XXXIV IMFP 2006, April 6th Susana Cabrera Page 1

A measurement of σ(ttbar) & a search for V+A structure in the

tWb vertex in CDF

Susana CabreraIFIC(CSIC-UV)

XXXIV International Meeting on Fundamental Physics

FROM HERA AND THE TEVATRON TO THE LHCApril 2-7; El Escorial, Madrid,SPAIN

XXXIV IMFP 2006, April 6th Susana Cabrera Page 2

Top pair production at the Tevatron

At √s=1.96 TeV:

85% qq 15% gg

At √s=14 TeV: 10% qq90% gg

σ = 833 ± 100 pb

Cacciari et al. JHEP 0404:068 (2004)Kidonakis & Vogt PRD 68 114014 (2003)

σ = 6.7 pb (MTOP=175GeV)

Very rare process One top pair each 1010 inelastic collisions at s = 1.96 TeV

High Pt lepton triggers are very important

XXXIV IMFP 2006, April 6th Susana Cabrera Page 3

Top pair decay: signatures at CDF.•t →Wb ~ 100% (Standard Model)•Main “usable” event topologies•tt llbb di-lepton 5% e+•tt lqqbb lepton+jets 30% e+•tt qqqqbb all hadronic 45%

q, l-

q’,

tt

p p

b

W+

W-

b

q, l+

q’,

tt

1st) Test the QCD calculationsof top pair production: By measuring top pair rate.

2nd) Test top decay: Measuring W helicity: look for V+A structure.

XXXIV IMFP 2006, April 6th Susana Cabrera Page 4

The Top dilepton channel event selection

q

q

W

W

e,µ

e,µ

b

b

t

t

Data used here

Central electron and muon triggersand plug electron plus MET

Two High Pt leptons: e(Et>20GeV) or µ (Pt>20GeV)3 dilepton channels: ee,eµ,µµReject cosmics, conversions,trileptons

Veto Z´s: ee & µµ 76< Mee,µµ< 106 GeV

MET>25 GeV: “L-cut”

NJETS>=2 (ET>15 GeV & |η|<2.5)

HT>200 GeV Opposite sign leptons.

CORRTT jetssEH },´,{

)/(8/ EEjetEEjetsig TTEET TT

oTT jetlEifGeVE 20)),(,(50 min

,,))(())(( ebbbWbWtt

XXXIV IMFP 2006, April 6th Susana Cabrera Page 5

ttbar Acceptance

))(()(2

2

1

1

2

2

1

12121021 MC

rec

DTrec

MCrec

DTrec

MCid

DTed

MCid

DTid

trgtrgtrgtrgzll E

E

E

E

E

E

E

EEEEEEC

q

q

W

W

e,µ

e,µ

b

b

t

t

•1M ttbar PYTHIA 6.1 MC•MTOP=175GeV•|zvtx|<60cm (95.5% CDF luminous region)

•Araw=0.873 +- 0.012% (ee : µµ: eµ) (22%:25%:53%)

•Acorr=0.732 +- 0.012 %

•(ee : µµ: eµ)

• (↓90.1%:↓78.1%:↓83.7%)

i

iRAWCORR CAA

21

XXXIV IMFP 2006, April 6th Susana Cabrera Page 6

The backgrounds: WW,WZ,Z→ττ

q

q

W

W

e,µ

e,µ

q

q

Z/*

e,µ

e,µ

Z

W

q

q(')

l

l

q,l

q('),ν

qq´→γ*/Z→ττ +(≥2 p)

σZττ=333 +- 33 pb

qq´→WW→lνlν+(≥2p)

σWW=12.4 +- 0.8 pb

qq´→WZ→(qq´)(ll´) qq´→WZ→(lν)(l´l´´)

σwz=3.65 +- 0.26 pb

ii

THEOBKGTOTMC

BKG

CUTSALLMCBKGMCDATA

jetsPREDBKG L

N

NSFCN /

221

MCZjets

DATAZjets

N

N

2

2

•Almost 100% MC driven: •Pythia (LO) for raw acceptances •Latest NLO predictions•MC shortcomings: jet multiplicity from extra QCD radiation

•NJET scale factors measured from Z boson data

XXXIV IMFP 2006, April 6th Susana Cabrera Page 7

Instrumental backgrounds: “Fakes” and DYOrigin: manly W(lv)+>=3 jets, others Wbb,Wcc,Wc, QCD

• Jets faking high Pt electrons: η, π0, π+, π-

• Tracks faking high Pt muons: Semileptonic B decays, punch-through

Methodology:• Fake lepton rates measured in high statistics jet triggered data Et (20-100) GeV

• FR(Pt)= Nl/Nd : l: electron or muon, d: fakeable object:• Jet with Ehad/Eem<0.125• High Pt isolated track with E/P<1

• Applied to “fakeable” events l1+d2+MET+>=2 jets in W+jets data.

q

qW

e,μ

q

q

e,µ

e,μ

Drell-Yan qq´→γ*/Z→ee,µµ +(≥2 p) Methodology: Use DATA in the high MET region inside the Z peak (76 <Mll<106) Use DY Pythia MC

•Correct non-DY sources of dileptons •To predict the number of events outside the Z mass window•To distribute them among different jet multiplicity bins: 0,1,>=2

ji

fakeableTiPREDICTED

BKGFAKE NPFRNj

,

)(

jIN

jOUT

jDY NNN

)( BkgZveto

DATAZveto

jZveto

jIN NNfN )(/ Bkg

METDATAMET

inoutMET

jMET

jOUT NNRfN

XXXIV IMFP 2006, April 6th Susana Cabrera Page 8

CONTROL REGIONS

NJETS=0,1 CONTROL REGIONS2 high Pt leptons, Z veto, high Met

SIGNAL REGION:NJETS>=2HT>200GeVOS leptons

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SUMMARY 750 pb-1

iii

BKGOBStt LA

NN

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σtt in 750 pb-1

pblumisyststattt 5.00.15.13.8

pbTHEOtt 7.6 M.Cacciari,et al. JHEP 404,68 (2004)

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KINEMATICS (I)

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KINEMATICS (II)

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Top dilepton candidate with one b-tag•Nrun=193051, Nevt=1108405•1st e: Pt=114.1 GeV Eta=-0.4•2nd e: Pt=60.3 GeV Eta=0.4•1st jet: Et=40.3 GeV Eta=0.0•2nd jet: Et=40.0 GeV Eta=1.1•MET Et=55.0 GeV Eta=1.1

XXXIV IMFP 2006, April 6th Susana Cabrera Page 14

From Lepton+Jets WG: 695pb-1 CDF

2 b tags

SIGNATURE1 Isolated lepton PT>20GeV/cMet>20GeV >=3 jets with Et>15GeV/c

>=1 b-tag HT >200 GeV

>=2 b-tags

156 158

53.0+-6.3 17.2+-1.9

33 467.2+-1.3 1.9+-0.5

BACKGROUNDSW+jets with HF (gbb, cc) Mistags Non W background

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CDF Summary XS (750 pb-1)

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Is it really SM Top ?

t W b

SM f V+A=0 f-=30% f0=70% f+=0%non SMf V+A = 1 f-=0 f0=70% f+=30%

12

||||cos

22

2*

Wtop

lb

b

bb

Mm

M

pp

EEpp

2*)cos1( )cos1( 2

XXXIV IMFP 2006, April 6th Susana Cabrera Page 17

Mlb

*222 cos12

1 WTOPb MMM

SAMPLES:L+jets single-btagged: 1D template, one pairing, one Mlb valueL+jets double-btagged: 2D template: 2 lepton-bjet pairing

2 entries per event: Mlb(1) vs Mlb(2) Dileptons Idem L+jets

Hipothesis: leading jets are b-jets

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Binned Log likelihood method

SM: f

Free parameters

Dilepton Templates:ttbar V+A,V-A(MC, Alpgen+Pythia)DY/WW/WZMC: (Alpgen+2p)FAKESfakeable events in W+jets data X FR(PT)

XXXIV IMFP 2006, April 6th Susana Cabrera Page 19

Pseudo-experiments: test robustness1. Hypothesis: f V+A= f TRUE [0,1]2. Fluctuate N expected with gaussians:

1. N PE,EXP BKGR = σ EXP BKGR L2. N PE,EXP V+A= ε V+A σ TTBAR f TRUE

3. N PE,EXP V+A= ε V+A σ TTBAR f TRUE

4. Compute expected fractions of BKGR, and signal V+A,V-A3. Fill ramdomly BKGR,V+A,V-A pseudo-templates with NPE,BKGR, N PE,V+A, NPE,V-A

PE events from BKGR,V+A,V-A templates.4. Perform fit: input f TRUE free parameters f V+A σttbar σBKGR, output f V+A FITTED

XXXIV IMFP 2006, April 6th Susana Cabrera Page 20

• 1000 PE´s with a fixed true value of fV+A

• For each pseudo-experiment:

• Get 95% CL by ‘scanning’ the likelihood function along the fV+A axis.

• After all 1000 PE’s at different f V+A true:

• a distribution of 95% CL’s.

• 95% CL contours: 1K PE scanning f TRUE in [0,1]

Confidence Level

SYSTEMATICS NOT YET INCLUDED

XXXIV IMFP 2006, April 6th Susana Cabrera Page 21

CONCLUSIONS & PROSPECTS

•11 years after the discovery and still elucidating..•We have measured the ttbar production cross section in the dilepton channel with 750 pb-1

•This result is combined with other results from l+jets channel achieving a precision of 12%, close to the theoretical error of the current QCD calculations of the top pair production.

•A technique to search for V+A top decay vertex in both dilepton and lepton plus jets top samples has been extensively tested using pseudoexperiment.

•Prospects:•Including systematics in the 95% CL.•Then we will open the box and measured fV+A with CDF data.

pblumisyststattt 5.00.15.13.8 CDF

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BACKUP SLIDES

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CDF Run II Detector

From Run I:•Solenoid•Central muon system •Central calorimeter

New For Run II:•Front-end DAQ•Trigger:Track (L1) and Displaced Track (L2)•Silicon Tracker (8 Layers) ( 2.0)•Central Outer Tracker ( 1.0)•Plug Calorimeters (1.0 3.6)•Extended Muon Coverage ( 1.5, gaps filled in)

CDF

||= 1.

||=2.

12 )/(%15.0 cGeV

PP

T

T

XXXIV IMFP 2006, April 6th Susana Cabrera Page 24

● 1995: Top quark discovered by CDF and D0

– Not a surprise: SM quark sector completed– But top mass was surprisingly high.

● 2006 (still elucidating!):

– Single top remains undiscovered.– Precision measurements:

● Pair production cross-sections: – Now CDF 12%

● Mtop (Moriond 2006)= 172.5 ± 2.3 GeV – Top properties (besides the top mass) studies

with almost 1 fb-1 . ● Is it really a Standard Model Top ?

11 years after the top quark discovery

5 orders of magnitude

XXXIV IMFP 2006, April 6th Susana Cabrera Page 25

• Delivered: 1.2fb-1

• Accelerator doing very well:– Record peak inst.

luminosity:• 1.79x1032/cm2s

(11/10/2005)

– If no further improvements:

• 4fb-1 in 2009

– Electron cooling on track• Could get 8fb-1 in 2009!

• Detectors doing well:– Upgrades finishing up to

deal with luminosity increase coming in 2006

• D0 installing silicon now

Where do I get this?

Data used here

XXXIV IMFP 2006, April 6th Susana Cabrera Page 26

Acceptance versus Top Mass

Raw acceptance changes by +0.055% per

Δ(MTOP) = 1 GeV or Δ(Acc) = -0.016%

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XS versus TOP MASS

At a top quark mass of 172.0 +- 2.7 GeV/c2 (CDF 750 pb-1, March 2006) The combined cross section is 7.53+-0.87 pb

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Lepton+Jets Channel: b TaggingCDF

HT>200GeV

2 b tags

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Nrun=193031 Nevt=7271850

Top dilepton candidate

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From Tevatron to the LHC

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e & at CDF Run II

Central e: ||<1.2

Et>20-25 GeV

EM cluster + Drift chamber track,Pt>10 GeV

Plug e: 1.2<||<2.0-2.8

EM cluster (+ Silicon track)

measured with Zee

Trigger : 100%, Et>30 GeV

ID : >[80-94]%

Drift chamber

Loose :

High Pt isolated track pointing to a gap in the - coverage ||<1.2

Tight :

pointing to a -stub ||<1.

measured with Z

Trigger : 88%-95%

ID : 85%-90%

•Veto cosmics using timing information and track information.

•Veto from jets (mostly b) using calorimeter-Isolation

•Veto e coming from photon conversions using track information.

e & μ in CDF

EEhad/Eem<0.055,Shower profiles, Track quality,showermax matching

Track quality|d0|<0.2,0.02 cmMIP: Eeom<2 GeVEhad<6 GeV

XXXIV IMFP 2006, April 6th Susana Cabrera Page 32

THE OUTLINE

• Top pair production.•The Dilepton channel.

•Event selection.•Signal acceptance.•Background estimates.•Main Systematics & XS measurement.

•The Lepton plus jets channel results•Latest XS results from CDF.

• Top properties.•Measurement of f V+A in the tWb vertex.

•Mlb observable. •Likelyhood technique. •Pseudoexperiments and expected limit.

• Conclusions & Prospects.

XXXIV IMFP 2006, April 6th Susana Cabrera Page 33

e & at CDF Run II

Central e: ||<1.2

Et>20 GeV

EM cluster + Drift chamber track,Pt>10 GeV

Plug e: 1.2<||<2.5

EM cluster (+ Silicon track)

Drift chamber

Loose :

High Pt isolated track

(drift chamber)

pointing to a gap in the - coverage

Tight :

pointing to a -stub

•Veto cosmics using timing and track information.

•Veto from jets (mostly b) using calorimeter isolation.

e & μ in CDF

E

ID:Ehad/Eem<0.055,Shower profiles..

ID:Track quality|d0|<0.2,0.02 cmMIP: Eeom<2 GeVEhad<6 GeV

XXXIV IMFP 2006, April 6th Susana Cabrera Page 34

W Helicity• Cos* in Lepton+Jets: (230pb-1)

– 2 Template analysis:• B tagging used

• Topological variables

– 2 fit is used for lepton matching (purity: 60%)

• Lepton PT in dilepton: (370pb-1)

Combined result:f+ = 0.04 ± 0.11 (stat) ± 0.06 (syst)0.0<f+<0.25 at 95%CL CDF combined (162pb-1):

0.0<f+<0.27 at 95%CL

f0 0.74 0.340.22