# Presentation: 500989

## Prospects for Higgs Searches at DØ

- Makoto Tomoto
- Fermi National Accelerator Laboratory
- (For the DØ Collaboration)
- La Thuile 2003, La Thuile, Italy

## Outline

- Introduction
- DØ Run II
- Run II Data
- W + jets
- Z + jets
- H  WW
- Summary

## Introduction

- Major goal of  the Tevatron Run II is the search for Higgs bosons.

**   ****Direct SM Higgs search at LEP:**
**    ****m****H****> 114.4 GeV at 95% C.L.**
**    ****( hep-ex/0211058)**

**    ****Indirect limit of the global SM fit:**
**      ****m****H**** < 196 GeV at 95% C.L.**
**      ****Fit minimum is at 85 GeV**
**      ****(hep-ex/0212036)**

**200 GeV**

## Production Process & Decay Channel 

- Production Process & Decay Channel

**H ********bb**

**H ******** WW**

- gg  H ... gg  H) ~ 1 pb
- For masses below ~140 GeV,
- 	Background hides H  bb signals
- For higher masses; mH> 120GeV,
- 	Combination with H  WW decay process can be useful
- HW, HZ  ... (HW/HZ)~ 0.1 pb
- 	Leptonic decays of W/Z help         background rejection
- Hqq ... (Hqq) ~ 0.1 pb
- 	Background too high
- Hbb ... (Hbb) ~ 5 fb
- 	SM extensions may enhance b/bb
- 	( =h,H,A)

## Tevatron Higgs Working Group Study

**The Higgs discovery potential for the Tevatron RunII has been evaluated.  **
**hep-ph/0010338**
**A joint effort of theorists and both experimental groups, CDF and DØ.**
**Simulation performed using a parameterized fast detector simulation.**

**Main conclusion :**
**Discovery at 3-5 ******** can be made,**
**Combine all channels.**
**Combine the data from both experiments, CDF and DØ**
**Must improve understanding of signal and background processes **
**	****and detector performance.**
**b-tagging, resolution of M****bb**

- LEP excluded
-  at 95% C.L.

**Results of studies with full simulations for selected signal process are consistent with SHWG expectations.**

**  ****Advanced analysis techniques are vital**

**  ****Largest luminosity required to discover Higgs**

## DØ Run II

- Tevatron Run II in progress
- Collider energy : 1.8 TeV  1.96 TeV
- 	  Higgs production cross section increases by 20~30%
- Target Luminosity : 6   11 fb1 or more
- Peak luminosity now better than Run I : 3.7 x 1031 cm1 s1
- DØ upgraded for Run II
- New tracking system fully working well.
- 	 Important for b-tagging
- D0 recorded over 80 pb-1 with full detector
- 	(Operating at >85% efficiency)
- Analysis in this talk based on 30~50 pb1
- (Collected from August 2002 to January 2003)

## Current Activity

- Study of the W/Z(lepton) + jets production
- First step towards W/Z (leptons) +H (bb) measurement
- The W/Z + b-jets can be related to W/Z+ jets properties
- Try to understand major background source from W/Z + di-jets
- Search for HWW(*) ( ee//e) decays
- Lot of interesting physics in WW(*) production
- Important to keep an eye
- Others
- Search for H  decays
- WH( l + bb)
- ZH( ll or bb)
-  b/bb, bb/  ( = h, H, A ; SUSY Higgs)

## Object Identification

**missing E****T**

**electron E****T**

**M****T**

**W p****T**

**Dot : Data**
**Line : MC**

- Z 

**W ********e******** **

- DØ Run II Preliminary

**Z ******

- W characteristics are represented by MC
- Clear mass peak of Z(ee) and Z()

**   ****: Data**
**   ****: MC**

- DØ RunII Preliminary

**Z ********ee**

- QCD

**   ****: Data**
**   ****: MC**

## W/Z + jets production

- First step towards W/Z (leptons) +H (bb) measurement.
- W/Z + b-jets properties can be related to W/Z + jets properties.
- Major background source to Higgs searches
- Analysis utilized 35 pb1
- Data samples triggered by lepton
- No bias for jets distribution.
- Basic Selection:
- Isolated lepton and large ET (for W)
- 2 high pT leptons and mll consistent with mz (for Z)
- Plus jets

## W+jets production (1)

- Selection
**W(******** e********) **
**Isolated e : p****T**** > 20 GeV  **
**|********| < 0.8**
**Missing E****T ****> 25 GeV **
**W(******** ********)**
**Isolated ******** : p****T**** > 25 GeV **
**|********| < 1.5**
**missing E****T****> 20 GeV**
**Jets**
**p****T**** > 20 GeV **
**|********| < 2.5**
- Compare PYTHIA MC with DATA
- Normalized by area
- Error includes stat. error and dominant syst. error from JES

**2****nd**** leading jets**

**1****st**** leading jets**

**GeV**

**GeV**

**W(e********)+jets**

- QCD BKG

- QCD BKG

**   ****: Data**
**   ****: MC**

**   ****: Data**
**   ****: MC**

## W+jets production (2)

**Di-jet Mass**

******R between di-jets**

**W(e********)+jets**

**W(e********)+jets**

- Reconstructed di-jet mass and R(=    2 + 2  ) between jets
- MC reproduces jet distributions well
- First step towards study of W(leptons)H( bb) decay process

- QCD BKG

- QCD BKG

**M****jj ****(GeV)**

******R****jj**

**   ****: Data**
**   ****: MC**

**   ****: Data**
**   ****: MC**

## W+jets production (3)

-    Di-jet mass and Rjj distribution for W( ) + jets event

**M****jj ****(GeV)**

******R****jj**

**Di-jet Mass**

******R between di-jets**

**W(********)+jets**
**        ****: Data**
**        ****:MC**

- QCD BKG

**W(********)+jets**
**        ****: Data**
**        ****:MC**

## Z+jets production (1)

-  Selections

** ****2 muons from Z(******** ********) **
**p****T**** > 15 GeV **
**|********| < 2**

** ****2 electrons from Z(******** ee)**
**p****T**** > 20 GeV  **
**|********| < 2.3**

** ****Jets**
**p****T**** > 20 GeV **
**|********| < 2.5**

**1****st**** leading jets**

**2****nd**** leading jets**

-  Compare PYTHIA MC
-    with DATA

-  Normalized by area

- Combined Z(ee)+jets and Z()+jets

-  Error includes stat. error
-    and dominant syst. error
-    from JES

## Z+jets production (2)

- Number of jets in Z + jets final states
- Reconstructed di-jet mass and R(=    2 + 2  ) between jets
- MC describes jet distributions well
- First step towards Z(leptons)H( bb) study

**Di-jet Mass**

******R between di-jets**

**#jets in Z+jets**

- Combined Z(ee)+jets and Z()+jets

##  b-tagging (1)

-  b-tagging (1)

- Next step in searches for Higgs would be b-jet identification
- Crucial to keep signal efficiency high and suppress non-b jets
- b-tagging efficiency determined by Impact Parameter (IP) resolution
- Measured IP resolution after 1st pass in SMT alignment

** ****IP resolution as the function of P****T**

** ****IP resolution**

**Mean = 0.9 ******** 2.2 ********m**
**Sigma = 36.3 ******** 1.8 ********m**
**Beam ~ 30 ********m**
****** ****IP resolution ~ 20 ********m**

##  b-tagging (2)

** ******** ****+ jet sample**

-  b-tagging (2)

- b-tagging explores IP significance method
- Lepton from semileptonic decay of b is very useful

**Positive IP**

**Negative IP**

** ****Impact Parameter > 0 **
****** ****track crosses jet axis after primary vertex**

- Jet

- Jet

- track

- track

- Resolution

- b enhanced

**Interaction point**

**Interaction point**

** ****Impact Parameter < 0 **
**track crosses jet axis before primary **
**   ****vertex**

- DØ Run II
- Preliminary

**Significance = IP/********IP**

## HWW(*)  lldecays

- Lot of interesting physics in WW production
- SM Higgs at high mass region (mH  > 120 GeV)
- 4th fermion family enhances SM Higgs cross section
- 	(factor ~8.5 for mH=100 – 200 GeV)
- Fermiophobic/Topcolor Higgs
- (Br(H WW)>98% for mH > 100 GeV)
- Non Higgs-related ... Tri-linear couplings, New Phenomena
- Look at ee//e plus missing ET events
- Backgrounds include Z/*, WW, tt, W/Z+jets, QCD
- Cannot directly reconstruct mass
- Transverse mass (mT) computed using mll and ET
- Opening angle between leptons (ll) is useful discriminating variable
- Two leptons from Higgs tend to move in parallel (small ll ) ,
- 	due to spin correlations in HWW decay products
- Leptons from Z/* , multijets are emitted back to back (large ll)

## HWW(*)  eefinal states

|  | Expected background  | DATA |
| --- | --- | --- |
| Lepton ID, pT>10, 20 GeV | 2748  42  245 | 2753 |
| mee < mH /2 | 264  18.6  4.3 | 262 |
| ET > 20 GeV | 12.3  2.5  0.7 | 11 |
| mT <mH + 20 GeV | 3.6  1.4  0.2 | 1 |
| ee < 2.0 | 0.7  1.4  0.1 | 0 |

**Event Selection**

**L=44.5 pb********1**
**Selection optimized**
**for m****H ****= 120 GeV**

**Expected **
**Backgrounds**

- (HWW e) x 50

**After basic kinematics cuts**

**After all selection but ********ee**** **

- signal = ~ 8%

## HWW(*)  efinal states

|  | Expected background  | DATA |
| --- | --- | --- |
| Lepton ID, pT>10, 20 GeV | 22  2.1  2.2 | 22 |
| ET > 20 GeV | 3.1  1.7  0.1 | 4 |
| (ET,jets)>0.5, |ET+pT|>50 GeV | 1.4 1.5 0.1 | 2 |
| e < 2.0 | 0.9  1.5  0.1 | 1 |

**Event Selection**

**After basic kinematics cuts**

**After all selection but ********ee**** **

- (HWWe) x 20

**Expected **
**Backgrounds**

**L=34 pb********1**
**Selection optimized **
**for m****H ****= 160 GeV**

- signal = ~ 12%

## Candidate of HWW(*)  ee

**E****T**

**e**

**e**

**E****T**

**e**

**e**

**e**

**p****T**** = 31.1 GeV**
**p****T**** = 27.3 GeV**
**E****T**** = 31.2 GeV**
**m****T**** = 106.8 GeV**
**M****ee**** = 36.1 GeV**
******ee****=1.43**

- Selection optimized for mH=160GeV

**e**

## Summary

- DØ is taking physics quality data.
- Background to Higgs production are under study.
- W+jets
- Z+jets
- WW
- More to come in the near future!!