Recent Results on Electroweak

1
Recent Results on Electroweak Related Physics
at DØ
Alan L. Stone Louisiana Tech University on
behalf of the DØ Collaboration

• Introduction Motivation
• W/Z ? muons
• W/Z ? electrons
• Z (ee) Search
• Top Quark Cross Section
• Conclusions

2
Motivation for Measuring W Z Production

• Test of SM couplings
• Constrain proton PDFs
• Probe effects of NLO QCD corrections
• Better understanding of our Experiment
• Efficiencies, Backgrounds, Luminosity
• Use these signals to tune triggers algorithms
• Improved luminosity measurement
• With sufficiently small statistical systematic
  uncertainties
• Normalize to other measurements
• Preliminaries to other Run II goals
• W boson mass
• Precision EW measurements
• Top Quark Studies
• (W or Z) Higgs

Previous Tevatron results
DØ Run IIa Prediction
3
W and Z Production Mechanics
Cross Sections increase by 10 from 1.8 to 1.96
TeV
O(as0)

• Distinctive lepton decay event signatures
• High PT isolated leptons (e or m)
• One high PT lepton Missing ET (W)
• Two high PT leptons (Z)

W?l? ? 1 Hz _at_ L 2?1032
4
W Z Cross Sections A Counting Experiment
Backgrounds
Measurements make use of data taken from
mid-August 02 through mid-January 03
Integrated Luminosity
Acceptance from Monte Carlo
Efficiencies from data where possible
5
The DØ Run II Detector
SMT

• Builds on the firm foundation of the Run I
  calorimeter central muon system
• Added magnetic tracking, silicon, new forward
  muon system, new electronics
• Electroweak analyses make use of the full
  detector capabilities

6
The Muon Detector

• Two regions Three layers of Scintillators and
  Drift Tubes
• Central and Forward
• A inside toroid magnet
• B C outside toroid magnet
• Muon rapidity coverage to 2
• Shielding reduces backgrounds by 50-100x

Mini drift tube Plane and Pixels (10m ? 10m)
7
Z ?mm Event Selection

• Event selection
• 2 tracked, oppositely-charged ms
• pTgt15 GeV hlt1.8
• Di-muon trigger
• At least one muon is isolated
• (DR)2(Dfmm)2(Dhmm)2 gt4.0
• Dt lt 9 ns in scintillator
• NO explicit mass requirement

• Background is small
• Z?bbar is 1 ? 1
• Z ?tt is 0.5 ? 0.1

PT(?1)40.97 GeV PT(?2)43.92 GeV M(??)86.04 GeV
8
Z?mm Efficiencies
mcontrol
(scaled)

• Acceptance from MC (401)
• Efficiencies from data

L1L2 central track
eL1 872
Z
L1 OR no L1
track OR no track
mprobe
(scaled)
etrk 821
e?ID 911
9
Z?mm Cross Section

• Drell-Yan Contribution
• Correct for 12?1 due to photon exchange and
  photon-Z interference determined from Pythia

1585 Candidates
10
W?mn Event Selection

• Trigger
• L1 Scintillator based single muon trigger (no
  pT cut)
• L2 At least one muon with pTgt5 GeV
• L3 At least one track with pTgt10 GeV

• Offline
• One isolated muon matched with central track
  pTgt20 GeV
• In fiducial region of the trigger ?lt1.6
• Muon corrected Missing transverse energy gt 20
  GeV
• No second muon in event (veto Z??? events)

PT(?)37.48 GeV MET35.5 GeV
11
W?mn Cross Section

• Backgrounds
• Z?bb, b??? where ? passes isolation cut
• 5.8, subtracted from above distributions
• Z??? 9
• W???????? 3.6
• QCD estimated from data

7352 Candidates in
12
The Calorimeter

• Liquid argon sampling
• Stable, uniform response, rad. hard
• LAr purity important (lt 0.7 ppm O2 equivalent)
• Uranium absorber (Cu/Fe for coarse hadronic)
• dense absorber hence can be compact
• Nearly compensated EM and hadronic response
• Linear response
• Hermetic with full coverage
• h lt 4.2 (? ? 2o)
• l int gt 7.2 (total)

13
W/Z?e Event Selection

• Trigger
• L1 1 calorimeter tower gt 10 GeV (or 2 gt 5 GeV)
• L3 Electron candidate gt 20 GeV, shower shape
  cut

• Electrons
• Isolated EM Cluster in the Calorimeter
• ETgt 25 GeV with large EM fraction
• Shower shape consistent with MC expectation
• Z?ee
• 70 GeV lt mee lt 110 GeV
• W?e?
• Missing transverse energy gt 25 GeV
• Matched with central tracks

e
PT(e)40.5 GeV MET43.2 GeV MT(W)83.7 GeV
14
Z?ee Cross Section
D0 Run II Preliminary

• Efficiencies per electron
• Trigger 982
• EMF, isolation 100
• Shower shape 861
• Track Matching 732

• Drell-Yan Contribution
• Small effect (1.7) in the mass window of 70 lt
  mee lt 110 GeV
• QCD Background
• Determined from data by fitting signal and
  background shape

Mee-(GeV)
1139 Candidates in
15
W?en Backgrounds

• Dominant background from QCD multijet events
• Estimated from data

loose
tight
Nloose NW Nb Ntight NWetrk Nbef
Solve for NW
tight loose track match
From QCD dijet sample
D0 Run II Preliminary
D0 Run II Preliminary

• Other backgrounds
• W????e??? (1.5 , MC)
• Z?ee (very small)

2.3/-1
MET(GeV)
pT(GeV)
16
W?en Cross Section
Background subtracted distributions compared to
MC Pythia
27370 Candidates in
17
Results
Z
W
C. R. Hamberg, W.L. van Neerven and T. Matsuura,
Nucl. Phys. B359 (1991) 343
CTEQ4M PDF
18
Direct Search for Z'?ee

• Search for Non-SM heavy particles that decay to
  lepton pairs
• Assumes Z' couples as Z
• Previous Tevatron Run I Limit
• MZ'gt690 GeV
• Event Selection
• 2 isolated EM objects
• hlt1.1 or 1.5lthlt2.5
• ETgt25 GeV
• No track requirement necessary

4585 Candidates in
D0 Run II Preliminary
M(ee) around the Z

• Backgrounds
• Drell-Yan dominates Meelt150 GeV
• Estimated from Pythia (CTEQ4L)
• QCD dominates Meegt150 GeV
• Fake electrons from jets
• Real electrons from heavy flavor
• Estimated from data

Data Bkg (QCDDY/Z) Bkg (QCD only) 10x Z'(600GeV)
Mee-(GeV)
19
Z Search

• Form limit from the ratio of cross sections for
  (?B)Z'/(?B)Z where many of the systematic errors
  in the efficiencies and luminosity measurements
  cancel
• Applied a binned likelihood approach with Poisson
  statistics
• Remaining Uncertainties
• K-factor 5
• Az/Az PDFs 2-3

D0 Run II Preliminary
M(ee)489.1 GeV
20
Motivation for Measuring Top Quark Production

• Run I Discovery
• ?tt
• Top mass
• W helicity in top events
• tt spin correlations
• Top pT
• Searches for new physics
• (X? tt and top decay)
• Run II With high precision we hope to answer
  questions such as
• Why is top so heavy?
• Is it or the third generation special?
• Is top involved with EWSB?
• Is it connected to new physics?

21
Top Quark Production Mechanics

• At the Tevatron, top quarks are produced in pairs
  
• Br(t?Wb)100

Run I Results 100 top events

• Prediction of 30 ?tt cross section increase
  from Run I (1.8 TeV) to Run II (1.96 TeV)

22
?(tt?lljets) Event Selection

• Event Selection
• Two high pT isolated ? or e
• ET (Z mass) cut
• 2 jets, ETgt20 GeV ? lt 2.5
• HT ?(ElT ,EjetT) cut
• Backgrounds
• WW, Z? tt determined with MC
• Z/g, Wjets and QCD from data

m-
ET
jet
Process Nevents
Z?mm Z?tt?mm WW?mm 0.40 0.23 0.02 0.02 0.001 0.001
QCDWjets 0.18 0.084
All BG 0.59 0.30
Expected Signal 0.3 0.02
All 0.89 0.30
Observed 2
D0 Run II Preliminary
ET (GeV)
mmjets
e
jet
e PT 20.3
m- PT 58.1
j PT 141.0
j PT 55.2
ET 91 GeV
Di-muon mass (GeV)
23
?(tt?lljets) Results
D0 Run II Preliminary
emjets
Jet1
Jet2
Sum of jet, electron PT (GeV)
emjets
D0 Run II Preliminary
Jet Multiplicity
24
?(tt?ljets) Event Selection (no m tag)

• Procedure
• Preselect a sample rich in Ws
• Evaluate QCD multi-jet (as f Njets )
• Estimate W4 jets assuming Berends Scaling
• Apply topological selection
• Event Selection
• Isolated lepton w/ PTgt20 GeV
• MET gt 20 GeV
• 4 jets with ETgt15 GeV hlt2.0 (2.5)
• Soft non-isolated muon tag veto
• Backgrounds
• multijet evaluated from data vs. Njets
• ejets due to fake jets (real po and g)
• mjets due to heavy flavor decays
• Estimate real W4 jets with scaling law

Estimation of QCD Background
D0 Run II Preliminary
a 0.145?0.02
Scaling of W for N jets 4 jets
D0 Run II Preliminary
25
?(tt?ljets) Results (no m tag)

• Topological Selection
• 1 jet with ETgt55 GeV
• METCalgt15 GeV
• hWlt2.0, ETWgt60 GeV
• HT(jets W)gt220 GeV
• HTgt180 GeV
• Aplanarity(W-jets)gt0.065

Analysis NW NQCD Bkg. Total Exp Signal Nobs
ejets 1.3?0.5 1.4?0.4 2.7?0.6 1.8 4
mjets 2.1?0.9 0.6?0.4 2.7?1.1 2.4 4
For s 7pb
26
?(tt?ljets) Event Selection (soft m tag)

• Preselection criteria
• Same as leptons jets except soft non-isolated
  muon is not vetoed
• 3 jets with ETgt20 GeV hlt2.0
• Aplanarity gt 0.04
• HTgt110 GeV
• Soft Muon Tag
• pT(m)gt4 GeV within DRlt0.5 of a jet

Analysis Bkg. Tot. Sig. Nobs
ejets 0.2?0.1 0.5 2
mjets 0.6?0.3 0.4 0
For s 7pb
D0 Run II Preliminary
D0 Run II Preliminary
27 ejets events
23 ?jets events
27
?(tt?X) Cross Section
Combined
Combined
28
Conclusions

• Four new W Z cross section measurements
• Assuming same SM Z couplings to quarks and
  leptons, Z' is excluded at 95 CL at mZ'lt620 GeV

• Combining the observation of the top decay
  channels an excess of 3s is observed, compatible
  with a signal expectation at the 35 CL

29
Backup Slides
30
Z?ee Candidate
No centrally matched track to second electron
PT(e1)45.1 GeV PT(e2)40.1 GeV M(ee)85.1 GeV
31
W?mn Backgrounds
Background estimated from data.

• Dominant background
• bbar, b?mn events where the muon passes
  isolation cuts

32
W??? Efficiencies
N0 1 local m 1 cal m
N1 2 local ?'s

• Dimuon events
• 1 ? w/central track match
• 1 Calorimeter ?
• Determine fraction of Calorimeter muons that meet
  local muon criteria

Muon reconstruction efficiency N1/(N0N1)
742
Trigger efficiencies
L1
L2
L3
33
Uncertainties
Z/W?e
Z???
W???