# Presentation: 684094

## Studies of tt Production (ll and l+jets) at DÆ

- Peter Tamburello
- University of Maryland/D**Æ**
- May 1997
- Outline
- Signatures and Backgrounds
- l+jets (b-tag)
- l+jets (kinematic)
- Dilepton
- Cross Section
- Kinematic Distributions

## Particle Masses

## A tt Event

- t

- t

- b

- b

- W+

- W-

- e-

- u

- d

- n*e*

- q

- q

## Run 1 Event Yields

- Integrated Luminosity 125 pb-1
- Some Cross Sections

## Single Lepton Events

- One isolated lepton (*e* or *m*)
*    **p**T *>20 GeV
-    |*h*| < 2.0 (1.7)  for *e *(*m*)
- Missing *E**T*
-     >20 GeV  for *l*+jets (b-tag)
- 		>25 GeV  for *l*+jets (kinematic)

## Jet Multiplicity in W Events

## b-tag Selection

- Tag *b*-quark jets by the presence of a muon in the jet cone
- Three or more jets
*    **p**T *>20 GeV*  *
-    |*h*| < 2.0
- Hadronic energy
*	**  **H**T* > 110 GeV,  *H**T** = **S** E**T**jet*
- Aplanarity
- 	 **A** > 0.04

## b-tag Results

*****assuming cross section from Laenen, Smith, Van Neerven
-   for *m**t *=170 GeV/c2

## W+jets Background Estimate

- The fraction of jets which contain a tagging muon is measured in multijet data.
- The tag-rate is parametrized as a function of jet *p**T* and *h.*
- The number of tagged jets is predicted using the multijet tag-rate.

## Muon Tag Rate per Jet

## Tests of theTag Rate

## Distribution Tests

## Tagged l+jets Events

## Tagged e+jets Events(No Trigger Bias)

## Kinematic Selection

- No muon tag
- Four or more jets
*	**p**T* > 15  GeV
-   |*h*| < 2.0
- Leptonic energy
*	**E**T**l*  > 60 GeV,  *E**T**l*  = *p**T**l** + E**T**n*
- Hadronic energy
*	**H**T* >180 GeV,  *H**T** = **S** E**T**jet*
- Aplanarity
- 	 **A** > 0.065

## Kinematic Results

*****assuming cross section from Laenen, Smith, Van Neerven
-   for *m**t *=170 GeV/c2

## Aplanarity vs. HT 

## Tests of the W+jets Model

- Þ 15%  uncertainty assigned

## Dilepton Selection

- Two isolated high *p**T* leptons (*e*,*m*)
*	**p**T *>15 (20) GeV*  *for *e**m, mm *(*ee*)
-   |*h*| < 2.5 (1.7)  for *e *(*m*)
- Missing *E**T*
- 	> 20 (25) GeV*  *for *e**m *(*ee*)
- Two or more jets
*	**p**T *>20 GeV
- 	|*h*| < 2.5
- Calorimeter energy
*	**H**T* = S *p*Tjet*  *> 100 GeV * *(*mm)*
*	**H**T**e* = S *p*Tjet + *p*T*e  *> 120 GeV*  *(*e**m, **ee)*

## Dilepton HT

## Dilepton Results 

*****assuming cross section from Laenen, Smith, Van Neerven
-   for *m**t *=170 GeV/c2

## en  Signature

- Selection
- One isolated electron
- Two or more jets, *p**T  *> 30 GeV
- Missing ET > 50 GeV
*e**n*  transverse mass > 115 GeV
- Sources of acceptance
- 50% *ee* and *e**m*
- 33% *e*+jets
- 17% *e**t*®hadrons

*****assuming cross section from Laenen, Smith, Van Neerven
-   for *m**t *=170 GeV/c2

## en  ET vs MT

## Summary of Event Counting

- * @ *m**t*=170 GeV/*c*2

## Sources of Uncertainty in Combined Cross Section

- Statistics			25%
- Efficiency
- (detector related)		13%
- Efficiency
- (generator related)	11%
- Backgrounds		  9%
- Luminosity			  5%
- Total				33%

## Cross Section by Channel

## Combined Cross Section

## Mass of the tt System

- 2C fit

-             3C fit
-   (*m**t*=173.3 GeV/*c*2)

## pT of the t-quark

- 2C fit

-             3C fit
-   (*m**t*=173.3 GeV/*c*2)

## Conclusions

- D**Æ** has measured the top quark cross section to be 5.6 ± 1.8 pb at the measured mass 172.0 ± 7.5 GeV/*c*2.
- The cross section is consistent across the various signatures and with the expected production modes.
- The kinematic properties of the events agree with expectations.

## Correction for Signal Events in the Inclusive Sample

- When the tag-rate is applied to the W+jets data we really calculate
- rather than
- We need to subtract off

## Tag Rate per Event for Signal Events

## Jet Multiplicity Scaling

## Jet Multiplicity for Signal Events

## Multijet Backgrounds

- False lepton signatures:
- jets mimic an electron shower
- jets contain a muon and yield little energy in the calorimeter
- The tag-rate underestimates the number of tagged jets in these events.
*E**T* bias in false *e* events
*bb* in `false’ *m* events

## Multijet Backgrounds (cont.)

-         The number of false lepton events in the sample of candidates is estimated by relaxing the lepton identification criteria.
-         This yields a sample with slighty more events containing true leptons from W decay and many more events containg false leptons.
-         We know from other data samples how much loosening the cuts increases the acceptance for both true and false leptons.
-         This information determines the expected number of true and false leptons in the sample.

## en  Results

*****assuming cross section from Laenen, Smith, Van Neerven
-   for *m**t *=170 GeV/c2