1. TOP CROSS SECTION MEASUREMENTS AT THE TEVATRON
SUSANA CABRERA
IFIC (CSIC-University of Valencia)
on behalf of
the CDF & D0 collaborations
In my talk I am going to present the most recent and interesting results of
Top pair production cross sections on behalf of the CDF and D0 collaborations.
XIV International Workshop
On Deep Inelastic Scattering
Tsukuba, APRIL 2006

2. Top pair production : from TEVATRON to LHC
At TEVATRON √s=1.96 TeV:
Cacciari et al.
JHEP 0404:068 (2004)
Kidonakis & Vogt
PRD (2003)
stt (theo)  6.7± 0.8 pb (MTOP=175GeV/c2)
σ tt(NLO THEO): 12% ACCURACY
At LHC √s=14 TeV:
10% qq vs 90% gg
R.Bonciani et al.
hep-ph/
CONNECTING: in order to reach a experimental precision of the same order of the current theoretical
Calculation we need to combine the different channels.
Experimental precision in σtt never achieved before
Exhaustive test of the QCD theory
We can find new physics in the top sample.
Now at the TEVATRON

3. OUTLINE & MAIN SIGNATURES
L(e,μ)+JETS CHANNEL:
Kinematics, NN
(CDF, L=760 pb-1)
Secondary vertex b-tag
(CDF, L=695 pb-1)
ALL HADRONIC:
Observed mass spectrum
(DØ, 360 pb-1)
tau+JETS
Missing Et + JETS & Secondary vertex b-tag
(CDF,L=311 pb-1)
DILEPTON CHANNEL:
ee,µµ,eµ
(CDF, L=750 pb-1)
e,µ + track with secondary vertex tag tagging
(DØ, L=370 pb-1)
Dilepton Inclusive (CDF, L=360 pb-1)
t →Wb ~ 100% (SM)
Main signatures
tt  llbb di-lepton % e+ bkgrd low
tt  lqqbb lepton+jets 30% e+ bkgrd moderate
tt  qqqqbb all hadronic 45% bkgrd high
An interesting and promising technique, not yet analized the full dataset.

4. THE DILEPTON CHANNEL. Final State from Leading Order Diagram
BACKGROUNDS:
Physics: WW,WZ, Z  tautau q W
e,µ  b t
Real MET from neutrinos
High E T jets from extra QCD radiation
MC DRIVEN
Instrumental
Fake leptons in W(→lν)+>=3 jets
DY/Z→ee/μμ with mismeasured MET.
CHALLENGE Determination relies on DATA.
SIGNATURE
2 high P T leptons, PT> 20 GeV
2 high E T jets from b-quarks
High Missing E T (MET) from neutrinos
ANALYSES STRATEGIES:
ee,µµ,eµ (CDF, L=750 pb-1)
S/B favorable, no btagging needed
TO BE SENSITIVE TO NEW PHYSICS
Looser lepton selection:
e,µ + track (DØ, L=370 pb-1)
Looser event selection:
Dilepton Inclusive (CDF, L=360 pb-1)
An interesting and promising technique, not yet analized the full dataset.

5. Dileptons: ee,μμ,eµ L=750 pb-1
CONTROL REGION NJETS=0,1
SIGNAL REGION N JETS≥2
Veto Z´s in 76<Mee,μμ<106
Jet Sig = MET/σ(MET)
Missing ET>25GeV
(away from any jet or lepton)
To enhance S/B:
HT > 200 GeV
( of ET, leptons, jets & MET)
To reduce fake leptons from W+(≥2jets)
Leptons oppositely charged
Bkg
19.3  4.3
ttbar(σ=6.7 pb)
36.1  1.3
Tota SM
55.4  5.1
Data (750 pb-1) 64
The techniquel is well established…
S/B favorable, even without using btaggging.
The analisis technique is simple and well established.
The backgrounds:
Instrumenal:
FAKE: fake rates measured
DY: DILEPTON EVENTS IN THE z WINDOW WITH HIGH MET,
MC to distribute them outside the window and in different jet multiplicity bins.
The statistical error still dominates the total error of the measurement,
Control region: 0,1 jet depleted of topdilepton signal,
The methods to predict backgrounds with dileptons and high MET work..
But close to the systematic, similar to the systematic of the best CDF result, l+jets with btagging

6. Dileptons: ee,μμ,eµ L=750 pb-1
Some kinematic comparisons of the top dilepton candidates with MC background prediction
And ttbar signal predicted by using the measured value
MAIN SYSTEMATICS
Jet Energy Scale
DY/Z →ee/μμ & fakes background method

7. e,μ+track vertex btag & eµ L= 370 pb-1
TO INCREASE ACCEPTANCE
Release lepton ID on second leg
PRICE TO PAY: MORE BACKGROUND
High MET, cut dependent on Meµ,track
Need to use b-tagging
At least 1 b-tagged jet
PRE-TAGGED
NJETS=1
PRE-TAGGED
NJETS>=2

8. e,μ+track vertex btag & eµ L= 370 pb-1
After b-tagging
e+track,1 jet
µ+track, 1jets
e+track >=2jets
µ+track, >=2jets
Bkg
1.57  0.77
ttbar(σ=6.7 pb)
1.55  0.03
0.92  0.02
6.59  0.07
4.74  0.06
Tota SM
Data (750 pb-1) 7 1 9 6
MAIN
SYSTEMATIC
Jet Energy Scale
AFTER BTAGGING s/b: 1.5 Njets=1 and almost 3 in Njets>=2

9. Global high-Pt dilepton analysis L=360 pb-1
Higher statistical power with
less purity
Jet multiplicity
Missing energy WW
ttbar
Ztt
DY(ee,mm) WZ ZZ
W+jets
W+g
New physics?
Fit ALL SM processes in the MET vs NJETS space
In ee,mm: σ
Stat+acc syst (fit)
Shape syst
σtt
6.6
σWW
15.4
σ Ztautau
282
 5.6
Another analysis performed in the dilepton channel is where we try to be as inclusive as possible so it has a high statistical power but less purity than in the previous coynting experiment, the idea is to look at the Missing E vs Jet Multiplicity plane and realize that ttbar is well separated from the other backgrounds. Not only this but the WW and Z-tau tau backgrounds can also be separated in this plane, so those 3 cross sections can be fitted for. Because the DY background is very large in the ee, mm channels, there is an additional cut related with the MET significance to cut it out. These are the templated used for the signal, ttbar, WW and Z-tau tau, this is where the data lies in the MET vs Njets plane and these are the obtained cross sections. The WW and Z-tau tau cross sections are in agreement with other analysis and the statistical uncertainty is less. The next step of this analysis will be to use this method to look for New physics.
NEX STEP: look for new physics

10. THE L+JETS CHANNEL Final State from BACKGROUNDS: Leading Order Diagram
Physics:
W+jets (Dominant)
Instrumental:
QCD multijets
-1 jet faking 1 high Pt lepton
-Missing ET from mismeasurements q W
e,µ  b t q´
ANALYSES STRATEGIES:
Event Kinematics S/B ~(1:5)
B-tagging S/B ~(3:1)
SIGNATURE
1 isolated lepton (e,µ) Pt>20 GeV
2 jets from b-quarks
2 jets from light quarks
High MET from neutrinos
CHALLENGES:
W+jets:
IF EVENT KINEMATICS:
MC driven: σ(W+jets) NOT precisely known
IF B-TAGGING:
W+HF(b,bb,c) (MC/DATA)
W+LF(MISTAGS)
QCD multijets:
Determination relies on DATA.
An interesting and promising technique, not yet analized the full dataset.

11. l+jets with kinematics & ANN
Backgrounds:
W+(>=n jets) (MAIN)
MC (ALPGEN+HERWIG) driven
QCD multijets (3.7%)
DATA driven
“l+jets” Event Selection:
To reduce QCD multijet:
If MET<30 GeV: 0.5<ΔΦ(MET,leading jet) <2.5
METHOD:
7 KINEMATIC & TOPOLOGICAL VARIABLES
HT, Aplanarity, min(Mjj), min(ΔRjj)
ηMAX, , Sum(Pz)/Sum(E T), E T(2nd-j)+ET(3rd-j)
ANN (Artificial Neuronal Network)
Maximize discrimination ttbar against W+jets
Take correlations into account
Dphi cut to reduce W+3 o mor jets

12. l+jets with kinematics: 760 pb-1
Binned
Likelihood
Fit
4% QCD
80% W+jets
15% ttbar
Main
systematics:
8.3% Jet Energy scale
10.2% W+jets Q2 scale
CDF Preliminary (760 pb-1)
Sample
Events
Fitted tt
(tt )
W +  3 Jets
2102
324.6  31.6
6.0  0.6  0.9 pb

13. L+jets secondary vertex tag L=695 pb -1
EVENT SELECTION:
>=1 b-tag
HT >200 GeV
CONTROL REGION
SIGNAL REGION
2 b tags

14. L+jets secondary vertex tag L=695 pb -1
s(tt) =  8.2 ± 0.6 (stat)
± 1.0 (syst)
± 0.5 (lumi) pb
SYSTEMATIC ERROR
DOMINATES
b-tagging
6.5%
luminosity
6.0%
PDF
5.8%
Jet Energy Scale
3.0%
ISR/FSR
2.6

15. MET+jets secondary vertex tag L=311 pb -1
EVENT SELECTION:
Multijet trigger:
4 high ET jets
High SumEt.
Veto high PT e or µ
≥ 1 btag
tau+jets
l+jets:µ,e not identified
CHALLENGE
Very small S/B !
1st) CONTROL REGION N JETS=3
Measure probability to get +btag
from QCD multijets and fake MET.
2nd) SIGNAL REGION NJETS≥ 4
Apply mistag probability to
sample before btagging.
3rd) Optimize S/B with KINEMATIC cuts
Met/ √SumEt ≥ 4 GeV ½
minΔΦ(Met,jets) ≥ 0.4 rad
S/B:1/5 BEFORE BTAGGING

16. MET+jets secondary vertex tag L=311 pb -1
After btagging : >=1btag
MAIN SYSTEMATICS:
8.2% Generator
10% Background predicion
S/B= N expected (ttbar)=56.5
(MTOP=178 GeV/c2)

17. ALL HADRONIC: OBSERVED MASS SPECTRUM with secondary vertex tagging
EVENT SELECTION:
6 JETS: 2 b-tagged jets ET>45 GeV
2 non b-tagged jets ET>20 GeV
2 jets ET>15 GeV
2-jet mass:
Mjj with 2 non-btg jets
BACKGROUND METHOD:
SHAPE: from pretagged multijet data
random jets as b-jets
Kinematic correlations: Pt-bjet, dRbb
RATE: Mjj < 65 GeV shape normalized to DATA
3-jet mass: Mbjj (1btg jet, 2 non-btg jets)
TIGHT
Aplanarity>0.05
Centrality>0.7
Sphericity>0.5
ΔR bb>1
LOOSE
No kinematic
cuts
MEDIUM
Aplanarity>0.05
Centrality>0.6
Sphericity>0.2
ΔRbb>1
REFUTAR

18. ALL HADRONIC: OBSERVED MASS SPECTRUM & secondary vertex tagging
Ncand
Nbackground
Eff(ttbar)
Loose
173 ± 13
140.4 ± 0.8
1.51 ± 0.03
Medium
86 ± 9
60.7 ± 0.5
1.17 ± 0.02
Tight
14 ± 4
5.6 ± 0.1
0.37 ± 0.03
MAIN SYSTEMATICS:
25% Bkg Model.
15% Jet Energy Scale.
18% b-tagging efficiency
σ(tt) = 12.1  ± 4.9 (stat) ± 4.6 (syst) pb

19. CDF & DØ SUMMARY 11% 32% 50% 2% 6% -2% CDF COMBINED σtt=7.3±0.9 pb
WEIGHT
11%
32%
50% 2% 6%
-2%
BEST SINGLE σMEASURED
CDF COMBINED
σtt=7.3±0.9 pb
15% improvement w.r.t best single σmeasured

20. σ ttbar vs √s & MTOP MTOP σttbar 169.3 7.73 ±0.89 pb 172.0 ± 2.7
174.7
7.34 ±0.85 pb

21. CONCLUSIONS Check of perturbative QCD
Importance of s tt measurements
Check of perturbative QCD
Starting point to measure top quark properties: mass, charge, W helicity…..window for NEW PHYSICS.
Background for Higgs and other searches.
Current CDF precision from combined result ~12% , reach the current accuracy of the NLO QCD calculations
CDF PRELIMINARY
(MTOP=175 GeV/c2)
Combining 6 measurements with data samples up to 760 pb-1 the systematic uncertainty dominates over the statistical.
New challenge with 1 fb-1 (combining CDF & D0 ) is to reduce the systematic uncertainties (Jet Energy Scale and b-tagging)
Measuring the cross section is crucial:
It is a check of perturbative QCD and
it is a window to New physics by comparing the cross section in the various decay channels.
It is also the starting point for all properties analysis,
Finally, ttbar is a background for various particle searches, so having the right cross section and everything implied in thos analysis is very important.