# Presentation: 795741

## Recent QCD and top results at D0̸ 

- Run II  jet production measurements
- Run II  forward proton detector
- Run II  top production
- Run I    new top mass measurement

- Boris Tuchming
- CEA Saclay

- QCD and top at Dzero

- QCD 03-Montpellier

## Tevatron - Run II

-  proton  vs antiproton  collisions
- Run I  1992-1996  s=1.8 TeV    L~120 pb-1

- Run II  s=1.96 TeV
- Main injector is new
- Bunch crossings 396 ns (Run I : 3.5 s)
- First physics collision spring  2001
- Available for physics  (summer 2003) L~120 pb-1
- ~20 times more data   in 2006

- QCD and top at Dzero

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## D0̸  detector upgraded for Run II

- D0̸  detector upgraded for Run II

- 396 ns -> new electronic for readout and trigger
- New software (C++)

- Muon system
- Drift chambers + scintillators
- (new forward detector)
- Iron Toroid 1.8 T
- Calorimeter Ar/U hermetic and partially compensated.
- MC Resolution :
- EM: (E)/E = 0.23/E1/2 + 0.2/E + 0.004
- Had: (E)/E = 0.40/E1/2 + 1.3/E + 0.03

- Preshower
- Solenoid 2T
- Scintillating fibers hit> 98%
- 8 axial and stereo layers
- 77k channels
- Sillicon vertex detector
- 4 layers  (single and double sided)
-  hit>97%

- QCD and top at Dzero

- QCD 03-Montpellier

## the  DØ detector

**Muon sytem**

**shielding**

**electroni****c**

- QCD and top at Dzero

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## the  DØ detector

**Central Calorimeter**

**Solenoid**

**Fiber Tracker**

**Silicon**

- QCD and top at Dzero

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## Jet production results

- Jet production at DØ Run II:
- Test of  pQCD at  new energies and with a large statistics
- Structure function at large x, especially gluon PDF
- Probe for new physics:  compositeness, W’, Z’, etc...
- At s=1.96 TeV   cross sections are enhanced by a factor of ~2 for  ET>400 GeV compared  to s=1.8 TeV

- QCD and top at Dzero

- QCD 03-Montpellier

## Inclusive jet cross-section :

- Jet definition
- Cone jet of R0.7
- New feature of cone algorithm/Run I
- usage of midpoints as new seed
- Use four-vectors instead of scalar quantities to build jet
- Quality requirements :
- |z vtx| < 50 cm.
- 5 charged tracks
- E̸T/Pt leading jet <0.7
- Central region  ||<0.5
- Jet shower shape cut to get rid of  background

- QCD and top at Dzero

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## One event  with high dijet mass

- pT = 432 GeV/c

- pT = 394 GeV/c

- Dijet Mass = 838 GeV

- Missing ET = 36 GeV

- 

- 

- 

- 

- 

- 

- 

- 

- QCD and top at Dzero

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## Jet energy scale

- Use gamma-jet events to calibrate the energy response.
- O offset due to noise and underlying event
- S  fraction of energy leakage out of the cone due to showering
- R calorimeter response due to non linearity, dead material and different response 0 vs 
- Uncertainty on the jet energy scale increases with energy due to the poor  statistic  of gamma-jet events

- QCD and top at Dzero

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## Jet energy resolution

- Measured using jet energy unbalance in back to back dijet events
- The knowledge of the resolution  allows to unsmear the data for the cross section measurements

- QCD and top at Dzero

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## Inclusive jet cross section

- Luminosity 34 pb-1
- Efficiencies are computed with the data.
- 10% uncertainty due to luminosity not shown here
- Uncertainty due to energy scale not shown
- Bin to bin correlation not shown

- QCD and top at Dzero

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## Inclusive jet cross section : comparison with NLO prediction

- Large uncertainties due to the jet energy scale.
- This will be improved very soon
- Data are in good agreement with NLO pQCD

- QCD and top at Dzero

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## Dijet mass

- Events with at least two jets
- The leading jets are required to be central ||<0.5
- Luminosity 34 pb-1
- Efficiencies are computed with the data.
- 10% uncertainty due to luminosity not shown here
- Uncertainty due to energy scale not shown
- Bin to bin correlation not shown

- QCD and top at Dzero

- QCD 03-Montpellier

## dijet mass : comparison with NLO prediction

- Large uncertainties due to the jet energy scale.
- This will be improved very soon
- Data are in good agreement with NLO pQCD

- QCD and top at Dzero

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## Forward proton detector

- Goal is to measure diffractive cross-sections by tagging the proton deflected at low angle.
- Diffractive production of W
- Diffractive production of Z
- Diffractive production of jets
- Double diffractive processes
- This will allow to probe the pomeron structure functions

- 9 momentum spectrometers composed of 18 Roman Pots

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## Forward proton detector alignment

- Alignment has been done using elastic scattering events
- Performance of detector is close to design (efficiency/resolution)
- Detector is ready to go for physics

- t=(p-p’)2
- p’=1-p

- QCD and top at Dzero

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## top production  at DØ Run II

- At Tevatron top quarks are mostly produced by pair
- Rate of electroweak production of single top is one order of magnitude lower
- Run I : with ~100 pb-1
- top discovery.
- Cross section measurement ~6 pb
- top mass measurement  m~O(8 GeV)

- Run II  pair production has a 30% higher  cross section.
- With  a few fb-1 we can cover many topics of top physics :
- Production cross section
- Mass m~O(2-3 GeV)
- Spin/polarization
- Branching ratio
- W helicity
- Non SM decay
- Single top
- At this earlier stage in Run II (50 pb-1) : rediscovery of top

- QCD and top at Dzero

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## Top decay channels

- Top decays into  Wb (100%). According to W branching ratio we look for :
- Dilepton + jets = pure  but very low rate (5%)
- 	Di-electron +  2  jets
- 	Di-muon  + 2  jets
- 	Electron-muon +  2  jets
- Lepton + jets =  rate is 30%
- 	Electron + 4 jets
- 	Muon + 4 jets
- All the channels may benefit from b-tagging technics :
- Secondary vertex tag, impact parameter tag, or soft lepton tag.
- Currently, only soft muon tag  has been used with the lepton+jets channels

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## Dilepton channels

- Requirement :
- Two high Pt isolated muons or electrons
- Large missing transverse energy E̸T
- At least 2 jets ET>20 GeV ||<2.5
- Cut on HT=jets,lepton ET

- Backgrounds :
- WW, WZ,   Z-> , Z/* and QCD (heavy flavour)
- Instrumental background determined with the data
- E̸T mismeasurement,
- jet faking electron
- Muon  from QCD (heavy flavour) seen as isolated
- Physical background (eg WW+jets, Z)   determined with the MC

- Example  e channel

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## Dilepton channels - results

- 33pb-1

- 42pb-1

- 48.2pb-1

- QCD and top at Dzero

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## lepton +jets channels

- Requirement
- One high Pt isolated muon or electron
- Large missing transverse energy E̸T
- At least 4 jets
- Dilepton veto

- Backgrounds :
- W+jets
- QCD jets faking electron
- Isolated muons from heavy flavour production

- l

- 

- E̸T

- W
- QCD

- Method :
- Use Berends scaling 		to extrapolate the number of W+4jets

- Then apply topological cuts :
- E̸T calo
- HT
- Pt leading jet
- Aplanarity

- QCD and top at Dzero

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## lepton +jets  + soft muon tag

- Requirement
- One high Pt isolated muon or electron
- Large missing transverse energy E̸T
- At least 3 jets

- Backgrounds :
- W+jets
- QCD

- l

- 

- E̸T

- Apply  loose topological cuts :
- E̸T calo
- HT
- Aplanarity
- Soft muon requirement

- 

-  untag events

- QCD and top at Dzero

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## lepton +jets channels - results

*Doubly b-tag event*

- QCD and top at Dzero

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## Top production summary

- We expect to improve those results and measure the top mass in the coming weeks thanks to  :
- Increase of luminosity (~2)
- Better tracking efficiency and resolution
- Usage  of  b-tagging

- QCD and top at Dzero

- QCD 03-Montpellier

## top mass measurement in lepton+jets channels at Run I

- Old published method (Run I - L=125 pb-1)
- use of a  multidimensionnal template to build a likelihood function.
- With this method one takes the  average resolutions for the measured quantities -> each event contributes with the same weight

- New method :
- use the same 91 events + additionnal selection criteria -> 22 events
- Build a signal and background probability for each events.
- Take into accounts  resolution effects on an event by event basis

*Transfer Functions*

- likelihood is used to estimate top mass, signal and background fraction

- QCD and top at Dzero

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## top mass measurement in lepton+jets channels at Run I

- Old published method (PRD 58 052001,1998)
*m**top* = 173.3 ± 5.6 (stat) ± 5.5 (syst) GeV	(3.2% stat error)
- New measurement
*m**top* = 180.1 ± 3.6 (stat) ± 4.0 (syst) GeV	(2.0% stat error)

- Total error with this new method is 3% to be compared to the published DØ+CDF combination:  2.9%

- QCD and top at Dzero

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## Summary

- QCD
- Dijet and inclusive cross sections agree with QCD NLO
- Expect to improve the results in the near future
- higher statistics
- jet energy scale improvement
- Extended angular coverage
- Diffraction : detector is ready to go for physics
- Top quark
- Rediscovery of top quark
- Competitive cross section and mass measurement coming soon
- Top quark physics in the coming months
- Top mass at Run I.
- With a more sophisticated analysis one is able to substantially reduce the uncertainty on the mass measurement.

- QCD and top at Dzero

- QCD 03-Montpellier