# Presentation: 661202

## W/Z Physics at DØ Nirmalya Parua(for the DØ Collaboration)State University of New York Stony BrookLes Recontres de Physique de la Vallée d’ Aosta Italy, Feb 29-March 6, 2004

**Plan of the talk:**
**  ****- ****Overview of Tevatron and DØ Detector**
**  ****- Introduction to W/Z Physics**
**  ****- Results**
**  ****- Summary and outlook**

## The RunII Tevatron

**Increased center of mass energy**
**1.8 Tev ******** 1.96 TeV**

**Increased luminosity**

## Overview of DØ Detector

**2 Tesla solenoid magnetic field for         central tracking system to facilitate  charge and momentum measurement.**
**Silicon and fiber tracker detector.**
**Add scintillator detector in muon system for faster trigger**
**Pre-shower detectors.**
**Pipelined 3 Level trigger**

## Overview of the DØ Detector

**Liquid argon sampling**
**Stable, uniform response, rad. hard, fine spatial seg****.**
**LAr purity important ( impurity <0.5 ppm)**
**Uranium absorber (Cu or Steel for coarse hadronic)**
**Compensating e/******** ******** 1, dense ******** compact****  **
**Uniform, hermetic with full coverage**** **
******< 4.2 (******** ******** 2****o****), ********int ********total) **

**Z**

**X**

**Y**

**Three layers of Scintillator detectors and Drift Tubes divided into two regions central and forward.**
**One layer inside toroid magnet and two layer outside.**
**Muon ********  coverage to ±2**
**Shielding reduces backgrounds by factor of 50-100 .**

## Particle identification

## Importance of W/Z Physics

**Test of standard model couplings**
**Help predicting and test higher order QCD corrections**
**Constrains PDFs**
**Helps in understanding detector performance, trigger studies, tuning up algorithms**
**Used to understand background processes of other physics processes like top and Higgs.**
**Can  used as independent measurement of integrated luminosity**
**Precision measurement of W mass in conjunction to top mass can be used to predict bounds on Higgs mass**

## Mechanism of W/Z production 

**W/Z production is dominated by q q**
**In RunII millions of Ws and 100ks of Zs will be produced**

**At DØ use leptonic decay modes**
**   ****Avoids high QCD background**
**   ****~11% Br per leptonic mode for W**
**   ****~3% Br per leptonic mode for Z**
**   ****Clean distinctive signature**
**   ****High PT leptons**
**For W : 1 high p****T**** lepton and high missing E****T**
**For Z : 2 high p****T**** leptons**

## Dataset

- Only a portion of the dataset collected so far has been used for the analysis reported here.

## Measurement of  .B  

**N****obs**** is the number of events observed**
**N****bkg**** is the estimated number of background events**
**A is the acceptance, derived from Monte Carlo**
****** ****is the efficiency, computed mostly from data**
**∫****L**** ****dt** ** is the total integrated luminosity**

## Z    + - 	

**Selection criteria:**
**Two central tracks:**
-      -  **loose ********-id**
**     ****- pT > 15 GeV**
**     ****- opposite charge**
-  **|********| <1.8**
** ****M ******** ********  ****> 30 GeV**
** ****Cosmic veto**
** ******** ****1 isolated**

## Z    + -

- Very low Backgrounds
-     QCD (0.6  0.3)%
-      Z     (0.5  0.1)%

******  ****= 19%**

-    σZ • Br(Z→μ+μ−) = 261.8 ± 5.0 ± 8.9 ± 26.2 pb
- 					     stat.   syst.   lumi.

## Ze+e-

**Selection Criteria:**
**Two Isolated electrons**
-      **- E****T ****> 25 GeV & |********| < 1.1**
**     ****- Electrons also must satisfy **
**       ****shower shape, Emfr cuts.**
-  **70< M****ee ****<110**

- • Data
-    MC

- ∫L = 42 pb-1

**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 bkg distributions**.

- Number of Z events after background subtraction 1139 ****42

**DØ:  ********Z**** • Br(Z********e+e-) = 275 ± 9 (Stat) ± 9 (syst) ± 28 (lumi)**  **pb**

-  Electron ID efficiency:
-   -Trigger 98+/-2 %
-   -EMF, isolation ~100 %
-  -Shower shape 86+/-1

## Z’ search in Z’  ee Channel

**Many extensions of SM predicts the Existence Z’**
**Look for bumps in the high mass region of the invariant mass distribution.**

**Two isolated electrons with same  criteria as Z ******** ee except with  extended ******** coverage for one electron and track match requirement for one CC electron**

- trk = 732 %

**Assuming SM coupling **
**Z’ < 719 GeV excluded at 95% C.L.**

**RunI limit was 670 GeV at DØ**

- ∫L = 122 pb-1

## Z→ +−

**Z ******** ********  (******** ********) ******** ( ******** **************** **+** **n******0****)        **

- Event signature is isolated high pT muon opposite to narrow hadronic jet

**Background**
**  ****- QCD ******** from bb or ********/K decay**
**  ****- W ******** ******** or ******** + jet **
**  ****- Z ******** ******

**M**** ******

- ∫L = 68 pb-1

- Handfull of events consistent with SM expectation

## We 

**Selection Criteria**
**One Isolated electrons**
-      **- E****T ****> 25 GeV & |********| < 1.1**
**     ****- Electrons also must satisfy **
**       ****shower shape, Emfr cuts.**
**     ****- Should have track matching**
- ETmiss > 25 GeV

**Backgrounds:**
**Dominant source QCD dijet events**
**   ****Estimated from data**
** ****W ******** ******** e******
**    ****Z ********  e e **
**    ****Both these two backgrounds are     estimated from MC**

## We 

**E****T ****Distribution of Electrons**

**Missing E****T ****Distribution of Electrons**

**Transverse Mass**

**∫****L**** = 42 pb****-1**

**Number of Candidate events after background subtraction 27370 ******** 898**

- σW • Br(w→e ) = **2844 ********21 (stat) ********128 (syst) **
**	****	****	******** ****284 (lumi****)**

## W

**Selection Criteria**
-    - pT(μ) > 20 GeV
-    - ETmiss > 20 GeV

**Number of Candidate events after background subtraction 8302**

- σW • Br(w→ μ) = **3226 ******** 128 (stat)  ******** ****100 (syst)  ******** 322 (lumi****)**

**∫****L**** = 17 pb****-1**

## Determination of R

-         σW • Br(W→l)              σZ • Br(Z→l+l−)

**Luminosity error cancels out**
**Common systematic uncertainty like PDF uncertainty cancels out**

**DØ RunII Preliminary:**
**  ****- ** **R****e ****= 10.34 ******** 0.59**
-   -  **R******** ****= 12.32 ******** 0.73**

## Summary and Outlook

**Preliminary result from W/Z cross sections available and are consistent with SM**
**Performance of detectors, triggers, details of simulations are being better understood**.
-  **Limit on Z’ mass already exceeded Run I Limit**
** ****More data is being collected**
** ****More precise results with ~ 200 pb****–1**** luminosity will be available soon.**