XXXIV IMFP 2006, April 6th Susana Cabrera Page 1 A measurement of σ(ttbar) & a search for V+A...
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Transcript of XXXIV IMFP 2006, April 6th Susana Cabrera Page 1 A measurement of σ(ttbar) & a search for V+A...
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
XXXIV IMFP 2006, April 6th Susana Cabrera Page 10
σtt in 750 pb-1
pblumisyststattt 5.00.15.13.8
pbTHEOtt 7.6 M.Cacciari,et al. JHEP 404,68 (2004)
XXXIV IMFP 2006, April 6th Susana Cabrera Page 13
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
XXXIV IMFP 2006, April 6th Susana Cabrera Page 16
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
XXXIV IMFP 2006, April 6th Susana Cabrera Page 18
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
XXXIV IMFP 2006, April 6th Susana Cabrera Page 23
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%
XXXIV IMFP 2006, April 6th Susana Cabrera Page 27
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
XXXIV IMFP 2006, April 6th Susana Cabrera Page 28
Lepton+Jets Channel: b TaggingCDF
HT>200GeV
2 b tags
XXXIV IMFP 2006, April 6th Susana Cabrera Page 31
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