# Presentation: 803786

## Recent Results on Electroweak & Related Physics at DØ

**    ****Louisiana Tech University**
***...******on behalf of the DØ Collaboration***

**Recent Results on Electroweak & Related Physics at DØ**

** ****Introduction & Motivation**
** ****W/Z ******** muons**
** ****W/Z ******** electrons**
** ****Z’ (ee) Search**
** ****Top Quark Cross Section**
** ****Conclusions**

## Motivation for Measuring W & Z Production

**Test of SM couplings**
**Constrain proton PDFs**
**Probe effects of NLO QCD corrections  **
**Better understanding of our Experiment**
**Efficiencies, Backgrounds, Luminosity**
**Use these signals to tune triggers & algorithms**
**Improved luminosity measurement**
**With sufficiently small statistical & systematic uncertainties**
**Normalize to other measurements**
**Preliminaries to other Run II goals**
**W boson mass  **
**Precision EW measurements**
**Top Quark Studies**
**(W or Z) + Higgs**

**DØ**
**Run IIa**
**Prediction**

**Previous Tevatron results**

## W and Z Production Mechanics

**W and Z Production Mechanics**

**Distinctive lepton decay event signatures**
** ****High P****T**** isolated leptons (e or ********)**
** ****One high P****T**** lepton + Missing  E****T ****(W)**
** ****Two high P****T ****leptons (Z)**

**Cross Sections**
**increase by ~10% from **
**1.8 to 1.96 TeV**

**W********l********  ******** ~1 Hz @ L = 2********10****32**

- O(as0)

**Notes:**
 Production dominated by qq annihilation
 Due to very large pp jj production, need to use leptonic decays
 W  l (BR ~ 11% per mode)	
 Z  ll (BR ~ 3% per mode)
 Distinctive event signatures 
 High PT isolated leptons (e or )
 One high PT lepton + Missing  ET (W)
 Two high PT leptons (Z)
 Low backgrounds
 Large samples
 Well understood EW vertex 
 Test QCD 

## W & Z Cross Sections: A Counting Experiment

**Measurements make use of data taken from mid-August ’02 through mid-January ‘03 **

**Integrated **
**Luminosity**

**Backgrounds**

**Acceptance from Monte Carlo**

**Efficiencies from data where possible**

## The DØ Run II Detector

**The D****Ø Run II Detector**

- SMT

**Builds on the firm foundation of the Run I calorimeter & central muon system**
**Added magnetic tracking, silicon, new forward muon system, new electronics **
**Electroweak analyses make use of the full detector capabilities**

## The Muon Detector

**Two regions & Three layers of Scintillators and Drift Tubes **
**Central and Forward **
**A – inside toroid magnet**
**B & C – outside toroid magnet**
**Muon rapidity coverage to ±2**
**Shielding reduces backgrounds by 50-100x**

**Mini drift tube**
**Plane and Pixels (10m ******** 10m)**

## Z  Event Selection 

**Event  selection**
**2 tracked, oppositely-charged ********’s **
**p****T****>15 GeV & |********|<1.8**
**Di-muon trigger**
**At least one muon is isolated**
**(********R)****2****=(****************2****+(****************2**** >4.0**
**|********t| < 9 ns in scintillator**
**NO explicit mass requirement**

**Background is small**
**Z********bbar is 1 ********1%**
**Z ************ is 0.5 ********0.1%**

**P****T****(********1****)=40.97 GeV**
**P****T****(********2****)=43.92 GeV**
**M(********)=86.04 GeV**

## Z Efficiencies 

******control**

******probe**

**L1+L2**
**central track**

**L1 OR no L1**

**Z**

** ****Acceptance from MC (40****±****1%)**
** ****Efficiencies from data**

**track OR no track**

******L1**** = 87****±****2%**

**********ID**** = 91****±****1%**

******trk**** = 82****±****1%**

**(scaled)**

**(scaled)**

## Z Cross Section

**Drell-Yan Contribution**
**Correct for 12********1%  due to photon exchange and photon-Z interference determined from Pythia **

**1585 Candidates**

## WEvent Selection

**Offline**** **
**One isolated muon matched with central track p****T****>20 GeV **
** ****In fiducial region of the trigger: |********|<1.6 **
** ****Muon corrected Missing transverse energy > 20 GeV**
** ****No second muon in event (veto Z************ events)**

**Trigger**
** ****L1: Scintillator based single muon trigger (no p****T**** cut)**
** ****L2: At least one muon with p****T****>5 GeV**
** ****L3: At least one track with p****T****>10 GeV**

**P****T****(********)=37.48 GeV**
**ME****T****=35.5 GeV**

## W Cross Section

**7352 Candidates in **

**Backgrounds**
** ****Z********bb, b************ where ******** passes isolation cut**
** ****5.8%, subtracted from above distributions**
** ****Z************: ~9%**
** ****W********************: 3.6%  **
** ****QCD estimated from data**

## Liquid argon sampling
Stable, uniform response, rad. hard
LAr purity important (< 0.7 ppm O2 equivalent)
Uranium absorber (Cu/Fe for coarse hadronic)
dense absorber hence can be compact 
Nearly compensated EM and hadronic response
Linear response
Hermetic with full coverage
 < 4.2 (  2o)
int total) 

**Stable, uniform response, rad. hard**
**LAr purity important (< 0.7 ppm O****2**** equivalent)**
**Uranium absorber (Cu/Fe for coarse hadronic)**
**dense absorber hence can be compact **
**Nearly compensated EM and hadronic response**
**Linear response**
**Hermetic with full coverage**
** ********< 4.2 (******** ******** 2****o****)**
******int ********total) **

**The Calorimeter**

## W/Ze Event Selection

**Electrons**
** ****Isolated EM Cluster in the Calorimeter**
** ****E****T****> 25 GeV with large EM fraction**
** ****Shower shape consistent with MC expectation **
**Z********ee**
** ****70 GeV < m****ee ****< 110 GeV**
**W********e******
** ****Missing transverse energy > 25 GeV**
** ****Matched with central tracks**

**Trigger: **
** ****L1: 1 calorimeter tower > 10 GeV (or 2 > 5 GeV)**
** ****L3: Electron candidate > 20 GeV, shower shape cut**** **

**P****T****(e)=40.5 GeV**
**ME****T****=43.2 GeV**
**M****T****(W)=83.7 GeV**

**W/Z*********e***** Event Selection**

***e******+***

## Zee Cross Section

**1139 Candidates in **

**M****e+e-****(GeV)**

**D0 Run II Preliminary**

**Drell-Yan Contribution**
**Small effect (1.7%) in the mass window of 70 < m****ee**** < 110 GeV**
**QCD Background**
**Determined from data by fitting signal and background shape**

**Efficiencies per electron**
** ****Trigger: 98±2%**
** ****EMF, isolation: ~100%**
** ****Shower shape: 86±1%**
** ****Track Matching: 73±2%**

## We  Backgrounds

** ****Dominant background from QCD multijet events**
** ****Estimated from data**

- tight = loose + track match

**Other backgrounds:**
** ****W****************e******** (1.5 %, MC)**
** ****Z********ee (very small)**

**From QCD dijet sample**

- loose

- tight

- 2.3+/-1%

**D0 Run II Preliminary**

**D0 Run II Preliminary**

**ME****T****(GeV)**

**p****T****(GeV)**

- N**loose** = N**W** + N**b**
- N**tight** = N**W********trk** + N**b********f**

- Solve for N**W**

## We Cross Section

**27370 Candidates in **

**Background subtracted distributions compared to MC Pythia**

## Results

- C. R. Hamberg, W.L. van Neerven and T. Matsuura, Nucl. Phys. B359 (1991) 343
- 						            CTEQ4M PDF

**Z**

**W**

## Direct Search for Z'ee

- M(ee) around the Z

**4585 Candidates in **

**Data**
**Bkg (QCD+DY/Z)**
**Bkg (QCD only)**
**10x Z'(600GeV)**

**Backgrounds**
**Drell-Yan dominates M****ee****<150 GeV**
** ****Estimated from Pythia (CTEQ4L)**
**QCD dominates M****ee****>150 GeV**
** ****Fake electrons from jets**
** ****Real electrons from heavy flavor**
** ****Estimated from data**

**D0 Run II Preliminary**

**Search for Non-SM heavy particles that decay to lepton pairs**
**Assumes Z' couples as Z**
**Previous Tevatron Run I Limit**
**M****Z'****>690 GeV**
**Event Selection**
**2 isolated EM objects**
**|********|<1.1 or 1.5<|********|<2.5**
**E****T****>25 GeV **
**No track requirement necessary**

**M****e+e-****(GeV)**

## Z’ Search

**M(ee)=489.1 GeV**

**Form limit from the ratio of cross sections for (********B)****Z'****/(********B)****Z**** where many of the systematic errors in the efficiencies and luminosity measurements cancel**
**Applied a binned likelihood approach with Poisson statistics**
**Remaining Uncertainties**
**K-factor 5%**
**A****z’****/A****z****  &  PDFs 2-3%**

**D0 Run II Preliminary**

## Motivation for Measuring Top Quark Production 

**Run I: Discovery**
******tt**
**Top mass**
**W helicity in top events**
**tt spin correlations**
**Top p****T**
**Searches for new physics **
** ****(X******** tt and top decay)**
**Run II: With high precision we hope to answer questions such as:**
**Why is top so heavy?**
**Is it or the third generation special?**
**Is top involved with EWSB? **
**Is it connected to new physics?**

**Motivation for Measuring Top Quark Production **

## Top Quark Production Mechanics

**Top Quark Production Mechanics**

**At the Tevatron, top quarks are produced in pairs ****	**
**Br(t****→Wb****)=100%**

**Run I Results ~100 top events**

**Prediction of ~30% ********tt ****cross section increase from Run I (1.8 TeV) to Run II (1.96 TeV) **

## (ttll+jets) Event Selection

******-**

**jet**

**jet**

***e******+***

| e+ | PT= 20.3 |
| --- | --- |
|  | PT= 58.1 |
| j | PT= 141.0 |
| j | PT= 55.2 |
| ET | 91 GeV |

**Di-muon mass (GeV)**

******(tt*********ll*****+jets) Event Selection**

**E****T**

**E****T ****(GeV)**

**Event Selection**
** ****Two high p****T**** isolated ******** or e **
** ****E****T**** (Z mass) cut**
** ≥ ****2 j****ets, E****T****>20 GeV & |********| < 2.5**
** ****H****T ****=********(E****l****T**** ****,E****jet****T****) cut**
**Backgrounds**
** ****WW, Z****→ tt determined with MC**
** ****Z/g*, W+jets and QCD from data**

| Process | Nevents |
| --- | --- |
| Z→
Z→→
WW→ | 0.40 ± 0.23
0.02 ± 0.02
0.001 ± 0.001 |
| QCD+W+jets | 0.18 ± 0.084 |
| All BG | 0.59 ± 0.30 |
| Expected Signal | 0.3 ± 0.02 |
| All  | 0.89 ± 0.30 |
| Observed | 2 |

- +jets

**D0 Run II Preliminary**

## (ttll+jets) Results

**Jet****1**

**Jet****2**

******(tt*********ll*****+jets) Results**

**D0 Run II Preliminary**

**D0 Run II Preliminary**

- e+jets

- e+jets

**Jet Multiplicity**

**Sum of jet, electron P****T ****(GeV)**

## (ttl+jets) Event Selection (no  tag)

**Scaling: # of W**
**for ****N**** jets****≥ 4**** jets**

**Procedure**
**Preselect a sample rich in W’s**
**Evaluate QCD multi-jet (as f [Njets] )**
**Estimate W+4 jets assuming Berends Scaling**
**Apply topological selection**
**Event Selection**
**Isolated lepton w/ P****T****>20 GeV**
**ME****T**** > 20 GeV**
**4+ jets with E****T****>15 GeV & |********|<2.0 (2.5)**
**Soft non-isolated muon tag veto**
**Backgrounds**
** ****multijet evaluated from data vs. N****jets**
**e+jets: due to fake jets (real ********o**** and ********)**
** ********+jets: due to heavy flavor decays**
** ****Estimate real W+4 jets with scaling law**

**D0 Run II Preliminary**

**D0 Run II Preliminary**

******

**Estimation of QCD Background**

## (ttl+jets) Results (no  tag)

| Analysis | NW | NQCD | Bkg. Total | Exp Signal* | Nobs |
| --- | --- | --- | --- | --- | --- |
| e+jets | 1.30.5 | 1.40.4 | 2.70.6 | 1.8 | 4 |
| +jets | 2.10.9 | 0.60.4 | 2.71.1 | 2.4 | 4 |

**Topological Selection**
**1+ jet with E****T****>55 GeV**
**ME****T****Cal>15 GeV **
**|********W****|<2.0, E****T****W****>60 GeV**
**H****T****(jets & W)>220 GeV**
**H****T****>180 GeV**
**Aplanarity(W-jets)>0.065**

** For ****= 7pb*

## (ttl+jets) Event Selection (soft  tag)

**Preselection criteria**
**Same as leptons + jets except soft non-isolated muon is not vetoed **
**3+ jets with E****T****>20 GeV & |********|<2.0**
**Aplanarity > 0.04**
**H****T****>110 GeV**
**Soft Muon Tag**
**p****T****(********)>4 GeV within ********R<0.5 of a jet**

| Analysis | Bkg. Tot. | Sig.* | Nobs |
| --- | --- | --- | --- |
| e+jets | 0.20.1 | 0.5 | 2 |
| +jets | 0.60.3 | 0.4 | 0 |

** For ****= 7pb*

**D0 Run II Preliminary**

**D0 Run II Preliminary**

*27 e+jets events*

*23 ****+jets events*

## (ttX) Cross Section

**Combined**

**Combined**

## Conclusions

** ****Combining the observation of the top decay channels an excess of 3******** is observed, compatible with a signal expectation at the 35% CL**

** ****Four new W & Z cross section measurements:**
** ****Assuming same SM Z couplings to quarks and leptons, Z' is excluded at 95% CL at m****Z'****<620 GeV**

## Backup Slides

## Zee Candidate

**No centrally matched**
**track to second electron**

**P****T****(e****1****)=45.1 GeV**
**P****T****(e****2****)=40.1 GeV**
**M(ee)=85.1 GeV**

## WBackgrounds 

**Background estimated from data.**

-  Dominant background:
-  bbar, b**** events where the muon passes isolation cuts

## W Efficiencies

**Dimuon events**
**1 ******** w/central track match**
**1 Calorimeter ******
**Determine fraction of Calorimeter muons that meet local muon criteria**

**L1**

**Trigger efficiencies**

**Muon reconstruction efficiency = N****1****/(N****0****+N****1****) = 74±2%**

**N****0**** = 1 local ********+ 1 cal ******

**N****1**** = 2 local ********'s**

**L2**

**L3**

## Uncertainties

- Z

- W

- Z/We