# Presentation: 295186

## Top Quark Results from DØ

- 40th Rencontres de Moriond
- Electroweak Interactions and Unified Theories

- Herbert Greenlee
- Fermilab
- Mar. 8, 2005

## Outline

- Overview.
- Top quark pair production cross section.
- Top quark mass.*
- Single top production.*
- * Updated in 2005.

- H. Greenlee

- Mar. 8, 2005

## The Top Quark 

- The Top Quark

- Discovered at Fermilab in 1995.
- Feb. 24, 1995 – papers submitted to PRL.
- Mar. 2, 1995 – First public presentations at fnal.
- Heavier by far than any other fundamental fermion.
*m**t *~ Electroweak scale.
- Top quark research.
- Does the top quark exist with properties predicted by SM?
- Are top-like signals seen at the Tevatron due exclusively to SM top quark?
- Measure fundamental parameters:
*m**t*, spin, couplings, etc.

- H. Greenlee

- Mar. 8, 2005

## Top Quark Pair Production & Decay

- Top quarks are pair-produced by the strong interaction.
- At the Tevatron, top quark pairs are produced nearly at rest.

- Top Quark Pair Signatures
-   Dilepton (*ee*, *e***, **)        5%
-   Lepton (*e*,**) + jets          30%
-   All jets                            44%
-   ’s                                   21%
-   All channels include 2 *b* quarks.
-   Goal: measure *tt* in as many channels
-     as possible, with and without *b*-tagging.

*B*(*t**Wb*) ~ 100% (according to SM)

- H. Greenlee

- Mar. 8, 2005

## Run II Integrated Luminosity

- D0 ~ 560 pb-1 on tape

- ~160 pb1

- ~230 pb1

- H. Greenlee

- Mar. 8, 2005

## Pair Production – Dilepton

- Signature:
- Two leptons            (*ee*, *e***, **).
- Missing *E**T*
- Two jets.
- Major backgrounds.
*WW.*
*Z*
*Z**ee*, **fake missing *E**T*).
- Fake leptons (*W*+jets, multijets).

- ~150 pb-1

***tt* = 14.3+5.1 (stat.)+2.6 (syst.)±0.9 (lum.) pb

- 

- 

- D0 Run II Preliminary

- H. Greenlee

- Mar. 8, 2005

## Pair Production – Lepton + jets

- Signature (preselection):
- One isolated lepton (*e* or **).     *p**T* > 20 GeV/c,  || < 1.1 (*e*) or 2.0 ().
- Missing *E**T**.                               **E**T** > *17 or 20* *GeV*.*
-  3 or  4 jets.                         *E**T* > 15 GeV,      || < 2.5.
- Major backgrounds.
*W* + jets.
- Jets misidentified as leptons.
- Final selection:
- Event topology.
- Lifetime *b*-tag.
- Secondary vertex *b*-tag.
- Impact parameter *b*-tag.
- Cross Section Likelihood fit.

- H. Greenlee

- Mar. 8, 2005

## Topological Lepton + Jets Results

- D0 Run II Preliminary

- :

- Combined* ****tt* = 7.2 +2.6 (stat.) +1.6 (syst.) ± 0.5 (lum.) pb

- -2.4

- -1.7

- H. Greenlee

- Mar. 8, 2005

## b-Tagged Lepton + Jets Results

- Secondary vertex b-tag jet multiplicity

- Single tagged

- Double tagged

- SVT: ***tt* = 8.2 +1.3 (stat.) +1.9 (syst.) ± 0.5 (lum.) pb

- 

- 

- IP: ***tt* = 7.2 +1.3 (stat.) +1.9 (syst.) ± 0.5 (lum.) pb

- 

- 

- ~160 pb-1

- :

- H. Greenlee

- Mar. 8, 2005

## Pair Production – All Jets

- Signature (preselection):
-   6 jets.  *E**T* > 15 GeV, 		|| < 2.5.
- One sec vertex *b*-tag.
- Major backgrounds:
- QCD multijets.
- Final selection:
- Neural network event shape analysis based on 13 variables.
- Final cut: NN > 0.75.

***tt* = 7.7 +3.4 (stat.) +4.7 (syst.) ± 0.5 (lum.) pb

- -3.3

- -3.8

- 160 pb

- 220            events observed
- 186514 expected background

- H. Greenlee

- Mar. 8, 2005

## Pair Production – Summary

- Top quark pair production has been observed in many channels.
- So far, all cross section measurements are consistent with each other and with SM top quark.

- H. Greenlee

- Mar. 8, 2005

## Tevatron Average Top Quark MassSummer 2004 Update (Run I)

- Run I Tevatron average top quark mass.
*m**t* = 178.04.3 GeV/*c*2.
- Updated April, 2004 to include improved D0 lepton+jets mass.

- H. Greenlee

- Mar. 8, 2005

## Top Quark Mass – Dilepton

- Mass estimator.
- Dilepton events can not be fully reconstructed.  Usually, candidate events are consistent with a continuum of top quark masses.
- But different mass hypotheses are not equally likely.
- Use Dalitz-Goldstein-Kondo (DGK) likelihood to find the top quark mass that is most consistent with each event.
- Likelihood fit.
- Likehood fit with signal and background templates to extract top quark mass and error.
- Event sample: 8 *e***, 5 *ee, *0 **.
- Expected background: (1.0 *e***, 0.9 *ee, *1.4* ***).

- H. Greenlee

- Mar. 8, 2005

## DGK Candidate Likelihoods

*e*** Candidates

*ee* Candidates

- Smearing

- No Smearing

- Likelihood Fit

*m**t* = 155 +14 ± 7 GeV/c2

- 

- D0 Run II Preliminary

- 8% CL with respect to 175 GeV/c2

- Best *m**t* extracted for each event

- H. Greenlee

- Mar. 8, 2005

## Top Quark Mass – Lepton + Jets

- Template method (updated winter 2005, ~230 pb-1).
- Event preselection: *W* + 4 jets.
- Final selection:
- Topological analysis: Low-mass-bias multivariate discriminator (four variables, not strongly energy-related).
- B-tagged analysis: Require one *b*-tagged jet (SVT tag).
- Top quark mass is estimated by fully reconstructing candidate events to top-antitop hypothesis (kinematic fit).
- 12 possible permutations (6 with *b*-tag, 2 with double *b*-tag).
- Mass estimator taken from best fit.
- Top quark mass result based on likelihood fit of data to signal and background templates.

- H. Greenlee

- Mar. 8, 2005

## Lepton + Jets Mass (Template Method)

- Topological Event Selection

- B-tagged Event Selection

*m**t** *= 170.6  4.2  6.0 GeV/c2

*m**t** *= 169.9  5.8 7.8 GeV/c2

- 

- H. Greenlee

- Mar. 8, 2005

## Lepton + Jets Mass (Errors)

- Lepton + jets top mass is currently systematic-limited.
- Largest systematic is Jet Energy Scale.

| Source | Topological (GeV/c2) | b-tagged (GeV/c2) |
| --- | --- | --- |
| Statistical |  |  |
| Jet Energy Scale | – |  |
| Jet Resolution |  |  |
| Gluon Radiation |  |  |
| Signal Model |  |  |
| Background Model |  |  |
| b-tagging |  |  |
| Calibration (fitting bias) |  |  |
| Trigger |  |  |
| MC Statistics |  |  |
| Total Systematic |  |  |

- H. Greenlee

- Mar. 8, 2005

## Calorimeter jets are corrected to have the same energy as hard scatter particle jets for the same jet algorithm (e.g. cone algorithm).  Correction is the same for all D0 analyses.

Offset correction (O).  Remove energy not associated with hard scatter (spectators, multiple interactions, calorimeter noise).  Luminosity dependent.
Showering correction (S).  Particles inside jet cone can deposit energy outside jet cone (or vice versa) due to showering in calorimeter or magnetic bend.
Response correction (R).
EM scale calibrated using resonances (Z, J/etc.).
EM scale transferred to hadron calorimeter using ET balance in “” + jet events. “Missing ET Projection” (MPF) method.
Largest correction and largest contributor to JES systematic error.
Parton level correction.  Correct particle jet energy to parton energy.  Pure MC based.
Not considered part of JES systematic error per se (included in modeling error).

- Offset correction (*O*).  Remove energy not associated with hard scatter (spectators, multiple interactions, calorimeter noise).  Luminosity dependent.
- Showering correction (*S).  *Particles inside jet cone can deposit energy outside jet cone (or vice versa) due to showering in calorimeter or magnetic bend.
- Response correction (*R*).
- EM scale calibrated using resonances (*Z*, *J/***etc*.*).
- EM scale transferred to hadron calorimeter using *E**T* balance in “” + jet events. “Missing *E**T* Projection” (MPF) method.
- Largest correction and largest contributor to JES systematic error.
- Parton level correction.  Correct particle jet energy to parton energy.  Pure MC based.
- Not considered part of JES systematic error per se (included in modeling error).

- Jet Energy Scale

- H. Greenlee

- Mar. 8, 2005

## Jet Energy Scale Error

- For Run I, jet energy scale systematic error was 2.5% + 0.5 GeV.
- For Run II top quark mass results (so far), jet energy scale error is (conservatively) 5% for jets with *E**T*>30 GeV  (newer version of JES has significantly reduced uncertainty).

- Run I
- 2.5% + 0.5 GeV

- H. Greenlee

- Mar. 8, 2005

## Top Quark Mass Summary

- D0 Run II results are consistent with Run I world average.

- H. Greenlee

- Mar. 8, 2005

## Single Top Quark Search

- Single top quarks produced by weak interaction are a direct probe of top quark weak couplings.
- Measure |*V**tb*| without assuming three-generation unitarity.
- Cross section is close to top quark pair production cross section (2.9 pb vs. 6.7 pb), but background is much larger because there are fewer jets.

*s*-channel
-  ~ 0.9 pb

*t*-channel
-  ~ 2.0 pb

- H. Greenlee

- Mar. 8, 2005

## Single Top

- Event selection:
- One isolated lepton (*e* or **).     *p**T* > 15 GeV/c,  || < 1.1 (*e*) or 2.0 ().
- Missing *E**T*.                               *E**T* > 15 GeV.
- Leading jet.                              *E**T* > 25 GeV,      || < 2.5.
- 13 additional jets.                   *E**T* > 15 GeV,      || < 3.4.
- 1 or 2 SVT *b*-tags.
- Major backgrounds.
*W*+jets.
- Top quark pairs.
- Jets misidentified as leptons.
- Multivariate analysis using 11 variables:
- Energy-related (*p**T*, *H**T*, etc.).
- Reconstructed top quark mass.
- Angular/spin-related.

- H. Greenlee

- Mar. 8, 2005

## Single Top Event Shape Analysis

- Multivariate analyses (all using same variables):
- Cut-based selection.
- Neural network (best so far, results are reported here).

- Signal vs. *W*+jets

-  Signal vs. *tt*

*s*-channel

*t*-channel

- H. Greenlee

- Mar. 8, 2005

## Single Top - Results

*s*-channel:*  *** < 6.4 pb (95% CL)
- (expected limit: 4.5 pb)

*t*-channel:*  *** < 5.0 pb (95% CL)
- (expected limit: 5.8 pb)

- Previous Limits (95% CL)
-   s-channel
-  < 17 pb (D0 Run I)
-  < 18 pb (CDF Run I)
-  < 13.6 pb (CDF Run II)
-   t-channel
-  < 22 pb (D0 Run I)
-  < 13 pb (CDF Run I)
-  < 10.1 pb (CDF Run II)
-   s+t combined
-  < 14 pb (CDF Run I)
-  < 17.8 pb (CDF Run II)

- Limit from 2D binned likelihood (NN vs. NN)

- H. Greenlee

- Mar. 8, 2005

## Summary and Conclusions

- So far, all top quark results are consistent with Standard Model predictions.
- Statistics are still limited.
- New results for top quark mass and single top production.
- The Tevatron is running very well.  D0 currently has 560 pb1 of data.  Ultimately expect 4-8 fb1.
- Many opportunities for new top results with greatly increased statistics.
- Single top production should be seen in Run II.

- H. Greenlee

- Mar. 8, 2005

## References

- Pair Production Cross Section.
- Dilepton:  http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/TOP/T03/T03.pdf
- L+jets (b-tag): http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/TOP/T04/T04.pdf
- All jets:  http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/TOP/T02/T02.pdf
- Top Quark Mass.
- Tevatron Average:  http://arxiv.org/pdf/hep-ex/0404010
- Dilepton:  http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/TOP/T11/T11.pdf
- L+jets:  http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/TOP/T12/T12.pdf
- Single Top.
- Cross Section  http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/TOP/T13/T13.pdf

- H. Greenlee

- Mar. 8, 2005