# Presentation: 022784

## Top Quark Production and Properties at DØ

**Top Quark Production and Properties at D****Ø**

**Introduction**
**Top quark production and decay**
**Top quark pair production cross section**
**Top quark production properties**
**Top Branching Ratio**
**W-helicity**
**Conclusion and outlook**

- The 2007 Europhysics Conference on High Energy Physics
- July 19 – 25, 2007, Manchester, England

- Gustavo Otero y Garzón, University of Illinois at Chicago
- for the DØ experiment

## Top Quark CV

**Discovered in 1995**** by DØ and CDF after a 2 decades hunt**

**Heaviest fundamental particle**** (m****t**** = 170.9 ± 1.8 GeV)**

**Strongest coupling to the Higgs**** (Yukawa coupling ********t**** ******** m****t**** ~ 1)**
- may help identify the EWSB mechanism and mass generation
- may serve as a window to new physics related to EWSB that might couple preferentially to top

**A unique laboratory: lifetime (5x10****-25**** s) shorter than the hadronization time makes it ****decay as a free quark**

**We still have ****a lot to learn**** about this particle**
- Indirect constraints from low energy data and statistically limited Tevatron data leave plenty of room for new physics
- Even if the top is “just a normal quark”, precision top measurements are stringent tests of the SM

## Top Quark Pair Production and Decay

******ttbar ****/ ********inel**** ****~**** 10****-10**
** ****High luminosity**
** ****High efficiency**** **

**Top quarks are mainly produced in pairs (strong interactions) at Tevatron energies**

**No hadronic bound states due to short lifetime**
**Electroweak decay**

**|V****tb****|~1**** **

## Top Quark Production Cross Section

**Test of pQCD at high Q****2**
**Sensitive to New Physics**
**Higher cross section → resonant state, anomalous couplings**
**Lower cross section  → non SM decay modes**

**Important to measure in different channels and with different techniques**
**Provides sample composition for top properties measurements**
**Gives input for searches for which top events are a dominant background**

## Cross Section Results (dilepton)

-  = 22%

**Combination of *****ee, ************ *****and***** e***********channels**
**Opposite sign leptons**
**Significant MET**
**≥ ****1 jet for***** e***********, ≥ 2 jets for *****ee***** and ********

- Improves from  PLB **626**, 55 (2005)

## Cross Section Results (lepton + jets)

**L = 0.9 fb****-1**

**= 1 tag**

**b-tagging**** **

**Kinematic**** **

-  = 15% (a), 19%(b)

- Update of  PRD **74**, 112004 (2006)

**1 isolated lepton (*****e, ***********) **
**High MET**
**b-tagged ( ≥ 3 jets), kinematic (≥ 4 jets**

- Update of  PLB **626**, 45 (2005)

**b-tag**

**Kine.**

## Cross Section Summary

**Experimental results reaching theoretical precision of 12% (expect 10% with 2fb****-1****)**

## Probing the W-t-b vertex

**t → Wb / Wq**
**W helicity fractions**

## Top Branching Ratio

**Top Branching Ratio**

**    ****This is true at 90%CL if the CKM matrix is unitary and for 3 quark generations**

**    ****Measurement: count b-jets (strongly dependent on R and tagging efficiencies)**

**0.9 fb****-1**

**N****jet****=3**

**N****jet****≥4**

## Top Branching Ratio (II)

**Assuming CKM unitarity |V****tb****| > 0.81 @ 95% CL**

**Result obtained from a binned likelihood fit to data for N****jet**** = 3 and N****jet**** = 4**
**Simultaneous fit to *****R***** and ********tt**

- Improves from PLB **639**, 616 (2006)

******= 12% !**

**0.9 fb****-1**
**e+jets**

## W Helicity

**Within the SM the top quark decays via the V-A charged current interaction**
**W-helicity measurement probes new physics associated with V+A current interactions**

**     ****Measure W-helicity through cos(****θ*********) distribution (l+jets and dilepton events) **

**     ****In the SM f****0****=0.7 and f****+****~10****-4**
**       **
**     ****Fix f****0**** to the SM prediction an look for f****+**** deviations**
**    **

- SM

**f****-**

**f****0**

**f****+**

## W Helicity (II)

- Improves from  PRD **75**, 031102 (R) (2007)

**Reconstruct top quark and W boson four vectors**
**Compare the cos(****θ*********) distribution in data to models with different f****+**
**Compute a likelihood L(f****+****) for the data to be consitent with S+B templates at 7 chosen f****+**** values**

**f******** = 0.017 ± 0.048(stat) ± 0.047(syst)**
**f****+**** < 0.14 @ 95%CL ****when f****0**** = 0.7**

**l +jets**** **

**dileptons**** **

## Conclusions and Outlook

**The Tevatron is still the only top quark factory**
**Comprehensive program on top quark measurements is underway**
**Entering an era of top precision measurements**
**Cross section measurements reaching theoretical precision**
**Agreement among channels and with different methods**
**Walking through the largely unexplored territory of top quark properties**
**Measurements are consistent with the SM**
**Analyses based on larger datasets very close **
**Eager to see what the expected 4fb****-1**** of data delivered by the end of the year will bring us!**

## Back up slides

## History of the Top quark

## Top Quark Discovery

**Fermilab’s RunI started the experimental hunt of the top quark and a long fruitful program in top physics**

## Top Quark Physics

**Still only available at the Tevatron**
**Precise measurements possible for the first time **
**Are they really SM particles?**

## The top quark mass

**Fundamental parameter of the Standard Model**

**Affects predictions of SM via radiative corrections**

**m****t**** can be related, with M****W****, to the Higgs mass**
*********m******W****** ************ m******t******2******, ln(m******H******)***

**Probing the EWSB mechanism (new physics?)**

**Precision measurement → 2 (8) fb****-1**** projection: ********m****t**** ~ 1.5 (1) GeV**

## Top Quark Pair Production

## The Tevatron Collider

**The Tevatron Collider**

**Proton-antiproton collider with ********s=1.96 TeV**
**36x36 bunches with 396ns between crossings**
**3 ****~**** collisions per bunch crossing**
***L*****inst**** > 2x10****32****cm****-2****s****-1**
**Expected 4-8 fb****-1**** integrated luminosity for RunII (0.11fb****-1**** in RunI)**

**p **

******p **

**1.96 TeV **

## The DØ Detector

**The D****Ø**** Detector**

**Vertex measurement and tracking**
**Crucial for b-tagging**
**6 barrels and 16 disks with 800000 channels**
**15 ********m position resolution**

**Silicon Microstrip Tracker**

## The DØ Detector

**The D****Ø**** Detector**

## The DØ Detector

**The D****Ø**** Detector**

## The DØ Detector

**The D****Ø**** Detector**

## The DØ Detector

**The D****Ø**** Detector**

## D0 Triggering

**2 MHz crossing frequency**

**Data acquisition limited to 100Hz and not enough resources to store all these events ******** 3 tiered trigger system used to select rare and interesting events.**

## Top Mass Measurements

**M****top**** = 170.5 ± 2.4 (stat + JES) ****+1.2****-1.1**** (syst) GeV**

**M****top**** = 173.7 ± 4.4 (stat + JES) ****+2.1****-2.0**** (syst) GeV**

**MATRIX ELEMENT METHOD** **(0.9 fb****-1****)**
- DØ’s most precise measurement

**IDEOGRAM METHOD (0.42 fb****-1****)**
- hep-ex/0702018  (accepted  for publication in PRD)

## Top Mass

**Improved measurements allows us to reach a 1.1% precision (D****Ø**** and CDF combined)**
**aim at < 1% with 8 fb****-1**

**The precise measurement of the top mass helps constrain the mass of the  SM Higgs and it is one of the most important measurements at the Tevatron**

- MH = 76+33-24 GeV, MH < 144 @95%CL