Transcript Document 7869148

A measurement of σ(ttbar) & a
search for V+A structure in the
tWb vertex in CDF
Susana Cabrera
IFIC(CSIC-UV)
XXXIV International Meeting on
Fundamental Physics
FROM HERA AND THE TEVATRON TO THE LHC
April 2-7; El Escorial, Madrid,SPAIN
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 1
Top pair production at the Tevatron
Cacciari et al.
JHEP 0404:068 (2004)
Kidonakis & Vogt
PRD 68 114014 (2003)
Very rare process
One top pair each
1010 inelastic collisions
at s = 1.96 TeV
At √s=1.96 TeV:
85% qq
15% gg
σ = 6.7 pb (MTOP=175GeV)
At √s=14 TeV:
10% qq
90% gg
σ = 833 ± 100 pb
XXXIV IMFP 2006, April 6th
High Pt lepton triggers
are very important
Susana Cabrera
Page 2
Top pair decay: signatures at CDF.
1st) Test the QCD calculations
of top pair production:
By measuring top pair rate.
q’,

q,
l+
•t →Wb ~ 100% (Standard Model)
•Main “usable” event topologies
•tt  llbb di-lepton
5% e+
•tt  lqqbb lepton+jets 30% e+
•tt  qqqqbb all hadronic 45%
W
t
+
p
b
p
b
t
W
-
q, l-
q’,

2nd) Test top decay:
Measuring W helicity: look for V+A structure.
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 3
The Top dilepton channel event selection
tt  (W  b)(W b )  (  b)(  b )   e, ,
Two High Pt leptons:
e(Et>20GeV) or µ (Pt>20GeV)
b
q
3 dilepton channels: ee,eµ,µµ
t
e,µ
W

Reject cosmics, conversions,trileptons

Veto Z´s:
W
e,µ
q
t
ee & µµ 76< Mee,µµ< 106 GeV
jetsig  E T /  E T  8  E T
b


  (ET jet  E T / E )
MET>25 GeV:
“L-cut”
E
 T  50 GeV if min ( E
 T , (l , jet))  20o
NJETS>=2
(ET>15 GeV & |η|<2.5)
HT>200 GeV HT  {E T , ´s, jets}CORR
Opposite sign leptons.
XXXIV IMFP 2006, April 6th
Data used here
Central electron and muon triggers
and plug electron plus MET
Susana Cabrera
Page 4
ttbar Acceptance
•1M ttbar PYTHIA 6.1 MC
•MTOP=175GeV
•|zvtx|<60cm (95.5% CDF luminous region)
b
q
t
q
t
e,µ
W


W
e,µ
b
•Araw=0.873 +- 0.012% (ee : µµ: eµ) (22%:25%:53%)
Cl1l2
DT
DT
EidDT1 EedDT2 Erec
E
2
 E z 0  ( Etrg1  Etrg 2  Etrg1 Etrg 2 )  ( MC MC )( MC1 rec
)
MC
Eid1 Eid 2 Erec1 Erec 2
i
ACORR   ARAW
 C1 2
i
•Acorr=0.732 +- 0.012 %
•(ee : µµ: eµ)
• (↓90.1%:↓78.1%:↓83.7%)
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 5
The backgrounds: WW,WZ,Z→ττ

q
e,µ

Z/*
σZττ=333 +- 33 pb

q

e,µ
e,µ
q

W

q
e,µ
W
q
Z
q(')
W
qq´→γ*/Z→ττ +(≥2 p)
l
l
q,l
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
q('),ν
•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
N
Z DATA
 2 jets
Z MC
 2 jets
N
N
PRED
BKG
  C1 2
i
MC ALL CUTS

N
/ MC
BKG

 SFDATA

2 jets
 N MC TOT
BKG

XXXIV IMFP 2006, April 6th
Susana Cabrera

THEO
   BKG
Li

Page 6
Instrumental backgrounds: “Fakes” and DY
q
Origin: manly W(lv)+>=3 jets, others Wbb,Wcc,Wc, QCD
•
•
Jets faking high Pt electrons: η, π0, π+, πTracks faking high Pt muons: Semileptonic B decays, punch-through
•
W
q
Methodology:
•
e,μ

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.
PREDICTED
N FAKE
BKG   FRi ( PT j )  N fakeable
i, j
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
j
DATA
Bkg
N INj  f Zveto
( N Zveto
 N Zveto
)
XXXIV IMFP 2006, April 6th
j
j
N DY
 N OUT
 N INj
Susana Cabrera
q

q
e,µ
e,μ
j
j
out / in
DATA
Bkg
N OUT
 f MET
 RMET
( N MET
 N MET
)
Page 7
CONTROL REGIONS
NJETS=0,1 CONTROL REGIONS
2 high Pt leptons, Z veto, high Met
XXXIV IMFP 2006, April 6th
Susana Cabrera
SIGNAL
REGION:
NJETS>=2
HT>200GeV
OS leptons
Page 8
SUMMARY 750 pb-1
N OBS  N BKG
 tt 
 Ai  Li
i
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 9
σtt in 750 pb-1
 tt  8.3  1.5stat  1.0 syst  0.5lumi pb
 ttTHEO  6.7 pb M.Cacciari,et al. JHEP 404,68 (2004)
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 10
KINEMATICS (I)
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 11
KINEMATICS (II)
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 12
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 13
From Lepton+Jets WG: 695pb-1
SIGNATURE
1 Isolated lepton
PT>20GeV/c
Met>20GeV
>=3 jets with Et>15GeV/c
CDF
BACKGROUNDS
W+jets with HF (gbb, cc)
Mistags
Non W background
>=1 b-tag
HT >200 GeV
>=2 b-tags
2 b tags
33
46
7.2+-1.3 1.9+-0.5
156
158
53.0+-6.3
17.2+-1.9
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 14
CDF Summary XS (750 pb-1)
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 15

t W b
Is it really SM Top ?
SM f V+A=0 f-=30%
non SMf V+A = 1 f-=0
 (1  cos * ) 2
f0=70%
f0=70%
f+=0%
f+=30%
 (1  cos 2   )
p  pb  E  Eb
2 M lb2
cos 
 2
1


2
| p |  | pb |
mtop  M W
*
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 16
Mlb
M 2b 


1
2
M TOP
 M W2 1  cos *
2

SAMPLES:
L+jets single-btagged: 1D template, one pairing, one Mlb value
L+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 17
Binned Log likelihood method
SM: f
Free parameters
Dilepton Templates:
ttbar V+A,V-A
(MC, Alpgen+Pythia)
DY/WW/WZ
MC: (Alpgen+2p)
FAKES
fakeable events in
W+jets data X FR(PT)
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 18
Pseudo-experiments: test robustness
1.
2.
3.
4.
Hypothesis: f V+A= f TRUE [0,1]
Fluctuate N expected with gaussians:
1. N PE,EXP BKGR = σ EXP BKGR L
2. 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-A
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.
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 19
Confidence Level
• 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.
SYSTEMATICS NOT
YET INCLUDED
• 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]
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 20
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
CDF
 tt  8.3  1.5stat  1.0 syst  0.5lumi pb
•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.
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 21
BACKUP SLIDES
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 22
CDF
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)
XXXIV IMFP 2006, April 6th
||=
1. Run II Detector
CDF
||=2.
PT
PT2
 0.15% (GeV / c ) 1
Susana Cabrera
Page 23
11 years after the top quark discovery
●
●
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%
5 orders of
magnitude
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 ?
●
–
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 24
• Delivered: 1.2fb-1
• Accelerator doing
very well:
– Record peak inst.
luminosity:
• 1.79x1032/cm2s
(11/10/2005)
– If no further
improvements:
Where do I get this?
Data used here
• 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
XXXIV
IMFP 2006,coming
April 6th in
Susana Cabrera
Page 25
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 26
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 27
Lepton+Jets Channel: b Tagging
CDF
2 b tags
HT>200GeV
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 28
Top dilepton candidate
Nrun=193031 Nevt=7271850
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 29
From Tevatron to the LHC
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 30
e & μ in
CDF
Central e: ||<1.2
Et>20-25 GeV
EM cluster + Drift chamber
track,Pt>10 GeV
Loose :
e &  at CDF Run II
Tight  :
Plug e: 1.2<||<2.0-2.8
EM cluster (+ Silicon track)
High Pt isolated track
pointing to a gap in the
- coverage ||<1.2
Drift
chamber
pointing to a  -stub
||<1.
 measured with Z 
measured with Zee
Trigger : 88%-95%
Trigger : 100%, Et>30 GeV
ID  : 85%-90%
ID  : >[80-94]%
Track quality
|d0|<0.2,0.02 cm
Ehad/Eem<0.055,Shower profiles,
MIP: Eeom<2 GeV
Track quality,showermax matching
Ehad<6 GeV
•Veto cosmics using timing information and track information.
E
•Veto  from jets (mostly b) using calorimeter-Isolation
•Veto e coming from photon conversions using track information.
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 31
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 32
e & μ in
CDF
Central e: ||<1.2
Et>20 GeV
EM cluster + Drift chamber
track,Pt>10 GeV
Loose :
e &  at CDF Run II
Plug e: 1.2<||<2.5
EM cluster (+ Silicon track)
Drift
chamber
High Pt isolated track
(drift chamber)
pointing to a gap in the
- coverage
Tight  :
pointing to a  -stub
ID:
Track quality
|d0|<0.2,0.02 cm
MIP:
Eeom<2 GeV
Ehad<6 GeV
ID:
Ehad/Eem<0.055,
Shower profiles..
E
•Veto cosmics using timing and track information.
•Veto  from jets (mostly b) using calorimeter isolation.
XXXIV IMFP 2006, April 6th
Susana Cabrera
Page 33
• Cosq* in Lepton+Jets: (230pb-1)
– 2 Template analysis:
• B tagging used
• Topological variables
W Helicity
• Lepton P
T
in dilepton: (370pb-
1)
– c2 fit is used for lepton
matching (purity: 60%)
Combined result:
f+ = 0.04 ± 0.11 (stat) ± 0.06 (syst)
CDF combined (162pb-1):
0.0<f+<0.25 at 95%CL
0.0<f+<0.27 at 95%CL
f0  0.74
XXXIV IMFP 2006, April 6th
Susana Cabrera
0.22
0.34
Page 34