top cross section measurements at the tevatron

DIS 2006, April 20th Susana Cabrera, IFIC(CSIC-UV)

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


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


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)


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:


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)


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


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


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


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


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)


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


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


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


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


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


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


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


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


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


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


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

top cross section measurements at the tevatron

Documents