Transcript Top Physics at CDF Sandra Leone INFN Pisa

Top Physics at CDF
Sandra Leone
INFN Pisa
Italo-Hellenic School of Physics 2004
The Physics of LHC: Theoretical Tools and Experimental Challenges
Lecce
Sandra Leone
May 20-24, 2004
LHC School Lecce
May , 2004
INFN Pisa
Outline
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Motivations for studying top
A brief hystory
Top production and decay
The Tevatron & CDF
Detector issues
Identification of final states
Cross section measurement
Single top production
Mass determination
Study of top properties
 What’s next?
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Motivations for Studying Top
 Only known fermion with a mass at the natural
electroweak scale.
 Similar mass to tungsten atomic # 74
35 times heavier than b quark.
Why is Top so heavy?
Is top involved in EWSB?
(Does (2  2 GF)-1/2  Mtop mean anything?)
Special role in precision electroweak physics?
Is top, or the third generation, special?
 New physics may appear in production (e.g. topcolor) or
in decay (e.g. Charged Higgs).
 The Fermilab Tevatron has been the only place, and will
be until the LHC turns on, to study the top quark.
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A Brief History of Top
 Top quark was expected in the
Standard Model (SM) of
electroweak interactions as a
partner of b-quark in SU(2)
doublet of weak isospin for the
third family of quarks
(weak isospin of b can be inferred
from the forward-backward
asymmetry in e+e-  bb)
 Anomaly free SM requires the
sum of the family charges to be
zero: given the b (and the tau
lepton) there should be a 2/3
charge quark
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A Brief History of Top
 1977-1994: increasing lower top mass limits
 First top evidence in 1994 in CDF data, 19 pb-1, 15 events
on a background of 6,
 2.8 s excess, not enough to claim discovery
 Confirmed in 1995 by CDF and D0 in first ~70 pb-1 of run
1 data (4.8 s excess).
 Final run 1 top analyses based on ~110 pb-1.
Production cross sections in many channels.
Mass: 174.3  5.1 GeV (CDF/DØ combined).
Study of several aspects of event kinematics.
Limits on single top production, rare/non-SM decays.
 Overall consistency with the standard model.
 But only ~100 analyzable top events
analyses statistics-limited.
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A more cultural perspective of Top…
About 5 orders of magnitude
range in quark masses!
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Production and Decay Basics
At the Tevatron top quarks
are mainly pair produced:
SM predicts: BR( t → Wb) ≈ 100%
b
t
W
t
W
85%
15%
NB: qq, gg fractions
reversed at LHC
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σtheory ≈ 7 pb
Event
topology
determined
by the
decay
modes of
the 2 W’s in
final state
b
b-jet: identify via
secondary vertex
or soft lepton tag
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Top Decay : add. Motivation for Studying Top
 In the SM, assuming V-A coupling with a CKM matrix parameter
|Vtb| = 1 for the
t  bW decay vertex, one gets (LO):
G(t  bW )  175 MeV (MT/MW)2
(MT,MW >> Mb)
 G(t  bW )  1.5 GeV  t(top)  4 x 10-25 s
 Non-perturbative QCD hadronization takes place in a time of order:
L-1QCD  (100 MeV)-1  10-23 s
 top decays before hadronizing, as free quark (no top hadrons,
no toponium spectroscopy)
 the top quark provides the first opportunity to study the decay
characteristics of a “bare” quark.
 t  Ws, t  Wd allowed but suppressed by factors of
 10-3 and  5 x 10-5 respectively
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t-tbar Final States
 Dilepton (ee, μμ, eμ)
 BR = 5%
 2 high-PT leptons + 2 b-jets + large missing-ET
 Lepton (e or μ) + jets
 BR = 30%
 single lepton + 4 jets (2 from b’s) + missing-ET
 All-hadronic
 BR = 44%
 six jets, no missing-ET
 τhad +X
e-e(1/81)
mu-mu (1/81)
tau-tau (1/81)
e -mu (2/81)
e -tau(2/81)
 BR = 23%
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Most favorable
channels for top
physics
mu-tau (2/81)
More challenging
backgrounds, but
measurements
still possible
e+jets (12/81)
mu+jets(12/81)
tau+jets(12/81)
jets (36/81)
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The Tevatron
Run II: s = 1.96 TeV
Started in spring 2001
After a commissioning period,
data “good for physics’’ since
February 2002
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Accelerator Improvements for Run II
 Energy upgrade: 1.8 → 1.96 TeV
30-40% increase in top cross section
 New Main Injector:
Improve p-bar production
 Recycler ring (commissioning):
Accumulate p-bars
 Luminosity upgrades: factor of ~ 3-4 so far
Increased from 6x6 bunches with 3.5ms between bunch
crossing in Run I to 36 p and pbar bunches 396 ns
between bunch crossing
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Tevatron Peak Luminosity
7 x 1031
Typical recent stores: 5 – 6 x1031
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Integrated Luminosity
delivered
~500 pb-1
Start of
physics-quality
data
on tape
~400 pb-1
Taking data
with > 85%
efficiency
Running stably
since Feb. ’02
Analyses use data till 10/2003
Results from first ~200 pb-1 (~ 2 x Run 1) presented here.
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The CDF Run II Detector
– new front-end,DAQ
– trigger
• new L1 track trigger
• new L2 secondary vertex
tracker (SVT)
– new silicon tracker
• 7 layers |h|<2
• 3-D reconstruction
– new central tracker
Inherited from Run I:
• Central Calorimeter
• Muon System (some new)
• Solenoid
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• Nstereo=Naxial=48
 Dpt/pt<0.001pt
– Time of flight
– new forward calorimeter
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CDF Run II Detector
Central+Plug
Calorimetry
|η|  3.6
Muon Chambers
|η|  1.5
10m
Central tracking
|η|  1.0
Silicon tracking
|η|  2.0
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“Physics at a hadron collider…
High pT lepton
High ET jet, photon
High Missing ET (MET)
…Is all about the trigger!’
Examine each pp collision (1.7 MHz)
Select few interesting events (<70 Hz)
Store for further offline analysis
Process
Keep 1 out of 25,000
Cross-section
Event Rate
Inelastic pp
60 mb 6 MHz
pp→bb (b pT>6 GeV, |η|<1)
10 μb 1 kHz
pp→WX→ℓνX
5 nb 0.4 Hz
pp→ZX→ℓℓX
0.5 nb 0.04 Hz
pp→tt→WWbb→ℓνbbX
2 pb 0.0002 Hz
pp→WH→ℓνbb (if MH=120GeV)
15 fb 0.0000015 Hz
Assume L =100x1030 cm-2s-1, ℓ=electron or muon
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Comparison of Cross Sections
 In Run 1, over 5 x 1012 total
collisions, one in every 1010
producing a ttbar event (c.f. one
every 2.5 X 106 producing a W
event)
In Run 1 About 500 ttbar
events produced per experiment
About 50 remaining after
all cuts, per experiment
Run 1 uncertainties
dominated by lack of
statistics
Note: LHC will be a top factory,
producing  2800 ttbar events
per hour at low luminosity (1033)
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Principles for Particle Identification
Beam direction
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Principles for Particle Identification
Electrons: (|h| < 2.8)
Muons: (|h| < 1)
A track in the Central Outer
Chamber
A track in the Central Outer
Chamber
Corresponding energy
deposition in EM calorimeter
Small Had/EM
shower shape consistent
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May , 2004 electron
Corresponding energy
deposition in calorimeter
consistent with MIP
Extrapolated track matching
signal in muon chambersSandra Leone
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Electron identification in W events
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•
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•
•
•
•
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Energetic: transverse energy ET > 20 GeV
Track pointing to EM: pT > 10 GeV/c
and matching EM energy: ET/pT < 2.0 when ET < 50 GeV
Well contained into detector: Track |z0| < 60 cm
Shower profile consistent with electron
Signal in strip chamber embedded in EM at shower max.
Shower contained into EM:
EHAD/EEM < 0.055 + 0.00045 * E
Track-to-shower match  3 cm
Topological W cut the electron is isolated:
Fractional calorimeter energy isolation < 0.1
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Muon identification in W events
• Energetic: Track pT > 20 GeV/c
• Well contained into detector: Track |z0| < 60 cm
• Track match to a muon chamber stub: 3, 5, and 6 cm
•
•
•
for CMU, CMP, and CMX, respectively
Cosmic ray veto
Small energy deposition in calorimeter, consistent with
MIP:
• EEM < 2 + max[0, 0.0115 * (p - 100)] GeV
• EHAD < 6 + max[0, 0.0280 * (p - 100)] GeV
Topological W cut the muon is isolated
Fractional calorimeter energy isolation < 0.1
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Principles for Particle Identification
Neutrino:
Jet:
No interaction seen
in the detector
Energy deposition in
Energy deposition in
em and had calorimeter em calorimeter
Missing Transverse
energy:
Projective tower
No associated track
geometry.
Fixed cone algorithm in
h-f, DR = 0.4
ET = Si ETi  ni
ET = - ET
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Photon:
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Method to identify a top signal
 Start from “counting events” passing cuts in all decay
channels
Optimization of signal region with respect to SM
background processes (control region)
 Background dominates the production of ttbar pairs by
several orders of magnitude
 How to separate signal from background:
Top events have very distinctive signatures
 Decay products (leptons, neutrinos, jets) have large pT’s
 Event topology: central and spherical
 Heavy flavor content: always 2 b jets in the final state!
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Dilepton Channel
Relatively clean:
jet
Top and a small amount of SM bkgds
l
Down side is small event samples
Signature:

Two high pT Isolated leptons
opposite sign

Veto Z, cosmic, conversion

D(ET,l/j)>20o, or ET>50 GeV

ET > 25

Two jets with ET>10 GeV

Total ET > 200 GeV (HT = Scalar
summed ET of jets, leptons, and
ET )
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n
b
p
p
b
jet
n
l
ET
Expect: S/B ~ 9
Dominant backgrounds:
Drell-Yan, W+jets (“fakes”)
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Dilepton Channel: ee, em, mm
HT
13 events on 2.4 ± 0.7 background
1 ee, 3 mm, 9 em
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Jet Multiplicity
ttbar
signal bin
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Dilepton Channel: lepton + track
Signature:
second lepton:
looser requirements for
Jet Multiplicity
 1 lepton+1 isolated track opposite sign
pT>20 GeV
|h| < 1
 Et > 25 GeV
 ≥ 2 central jets (|h| < 2, ET>20)
Sensitive also to t lepton final states
(~20% from t)
19 events on 7.1 ± 1.2 background
11 e-track, 8 m-track
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ttbar
signal bin
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Dilepton background overview
Instrumental backgrounds

Drell-Yan (ee, mm)
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False ET from mismeasured leptons, jets
Fake leptons
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W+jets with jet misidentified as lepton
Use data whenever possible
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Physics backgrounds
Diboson (WW/WZ/ZZ) and Z  tt

Real leptons, ET, jets

Evaluate using MC

Determine bkgd in 0j, 1j bins to give confidence in
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signal bin prediction
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
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Dilepton: DY (ee, mm) background
• Large cross section but no
intrinsic ET
 False Et
 Detector coverage isn’t 4π
 Reconstruction isn’t perfect
 Tails of ET resolution critical
 Simulation doesn’t accurately
model this
 Extra cuts in “Z window”
 Estimate residual contamination:
 Use loose Z data to normalize
(subtract expected non-Z’s)
 Use MC to distribute inside outside Z window, across jet
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bins
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ET > 25
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 Determine lepton fake rate from
jets in j50 sample
 Cross-check fake rates in other
samples with jets
 Apply fake rates to jets in W+jets
data sample
LTRK
Obsv
Pred
DIL
Obsv
Pred
j20
74
70 +/- 14
j20
51
49 +/- 6
j100
384
304 +/- 77
j70
75
65 +/- 9
inc lep
231
189 +/- 37
j100
69
114 +/- 31
ET of observed (predicted)
fake tracks in green (black)
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# observed (predicted) fakes
Dilepton: Fake lepton backgrounds
Jet20
Jet70
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Dilepton:Z -> tt, diboson bkgds
 In both cases:
Real missing energy
Jets from decays or
initial/final state
radiation
 Estimates derived using
PYTHIA, ALPGEN+HERWIG
MC, normalized to
theoretical xsecs
 Correct for underestimation
of extra jets in MC
 Determine jet bin reweighting
factors for Z  tt from
Zee, mm data
 Reweight WW similarly
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Why Measure the ttbar Cross Section?
 Basic engineering number, absolute measurement ( very
difficult!) , starting point for all top physics.
 Requires detailed understanding of backgrounds and
selection efficiencies.
 Test of SM
Latest calculations: NNLO + NNNLL
Departures from QCD prediction could indicate
nonstandard production mechanisms, i.e. production
through decays of SUSY states.
_
s (t t ) 
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N obs - N back
  A   Ldt
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Dilepton: tt acceptance
 Determine from PYTHIA
MC (mt=175 GeV)
 Apply trigger
efficiencies, lepton ID
MC correction factors,
luminosity weights for
different detector
categories
 DIL: (0.62 +/- 0.09)%
 LTRK: (0.88 +/- 0.14)%
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Dilepton:Lepton ID, track efficiencies
 Lepton efficiencies
Determined using second leg of Z’s (see
Sidoti’s talk)
Get efficiency for data and MC, estimate
difference and derive scale factor (0.95 is
typical)
 Lepton track efficiency
Use W sample selected w/o tracking
requirement
Compare track efficiency with efficiency for
W track in top Monte Carlo simulation
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Dilepton:Signal acceptance systematics (I)
Must take into account many effects potentially
contributing to the acceptance uncertainty:
Systematic
Lepton ID efficiency
- Variation of data/MC SF with isolation
Track-lepton efficiency
- iso efficiency difference btw W+2j data and ttbar
MC
Jet Energy Scale
- vary jet corrections by ±1σ, for evts w/≥2 jets
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LTRK(%)
DIL(%)
5
5
6
-
6
5
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Dilepton:Signal acceptance systematics (II)
Signal Systematics Continued
Initial- and Final-state radiation
- ISR: difference from no-ISR sample
- FSR: parton-matching method, different PYTHIA
tune
Parton Distribution Functions (PDF’s)
- default CTEQ5L vs MRST PDF’s, different as
samples
Monte Carlo Generators
- compare acceptance of PYTHIA to HERWIG
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LTRK(%)
DIL(%)
7
2
6
6
5
6
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Dilepton: Systematic Uncertainty on
Background Estimate
Systematic
Lepton (track) efficiency – same as signal
Jet Energy Scale – same procedure on bkgnd acc.
WW, WZ, ZZ estimate
- Compare WW+0p+(njet scaling) to WW+2p
Drell-Yan Estimate
- Absolute scale (data driven), Monte Carlo shape
Fake Estimate
- J20, J50, J70, J100 x-check
- DIL: shape of HT, effect of MET cut for fake rates
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LTRK(%)
DIL(%)
5 (6) 5(-)
10 18-29
20
20
30
51
12
41
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Dilepton:Events expected, observed
Compare number of events observed with total
expected from tt ( s = 6.7 pb) and backgrounds
LTRK
Good agreement in 0j,
1j bins (~all bkgd)
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DIL
s(tt) measured here
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Dilepton:Cross section results
_
s (t t ) 
N obs - N back
  A   Ldt
DIL:
s (tt )  8.4 -32..27 ( stat ) -11..15 ( syst )  0.5(lum ) pb
LTRK:
s (tt )  7.0
2.7
- 2.3
( stat)
1.5
-1.3
( syst )  0.4(lum ) pb
SM: s = 6.7 pb (mt = 175 GeV/c2)
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Dilepton:Cross checks
 Identical analysis techniques reproduce the expected W
and Z cross sections
 Number of like-sign events (fakes-dominated sample)
observed agrees with prediction in all jet bins
 Number of events containing b-quark evidence (DIL: 7,
LTRK: 10) consistent with that expected from top
 Cross section stable over a range of jet, lepton ET
thresholds
 Measure cross section with “tight-tight” subset of LTRK:
s (tt )  8.5-43..55 ( stat ) -11..48 ( syst )  0.5(lum ) pb
Good agreement with DIL, LTRK
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Dilepton: Combining the cross sections
 The two analyses
are complementary
 Combining them
reduces the largest
uncertainty
(statistics)
 Strategy: divide
signal, bkgd into
three disjoint
regions
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Dilepton:Combination technique (I)
•Need to divide into 3 subgroups:
•Acceptance
•Background
• MC used to determine
acceptance overlap
• event by event weighting
•Distribute bkgd according to
expected S/B in 3 regions, vary
for systematic
•We are NOT averaging two
analyses, we are combining three
distinct analyses
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Dilepton:Combination technique (II)
•For three regions, minimize combined c2 or maximize combined likelihood
(i.e. lower green bar until 68.28% is included in area ) -> identical results
•Be conservative with systematics between regions
•Treat as 100% correlated, distribute to give largest total systematic
s (tt )  7.0
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2.4
- 2.1
( stat)
1.6
-1.1
( syst )  0.4(lum ) pb
12% reduction in
stat error w/r/t
LTRK Sandra Leone
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Dilepton Kinematics: lepton + track
RunI: had seen hints of discrepancy in kinematic distributions:
Ht:Scalar summed ET of jets,
leptons, and missing ET
Missing ET
Leptons transverse momentum
With higher statistics in Run II
Data follow SM expected
distribution of top + bkgd
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Dilepton:Flavor distribution
Every lepton in DIL is an electron or muon
channel
Expected
(scaled to 13
total obsv’d)
Observed
ee
3.3+/- 0.5
1
mm
2.8+/- 0.5
3
em
6.8+/- 0.8
9
 Flavor distribution is consistent with expectation
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Dilepton event display
•2 electrons (ET1=73 GeV, PT2=63 GeV)
• Missing ET = 59 GeV
jet
• 2 central jets + 1 forward jet
jet
n
e2
e1
e1
e1
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May , 2004
e1e2
jet
jet
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tt Dilepton decays with thad
•We look for anomalously large or small t
lepton rate in top decay
•One W decays into t , the other into m or e
t decay mode: semileptonic, 1 charged
hadron (~50%) or 3 charged hadrons (~15%)
•Signature: high PT e or m, large ET,  2
jets and a object passing t identification
cuts:
 PTt > 15 GeV/c
Isolated S PT < 1 GeV/c
electron and muon removal
•HT>205 GeV is requested to reduce the
background.
•Use W-> t n to understand t ID
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May , 2004
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tt Dilepton decays with thad
Results:
Sample
Ele + thad
Muon+ thad
0.59 ±0.05 ±0.10
0.47 ±0.04 ±0.07
2
0
Bckground 0.77 ±0.12 ±0.13
ttbar
Obs. Data
0.53 ±0.08 ±0.08
In 193 pb -1 expect 1.03
signal events. Background
dominated by jets faking t’s.
Test for anomalous t contribution in ttbar decays wrt SM.
rt = 1 if SM is correct
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Single lepton channel (Lepton + jets)
Signature:
One
jet
Veto
Z, cosmic, conversion,
dilepton
ET
l
high pT Isolated lepton
ET
n
b
> 20 GeV
3
or more jets with ET>15 GeV
|h| < 2.0
S/B
~ 1/6
p
b
p
jet
l
jet
W + 1 jet
W + 2 jets
W + 3 jets
W+>=4 jets
Secondary vertex
sample (~162 pb 1)
15314
2448
387
107
Soft lepton sample
(~126Lecce
pb-1)
LHC School
11780
1867
295
84
May , 2004
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Single lepton channel: counting events
 Require ≥ 1 b-tagged jets to reduce background
 Motivation of using b-tags:
Reduce the backgrounds, especially W + light flavor jets
events while keeping ttbar signal efficiently
B tag improves S/B from 1/6 3/1
 Count ttbar candidate events
 Predict rates for SM non-top processes in
tagged W+jets, excess in ≥3 jets is top
 Use data as much as possible to determine
background contamination (non-W QCD, fake tags)
 Use MC when necessary (diboson, W+heavy flavor)
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Lepton + jets Channel: HT cut
 HT is the scalar sum of ET of jets, leptons, and ET
 HT is a powerful discriminator for Top signal
 HT > 200 GeV keep 96% of signal and reject 38% of
background
An HT cut increases the
systematics due to Top
mass dependence, Energy
scale and Heavy flavor
fraction, but still small
compared with other syst
(SF, lum)
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Tagging Tools: Vertexing and Soft Muons
B hadrons in top signal events:
are long-lived and massive
Vertex of displaced tracks
55%
0.5%
LHC School Lecce
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may decay semileptonically
Identify low-pt muon from decay
Top Event Tag Efficiency
False Tag Rate (QCD jets)
15%
3.6%
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Double b-tagged
Lep+Trk event at
CDF
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May , 2004
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Vertex Tagging Algorithm
 Take advantage of the long lifetime of B hadrons: t(b)  1 ps
(ct  450 mm)  B hadrons travel Lxy  3mm before decay
 Select good quality tracks with large impact parameter.
 Try to reconstruct a vertex with  2 traks
 first pass searches for at least 3 tracks with loose kin and Pt >2
GeV/c
 second pass looks for 2 tracks vertices with tighter cuts on tracks
quality and Pt> 3GeV/c
 A jet containing a vertex is considered b-tagged if has large (positive)
decay length significance: Lxy/sLxy > 3 (typically sLxy  150 mm)
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Vertex b-Tagging Efficiency
 Use events with back-to-back jets,
non-isolated electron, require away
jet to be tagged (enriched in heavy
flavor)
 The efficiency of b-tagging
determined by the ratio of the
number of double tagged to single
tagged events, in data and MC
 Define a Scale Factor between data
and MC tagging efficiency (usually
DATA <  MC)
SF = 0.81  0.07
 SF < 1 due to # of good vertex tracks
higher in MC
 Check in generic jets the ET
dependence of the b-tagging
efficiency
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Efficiency for tagging at
least one jet in a ttbar
event (L+>=3 jets, including
data-MC scaling):
 = 53
4%
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Vertex Tagging Background
 Most from W+heavy flavor and W+mistags
 In generic jets heavy flavor pairs are produced by both
direct production and gluon splitting. In W+jets by gluon
splitting only.
The fake background is measured with inclusive jet data
using negative decay-length tags
Assume that positive mistag rates are well described
by negative (Lxy <0) tag rates
The fraction of Wbb, Wcc events is determined from
MC and scaled to the observed number of W events in
each jet multiplicity bin
The (smaller) QCD (non-W) background is evaluated
from data: lepton isolation vs Missing ET method
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(standard),
see Sidoti’s talk
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Lecce
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Lepton + jets with 1 vertex tag: HT
HT distribution of W +  3 jets with  1 b vertex tag
with expected ttbar and background contributions
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2nd Part
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May , 2004
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Outline










Motivations for studying top
A brief hystory
Top production and decay
The Tevatron & CDF
Detector issues
Identification of final states
Cross section measurement
Single top production
Mass determination
Study of top properties
 What’s next?
LHC School Lecce
May , 2004
Yesterday
Introductory
Part
Dilepton channel
Single Lepton
channel
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t-tbar Final States
 Dilepton (ee, μμ, eμ)
 BR = 5%
 2 high-PT leptons + 2 b-jets + large missing-ET
 Lepton (e or μ) + jets
 BR = 30%
 single lepton + 4 jets (2 from b’s) + missing-ET
 All-hadronic
 BR = 44%
 six jets, no missing-ET
 τhad +X
e-e(1/81)
mu-mu (1/81)
tau-tau (1/81)
e -mu (2/81)
e -tau(2/81)
 BR = 23%
LHC School Lecce
May , 2004
Most favorable
channels for top
physics
mu-tau (2/81)
More challenging
backgrounds, but
measurements
still possible
e+jets (12/81)
mu+jets(12/81)
tau+jets(12/81)
jets (36/81)
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Important Background issue: “fakes”
 Dilepton channel:
Fake electron: prob < 0.1 % per jet
A jet with track multiplicity = 1 and a energy
deposition consistent with that of an electron (I.e.
hadrons that shower early in the detector)
– Low Hadronic energy
– In the tail of the jet fragmentation function
Fake muon:
A track part of a jet, being mismesured in h,
although has the same f of the jet can appear to
be isolated because of the error in h
– V ery rare, but can happen
LHC School Lecce
May , 2004
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You cannot trust MC to predict detector
effects involving distribution tails
Is our detector/simulation particularly bad?
No! You always have at some level this kind of effects
We are looking for
very rare events
We are afflicted by very
rare backgrounds
What can we do to avoid this?
Optimize our cuts to reduce the bckgnd
contamination, optimize with available data
the simulation, but in the end we’ll have to
wait for data in order to study all the Sandra Leone
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instrumental effects
May , 2004
L +  3jets ( 1 b-tag)
 “Fake tags”:
 The probability to have a negative Lxy is obtained from generic
jet data (~ 0.5% per jet)
 it is parametrized as a function of the jet ET, jet silicon track
multiplicity, h
 it is used to estimate the mistag rate:
 Negative- non-physical tags = positive mistags
 This probability matrix is then applied to W+jets events to
obtain the background estimate in our sample
LHC School Lecce
May , 2004
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Lepton + jets with  1 vertex tag: results
Vertex tag
Mis tag from
Generic jet data
Method II:
based on MC
MET vs Isolation
method: data driven
top signal
region
Theoretical value
SVX = 57 events on
Number of jets per event:
LHC School Lecce
May , 2004
23.4 ± 3.0 bkgnd,
before HT cut
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Summary of secondary vertex counting
LHC School Lecce
May , 2004
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Summary of secondary vertex counting
Require HT > 200 GeV for 3,  4 jets bins only
Events before
tagging
Total
background
Observed
positive tags
LHC School Lecce
May , 2004
W + 1 jet
W+
2 jets
W+
3 jets
W+
 4 jets
15414
2448
179
91
141.8  18.9 66.0  8.9
160
73
13.8  2.0
21
27
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Tagged Jets Properties
The tagged events contain b quarks, as
seen by the decay length Lxy distribution
LHC School Lecce
May , 2004
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Tagged Jets Properties
LHC School Lecce
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The tagged jets are kinematically
consistent with the expectation from
top, as seen by their ET distribution
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Additional cross check:
HT in 1 and 2 jet bins
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May , 2004
Expected ttbar and background
contributions are shown
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Additional cross check: Z + jets
Results from the secondary vertex -tagged Z+jets selection (no
HT requirement). Background is estimated from the positive tag
rate of generic jet sample applied to the Z+jets pre-tag sample
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May , 2004
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L+jets cross section: counting
 High pt lepton plus missing ET
 N(jet) 3
  1 jet SEC(ondary) VTX
 HT>200 GeV
13.82.0 events
48 events
s (tt ) 
N (W   3 jets) - N ( BKG )
A     Ldt

A: ttbar acceptance
: b-jet eff. for ttbar
A   : 3.80 0.03(stat.)  0.45(sys) %
+1.0
(stat.)
(sys.)
s (tt)  5.6 +1.2
- 1.1
- 0.7
LHC School Lecce
May , 2004
pb
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L+jets with  2 vertex tags
Low statistics but low background:
8 events observed, 0.99 ± 0.26 bkgd expected
b-tagging efficiency
and its uncertainty
enters twice here
top signal
region
2 jet bin:
8 events observed,
2.36±0.64 bkg.
1.3 ttbar
LHC School Lecce
May , 2004
Number of jets per event:
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Summary of double tag counting
LHC School Lecce
May , 2004
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Soft Lepton Tagging
 Leptons from semi-leptonic decays of B have a softer pT
spectrum than W/Z leptons and are less isolated
 Soft Muon Tagger based on a “Global c2” to identify lowpt muons:
 “Global c2” combines information from muon matching variables
 There is no calorimetry or isolation requirements
 b-tagging with SLTm:
 Track with dR slt-jet<0.6,
|dZ slt-Zvtx|<5cm & pT>3 GeV/c
are considered by the SLTm tagger
 An event is tagged if at least
one SLTm tag is found
LHC School Lecce
May , 2004
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Lepton + jets: Soft Lepton Tag counting
Alternative way to secondary vertex b-tag (bmnX, bc  mnX)
SLT = 18 events on 12.7 ± 2.5 bkgnd
Predicted by fake rate
matrix made by photon
+ jets sample.
MET vs isolation method
Predicted by data in/out
of Z-mass window.
top signal
region
LHC School Lecce
May , 2004
Number of jets per event:
From MC and theory
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Lepton + Jets: Soft Lepton Tag Results
LHC School Lecce
May , 2004
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Soft Lepton Tag Properties
Comparison of the SLTm pT distribution
in W +  3 jets for SLtagged events
and for expectations from background
and ttbar (scaled to the measured
LHC School Lecce
cross
May
, 2004section of 4.1 pb).
Jet ET distribution in W +  3 jets for
SLtagged events and expectations
from background and ttbar (scaled to
Sandra
the measured cross section
ofLeone
4.1 pb).
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Top cross section summary
√s-Dependence:
 Main data driven systematics
(jet energy scale, ISR, btag) scale
with 1/N :
LHC School Lecce
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 RunII(2fb-1) dstt/stt <10%
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Lepton + jets event display
Vertex display of double
tagged event with an
electron plus 3 jets
LHC School Lecce
May , 2004
Lego plot
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All Hadronic channel
Signature:
• 6 or more jets
• kinematical selection
•  1 vertex tag.
Background estimate based
on data.
Observe 62 tags excess
over 264 background in
signal region.
s tt  7.8  2.5( stat )
LHC School Lecce
May , 2004
4.7
- 2.3
( syst )  7.8
5.3
-3.4
pb
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Top Mass Measurement
Mtop is a precision electroweak parameter that helps
constrain the mass of the Higgs.
LHC School Lecce
May , 2004
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Top Mass Measurement Challenges
Many combinations of leptons and jets:
which one is correct?
l
+
Choose assignment kinematically most
consistent with top
Use all combinations, but weight them
Link observables to parton-level
energies
go through generation-physics
effects-detector effects
Largest uncertainties come from
this difficult relation
Final likelihood fit methods
W
b-jet
n
X
t
Constraints
t
W-
jet
jet
b-jet
Derive mass templates from top MC and fit data to most likely template
Add event kinematic information, possibly including matrix element
LHC School Lecce
May , 2004
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Jet Energy Corrections
 Jets may be mismeasured due to a variety of effects:
 Calorimeter nonlinearities
 Curvature of low-momentum charged particles by the magnetic field
 Reduced cal. response at boundaries between modules and cal.
subsystems
 Contributions from underlying event
 Out-of-cone losses (due to fragmentation or final state radiation)
 Undetected energy carried by muons or neutrinos
 The correction factor depends on jet ET and h and is meant
to reproduce the average jet ET correctly, (not to reduce
the jet fluctuations around this mean)
 A set of corrections was developed for generic jets.
 Absolute corrections (gamma-jet balancing)
 Relative corrections (central-forward calorimeters, dijet balancing)
 A set of top-specific corrections is also applied (to account,
for instance, for the presence of neutrinos in b-jets)
LHC School Lecce
May , 2004
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Lepton +  4 jets with sec. vertex b-tag:
2 mass fitter:
c
template method
Finds top mass that fits event best

Datasets
Data
tt MC
All event info into one number
 12 parton/jet matching assignments
possible, 2 longit. neutrino possible,
use b-tag to reduce permutations
 test for consistency with top using
kinematic constraints
 choose combination with lowest χ2

Wbb MC
Mass
fitter
Templates
Likelihood fit:
 fit resulting mass distribution to MC
background + top signal templates at
different values of Mtop
 Best signal + bkgd templates to fit data
LHC School Lecce

Constraint
on background normalization
May
, 2004
Likelihoo
d
fit
Result
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Top mass: Lepton + 4 jets with SVX tag
28 vertex-tagged events
+7.1(stat.) 6.5(syst.) GeV/c2
Likelihood fit result: mtop = 174.9 -7.7
LHC School Lecce
May , 2004
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Signal Templates
140
150
160
170
180
190
200
210
220
Parametrization:
Build signal probability d.f. as
a function of generated mass.
LHC School Lecce
May , 2004
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Top Mass Uncertainties, L + 4jets
CDF Run II Preliminary
Source
Uncertainty (GeV/c2)
Statistical
+7.1-7.7
Jet energy scale
6.3
Final State Radiation
0.9
Parton Dis. Functions
0.2
We estimate all
Initial State Rad.
0.4
systematics using
Other MC modeling
0.7
large ensembles of
Generator
0.4
pseudoexperiments
Backgrounds
0.8
b-tagging
0.1
in Monte Carlo.
Total systematic
6.5
LHC School Lecce
May , 2004
Dominated by
calorimeter
energy scale
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Uncertainty due to Jet Energy Scale
CDF Run II Preliminary
Source
Uncertainty
(GeV/c2)
Relative to central (h response)
3.0
Central Calorimeter Response (g-jet)
4.6
Abs. Scale (non-linear.,fragm,underl)
2.2
Corrections to partons (out-of-cone)
2.3
Total
6.3
LHC School Lecce
May , 2004
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Top Mass: dilepton channel
6 dilepton cand. in 125.8 pb-1
•system is underconstrained
•scan over n directions, weight
by pT of ttbar
•For each lepton-b pair
assignment:
Calculate best raw mass,
from most probable
combination
•Likelihood fit to signal and
background templates
Likelihood fit result:
LHC School Lecce
May , 2004
2
175.0 -17.4
(stat)

7.9(syst)
GeV/c
16.9
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Top Mass Uncertainties, dilepton
Source
Uncertainty
(GeV/c2)
Jet Energy Scale
7.1
Dominated by
calorimeter
energy scale
Generators+PDF+ISR+FSR 3.5
Background
1.3
Total
7.9
LHC School Lecce
May , 2004
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How to improve JES
 Most improvements of jet energy scale: using W  jj in
tt and Zbb
Run 1 Results
The best sample
to measure the
top mass may
become the double
b-tagged lepton +
jets sample or the
tagged dilepton
sample ( S/B ~10)
M(Z)bb

M(W)  jj
Some improvement from the sample where a jet recoils
against a reconstructed Z (now we use jet + photon)
LHC School Lecce
May , 2004
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
Other methods to measure Mtop:
Dynamical likelihood
Use the leading order ttbar matrix
element, convoluted with transfer
functions, to make an event by event
likelihood distribution as a function of
top mass
 Combines all combinations and events
according to their power to
distinguish the top mass
2 4
2
L (M top )   
Μ F ( z1 , z2 ) f ( pt )w(x, y t ;a )dx
comb nsol Flux
b jets
w jets
i
 Transfer functions: given a jet, return
the probability that it came from partons
of various energies
 Models the shape of the response
curve, not just the mean
 Mapping function: account for
background and for the dependence of
the transfer functions on top mass
 Similar to the recent l+jets analysis from
D0, but a CDF original measurement
 Proposed in 1988 by K. Kondo (J. Phys.
Soc. 56, 4126)
LHC School Lecce
May , 2004
Transfer functions for various ET bins
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Dynamical Likelihood Method Results
Official current CDF Run 2 top mass value
Best precision, Systematics dominated!!!
M
LHC School Leccetop
May , 2004
2
= 177.8 +4.5 -5.0 (stat.) ± 6.2(syst.) GeV/c
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Other methods to measure Mtop:
Multivariate Templates
 Reduce jet systematics (while
increasing stat. error) by
calibrating jet energy scale
event by event with W mass
 Improve signal/background
separation by using other
kinematic variables (sum of four
leading jet PT’s) in addition to
reconstructed top mass
 Estimate the probability to pick
correct combination event-byevent and reweight events.
 Use nonparametric techniques
(kernel density estimation) to
make multivariate templates
 Fit background fraction in data
LHC School Lecce
May , 2004
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Multivariate Templates Method: Results
Fitted background fraction 34 ± 14%
Mtop = 179.6 +6.4 -6.3 (stat.) ± 6.8(syst.) GeV/c2
LHC School Lecce
May , 2004
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Single top physics (EW Production)
The dominant single-top EW
production processes are:
s-channel:W* process
s(W*tb+X) = 0.88 pb
Cross section smaller than top
pair production, but:
– can provide a direct,
independent measurement of
the Wtb vertex (s a |Vtb|2)
–sensitive to new physics:
t-channel: W-gluon fusion process
s(Wgtb+X)= 1.98 pb
LHC School Lecce
May , 2004
• t-channel:anomalous couplings,
FCNC
• s-channel: new charged gauge
bosons
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Single top search
 Single Top Signature
High-pT electron or muon
Missing transverse energy ET
2 jets
t-channel:
– 1 b-jet + 1 light-quark jet + 1 soft b-jet (from
gluon splitting) which is rarely seen
s-channel: 2 b-jets
 Final state is W + 2 jets
 Strategy:
 isolate W+ exactly 2 jets and tag one jet
 likelihood Fit to Q*h (t-channel)
 Q: charge of lepton, h : pseudorapidity of forward jet

likelihood Fit to Ht (combined)
LHC School Lecce
May , 2004
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Single top search
Fit to the data (combined search):
t-channel search:
Using 162 pb-1 of data:
Uncertainty
2fb-1
d| V tb|
26%
28%
14%
st(t-channel)<8.5pb @95% C.L. (th. 2 pb)
ds (tbX)
st(combined)<13.7pb @95%C.L.(th. 2.9 pb) dG (tWb)
LHC School Lecce
May , 2004
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Measurement of Top Quark Production
and Decay Properties
 Once a top signal has
been re-established
the natural path will
be to measure its
properties in order to
confirm SM or find
deviations from it
 Is top quark
adequately described
by the Standard
Model?
LHC School Lecce
May , 2004
Top spin polarization
Production Cross Section
Resonance production ?
Production kinematics
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Test for new physics in tt production:
Search for X -> ttbar
 Studies of the ttbar invariant mass spectrum provide a
general search for heavy objects decaying to top pairs
 Are dynamical ttbar condensates in models such as
Topcolor responsible for EWSB? We can look for the
predicted resonances in the Mttbar spectrum. Topcolor is
needed in technicolor theories to explain the large t – b
mass difference.
 The particle manifestation of Topcolor are:
 the topgluon, gT: a vector boson that preferentially
couples to the third generation
the topcolor Z’: a neutral gauge boson resulting from
the additional U(1) symmetry needed to keep the large
mass split Hill, Phis Lett B 345, 483 (1995)
LHC School Lecce
May , 2004
Hill,Parke, Phys. Rev. D 49, 4454 (1994)
Harris et al, FNAL-FN-687 (1999)
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Test for new physics in tt production
ttbar invariant mass spectrum can be used to set limits on X -> ttbar
Model independent search for a narrow resonance Xttbar
excludes a narrow, leptophobic X boson with mX < 560 GeV/c2 (CDF)
LHC School Lecce
May , 2004
N.B. LHC will be relatively insensitive to new colour
singlet gauge bosons (such as Z’) because gluon
fusion will be the dominant production mechanism
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W Helicity Measurement in top decay
 Top decays before it can hadronize, because
width Γt ~ 1.4 GeV > ΛQCD.
Decay products preserve information about the
underlying Lagrangian.
Unique opportunity to study the weak interactions of
a bare quark, with a mass at the natural electroweak
scale!
 SM Prediction:
W helicity in top decays is fixed by Mtop, MW, and V-A
structure of the tWb vertex.
W helicity reflected in kinematics W decay products
LHC School Lecce
May , 2004
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W Helicity Measurement, contd.
The angular dependence of the semileptonic decay in the
W rest frame is given by
3
3
3
w(cos  l - b )  F-  (1 - cos  l -b ) 2  F0  (1 - cos 2  l -b )  F  (1  cos  l -b ) 2
8
8
8
left
long.
SM (V-A) predictions (for mb=0):
2
1
F- 
 0.3 F0 
 0.7
1  2
1  2
where = MW2/Mtop2
LHC School Lecce
May , 2004
[V+A: 70% long., 30% r.h.]
right
F  0
parameter to measure
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Helicity Measurement: dilepton channel
Helicity affects lepton PT in lab frame
F0 < 0.52 @ 95% CL
LHC School Lecce
May , 2004
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Helicity measurement: L + jets channel
F0 = 0.88+0.12-0.47 (stat. + syst.)
F0 > 0.24 @ 95% CL
LHC School Lecce
May , 2004
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Helicity measurement: combined
Combined L+jets and
dilepton channels:
F0 = 0.27+0.35-0.21 (stat. + syst.)
F0 < 0.88 @ 95% CL
LHC School Lecce
May , 2004
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Top Branching fractions I
 Dilepton and lepton + jets cross-section values
assume t→Wb always, never t→Xb.
 Ratio of measured s’s (Rs=sℓℓ/sℓ) tests that
assumption:
Should be unity
Ratios are good:
Many systematic uncertainties cancel
Independent of theory value of s (PDF’s, mt)
Sensitivity to non-SM decays of top, e.g. t →
H+b, different tan  gives different mix of ℓℓ/ℓj
LHC School Lecce
May , 2004
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s ratio, non-SM limits
 Create probability
distribution for Rs:
Rs = 1.45+0.83-0.55,
0.46<Rs<4.45 @ 95%CL
 Set limits on non-SM
BR’s of top assuming
“simple” model: detect
non-SM decays with
same efficiency as SM:
BR < 0.46 for additional
t→Xb all-hadronic decay
LHC School Lecce
May , 2004
Preliminary
w/o cancellation
w/cancelllation
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Top Branching fractions II
 Cross section measurements assume t→Wb
always, never t→Wq.
 b-tag rates in tt events test that assumption,
depend on B(t→Wb) = b, tag efficiency = :
 Measure from the ratio of tag rates the product b:
assume  and measure b, or vice-versa.
LHC School Lecce
May , 2004
Sandra Leone
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Measuring b, 
 Find most likely b 
in SVX-tagged
sample:
b  = 0.25+0.22-0.18,
→ b = 0.54+0.49-0.39,
 consistent with
measurements in
calibration samples
 b > 0.12 @ 95% CL
LHC School Lecce
May , 2004
Sandra Leone
INFN Pisa
Search for rare decays
 Look for FCNC decays t  gq and t  Zq (q=c,u)
(BR  10-10 – 10-12)
 Any sign of such decays would indicate new physics
 Run 1 Result: assume that one of the 2 top quarks
decays according to SM t -> Wb
 t  gq: look for lepton+g+2jets+Et and g+4jets
1 event seen, W+jets background  1
BR (t  gq ) < 3.2 % (95%CL)
 t  Zq: look for ℓ+ℓ- + 4jets and 3 leptons +2jets
1 event seen, expected background  1
BR (t  Zq) < 33% (95%CL)
LHC School Lecce
May , 2004
Sandra Leone
INFN Pisa
Summary of top measurements and
future expectations
Top quark
Property
Mass
(CDF+D0)
stt
W helicity
|Vtb|
(run 1)
Run 1
Precision
Exp. Run 2 LHC
2.4 %
1.2%
1%
25%
10%
5%
0.4
0.96+0.16-0.12 (3gen) 15%
0.09
2.5%
0.01
1.5%
-
26%
14%
5%
2%
2x10-3
0.02
2x10-5
2x10-4
Current best
measurement
178.0 ± 4.3
(run 1)
6.5 +1.7-1.4
0.91 ± 0.37 ± 0.13
>0.78 at 95% CL
s(single top) < 13.7 pb at 95% CL
|Vtb|
BR(t->gq)
BR(t->Zq)
LHC School Lecce
May , 2004
<0.03
<0.30
0.03
0.30
Sandra Leone
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Conclusions and Outlook
 The top quark is back!
 First Run II measurements of
cross section & mass are
available and will improve
rapidly.
 Other analyses (W helicity,
single top…) are making
excellent progress.
 It is the start of a program of
precision top physics—and
hopefully top surprises—at the
Tevatron.
 We still expect 50x more data
compared to Run I in hand
before LHC starts!
LHC School Lecce
May , 2004
DØ / CDF
Run 2a
Goal
Run II with first 2 fb-1 will
provide a chance to:
Measure
Mw to < ±40 MeV/c2
Measure
Mtop to <Sandra
±3 Leone
GeV/c2
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The Road Ahead






Search for top  H+
Precise measurement of Mtop
Single top production, measure Vtb
ttbar resonant production, strong EWSB
Searches for rare decays
Is top *the connection* to new physics?
 you are invited to join CDF to
experience the real data before LHC
turns on!
LHC School Lecce
May , 2004
Sandra Leone
INFN Pisa