# Presentation: 827684

## Studies of W+jets Events and Top Quark Identification in the Electron + jets Channel at D

- Ariel Schwartzman

- Universidad de Buenos Aires / Fermilab

- For the D Collaboration

- Top quark production and decay
-  Survey of Run 1 results
- Performance and studies of e+jet events in Run 2 data
-  Summary

- DPF 2002, Williamsburg, Virginia

- 23 - 28 May, 2002

- Ariel Schwartzman

- DPF 2002

## Top Quark Production and Decay

- _Top-antitop quark pair production_

- _Single top quark production_

- (Drell-Yan)

- (W-gluon fusion)

- (90%)

- Top decays into Wb
- top lifetime = 0.4 10-24 s
- event topology depends on W decay mode

- _Event requirements_:
-  High pT Electron (leptonic W decay)
-  Large Missing ET (neutrino)
-  4 or more Jets with large ET
-   Jets from b-quarks

- (10%)

- Ariel Schwartzman

- DPF 2002

## Survey of Top Quark Measurements

_**Cross Section**_
- test of QCD predictions
- any discrepancy indicates possible new physics
- Run 1  = 5.7  1.2  1.0 pb @ 1.8 TeV (L = 0.1 fb-1) (PRL 79,1203 (1997))

_**Mass**_
-  D Run 1: mt = 172.1  5.2  4.9 GeV

_**Spin and Helicity**_
-  Produced W’s are 70%  longitudinal,
-    30% left-handed
-  Angular distribution of lepton sensitive to W helicity
-  Event by event spins of t and t are correlated

- (PRD 58, 052001 (1998))

- Ariel Schwartzman

- DPF 2002

## The D Detector

- The D Detector

**Tracking: Silicon and Fiber in 2T magnetic field                         **
**Calorimeter supplemented with preshower detectors     **
**Improved muon system, entirely new DAQ and  trigger systems****  **

- Ariel Schwartzman

- DPF 2002

## The D Tracking Detectors

- The D Tracking Detectors

_**Silicon Tracker**_
- - ~800,000 readout channels
- - 6 barrels (4 layers) with                     interspersed small diameter disks
-  4 external large area large diameter disks for  forward tracking  (2 < || < 3)
-  Single (axial) and double sided (axial+stereo) detectors
-  3D track reconstruction capabilities

- - 20 cm < r < 51 cm
- - 8 layers of axial and stereo 830 m  scintillating fibers
- - Visible Light Photon Counter (VLPC)
- - ~77k readout channels
- - tracking trigger

_**Fiber Tracker**_

- Ariel Schwartzman

- DPF 2002

## The D Calorimeter

**  ****3-jets event**
-  ETjet1~310GeV
-  ETjet2~240GeV
-  ETjet3~110GeV
-  Missing ET~8GeV

- _Preserve excellent Run 1 calorimetry_:
- LAr sampling: stable, uniform response, rad. hard.,                                             fine spatial segmentation
- Uniform, hermetic, full coverage (|| < 4.2)
- Compensating (e/~1)
- Good energy resolution

- Ariel Schwartzman

- DPF 2002

## e+jet events: triggers

- Fully efficient at pT 1.5 pT threshold

- Fully efficient at pT 5 pT threshold

- L1 EM Trigger turn-ons

- L1 Jet Trigger turn-ons

- Main trigger:
- EM tower pT>10 GeV/c and 1 additional calorimeter trigger tower pT>7 GeV/c

- Ariel Schwartzman

- DPF 2002

## Selecting good e + jet events 

- Select events with electron, jets and missing ET
- Identified electrons are required to have EM
-   shape consistent with electron showers and a
-   matched track.

- Separate top events from W+jets events
- and multijet events with misidentified electron
- and mismeasured missing ET

- Misidentified electron background estimated
- from data:
-  probability for signal electron
-  probability for misidentified electron

- Signal / Background still very low

- Ariel Schwartzman

- DPF 2002

## W + jet events

_**Reject background**_:
-  topological variables
-  missing ET significance
-  b-tagging

**DØ Run2 Preliminary**

*p**T** > 10 GeV/c*

*p**T** > 15 GeV/c*

*p**T** > 20 GeV/c*

*(W**** e****)+N**Jets** Events*

*N**Jets*

- Ariel Schwartzman

- DPF 2002

## Topological variables 

- Correlation between Aplanarity (A)
- and HT
-  Aplanarity: 3/2 of smallest eigenvalue
-    of the normalized momentum tensor:
-  spherical events,  A = 1/2
-  planar events,  A = 0
- HT =  ET(jets)

- Ariel Schwartzman

- DPF 2002

## Missing ET Significance

- Calculate, for each event, the probability
- that the measured missing ET in that event
- is consistent with resolution effects
- considering:
-  Energy resolution of e and jets
-  Mis-vertexing

- Multijet event

- W+jets event

- Ariel Schwartzman

- DPF 2002

## b-tagging 

- b-tagging

- 2 b-quarks in each tt event
- 2 ways of identifying them:
**1 – **_**detect leptons in jets**_

**2- **_**Lifetime tag**_**:**

- Ariel Schwartzman

- DPF 2002

## b-tagging (muon in jet) 

- Decay muon from heavier b quark has
-   high pT relative to the jet direction.
- b-Tagging method used in Run1

- Ariel Schwartzman

- DPF 2002

## b-tagging (lifetime) 

- b-tagging (lifetime)

- Tag long lived hadrons in jets by
- reconstructing secondary vertices
- displaced from the primary interaction.

- Not fully optimized yet.
- Expect improvements combining
- other tagging algorithms using
- multivariate techniques

- c-jet efficiency < 20%

- light-jet efficiency < 2.5%

- b-jet efficiency = 55 %

**Jet p****T****  [GeV/c]**

**Jet p****T****  [GeV/c]**

**Jet p****T****  [GeV/c]**

- Ariel Schwartzman

- DPF 2002

## b-tagging (lifetime) 

- b-tagging (lifetime)

**D0 Run 2 Preliminary**

- di-jet sample
-  track pT > 1.5 GeV/c, |dca|<1.0mm
-   associated with jet sample (dR<0.7)
-    pTrel > 1.0 GeV/c  Enhanced in b-jet content

**Tracking resolution**

**Heavy flavor**

**DCA/resolution**

- Ariel Schwartzman

- DPF 2002

## b-tagging (lifetime) 

- b-tagging (lifetime)

- Track probability: derived from impact parameter
- resolution:

- di-jet sample

**** -jet sample

**D0 Run 2 Preliminary**

**D0 Run 2 Preliminary**

**DCA/resolution**

**probability**

**probability**

**R(s)**

- Ariel Schwartzman

- DPF 2002

## b-tagging (lifetime) 

- b-tagging (lifetime)

- di-jet sample

**** -jet sample

- Jet probability: combine track probabilities within jets

**probability**

**probability**

**D0 Run 2 Preliminary**

**D0 Run 2 Preliminary**

- Ariel Schwartzman

- DPF 2002

## Summary 

- Demonstrated D detector is performing well.
-  Upgraded detector in Run 2
-  new tracking and muon system (b-tagging)
-  new trigger
-  Several methods to isolate top signal.
- Expect 2 fb-1 by the end of 2004
-  (~1600 top candidates in the e+jet channel)
- Improve Run 1 measurements:
-  cross section error ~8%
-  mass ~ 2GeV
-  spin, helicity, resonances, ...

- Ariel Schwartzman

- DPF 2002