# Presentation: 714735

## Measurement of the Mass of the Top Quark in Dilepton Channels at DØ

**Jeff Temple**
- University of Arizona
- for the DØ collaboration
- DPF 2006

## tt     dilepton decays
Event selection
Reconstructing top mass
Matrix weighting
Neutrino weighting

Combined dilepton result for 370 pb-1 sample
Result in e channel for 835 pb-1 sample

- tt     dilepton decays
- Event selection
- Reconstructing top mass
- Matrix weighting
- Neutrino weighting
- Combined dilepton result for 370 pb-1 sample
- Result in e channel for 835 pb-1 sample

**Overview**

## Fermilab Accelerators

- 5 accelerators
- Collide protons and antiprotons at √s=1.96 TeV
- Collisions at CDF, DØ

## DØ Detector

**DØ Detector**

- Silicon, Fiber Trackers within 2 T solenoid
- LAr calorimeter
- 3 layers of muon scintillators and wire chambers
- Toroid between 1st and 2nd layers

## Dilepton Decay Channel

**Dilepton Decay Channel**

- 2 high-pT  jets
- 2 high-pT leptons
- Significant missing transverse energy (MET) from neutrinos

- t

- p

- p

## Cons:
Small branching fraction

Neutrinos not measured directly!

- Small branching fraction
- Neutrinos not measured directly!

**Dilepton Channel Pros and Cons**

- Pros:
- Small background yields
- Z    ee, 
- Z    
- WW, WZ
- Instrumental Fakes
- Few Jet Combinations

## Dilepton Event Selection

- 2 leptons
- pT>15 GeV
- 2 jets
- pT>20 GeV
- Channel-specific cuts
*(“b-tagging”: require one jet to be identified with a secondary vertex)*

*	**ee channel*
-   Reject 80 GeV<Mee<100 GeV
-   if Mee<80 GeV, MET>40 GeV
-   if Mee>100 GeV, MET>35 GeV
-   Sphericity > 0.15

*	**** **channel*
-  Z fitter
-  MET > 35 GeV
-   increase MET requirement
-    based on MET- angle

*	**e**** channel*
-   MET > 25 GeV
-   Energy of jets + leading lepton
-    >140 GeV
-   Cut on electron shower shape

## Select one lepton + one isolated central track

Increases dilepton acceptance

Veto events satisfying ee, e,  selection

**Additional Selection: Lepton+Track**

- Select one lepton + one isolated central track
- Increases dilepton acceptance
- Veto events satisfying ee, e,  selection

*Lepton + Track selection*
- 1 lepton
- pT > 15 GeV
- 1 isolated track
- pT> 15 GeV
- 2 jets
- pT>20 GeV
- at least one b-tagged jet
- MET> 15-35 GeV

## ~370 pb-1 of data
Slightly different selection criteria for 		matrix weighting, neutrino weighting  

- ~370 pb-1 of data
- Slightly different selection criteria for 		matrix weighting, neutrino weighting

## Reconstructing Top Mass

- 18 independent kinematic variables
- 12 measured directly (jets, charged leptons)
- Also measure METx, METy
- 2 constraints from mW+, mW-
- 1 constraint from mt=mt

- t

- t

- W+

- W-

- p

- p

- b

- b

**e****+**

******-**

******e**

********

- Insufficient constraints to determine mt!

## Assigning Event Weight

*Assume *mt
- Assign weight W(mt) to mass assumption based on agreement with observables
- matrix weighting
- neutrino weighting
- Repeat for many values
- 	of mt
- Use weights
- 	to determine mass

- Sample event weight distribution
- for Monte Carlo 175 GeV top

## Matrix Weighting (MWT)

- f(x) = parton distribution function
- P(E|mt)=probability that lepton has energy E in rest frame of top quark:

**Weight based on consistency of observed lepton energy with m****t**** hypothesis**

*R. H. Dalitz and G.R. Goldstein, Phys. Rev. D ****45****, 1531 (1992)*

## Ignore METx, METy
Assume mt, 
Repeat for many rapidity assumptions

**Neutrino Weighting (********WT)**

- Ignore METx, METy
- Assume mt, 
- Repeat for many rapidity assumptions

**Weight based on agreement between assumed total neutrino momentum and observed MET**

## Extract mass information from event weights

- Collection of event weight info is a *template*
- Form templates for signal, backgrounds
- Monte Carlo for different values of mt
- Monte Carlo/Data for backgrounds

## Determine mt with maximum likelihood fit

Use ensemble tests to calibrate fit minimum

- Use ensemble tests to calibrate fit minimum

**Likelihood Fit**

- Slope=1.00
- Offset=1.9 GeV

- Slope=0.99
- Offset=1.7 GeV

## open circles: WT
dilepton
lepton + track
closed circles: MWT
dilepton
b-tagged 
no b-tagged
Combined measurement:

- open circles: WT
- dilepton
- lepton + track
- closed circles: MWT
- dilepton
- b-tagged
- no b-tagged
- Combined measurement:

## Largest uncertainty:  Jet Energy Scale (JES)
Create ensembles with jet energies varied by their uncertainties, and compare to original templates
Similar approach for other uncertainties 

- Largest uncertainty:  Jet Energy Scale (JES)
- Create ensembles with jet energies varied by their uncertainties, and compare to original templates
- Similar approach for other uncertainties

- Combined JES uncertainty:
- 4.3 GeV

## Systematic Uncertainties

## mt=178.1±8.3 GeV
Submitted to PRL
hep-ex/0609056

Consistent with previous measurements

Significant improvement on Run I dilepton uncertainty

**Combined Result for 370 pb****-1**

**m****t****=178.1±8.3 GeV**
- Submitted to PRL
- hep-ex/0609056
- Consistent with previous measurements
- Significant improvement on Run I dilepton uncertainty

## MWT, WT analyses for e channel only

**Event Selection for 835 pb****-1**

- MWT, WT analyses for e channel only

*e**** event selection*
- 1 muon, pT >15 GeV
- 1 electron, pT > 15 GeV
- 2 jets, pT> 15 GeV
- Energy of jets + leading lepton
-  > 120 GeV

**OBSERVED:  28**

## MWT Measurement

- -log (likelihood)

- Event Weight Peaks vs. Top Mass

- DØ Run II preliminary

- DØ Run II preliminary

## 3 approaches
Coarse binning
same as previous WT analysis
6 bins, not 10
peak value 
à la MWT
Modified fitting procedure
Mean + RMS

- 3 approaches
- Coarse binning
- same as previous WT analysis
- 6 bins, not 10
- peak value
- à la MWT
- Modified fitting procedure
- Mean + RMS

## Each method shows good agreement between input, output top mass

- 6 bins

- peak value

- mean+RMS

- DØ Run II preliminary

- DØ Run II preliminary

- DØ Run II preliminary

- Slope=0.99
- Offset=-0.6 GeV

- Slope=0.99
- Offset=0.2 GeV

- Slope=0.95
- Offset=-0.4 GeV

## 6 bins:  	    mt = 173.6 ± 6.7 (stat.) +5.1-4.0  (syst.) GeV
peak:  	    mt = 165.7 ± 9.7 (stat.) +4.4-4.7 (syst.) GeV
Mean+RMS: mt = 171.6 ± 7.9 (stat.) +5.1-4.0 (syst.) GeV

**6 bins:  ****	****    m****t**** = 173.6 ± 6.7 (stat.) ****+5.1****-4.0****  (syst.) GeV**
**peak:  ****	****    m****t**** = 165.7 ± 9.7 (stat.) ****+4.4****-4.7**** (syst.) GeV**
**Mean+RMS: m****t**** = 171.6 ± 7.9 (stat.) ****+5.1****-4.0**** (syst.) GeV**

- 6 bins

- peak value

- mean+RMS

- DØ Run II preliminary

- DØ Run II preliminary

- DØ Run II preliminary

## New results consistent with one another and world average top mass

Uncertainties in e channel comparable to combined uncertainty for 370 pb-1

**Results for 835 pb****-1**

- New results consistent with one another and world average top mass
- Uncertainties in e channel comparable to combined uncertainty for 370 pb-1

## Combined 370 pb-1 Run II dilepton measurement improves upon Run I result
835 pb-1 result in e channel provides further reduction in uncertainty
 ee,  analyses proceeding
 WT converging on preferred mass extraction method
Approaching point where measurement is limited by systematics!

- Combined 370 pb-1 Run II dilepton measurement improves upon Run I result
- 835 pb-1 result in e channel provides further reduction in uncertainty
-  ee,  analyses proceeding
-  WT converging on preferred mass extraction method
- Approaching point where measurement is limited by systematics!

## Run II luminosity
Sample event weights
Individual mass measurements 
Pull Distributions

- Run II luminosity
- Sample event weights
- Individual mass measurements
- Pull Distributions

## 
Run II started in April, 2002
Data collection efficiency > 85%
370 pb-1 collected by fall 2004
835 pb-1 collected by fall 2005
(compare to 110 pb-1 for Run I)

- Run II started in April, 2002
- Data collection efficiency > 85%
- 370 pb-1 collected by fall 2004
- 835 pb-1 collected by fall 2005
- (compare to 110 pb-1 for Run I)

## Correct assumption of mt,  leads to non-zero weight
Incorrect assumptions can also produce valid solutions!

- Correct assumption of mt,  leads to non-zero weight
- Incorrect assumptions can also produce valid solutions!

## Object momenta not measured perfectly
Repeat weight calculations, smearing over detector resolutions
~150 smears “stabilizes” weight distribution

- Object momenta not measured perfectly
- Repeat weight calculations, smearing over detector resolutions
- ~150 smears “stabilizes” weight distribution

## Individual Channel Results – WT 370 pb-1

## Pull Distributions – 370 pb-1

**Pull Distributions – 370 pb****-1**

## Ensemble Checks – 835 pb-1

**Ensemble Checks – 835 pb****-1**

- WT Peak Calibration

- MWT Pull Widths

- WT 6-bin
- Expected Error