# Presentation: 936558

## 

DØ Search for the Higgs Boson             in Multijet Events

Alex Melnitchouk
University of Mississippi

For the DØ Collaboration

**DØ***** *****Search for the Higgs Boson             in Multijet Events**
- Alex Melnitchouk
**University of Mississippi**
**For the DØ Collaboration**

-     PANIC 05         Santa Fe, NM, October 2005

## Introduction

- Two analyses are presented in this talk:                                           **ZH********bb**** **and** ****b(b)h****b(b)bb**** **
**ZH********bb**** **: a DØ Search for the Standard Model Higgs
**b(b)h****b(b)bb**** **: SUSY Higgs Search at DØ
- In both analyses final state contains b-jets
-     need good understanding of calorimeter response and b-tagging

## DØ Calorimeter and Tracking System

- Uranium/Liquid Argon                                                                                                Sampling Calorimeter
- Three modules:                                                                                                                 -- central calorimeter (CC)                                                                                             -- two end-cap calorimeters (EC)

**Unit cell**

- End-View

- Fiber Tracker end-view

- One-Quarter  r-z View                of Tracking System

- Central Fiber and Silicon Miscrostrip Tracker

**      ****Silicon Microstrip Tracker**

## SM Higgs boson production

## Low Mass Region Higgs Searches. Why ZH  bb

**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 **
- Cross-Sectin x Branching Fraction      0.01 pb                                               (almost as large as                                         **qq’ **** W* **** WH **** l********bb** )
- Tag b-jets
- Disentangle H  bb peak in di-b-jet mass spectrum

**H ********bb**

**Excluded at LEP**

**B(****Z********)**=20%

## 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

## Example: ET,HT Asymmetry

- In signal events there is                                                                                           PT balance between Higgs and Z-boson
-  PT balance between and  bb
- 	  Asymmetry(ET,HT) = (ET – HT)/(ET+HT)                                                             	    peaks at 0
- Instrumental                                                                                                               background                                                                                                 (calorimeter mismeasurements                                                                                              of multijet events)  Large Asymmetry(ET,HT)

- Jet1

- Jet2

- ET

- HT

- 

- 

- ET in QCD
-                event with mis-measured jet

**b**

**b**

## Background Estimation

- Physics bkgd. from MC

- signal

- sideband

- sideband

-       Data:   2140 Expected:  2125

## ZHbb: Distributions before b-tagging

- ZHbb: Distributions before b-tagging

- Total
- Data : 2140
- Expect : 2125

## Singly b-tagged events

- Total
- Data : 132
- Expect : 145

## ZHbb: Doubly b-tagged events

- Total
- Data : 9
- Expect : 6.4

## Results (double b-tag)

| 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 (%)

## SUSY Higgs 

- SUSY Higgs sector consists of  **more than one Higgs particle**
-  e.g. Minimal Supersymmetric Model (MSSM) :
- 	two complex scalar Higgs doublets
-    two VEV’s v1 and v2 (tan=v1/v2)
-    5 Higgs particles : h0, H0, A0, H+, H-
**        ****In this talk: Search for Neutral Higgses **
- At large tan Higgs coupling to down-type quarks i.e. b-quarks is enhanced with respect to the Standard Model:  at tree level ~tan                                                    production cross-section rises as tan2
- CP-conservation in the Higgs sector is assumed                                                            Mass degeneracy  (100-130 GeV: h0,H0,A0 ; higher mass: h0,A0 or H0,A0)                                                                                              	 Total signal cross-section is assumed to be                                                          	     twice that of the A  boson

## 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**

## 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

## b(b)h: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%)
- Systematic 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

## Expected Improvements in b-tagging 

- Continue improving        b-tagging (Neural Net)
- the NN tagger combines the 3 b-tagging algorithms used in DØ
** **

- Layer Zero of the Silicon Tracker Upgrade
- Layer Zero detector is scheduled to be installed in spring 2006
** **

## Tevatron Performance

## Summary

- Standard Model and SUSY Higgs searches in multijet events                       at the Tevatron/DØ Run II have started
- Upgraded accelerator and DØ are performing well,                               more data are being accumulated (1 fb**-1** on tape !)
- Work is in progress on improving b-tagging                                    (algorithms, silicon tracker upgrade)
- Stay tuned for new results

## Next Slide is a Backup Slide

## Multi-b-jet background estimation

- Cross-check of bkgd. estimation method

- Tag Rate Function

- Correct TRF for HF contamination (~ 8%)