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
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
1
Motivation for
Measuring W & Z
Production
•
•
•
•
Test of SM couplings
Constrain proton PDFs
Probe effects of NLO QCD corrections
Better understanding of our Experiment
Previous Tevatron results
– 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
24 April 2003 - DIS03 - St. Petersburg, Russia
DØ
Run IIa
Prediction
Alan L. Stone - Louisiana Tech University
2
W and Z Production Mechanics
p
p
q
q
l
W(Z)
O(as
0)
n (l)
  pp  W  X  n  X   2 nb
  pp  Z  X     X   0.2 nb
6%
W  10
.
 en e , n  , or n 
8.5%
6
 qq ,
.4%
Z0 3
 e  e - ,     , or   
Cross Sections
increase by
~10% from
1.8 to 1.96 TeV
0%
20
.
nn
9%
69
.
 qq
Distinctive lepton decay event signatures
• High PT isolated leptons (e or )
• One high PT lepton + Missing ET (W)
• Two high PT leptons (Z)
24 April 2003 - DIS03 - St. Petersburg, Russia
Wln  ~1 Hz @ L = 21032
Alan L. Stone - Louisiana Tech University
3
W & Z Cross Sections: A Counting Experiment
Nobs  Nbkg
 B 
A  Ldt
Backgrounds
Integrated
Measurements make use of
data taken from mid-August
’02 through mid-January ‘03
Luminosity
Acceptance
from Monte
Carlo
Efficiencies
from data
where
possible
s  1.96 TeV
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
4
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
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
5
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)
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
6
Z  Event Selection
Event selection
•
•
•
•
•
•
•
2 tracked, oppositely-charged ’s
pT>15 GeV & |h|<1.8
Di-muon trigger
At least one muon is isolated
(DR)2=(Df2+(Dh2 >4.0
|Dt| < 9 ns in scintillator
NO explicit mass requirement
Background is small
•
•
Zbbar is 1  1%
Z  is 0.5  0.1%
PT(1)=40.97 GeV
PT(2)=43.92 GeV
M()=86.04 GeV
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
7
Z
Efficiencies
control
• Acceptance from
L1+L2
central track
MC (40±1%)
• Efficiencies from
data
Z
(scaled)
L1 = 87±2%
L1 OR no L1
track OR no track
probe
(scaled)
trk = 82±1%
ID = 91±1%
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
8
Z Cross Section
Drell-Yan Contribution
• Correct for 121%
due to photon
exchange and
photon-Z
interference
determined from
Pythia
1585 Candidates
1
Ldt

32
pb

 ( Z ) x B( Z   )  264  7 stat  17 sys  26lum pb
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
9
Wn Event Selection
Trigger
• L1: Scintillator based single
muon trigger (no pT cut)
• L2: At least one muon with
pT>5 GeV
• L3: At least one track with
pT>10 GeV
Offline
•One isolated muon matched with central track pT>20 GeV
• In fiducial region of the trigger: |h|<1.6
• Muon corrected Missing transverse energy > 20 GeV
PT()=37.48 GeV
MET=35.5 GeV
• No second muon in event (veto Z events)
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
10
Wn Cross Section
Backgrounds
• Zbb, bn where  passes isolation cut
 5.8%, subtracted from above distributions
• Z: ~9%
• Wnnnn: 3.6%
• QCD estimated from data
7352 Candidates in
1
Ldt

17
pb

 (W ) x B(W  n )  3226  128stat  100sys  323lum pb
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
11
The Calorimeter
•
Liquid argon sampling
 Stable, uniform response, rad. hard
 LAr purity important (< 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| < 4.2 (  2o)
 l int > 7.2 total) y
•
•

j
x
Z
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
12
W/Ze Event
Selection
Trigger:
• L1: 1 calorimeter tower > 10 GeV (or 2 > 5 GeV)
• L3: Electron candidate > 20 GeV, shower shape cut
Electrons
• Isolated EM Cluster in the
Calorimeter
• ET> 25 GeV with large EM fraction
• Shower shape consistent with MC
expectation
Zee
• 70 GeV < mee < 110 GeV
Wen
• Missing transverse energy > 25 GeV
• Matched with central tracks
PT(e)=40.5 GeV
MET=43.2 GeV
MT(W)=83.7 GeV
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
13
Zee Cross
Section
D0 Run II Preliminary
Efficiencies per electron
• Trigger: 98±2%
• EMF, isolation: ~100%
• Shower shape: 86±1%
• Track Matching: 73±2%
Drell-Yan Contribution
• Small effect (1.7%) in
the mass window of
70 < mee < 110 GeV
QCD Background
• Determined from data
by fitting signal and
background shape
Me+e-(GeV)
1139 Candidates in
1
Ldt

42
pb

 ( Z ) x B( Z  ee)  294  11stat  8sys  29lum pb
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
14
Wen Backgrounds
• Dominant background from QCD multijet events
• Estimated from data
Nloose = NW + Nb
Ntight = NWtrk + Nbf
Solve for NW
loose
tigh
t
tight = loose + track match
From QCD dijet sample
D0 Run II Preliminary
D0 Run II Preliminary
Other backgrounds:
2.3+/-1%
• Wnennn (1.5 %, MC)
• Zee (very small)
pT(GeV)
24 April 2003 - DIS03 - St. Petersburg, Russia
MET(GeV)
Alan L. Stone - Louisiana Tech University
15
Wen Cross
Section
Background subtracted distributions
compared to MC Pythia
27370 Candidates in
1
Ldt

42
pb

 (W ) x B(W  en )  3054  100stat  86sys  305lum pb
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
16
Results
Z
W
C. R. Hamberg, W.L. van Neerven and T. Matsuura, Nucl. Phys. B359 (1991) 343
CTEQ4M PDF
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
17
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'>690 GeV
• Event Selection
–
–
2 isolated EM objects
• |h|<1.1 or 1.5<|h|<2.5
• ET>25 GeV
No track requirement necessary
Backgrounds
Drell-Yan dominates Mee<150 GeV
• Estimated from Pythia (CTEQ4L)
4585 Candidates in
1
Ldt

50
.
0
pb

D0 Run II Preliminary
M(ee) around the Z
Data
Bkg (QCD+DY/Z)
Bkg (QCD only)
10x Z'(600GeV)
QCD dominates Mee>150 GeV
• Fake electrons from jets
• Real electrons from heavy flavor
• Estimated from data
24 April 2003 - DIS03 - St. Petersburg, Russia
Me+e-(GeV)
Alan L. Stone - Louisiana Tech University
18
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
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
19
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)
W helicity
Top Mass
l
Top Width
Production
cross-section
Resonance
production
Production
kinematics
Top Spin
W+
Top Charge
CP violation
p
n
t
b
X
Run II: With high precision we Top Spin
_
_
Polarization
hope to answer questions
p
t
such as:
• Why is top so heavy?
Rare/non SM Decays
W
• Is it or the third generation
Branching Ratios
special?
• Is top involved with EWSB?
|Vtb|
• Is it connected to new
physics?
24 April 2003 - DIS03 - St. Petersburg, Russia
Anomalous
Couplings
_
b
q
_
q’
Alan L. Stone - Louisiana Tech University
20
e-e(1/81)
Top Quark Production
Mechanics
mu-mu (1/81)
tau-tau (1/81)
e -mu (2/81)
e -tau(2/81)
mu-tau (2/81)
e+jets (12/81)
mu+jets(12/81)
•
At the Tevatron, top quarks are
produced in pairs
–
tau+jets(12/81)
jets (36/81)
Br(t→Wb)=100%
Lepton+jets
(soft muon tag)
Lepton+jets
(topological)




e+jets, +jets
Br = 14.7%
Efficient
Not very pure
Run I Results ~100 top events
dileptons
e+jets/, +jets/
Br = 14.7%
Pure
Not very efficient

ee, e, 
Br = 2.5 and 1.2%
Pure and efficient
Low branching
• Prediction of ~30% tt cross section increase
from Run I (1.8 TeV) to Run II (1.96 TeV)
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
21
(ttll+jets) Event Selection
Event Selection
• Two high pT isolated  or e
• ET (Z mass) cut
• ≥ 2 jets, ET>20 GeV & |h| < 2.5
• HT =(ElT ,EjetT) cut
Backgrounds
• WW, Z→ tt determined with MC
• Z/g*, W+jets and QCD from data
ET (GeV)
D0 Run II Preliminary
+jets
Process
Nevents
Z→
Z→→
WW→
0.40 ± 0.23
0.02 ± 0.02
0.001 ± 0.001
QCD+W+jets
0.18 ± 0.084
All BG
0.59 ± 0.30
Expected Signal
0.3 ± 0.02
All
0.89 ± 0.30
Observed
2
Di-muon mass (GeV)
24 April 2003 - DIS03 - St. Petersburg, Russia
e+
e+
PT= 20.3

PT= 58.1
j
PT= 141.0
j
PT= 55.2
ET
91 GeV
Alan L. Stone - Louisiana Tech University
22
(ttll+jets) Results
1
Ldt

33

48
pb

D0 Run II Preliminary
e+jets

e
ee
Z→tt→ll
WW→ll
0.02 ± 0.01 0.02 ± 0.02
0.001± 0.001 0.00 ± 0.00
0.02 ± 0.02
0.001 ± 0.001
Z→ll
DY→ll
QCD,W+jets
--0.05 ± 0.01
0.20 ± 0.12
0.20 ± 0.21
0.18 ± 0.18
0.98 ± 0.48
All BG
0.07 ± 0.01
0.60 ± 0.30
0.60 ± 0.30
Expected Signal
0.50 ±0.01
0.3 ± 0.04
0.25 ± 0.02
Observed
1
2
4
Jet1
Jet2
Sum of jet, electron
PT (GeV)
e+jets
D0 Run II Preliminary
Jet Multiplicity
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
23
(ttl+jets) Event Selection (no  tag)
Procedure
•
•
•
•
Estimation of QCD Background
Preselect a sample rich in W’s
Evaluate QCD multi-jet (as f [Njets] )
Estimate W+4 jets assuming Berends Scaling
Apply topological selection
Event Selection
•
•
•
•
Isolated lepton w/ PT>20 GeV
MET > 20 GeV
4+ jets with ET>15 GeV & |h|<2.0 (2.5)
Soft non-isolated muon tag veto
D0 Run II Preliminary
Backgrounds
•
•
multijet evaluated from data vs. Njets
• e+jets: due to fake jets (real po and g)
• +jets: due to heavy flavor decays
Estimate real W+4 jets with scaling law
  0.1450.02
D0 Run II Preliminary

( W  (n  1) jets )
24 April 2003 - DIS03 - St. Petersburg, Russia
( W  n jets )
Scaling: # of W
for N jets≥ 4 jets
Alan L. Stone - Louisiana Tech University
24
(ttl+jets)
Results (no  tag)
+jets Candidate Event
+jets Candidate Event
-
Topological Selection
•
•
•
•
•
•
1+ jet with ET>55 GeV
METCal>15 GeV
|hW|<2.0, ETW>60 GeV
HT(jets & W)>220 GeV
HT>180 GeV
Aplanarity(W-jets)>0.065
Jet 1
Jet 4
NW
Jet 3
Jet 2
1
Ldt

40

49
.
5
pb

Analysis
Jet 5
NQCD
Bkg.
Total
Exp
Signal*
Nobs
e+jets
1.30.5 1.40.4 2.70.6
1.8
4
+jets
2.10.9 0.60.4 2.71.1
2.4
4
24 April 2003 - DIS03 - St. Petersburg, Russia
* For  = 7pb
Alan L. Stone - Louisiana Tech University
25
(ttl+jets) Event Selection (soft  tag)
Preselection criteria
•
•
•
•
Same as leptons + jets except soft nonisolated muon is not vetoed
3+ jets with ET>20 GeV & |h|<2.0
Aplanarity > 0.04
HT>110 GeV
Soft Muon Tag
•
pT()>4 GeV within DR<0.5 of a jet
D0 Run II Preliminary
27 e+jets events
24 April 2003 - DIS03 - St. Petersburg, Russia
1
Ldt

40

49
.
5
pb

Analysis
Bkg.
Tot.
Sig.*
Nobs
e+jets
0.20.1
0.5
2
+jets
0.60.3
0.4
0
* For  = 7pb
D0 Run II Preliminary
23 +jets events
Alan L. Stone - Louisiana Tech University
26
(ttX) Cross Section
Combined
Combined
21.0
σ(t t)  29.9-15.7
(stat) -14.1
6.1 (sys)  3.0(lum) pb
4.3
4.1
σ(t t)  5.8-3.4
(stat)-2.6
(sys)  0.6(lum) pb
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
27
Conclusions
 Four new W & Z cross section measurements:
σ(Z)x B(Z  μμ)  264  7stat  17sys  26lum pb
σ(W)x B(W μν) 3226128stat 100sys 323lum pb
σ(Z)x B(Zee)29411stat 8sys 29lum pb
σ(W)x B(W  eν )  3054  100stat  86sys  305lum pb
 Assuming same SM Z couplings to quarks and
leptons, Z' is excluded at 95% CL at mZ'<620 GeV
 Combining the
observation of
the top decay
channels an
excess of 3 is
observed,
compatible with a
signal expectation
at the 35% CL
-3.5 (sys)  0.8(lum) pb
σ(t t)  8.5-3.6
(stat)
4.5
6.3
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
28
Backup Slides
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
29
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
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
30
Wn Backgrounds
Background
estimated
from data.
Dominant background:
• bbar, bn events where the muon
passes isolation cuts
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
31
Wn
Efficiencies
N0 = 1
local  +
1 cal 
• Dimuon events
N1 = 2
local 's
– 1  w/central track match
– 1 Calorimeter 
• Determine fraction of
Calorimeter muons that
meet local muon criteria
Muon reconstruction efficiency = N1/(N0+N1) = 74±2%
Trigger efficiencies
L1
L2
24 April 2003 - DIS03 - St. Petersburg, Russia
L3
Alan L. Stone - Louisiana Tech University
32
Uncertainties
Z/We
Z
Wn
24 April 2003 - DIS03 - St. Petersburg, Russia
Alan L. Stone - Louisiana Tech University
33