m****H**** > 114.4 GeV** (95% C.L.)
- Analysis of precision EW measurements combined with Fermilab’s top quark mass measurement suggest
**	****	****	****m****H**** < 280 GeV** (95% C.L.)
- Central value: **m****H**** = 126 ****+73****−48**** GeV**
- Hint (~2) for a Higgs around 115 GeV at LEP

- Higgs seems to be relatively light
- Until about 2008, the Tevatron is the only place to search for Higgs, and with good chances
- Mass range favorable to Tevatron’s reach

## CDF Run I combined limits

- Results based on 106 pb-1 of Run I data: **CDF Collaboration, hep-ex/0503039**

- The 95% C.L. upper limits on (VH)×B(Hbb) for each of the channels and their combination

## The upgraded DØ detector in Run II

- Silicon detector
- Fiber tracker

- Upgraded
- Muon system, cal. electronics
- DAQ, (track) trigger system
- Displaced-vtx trigger

## ... and how it works

## Data sets

- Depending on the analysis/final states,
- the following results are based on data
- taken till June 2004

## SM Higgs boson production

## The SM Higgs boson decays

- Tevatron can explore bb and WW(l+l-) decay modes

**Excluded at LEP**

## Higgs search strategies: low mass region

**M****H**** < 135 GeV:  H ******** bb**
- Higgs produced in gluon fusion has too large QCD/bb background
- Search for (W/Z)H production where W/Z decay leptonically
**qq’ ******** W* ******** WH ******** l********bb**
**Bkgd: Wbb, WZ, tt, single top**
**qq ******** Z* ******** ZH ******** l****+****l****-****bb**
**Bkgd: Zbb, ZZ, tt**
**qq ******** Z* ******** ZH ******** ********bb**
**Bkgd: QCD, Zbb, ZZ, tt**
- Identify leptons (e/) and missing transverse energy from neutrinos
- Tag b-jets
- Disentangle H  bb peak in di-b-jet mass spectrum

## Higgs search strategies: high mass region

**M****H**** > 135 GeV:    H ******** W****+****W****-**
- Search for gluon fusion and leptonic decays of W’s
**gg ******** H ******** W****+****W****-**** ******** l****+********l****-******
- Bkgd: Drell-Yan, WW, WZ, ZZ, tt, tW, 
- Initial signal/bkgd. ratio ~10-3 !
- Identify leptons (electrons/muons) and missing transverse energy from neutrinos
- Explore angular correlations to separate signal from background

## WH searches: W(e)+jets production (1)

- Event selection include
- Isolated e pT > 20 GeV, || < 1.1
- Missing ET > 25 GeV
- ≥ two jets: ET > 20 GeV, || < 2.5

- Good understanding of data

- Simulations with Alpgen plus Pythia through detailed detector response
- Cross sections normalized to MCFM NLO calculations

## WH searches: W(e)+jets production (2)

- Untagged (control) sample

- Good overall understanding of data

- Bkgd. other than W+jets is small

- Data and MC agree
- within JES uncertainties

## W(e)+jets/bb: angular correlations

- W(e)+jets/bb: angular correlations

- Correlations between leading two jets in R – a measure of distance in  space
- Sensitive to parton radiation processes
- Reduced sensitivity to jet energy scale

- Again, good agreement
- between data and MC

- Several processes “show up”

- Sample with at least
- one b-tagged jet

- Untagged sample

## W(e)H(bb) searches

- No excess of data in the Higgs boson search mass window

- Di-jet mass distribution in events with exactly two jets that are tagged as b’s
- Observe **6 evts**., expect **4.4±1.2**
- Estimated bkgd. sample composition

## W(e)H(bb) limits

- In the absence of a signal, 95% C.L. limits are set on Higgs boson production cross section times branching ratio to b-quarks

- Published in PRL

## Are we close to the performance goal set in the past ?

|  | DØ’04 analysis
We | Prospective study
We |  |
| --- | --- | --- | --- |
| Mass window | [85,135] | [100, 136]    Ratio |  |
| Mass resolution  | 14 +/- 1% | 10-15% |  |
| Signal (S) | 0.049  | 0.145 |  |
| Background (B) | 1. 07 | 1.7 |  |
| S/B | 0.047   | 0.11 | 2.4 |
| S/B | 0.046  | 0.085 | 1.8 |

- We are currently missing a factor of 2.4 in sensitivity
- Prospective studies assumed
- Larger ECAL coverage (+30%), better EM-ID (+40%), b-tagging efficiency (+50% for 2 tags), mass resolution (30% less bkgd.)
- Factor 2 in S/B  2.4/2 = 1.2 difference (only) in sensitivity
- Advanced analysis techniques
- All the missing factors can be recovered

## Improvements in the object ID

- Jet energy resolution (using track-jet algorithm)
- Subtract expected energy deposition in calo.
- Add the track momentum
- Add the energy of out-of-cone tracks
- Improve the jet energy resolution by ~10% 
- b-tagging capability
- Improvements would mainly come from L0 of the Silicon Tracker

- Improved SV
- algorithm

**Photon+jet data**

## b-tagging optimization: example of Zbb

- Optimize mistag rate vs. b-tag efficiency to obtain best significance S/√B
- Current operating point is at 0.5% for mistag rate (per jet)
- Corresponds to > 10-4 reduction in Z+jj rates, while Z+bb/Z+jj is ~ 1/50 only
- After b-tagging the bkgd. to ZH is dominated by Zbb production
- Optimize against Z+jj background

***Per jet efficiencies***

- Optimal point is ~ 3.5% mistag rate

- Gain a factor of 1.6 in efficiency !
- Further improvements to be made using event likelihood

## ZHbb searches

- Missing ET from Z and 2 b jets from Hbb
- Large missing ET > 25 GeV
- 2 acoplanar b-jets with ET 20 GeV, || < 2.5
- Backgrounds
- “physics”
- W+jets, Z+jets, top, ZZ and WZ
- “instrumental”
- QCD multijet events with mismeasured jets
- Huge cross section & small acceptance
- Strategy
- Trigger on events with large missing HT
- HT defined as a vector sum of jets’ ET
- Estimate “instrumental” background from data
- Search for an event excess in di-b-jet mass distribution

## More selection variables

- Suppress “physics” background
- In addition to missing ET > 25 GeV and two jets with ET 20 GeV
- Veto evts. with isolated tracks     reject leptons from W/Z
- HT = |pT(jets)| < 200 GeV          for tt rejection
- Reduce “instrumental” background
- Jet acoplanarity (dijet) < 165
- Various missing energy/momentum variables
- ET                calculated using calorimeter cells
- HT = – |pT(jet)|                 ... jets
- PTtrk = – |pT(trk)|              ... tracks
- PT,2trk = – |pT(trk in dijet)| ... tracks in jets
- Form various asymmetries
- Asym(ET,HT) = (ET – HT)/(ET+HT)
- Rtrk = |PTtrk – PT,2trk|/PTtrk
- 	 In signal like events they all peak at ~ 0 and are aligned

## Asymmetry distributions

- signal

- sideband

- sideband

## Event Selection: instrumental background estimation

- Physics bkgd. from MC

- signal

- sideband

- sideband

## Distributions before b-tagging

- Total
- Data : 2140
- Expect : 2125

## Singly b-tagged events

- Total
- Data : 132
- Expect : 145

## Doubly b-tagged events

- Total
- Data : 9
- Expect : 6.4

## Results

| Mass (GeV)
Window | 105
[70,120] | 115
[80,130] | 125
[90,140] | 135
[100,150] |
| --- | --- | --- | --- | --- |
| Data | 4 | 3 | 2 | 2 |
| Acceptance (%) | 0.29  0.07 | 0.33  0.08 | 0.35  0.09 | 0.34  0.09 |
| Total bkgd. | 2.75  0.88 | 2.19  0.72 | 1.93  0.66 | 1.71  0.57 |
| Expected limit (pb) | 8.8 | 7.5 | 6.0 | 6.5 |
| Limit @95% C.L. (pb) | 12.2 | 9.3 | 7.7 |  8.5 |

| Source | Sig  | bkgd |
| --- | --- | --- |
| Jet ID | 7 | 6 |
| JES | 7 | 8 |
| Jet energy resolution | 5 | 3 |
|   b-tagging | 22 | 25 |
| Instrumental bkgd. | - | 2 |
| Bkgd Cross Section | - | 17 |
| Total | 26 | 33 |

- Systematic uncertainty (%)

- Bkgd. composition (%)

## H  W+W-  l+l- decays; l = e,  (1)

- H  W+W-  l+l- decays; l = e,  (1)

- Event selection include
- Isolated lepton
- pT(l1) > 15 GeV, pT(l2) > 10 GeV
- Missing ET > 20 GeV
- Scaled missing ET > 15 (suppress evts. with mismeasured jet energy)
- Veto on
- Z resonance
- Energetic jets
- Data correspond to integrated lumi. of
- ~ 325 (ee), 320 (e) and 300 () pb-1

**Data vs MC after**
**evt. preselection**

## H  W+W-  l+l- decays; l = e,  (2)

- Employ spin correlations to suppress the bkgd.
- (ll) variable is particularly useful
- Leptons from Higgs tend to be collinear

- H  W+W-  l+l- decays; l = e,  (2)

- Good agreement between data and MC in all
- final states, and all variables examined so far

## H  W+W-  l+l- decays; l = e,  (3)

- H  W+W-  l+l- decays; l = e,  (3)

- Signal acceptance is ~ 0.04 – 0.18 depending on Higgs mass/final state

| Diboson | W+jet/ | Z/* | tt+multijet |
| --- | --- | --- | --- |
| 11.7±0.2 | 2.1±0.7 | 3.3±0.7 | 0.64±0.1 |

| Total | Data |
| --- | --- |
| 17.6±1.0 | 20 |

## W+W-  l+l- non-resonant production

- Has its own physics value
- Test non-Abelian structure of the SM
- Sensitive to trilinear couplings, resonance production, etc.

- W+W-  l+l- non-resonant production

## Slide 35

## Searches for SUSY Higgs bosons: motivation

- In MSSM there are two Higgs doublet fields
- Hu (Hd) couple to up- (down-) type fermions
- The ratio of their VEV’s
*	*tan = <Hu>/<Hd>
- 5 Higgs particles after EWSB
- 	h, H, A, H+, H-
- h is ‘guaranteed’ to be light
*     *mh <~ 130-140 GeV
- At large tan, A coupling to down-type quarks, i.e. b’s, is enhanced wrt SM
- At tree level ~ tan  production cross section rise as tan2
- CP conservation is assumed in the analysis

## MSSM scenarios

- Function of various SM/SUSY parameters: Xt=At-cot, , Mg, Mq, etc.

- Loop level corrections to cross section and BR

- with

- M. Carena, S. Mrenna, C. Wagner

## Mass relations and production cross sections

- Mass degeneracy, doubling of A production cross section

## Higgs boson production in association with b quarks

- Two ways to calculate b(b) processes

- Both calculations are available at NLO and agree within uncertainties

**gb********bh**

**gb********bh**
**gg,qq********bbh**

## Motivation
Benchmark for SUSY Higgs boson production via gbbh
Probes PDF of the b-quark
Background to ZH production

Examples of ZQ (Zj) LO diagrams

Measure cross section ratio
(Z+b)/(Z+j)
Many uncertainties cancel

- Benchmark for SUSY Higgs boson production via gbbh
- Probes PDF of the b-quark
- Background to ZH production
- Examples of ZQ (Zj) LO diagrams
- Measure cross section ratio
******(Z+b)/********(Z+j)**
- Many uncertainties cancel

- Data correspond to integrated lumi. of 184 (ee), 152 () pb-1
- Event selection include
- Isolated e: pT >15/20 GeV
- 			  || < 2.5/2.0
- Jet ET > 20 GeV, || < 2.5
- At least one b-tagged jet
- Z peak for signal, side bands for bkgd. evaluations
- Simulations performed with Pythia or Alpgen plus Pythia passed through detailed detector response
- Cross sections normalized to data
- Relative b- and c-quark content as given by MCFM NLO calculations

- Benchmark: Z(ee/)b associated production (1)

## Benchmark: Z(ee/)b associated production (2)

- Decay length significance of sec. vertices in transverse plane for b-tagged jets

- Measure cross section ratio Z+b/Z+j
**  ****0.021 ± 0.004 (stat)               (syst)**
- Prediction:   0.018±0.004
-     **J.Campbell, R.K.Ellis, F.Maltoni, S.Willenbrock, Phys. Rev. D69 (2004) 074021**
- Systematics studies

| Source
(dominant) | Uncertainty
 (%) |
| --- | --- |
| Jet energy scale | +5.8  -6.9 |
| Bkgd. estimation | +5.7  -5.2 |
| Jet tagging | +4.6  -5.1 |
| Z+(QQ) vs Z+QQ | +1.7  -5.4 |
| (Z+c)/(Z+b) | +2.8  -2.8 |
| Total | +10.4  -11.8 |

**+ 0.002**
**–** **0.003**

- Heavy flavor component in b-tagged
- candidate events is clearly seen !

- Accepted for publication in PRL

## SUSY Higgs boson search

- Multijet trigger
- L1: 3 jets of > 5 GeV, L2: HT > 50 GeV, L3: 3 jets with ET > 15 GeV
- Offline: at least 3 b-tagged jets
- pT and  cuts optimized for Higgs mass and # of required jets
- Look for excess in di-jet mass
- Signal rates and kinematics are normalized to NLO calculations
- Bkgd. shape determined from doubly b-tagged data by applying tag rate function to non-b-tagged jets

## Multi-b-jet background estimation

- Cross-check of bkgd. estimation method

- Tag Rate Function

- Correct TRF for HF contamination (~ 8%)

## Cross-check of bkgd. method: doubly b-tag sample

- Jet tag rate is estimated from data
- Singly b-tag + TRF di-jet spectrum agrees with doubly b-tag sample
- Additional cross-check is done with ALPGEN MC
- Normalization of MC HF multi-jet processes (mainly bbjj + some bbbb) is left as a free parameter in the fit
- HF bkgd. agrees within with ALPGEN within ~10%

## Signal acceptance and systematics

- Signal acceptance is ~ 0.3–1% depending on mA and final state
- 	Acceptance breakdown (%)
- Systematics on signal efficiency is 21% total:
- b-tagging (15%), JES/resolution (9%), signal simulation (5%), trigger (9%), luminosity measurement (6.5%)
- Systematics uncertainties for background estimation ~ 3%

## Results

- Expected and measured 95% C.L. upper limits on the signal cross section

- The 95% C.L. upper limits on tan as a function of mA and for two scenarios of MSSM

- With 5 fb-1 of data, assuming the current performance, can probe tan values down to 20-30 depending on the mass, model

- No mixing in stop sector: Xt = 0
- 		Xt = At – cot, At – tri-linear coupling,  = – 0.2 TeV
- Maximal mixing: Xt = √6×MSUSY, MSUSY= 1 TeV

## Summary

- Hunting for Higgs at the Tevatron/DØ Run II has begun !
- Upgraded accelerator and DØ are performing well and contribute to world class results
- In coming years, the Tevatron Collider at Fermilab offers a real opportunity to find the Higgs boson
- If we are fortunate, and smart
- If not, we will exclude a very interesting region
- The low mass region will be in Tevatron’s domain for many years and will complement LHC’s reach
- Search for Higgs particles forms a central part of the DØ physics programme