TOP CROSS SECTION MEASUREMENTS AT THE TEVATRON
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Transcript of TOP CROSS SECTION MEASUREMENTS AT THE TEVATRON
DIS 2006, April 20th Susana Cabrera, IFIC(CSIC-UV) Page 1
XIV International Workshop
On Deep Inelastic Scattering
Tsukuba, 20-24 APRIL 2006
TOP CROSS SECTION MEASUREMENTSAT THE TEVATRON
SUSANA CABRERA
IFIC (CSIC-University of Valencia)
on behalf of
the CDF & D0 collaborations
CDF
DIS 2006, April 20th Susana Cabrera, IFIC(CSIC-UV) Page 2
Top pair production : from TEVATRON to LHC
tt (theo) 6.7± 0.8 pb (MTOP=175GeV/c2)
At TEVATRON √s=1.96 TeV: Cacciari et al. JHEP 0404:068 (2004)Kidonakis & Vogt PRD 68 114014 (2003)
At LHC √s=14 TeV: 10% qq vs 90% gg R.Bonciani et al. hep-ph/9801375
)%3%51(840)( PDFscalepbtheott
σ tt(NLO THEO): 12% ACCURACY
•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
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OUTLINE & MAIN SIGNATURES
t →Wb ~ 100% (SM)Main signaturestt llbb di-lepton 5% e+ bkgrd lowtt lqqbb lepton+jets 30% e+ bkgrd moderatett qqqqbb all hadronic 45% bkgrd high
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)
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)
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THE DILEPTON CHANNEL.
q
q
W
W
e,µ
e,µ
b
b
t
t
BACKGROUNDS:•Physics: WW,WZ, Z tautau
•2 high P T leptons, PT> 20 GeV•2 high E T jets from b-quarks•High Missing E T (MET) from neutrinos
•ee,µµ,eµ (CDF, L=750 pb-1)S/B favorable, no btagging needed
TO BE SENSITIVE TO NEW PHYSICSLooser lepton selection:•e,µ + track (DØ, L=370 pb-1)Looser event selection:•Dilepton Inclusive (CDF, L=360 pb-1)
Final State from Leading Order Diagram
SIGNATURE
•Instrumental• Fake leptons in W(→lν)+>=3 jets• DY/Z→ee/μμ with mismeasured MET.• CHALLENGE Determination relies on DATA.
•Real MET from neutrinos•High E T jets from extra QCD radiation•MC DRIVEN
ANALYSES STRATEGIES:
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Dileptons: ee,μμ,eµ L=750 pb-1
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
CDF
•To enhance S/B:
•HT > 200 GeV
• ( of ET, leptons, jets & MET)
•To reduce fake leptons from W+(≥2jets)
•Leptons oppositely charged
SIGNAL REGION N JETS≥2
CONTROL REGION NJETS=0,1
•Veto Z´s in 76<Mee,μμ<106 Jet Sig = MET/σ(MET) •Missing ET>25GeV (away from any jet or lepton)
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Dileptons: ee,μμ,eµ L=750 pb-1CDF
pblumisyststattt 5.00.15.13.8
MAIN SYSTEMATICS•Jet Energy Scale• DY/Z →ee/μμ & fakes background method
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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
PRE-TAGGEDNJETS=1
PRE-TAGGEDNJETS>=2
At least 1 b-tagged jet
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e,μ+track vertex btag & eµ L= 370 pb-1
e+track,1 jet µ+track, 1jets e+track >=2jets µ+track, >=2jets
Bkg 1.97 +0.91 -0.85 1.57 0.77 2.83 +0.87 -0.64 2.00 +0.51 -0.49
ttbar(σ=6.7 pb) 1.55 0.03 0.92 0.02 6.59 0.07 4.74 0.06
Tota SM 3.53+0.99 -0.86 2.49+0.83
-0.77 9.4+0.99 -0.85 6.74+0.67
-0.64
Data (750 pb-1) 7 1 9 6
After b-tagging
.)(6.0)()(6.8 1.11.1
9.17.1 pblumisyststattt
MAIN SYSTEMATICJet Energy Scale
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Global high-Pt dilepton analysis L=360 pb-1
– Fit ALL SM processes in the MET vs NJETS space
– In ee,:
METsig
MET
ET 2.0
CDF
Jet multiplicity
Mis
sing e
nerg
y
WW ttbar
ZDY(ee,)
WZZZ
W+jets
W+ New physics?
σ Stat+acc syst (fit)
Shape syst
σtt 6.6 +2.2 -1.9+0.5 -0.2
σWW 15.4 +5.1 -4.4+0.8 -0.2
σ Ztautau 282 +49 -44 5.6
Higher statistical power with less purity
NEX STEP: look for new physics
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THE L+JETS CHANNEL
q
q
W
W
e,µ
q
b
b
t
t
q´
•1 isolated lepton (e,µ) Pt>20 GeV •2 jets from b-quarks•2 jets from light quarks•High MET from neutrinos
BACKGROUNDS:Physics:W+jets (Dominant)Instrumental: QCD multijets-1 jet faking 1 high Pt lepton-Missing ET from mismeasurements
Final State from Leading Order Diagram
SIGNATURE
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.
ANALYSES STRATEGIES:•Event Kinematics S/B ~(1:5)•B-tagging S/B ~(3:1)
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l+jets with kinematics & ANN
“l+jets” Event Selection:• To reduce QCD multijet:
If MET<30 GeV: 0.5<ΔΦ(MET,leading jet) <2.5
Backgrounds:• W+(>=n jets) (MAIN) MC (ALPGEN+HERWIG) driven•QCD multijets (3.7%) DATA driven
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
CDF
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l+jets with kinematics: 760 pb-1
Sample Events Fitted tt (tt )
W + 3 Jets 2102 324.6 31.6 6.0 0.6 0.9 pb
Main systematics:•8.3% Jet Energy scale•10.2% W+jets Q2 scaleCDF Preliminary (760 pb-1)
Binned LikelihoodFit
CDF
•4% QCD•80% W+jets•15% ttbar
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L+jets secondary vertex tag L=695 pb -1
2 b tags
EVENT SELECTION:• >=1 b-tag • HT >200 GeV
156 158
53.0+-6.3 17.2+-1.9
CDF
CONTROL REGION SIGNAL REGION
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L+jets secondary vertex tag L=695 pb -1
(tt) = 8.2 ± 0.6 (stat) ± 1.0 (syst) ± 0.5 (lumi) pb
CDF
b-tagging 6.5%
luminosity 6.0%
PDF 5.8%
Jet Energy Scale 3.0%
ISR/FSR 2.6
SYSTEMATIC ERRORDOMINATES
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MET+jets secondary vertex tag L=311 pb -1CDF
EVENT SELECTION:•Multijet trigger:
• 4 high ET jets •High SumEt.
•Veto high PT e or µ•≥ 1 btag
3rd) Optimize S/B with KINEMATIC cuts Met/ √SumEt ≥ 4 GeV ½ minΔΦ(Met,jets) ≥ 0.4 rad
1st) CONTROL REGION N JETS=3Measure probability to get +btag
from QCD multijets and fake MET.
2nd) SIGNAL REGION NJETS≥ 4Apply mistag probability to
sample before btagging.
tau+jetsl+jets:µ,e not identified
CHALLENGEVery small S/B !
S/B:1/5 BEFORE BTAGGING
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MET+jets secondary vertex tag L=311 pb -1CDF
MAIN SYSTEMATICS:8.2% Generator 10% Background predicion
S/B=1.14 N expected (ttbar)=56.5
.)()(2.18.5 9.07.0 pbsyststattt
(MTOP=178 GeV/c2)
After btagging : >=1btag
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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
BACKGROUND METHOD:SHAPE: from pretagged multijet data •random jets as b-jets•Kinematic correlations: Pt-bjet, dRbbRATE: Mjj < 65 GeV shape normalized to DATA
2-jet mass: Mjj with 2 non-btg jets
3-jet mass: Mbjj (1btg jet, 2 non-btg jets)
MEDIUMAplanarity>0.05Centrality>0.6Sphericity>0.2ΔRbb>1
LOOSENo kinematic cuts
TIGHTAplanarity>0.05Centrality>0.7Sphericity>0.5ΔR bb>1
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ALL HADRONIC: OBSERVED MASS SPECTRUM & secondary vertex tagging
MAIN SYSTEMATICS:25% Bkg Model.15% Jet Energy Scale.18% b-tagging efficiency
σ(tt) = 12.1 ± 4.9 (stat) ± 4.6 (syst) pb
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
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CDF & DØ SUMMARY
CDF COMBINEDσtt=7.3±0.9 pb 15% improvement w.r.t best single σmeasured
WEIGHT
11%
32%
50%
2%
6%
-2%
CDF
BEST SINGLE σMEASURED
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σ ttbar vs √s & MTOP
MTOP σttbar
169.3 7.73 ±0.89 pb
172.0 ± 2.7 7.53 ±0.87 pb
174.7 7.34 ±0.85 pb
CDF
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CONCLUSIONS
• Importance of 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
pblumsyststattt )(4.0)(6.0)(5.03.7 CDF PRELIMINARY (MTOP=175 GeV/c2)
pbtt 9.03.7
•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)
pbPDFscalekinttTHEO )(45.0)(20.0)(42.077.6