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Page 1Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Measurement of the Forward-Backward
Asymmetry in Top Quark Pair Production
in pp Collisions at 1.96 TeV
Glenn Strycker
University of Michigan
On behalf of the CDF Collaboration
Page 2Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Measurement of the Forward-Backward
Asymmetry in Top Quark Pair Production
in pp Collisions at 1.96 TeV
Glenn Strycker
University of Michigan
On behalf of the CDF Collaboration
1999-2003 2003-2010
Page 3Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Outline
 People Involved
 What is Afb – why study it?
 Theoretical Predictions
 Previous Measurements
 FNAL and CDF Overview
 Measurement Techniques
 Correction Procedure
 Conclusion
Page 4Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
People Involved
University of Michigan:
University of California, Davis:
Also much work done by the Karlsruhe group:
J. Wagner, T. Chwalek, W. Wagner
Glenn Strycker Dan Amidei Monica Tecchio
Tom Schwarz Robin Erbacher
Page 5Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
The Top Forward-Backward Production Asymmetry
has already generated a lot of interest…
Asymmetries in tt Production
Page 6Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetries in tt Production
Page 7Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetries in tt Production
Page 8Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetries in tt Production
Page 9Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Simple Asymmetries in the tt System
t
p p
t
(cos 0) (cos 0)
(cos 0) (cos 0)
t t
t t
N N
fb N NA
 
 
  
  
Forward-Backward Asymmetry:
(cos 0) (cos 0)
(cos 0) (cos 0)
t t
t t
N N
C N NA
 
 
  
  
Charge Asymmetry:
(cos 0) (cos 0)tt
N N    C fbA A
Assuming CP parity holds,
 
Page 10Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
t
θ
 Use θ, Cos(θ), rapidity (y), Delta y, etc. to measure Afb
 Which variable do we use?
 Which reference frame should we use?
 Possible to do the more complicated measurement
cos
fbdA
d θ
p
t
Asymmetries in tt Production
Page 11Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
t
 Use θ, Cos(θ), rapidity (y), Delta y, etc. to measure Afb
 Which variable do we use?
 Which reference frame should we use?
 Possible to do the more complicated measurement
cos
fbdA
d θ
p
t
Asymmetries in tt Production
1
ln
2
L
L
E p
E p
 
  
 y
Page 12Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
 In lowest order QCD, top quark production is symmetric
 NLO QCD predicts a small asymmetry
 Asymmetries are important tools for understanding weak
interactions, as well as for searching for additional (chiral) couplings
t
p p
t
Asymmetries in tt Production
Page 13Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
+
+
tt
ttj
Ctt = -1 Ctt = +1
Ctt = -1 Ctt = +1
Halzen, Hoyer, Kim; Kuhn, Rodrigo
Dittmaier, Uwer, Weinzier; Almeida, Sterman, Vogelsang
Afb ~ +10-12%
Afb (NLO)
Afb (NNLO)
Consensus working value Afb ~ 5 ± 1.5%
~ -7%
~ -1%
Page 14Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
+
+
tt
ttj
Ctt = -1 Ctt = +1
Ctt = -1 Ctt = +1
Halzen, Hoyer, Kim; Kuhn, Rodrigo
Dittmaier, Uwer, Weinzier; Almeida, Sterman, Vogelsang
Afb ~ +10-12%
Afb (NLO)
Afb (NNLO)
~ -7%
~ -1%
Note that ggtt results in no asymmetry, so our
measurement will be diluted by 15% before any other effects
Consensus working value Afb ~ 5 ± 1.5%
Page 15Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Page 16Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
c = βcos(θ)
Page 17Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Vector coupling
mG = mass of axigluon
ΓG = width of axigluon
Axial coupling
Page 18Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
gluon exchange
gluon propagator
Page 19Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Interference Term
Page 20Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Note that there is a
symmetric part and
an asymmetric part
Page 21Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Pole term for axigluon
Page 22Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Again, note that there
is a symmetric part and
an asymmetric part
Page 23Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Since we observe no pole in
Mtt, this constrains the possible
values for gA and gV that give a
positive asymmetry
Page 24Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Lagrangian for Axigluon+Gluon
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Allows us to restrict regions
for coupling parameters that
won’t change tt cross section
Page 25Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Recent Measurement History
Previously Blessed Results (1.9 fb-1):
Afb cos(θ) method: Afb = 0.17 ± 0.08 (Davis/Michigan)
Afb ∆y method: Afb = 0.24 ± 0.14 (Karlsruhe)
Phys. Rev. Lett. 101, 202001 (2008)
D-zero Results (0.9 fb-1):
Afb = 0.12 ± 0.08 ± 0.01
Page 26Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Fermilab and CDF
Fermilab and CDF...
Page 27Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Fermilab
 Tevatron
~4 miles around
 1.96 TeV Collisions
 Over 6 fb-1 data
integrated luminosity
 We're still the most
energetic and highest
luminosity hadron
collider on Earth!
Page 28Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Fermilab
 Tevatron
~4 miles around
 1.96 TeV Collisions
 Over 6 fb-1 data
integrated luminosity
 We're still the most
energetic and highest
luminosity hadron
collider on Earth!
Page 29Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Fermilab
Page 30Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Event Reconstruction
Page 31Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Event Reconstruction
Page 32Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Event Reconstruction
Page 33Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Event Reconstruction
Page 34Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Event Reconstruction
Reconstruction
 Match jets to b, b, up, down
 Constrain fit W mass to 80.4 GeV/c2
 Constrain fit for a 175.0 GeV/c2 Top
 Float momenta of jets within known
resolution
 Use combination with lowest χ2
?
?
2
2
2
2
2
2
2
2
2
2,,
2
2,,
)()()()(
,
)(
,
)(2
top
topbl
top
topbjj
w
wl
w
wjj
j
fitUE
j
measUE
j
i
fiti
t
measi
t
MMMMMMMM
yxj
pp
jetslep
pp











 



Page 35Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Top Pair Production and Decay
Top signals
isolated using
secondary vertex
= “b-tag”
Lepton + Jets mode:
qq → g → tt → (W+b)(W-b) → (l+νb)(qqb) → l+ + Et + 4j + ≥ 1 “tagged” jet
Vertex for b-quarks
is roughly ½ mm
Charm jets are only
tagged ~1/10 the time
Page 36Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Selection
 Measurement is performed using
semi-leptonic top pair decays
 3.2 fb-1
of CDF data
 ≥ 4 jets (Et > 20 GeV and |η| < 2)
 ≥ 1 b-tagged jet
 1 electron (Et > 20 GeV and |η| < 1) –or–
1 muon (Pt > 20 GeV/c and |η| <1)
 Missing Transverse Energy > 20 GeV
776 Candidate Events
Data Sample
Page 37Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Selection
 Measurement is performed using
semi-leptonic top pair decays
 3.2 fb-1
of CDF data
 ≥ 4 jets (Et > 20 GeV and |η| < 2)
 ≥ 1 b-tagged jet
 1 electron (Et > 20 GeV and |η| < 1) –or–
1 muon (Pt > 20 GeV/c and |η| <1)
 Missing Transverse Energy > 20 GeV
Data Sample
776 Candidate Events
Page 38Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
 Hadronic top decay is more accurately
reconstructed, so we will measure yhad
 Get hadronic top charge from lepton
 Invoke CP invariance and multiply
hadronic-only distribution by -1 * lepton
charge to find equivalent top rapidity in
each event
 Measure Afb using -Qlep• yhad in the lab
frame, count forward and backward
events
 Correct Afb back to parton level
backward forward
→ W–
b → l ν b
→ W+
b → j j b
t t
Measurement Techniques
Page 39Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Selection
 Measurement is performed using
semi-leptonic top pair decays
 3.2 fb-1
of CDF data
 ≥ 4 jets (Et > 20 GeV and |η| < 2)
 ≥ 1 b-tagged jet
 1 electron (Et > 20 GeV and |η| < 1) –or–
1 muon (Pt > 20 GeV/c and |η| <1)
 Missing Transverse Energy > 20 GeV
Backgrounds
 Use W+Jets tagged backgrounds to model
the ttbar tagged backgrounds present in the
semi-leptonic sample
Data Sample
167 ± 34 Background Events
Page 40Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Data Sample
Say number of events
and number of events in
backgrounds.
Process
W+HF Jets 86.56 ± 27.40
Mistags (W+LF) 27.43 ± 7.70
Non-W (QCD) 33.44 ± 28.06
Single Top 7.82 ± 0.50
WW/WZ/ZZ 7.57 ± 0.74
Z+Jets 4.78 ± 0.59
Top 569.08 ± 78.81
Total Prediction 736.64 ± 89.22
776 Candidate Events
167 ± 34 Predicted Background
Selection
 Measurement is performed using
semi-leptonic top pair decays
 3.2 fb-1
of CDF data
 ≥ 4 jets (Et > 20 GeV and |η| < 2)
 ≥ 1 b-tagged jet
 1 electron (Et > 20 GeV and |η| < 1) –or–
1 muon (Pt > 20 GeV/c and |η| <1)
 Missing Transverse Energy > 20 GeV
Backgrounds
 Use W+Jets tagged backgrounds to model
the ttbar tagged backgrounds present in the
semi-leptonic sample
Page 41Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Raw Asymmetry
Signal MC is MC@NLO, normalized so signal+background = number of data events
Page 42Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Raw Asymmetry
Signal MC is MC@NLO, normalized so signal+background = number of data events
We wish to correct
for this shape
Page 43Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Background Subtraction
 Background has a known asymmetry that modifies the top Afb measurement
 Check anti-tagged data and MC for consistency and cross-section
 Subtract backgrounds from our data
Backgrounds in W+Jets
Page 44Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Background Subtraction
 Background has a known asymmetry that modifies the top Afb measurement
 Check anti-tagged data and MC for consistency and cross-section
 Subtract backgrounds from our data
Page 45Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Background Subtraction
 Background has a known asymmetry that modifies the top Afb measurement
 Check anti-tagged data and MC for consistency and cross-section
 Subtract backgrounds from our data
Page 46Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Background Subtraction
• We subtract off this total background from the data shape, 167
events out of 776 data events, which has Afb = -0.059 ± 0.0079
• Negative values mostly due to electroweak processes (W+HF)
 Background has a known asymmetry that modifies the top Afb measurement
 Check anti-tagged data and MC for consistency and cross-section
 Subtract backgrounds from our data
Page 47Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Background Afbs
Page 48Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetry Correction Technique
 We optimize for number of bins and
bin spacing
 Reconstruction of the event causes
smearing between bins. We use a
matrix unfolding technique to correct
for this effect.
 Acceptance efficiencies also bias our
sample, so an analogous correction is
made using an acceptance matrix
Sij = Nij
recon/Ni
truth
Aii = Ni
sel/Ni
gen
Ncorrected = A-1•S-1•Nbkg-sub
Bin smearing
Page 49Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Sij = Nij
recon/Ni
truth
Aii = Ni
sel/Ni
gen
Ncorrected = A-1•S-1•Nbkg-sub
Corrected Asymmetry Measurement
Page 50Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Test of Method
 We make control samples with known
asymmetry by reweighting Pythia Cos(θ)
signal in the ttbar frame by
1+A•Cos(θtt)
 Propagate these changes to obtain
appropriate factors for reweighting Ypp
 Check corrected measurement vs
known Afb for Cos(θtt) signal
Page 51Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Corrected Asymmetry Measurement
Raw Afb = 9.8 ± 3.6%
Bkg-sub Afb = 14.1 ± 4.6%
Final Corrected Afb = 19.3 ± 6.5%
Page 52Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Corrected Asymmetry Measurement
Raw Afb = 9.8 ± 3.6%
Bkg-sub Afb = 14.1 ± 4.6%
Final Corrected Afb = 19.3 ± 6.5%
~3σ significance
from LO QCD (A=0%)
Page 53Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Corrected Asymmetry Measurement
Raw Afb = 9.8 ± 3.6%
Bkg-sub Afb = 14.1 ± 4.6%
Final Corrected Afb = 19.3 ± 6.5%
>2σ significance
from NLO (A~5%)
Page 54Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
nominal bkg QCD difference Wbb difference new uncertainty
0.187 0.200 0.013 0.178 -0.009 0.013
Sigma is the max abs value of these two differences
Background subtraction is our largest component
of the systematic error. We have two main
sources: background shape and size.
For the shape uncertainty:
 Take the QCD (or WHF) contribution of the
background and rescale it to have the method II
number of events.
 Make a distribution using “Reweighted Signal +
Modified Bkg”
 Subtract off the original Method II background,
unfold, and measure an Afb.
 Compare with nominal value
Background Shape Systematic Uncertainty
Page 55Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Calc. Uncert.
Background Shape 0.011
Background Size 0.018
ISR/FSR 0.008
JES 0.002
PDF 0.001
MC Generator 0.003
Shape / Unfolding 0.006
Total Uncertainty 0.024
Vary simulated data by ±σ and calculate Afb difference with known sample
Systematic Uncertainties and Final Result
Page 56Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Systematic Errors and Result
Calc. Uncert.
Background Shape 0.011
Background Size 0.018
ISR/FSR 0.008
JES 0.002
PDF 0.001
MC Generator 0.003
Shape / Unfolding 0.006
Total Uncertainty 0.024
Final Corrected Afb = 19.3 ± 6.5 ± 2.4%
Vary simulated data by ±σ and calculate Afb difference with known sample
Page 57Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Calc. Uncert.
Background Shape 0.011
Background Size 0.018
ISR/FSR 0.008
JES 0.002
PDF 0.001
MC Generator 0.003
Shape / Unfolding 0.006
Total Uncertainty 0.024
Final Corrected Afb = 19.3 ± 6.5 ± 2.4%
Vary simulated data by ±σ and calculate Afb difference with known sample
Statistical error dominates total error
Systematic Uncertainties and Final Result
Page 58Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Corrected Asymmetry Measurement
Raw Afb = 9.8 ± 3.6%
Bkg-sub Afb = 14.1 ± 4.6%
Final Corrected Afb = 19.3 ± 6.5% ± 2.4%
Page 59Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
q q t t
F F
C
2
G
2 2 2 2
G G G
2
2 2 2 2
G G G
T C πβ2 2 2dσ
S Ndcos θ 2s
2s (s-m ) q t 2 2
V V(s-m ) +m Γ
q t s
A A (s-m ) +m Γ
q 2 q 2 t 2 2 2
V A V
t 2 2 2 q q t t
A V A V A
= α 1+c + 4m
+ [g g (1+c + 4m )
+2g g c]+
×[((g ) +(g ) )((g ) (1+c + 4m )
+(g ) (1+c + 4m ))+8g g g g c]
{
}
Back to theory…
Theoretical Predictions
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Page 60Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
2
2 2 2 2
2
2 2 2 2
2 2 2
cos 2
2 ( ) 2 2
( )
( )
2 2 2 2 2
2 2 2
1 4
[ (1 4 )
2 ]
[(( ) ( ) )(( ) (1 4 )
( ) (1 4 )) 8 ]
{
}
q q t t
F F
C
G
G G G
G G G
T Cd
S Nd s
s s m q t
V Vs m m
q t s
A A s m m
q q t
V A V
t q q t t
A V A V A
c m
g g c m
g g c
g g g c m
g c m g g g g c





  
  
  
  
 
   
   
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Page 61Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
2
2 2 2 2
2
2 2 2 2
2 2 2
cos 2
2 ( ) 2 2
( )
( )
2 2 2 2 2
2 2 2
1 4
[ (1 4 )
2 ]
[(( ) ( ) )(( ) (1 4 )
( ) (1 4 )) 8 ]
{
}
q q t t
F F
C
G
G G G
G G G
T Cd
S Nd s
s s m q t
V Vs m m
q t s
A A s m m
q q t
V A V
t q q t t
A V A V A
c m
g g c m
g g c
g g g c m
g c m g g g g c





  
  
  
  
 
   
   
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Page 62Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
2
2 2 2 2
2
2 2 2 2
2 2 2
cos 2
2 ( ) 2 2
( )
( )
2 2 2 2 2
2 2 2
1 4
[ (1 4 )
2 ]
[(( ) ( ) )(( ) (1 4 )
( ) (1 4 )) 8 ]
{
}
q q t t
F F
C
G
G G G
G G G
T Cd
S Nd s
s s m q t
V Vs m m
q t s
A A s m m
q q t
V A V
t q q t t
A V A V A
c m
g g c m
g g c
g g g c m
g c m g g g g c





  
  
  
  
 
   
   
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Page 63Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
2
2 2 2 2
2
2 2 2 2
2 2 2
cos 2
2 ( ) 2 2
( )
( )
2 2 2 2 2
2 2 2
1 4
[ (1 4 )
2 ]
[(( ) ( ) )(( ) (1 4 )
( ) (1 4 )) 8 ]
{
}
q q t t
F F
C
G
G G G
G G G
T Cd
S Nd s
s s m q t
V Vs m m
q t s
A A s m m
q q t
V A V
t q q t t
A V A V A
c m
g g c m
g g c
g g g c m
g c m g g g g c





  
  
  
  
 
   
   
Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
Page 64Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
2
2 2 2 2
2
2 2 2 2
2 2 2
cos 2
2 ( ) 2 2
( )
( )
2 2 2 2 2
2 2 2
1 4
[ (1 4 )
2 ]
[(( ) ( ) )(( ) (1 4 )
( ) (1 4 )) 8 ]
{
}
q q t t
F F
C
G
G G G
G G G
T Cd
S Nd s
s s m q t
V Vs m m
q t s
A A s m m
q q t
V A V
t q q t t
A V A V A
c m
g g c m
g g c
g g g c m
g c m g g g g c





  
  
  
  
 
   
   
Frampton, Shu, and Wang – [hep-ph] arxiv:0911.2955 IPMU-09-0135
Page 65Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Future Plans
• dA/dM (Afb vs Mtt)
• dA/dy (Afb as a function of y)
• increase data set
• understand the systematic
uncertainties better
• submit thesis!
Page 66Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Conclusions
 Previous results (1.9 fb-1):
Afb cos(θ) method: Afb = 17 ± 8%
 Current results (3.2 fb-1):
Afb -Q*yhadtop method: Afb = 19.3 ± 6.5 ± 2.4%
 Are measured values are higher than the 5% SM
prediction, but consistent within ~2 sigma
lab
lab
Page 67Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
and while our current asymmetry
measurement is quite exciting…
Page 68Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Stay tuned for more
exciting asymmetries!
Page 69Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
The End
Page 70Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Backup Slides
Backup
Slides
Page 71Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Measurement of Forward-Backward
Asymmetry in Top Quark Pair Production
in ppbar Collisions at 1.96 TeV
Glenn Strycker
University of Michigan
On behalf of the CDF Collaboration
1999-2003
2003-2010
Page 72Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Previous Methods
 Earlier Michigan/Davis measurements counted forward and
background events from a Cos(θ) distribution in the lab frame.
 Karlsruhe group investigated ∆y, the difference in rapidity
between the top and antitop. This quantity is Lorentz-invariant,
so the corresponding Afb is a measurement in the ttbar frame.
Page 73Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Fermilab
Page 74Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
CDF
The CDF Detector
Page 75Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
CDF
Here are several important components of CDF
Page 76Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
CDF
Construction of CDF
Page 77Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
CDF Geometry
y=0 y=2
Rapidity Geometry of CDF
y=1
1
ln
2
L
L
E p
y
E p
 
  
 
Page 78Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Various Plots of Observables
Page 79Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Jet Energies
Page 80Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Jet Rapidities
Page 81Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Validation Plots for Fitter Variables
Overflow bin
Page 82Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Validation Plots for Fitter Variables
Page 83Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Reconstructed W Quantities
Page 84Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Reconstructed B Quantities
Page 85Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Background Systematics – Size
nominal afb less less difference more more difference new uncertainty
0.187 0.176 -0.011 0.200 0.013 0.012
What if our Method II numbers are off?
Then we will be subtracting off the wrong
amount of background.
Use our reweighted dataset for three cases:
1) subtract off background + find Afb
2) subtract off 0.75x background
3) subtract off 1.25x background
 Use |more-less|/2 if the
more/less values are on either
side of the nominal value
 Use |max difference/2| if
values are on the same side
Page 86Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
“Reweight Function” Systematic
nominal Cos+Cos^6 diff Cos+Sin^2 diff Cos^5 diff Cos^3 diff new uncertainty
0.187 0.185 -0.002 0.188 0.001 0.195 0.007 0.193 0.006 0.004
What if our reweighted shape
does not match the data? 1+A*Cos()
Cos^6
Sin^2
Cos^5
Cos^3
Cos() Rapidity Y
Uncertainty (for now) is the max abs value of these differences divided by 2
A = 0.20A = 0.20
If we are reweighting central or outer bins more heavily
than we should, our unfolding technique will have error.
To find the uncertainty...
 Use several functions, each with an “A” parameter
 Adjust A to get a given true asymmetry in the lab frame
 Compare the unfolded Afb for each function with our
nominal 1+A*cos(θtt) reweighted MC.
Page 87Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Signal Systematics
What if our MC parameters are incorrect, introducing error into our unfolding
matrices? To measure this uncertainty we use the following method:
 generate MC with more/less of various components
 reweight by same A parameter as used earlier
 use our original unfolding matrices to find a corrected Afb
 compare to our original nominal value.
nominal afb less more less difference more difference new uncertainty
ISR 0.187 0.191 0.186 0.004 -0.001 0.003
FSR 0.187 0.172 0.178 -0.015 -0.009 0.007
JES 0.187 0.181 0.189 -0.006 0.002 0.004
PDF 0.187 0.183 0.189 -0.004 0.002 0.002
TopMC 0.187 0.175 -0.012 0.012
Page 88Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Verify Correction Procedure
Now that we've subtracted background we
can calculate our smear and acceptance
matrices from truth info in MC.
How do we know that this unfold
method is returning reasonable
values? We test our method on MC
reweighted to have an asymmetry.
We will calculate the corrected
measurement and compare with the
truth asymmetry of the signal MC
Page 89Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Reweighted Shapes
To test unfolding method, we use “fake”
data samples with different forms of
known asymmetry AFB.
Simplest case:
add linear (1+AFBcos(θtt)) term to Pythia
MC true cos(θ) distribution in ttbar
frame.
 Define weights:
 Ni is the content of bin i-th
 ni is the content of ith
bin assuming a
AFBcos(θi) distribution
 Reweight events in reconstructed
cos(θ) using wi
i
ii
N
nN
iw 

Page 90Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Test of method
Propagation of Afb from ttbar frame to lab frame rapidity
 We make control samples with known
asymmetry by reweighting Pythia Cos(θ)
signal in the ttbar frame by
1+A•Cos(θtt)
 Propagate these changes to obtain
appropriate factors for reweighting Ypp
 Check corrected measurement vs
known Afb for Cos(θtt) signal
Page 91Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Correction Bias Check
 First plot (truth info precut) shows the dilution of true Afb caused
by changing reference frames (center of momentum → lab frame)
 Second plot shows the statistical fluctuations across subsets of
the MC sample – MC is consistent with itself
Page 92Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Nominal Comparison – Reweighted MC
We use the standard technique of simulating
the entire analysis and varying the model
assumptions to understand their effect and
calculate the systematic uncertainty.
In this analysis, in order to find the correct
uncertainties we must first generate a
sample having a known asymmetry.
black = ttbar MC
blue = 0.20 reweight
Rapidity Y
 reweight cos(θtt) distribution → rapidity distribution as
explained on previous slides
 choose our “A” parameter such that our final corrected rapidity
Afb in the reweighted sample is closest to that observed in the
corrected data measurement.
Page 93Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Test of 1+A•Cos(θ) Hypothesis
The simplest form of an asymmetry is 1+A•cos(θtt)
 Use Pythia MC (initial Afb=0) and reweight by 1+A•cos(θtt) in the rest frame
 Propagate reweights to lab frame and rapidity variable to make a template
 Set up binned likelihood fit to data rapidity distribution using signal (templates)
plus background contribution (Gaussian-constrained to background parameters)
 The good fit to the data supports the linear hypothesis
Page 94Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Rapidity Template Fit
We can test our 1+Acos(θtt) linear hypothesis by
comparing template models with the data
 Set up binned likelihood fit to data rapidity
distribution using signal (templates) plus
background contribution (Gaussian-constrained
to M24U parameters)
 Find minimum of -log(likelihood) vs Afb
tt
 From template generation, we found the dilution
factor to be Afb
pp/Afb
tt = 0.5593 ± 0.0023,
which we use to convert results to ppbar frame
Page 95Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Rapidity Template Fit
-log(likelihood) fit is tested with PE's
and is found to have good coverage
Here is the best fit – 30% asymmetry
in the ttbar frame
Page 96Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetry Analogy
 Who has the better football team?
 Which metric should we use? (overall record, scores, Heisman winners?)
 Are the differences (asymmetries) statistically significant?
-VS-
Afb = Asymmetry“Forward-Backward” →
A“Football”
Page 97Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetry Analogy
-VS-
21 Wins
1887 1888(2)
1898 1899
1900 1902
1908 1942
1978 1981
1985 1986
1991 1994
1997 1999
2003 2006
2007 2009
15 Wins
1909 1943
1979 1980
1982 1987
1988 1989
1990 1993
1998 2002
2004 2005
2008
1 Tie
1992
Asymmetry is
A = 16.2% ± 15.8%
LARGE Asymmetry, but
*NOT Statistically Significant!
*In fact, A>1910 = 3.6% ± 18%
Afootball
=
WinsMichigan
− WinsND
Total GamesPlayed
Information from Wikipedia and http://grfx.cstv.com/photos/schools/nd/sports/m-footbl/auto_pdf/07fbguidehistory.pdf
Page 98Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetry Analogy
Information from Wikipedia and http://grfx.cstv.com/photos/schools/nd/sports/m-footbl/auto_pdf/07fbguidehistory.pdf
“Score Asymmetry” is
Ascore = 11.6% ± 2.7%
only a medium-sized asymmetry,
but a 4.4 sigma significance
A=
Points Michigan
− PointsND
Total Points Scored
– To be unbiased, we should look at
MC and choose our “best” variable
before using it to measure data. Notre Dame won by... Michigan won by...
Page 99Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Asymmetry Analogy
 Choosing the “correct” variable isn't trivial
 There are additional convolutions, smearing, acceptance
effects, and systematic errors to take into account
 Maybe physics is easier than football! (At least we have MC...)
Afb = Asymmetry“Forward-Backward” →
A“Football”
Page 100Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Vary simulated data by ±σ and calculate Afb difference with known sample
Calc. Uncert.
Background Shape 0.011
Background Size 0.018
ISR/FSR 0.008
JES 0.002
PDF 0.001
MC Generator 0.003
Shape / Unfolding 0.006
Total Uncertainty 0.024
Systematic Errors and Result
Final Corrected Afb = 19.3 ± 6.5 ± 2.4%
Hmm... statistical error
dominates total error
Page 101Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Simple Asymmetries in the tt System
t
p p
t
( 0) ( 0)
( 0) ( 0)
t t
t t
N y N y
fb N y N yA
  
  
Forward-Backward Asymmetry:
( 0) ( 0)
( 0) ( 0)
t t
t t
N y N y
C N y N yA
  
  
Charge Asymmetry:
( 0) ( 0)tt
N y N y   C fbA A
Assuming CP parity holds,
 
Page 102Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Fermilab
This slide should have last weeks
data, total data, estimates of
future integrated luminosity, etc.
Page 103Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Publications Referencing Our Results
ArXiv:0906.5541 – Ferrario and Rodrigo
Page 104Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
 Forward-backward Afb – does the top preferentially go one way or the other?
 Count events from rapidity distribution in lab frame and calculate:
Afb = (forward-backward)/total
 QCD at lowest order predicts 0 asymmetry
 NLO processes predicted an asymmetry Afb = 5±1.5% lab
 Unexpected new top production processes with chiral or axial couplings may
cause an additional asymmetry
+
+
t
p p
t
Asymmetries in tt Production
Page 105Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Theoretical Predictions
2
2 2 2 2
2
2 2 2 2
2 2 2
cos 2
2 ( ) 2 2
( )
( )
2 2 2 2 2
2 2 2
1 4
[ (1 4 )
2 ]
[(( ) ( ) )(( ) (1 4 )
( ) (1 4 )) 8 ]
{
}
q q t t
F F
C
G
G G G
G G G
T Cd
S Nd s
s s m q t
V Vs m m
q t s
A A s m m
q q t
V A V
t q q t t
A V A V A
c m
g g c m
g g c
g g g c m
g c m g g g g c





  
  
  
  
 
   
   
Page 106Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
Event Reconstruction
Page 107Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24
 Are more top quarks produced in the direction of the proton?
 First order measurement – simply count numbers produced forward and backward
 To increase sensitivity, account for acceptance and reconstruction effects
t
p p
t
Asymmetries in tt Production

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Notre Dame Afb Seminar 2009

  • 1. Page 1Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Measurement of the Forward-Backward Asymmetry in Top Quark Pair Production in pp Collisions at 1.96 TeV Glenn Strycker University of Michigan On behalf of the CDF Collaboration
  • 2. Page 2Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Measurement of the Forward-Backward Asymmetry in Top Quark Pair Production in pp Collisions at 1.96 TeV Glenn Strycker University of Michigan On behalf of the CDF Collaboration 1999-2003 2003-2010
  • 3. Page 3Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Outline  People Involved  What is Afb – why study it?  Theoretical Predictions  Previous Measurements  FNAL and CDF Overview  Measurement Techniques  Correction Procedure  Conclusion
  • 4. Page 4Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 People Involved University of Michigan: University of California, Davis: Also much work done by the Karlsruhe group: J. Wagner, T. Chwalek, W. Wagner Glenn Strycker Dan Amidei Monica Tecchio Tom Schwarz Robin Erbacher
  • 5. Page 5Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 The Top Forward-Backward Production Asymmetry has already generated a lot of interest… Asymmetries in tt Production
  • 6. Page 6Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetries in tt Production
  • 7. Page 7Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetries in tt Production
  • 8. Page 8Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetries in tt Production
  • 9. Page 9Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Simple Asymmetries in the tt System t p p t (cos 0) (cos 0) (cos 0) (cos 0) t t t t N N fb N NA           Forward-Backward Asymmetry: (cos 0) (cos 0) (cos 0) (cos 0) t t t t N N C N NA           Charge Asymmetry: (cos 0) (cos 0)tt N N    C fbA A Assuming CP parity holds,  
  • 10. Page 10Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 t θ  Use θ, Cos(θ), rapidity (y), Delta y, etc. to measure Afb  Which variable do we use?  Which reference frame should we use?  Possible to do the more complicated measurement cos fbdA d θ p t Asymmetries in tt Production
  • 11. Page 11Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 t  Use θ, Cos(θ), rapidity (y), Delta y, etc. to measure Afb  Which variable do we use?  Which reference frame should we use?  Possible to do the more complicated measurement cos fbdA d θ p t Asymmetries in tt Production 1 ln 2 L L E p E p       y
  • 12. Page 12Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24  In lowest order QCD, top quark production is symmetric  NLO QCD predicts a small asymmetry  Asymmetries are important tools for understanding weak interactions, as well as for searching for additional (chiral) couplings t p p t Asymmetries in tt Production
  • 13. Page 13Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions + + tt ttj Ctt = -1 Ctt = +1 Ctt = -1 Ctt = +1 Halzen, Hoyer, Kim; Kuhn, Rodrigo Dittmaier, Uwer, Weinzier; Almeida, Sterman, Vogelsang Afb ~ +10-12% Afb (NLO) Afb (NNLO) Consensus working value Afb ~ 5 ± 1.5% ~ -7% ~ -1%
  • 14. Page 14Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions + + tt ttj Ctt = -1 Ctt = +1 Ctt = -1 Ctt = +1 Halzen, Hoyer, Kim; Kuhn, Rodrigo Dittmaier, Uwer, Weinzier; Almeida, Sterman, Vogelsang Afb ~ +10-12% Afb (NLO) Afb (NNLO) ~ -7% ~ -1% Note that ggtt results in no asymmetry, so our measurement will be diluted by 15% before any other effects Consensus working value Afb ~ 5 ± 1.5%
  • 15. Page 15Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
  • 16. Page 16Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) c = βcos(θ)
  • 17. Page 17Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) Vector coupling mG = mass of axigluon ΓG = width of axigluon Axial coupling
  • 18. Page 18Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) gluon exchange gluon propagator
  • 19. Page 19Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) Interference Term
  • 20. Page 20Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) Note that there is a symmetric part and an asymmetric part
  • 21. Page 21Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) Pole term for axigluon
  • 22. Page 22Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) Again, note that there is a symmetric part and an asymmetric part
  • 23. Page 23Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) Since we observe no pole in Mtt, this constrains the possible values for gA and gV that give a positive asymmetry
  • 24. Page 24Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Lagrangian for Axigluon+Gluon Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009) Allows us to restrict regions for coupling parameters that won’t change tt cross section
  • 25. Page 25Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Recent Measurement History Previously Blessed Results (1.9 fb-1): Afb cos(θ) method: Afb = 0.17 ± 0.08 (Davis/Michigan) Afb ∆y method: Afb = 0.24 ± 0.14 (Karlsruhe) Phys. Rev. Lett. 101, 202001 (2008) D-zero Results (0.9 fb-1): Afb = 0.12 ± 0.08 ± 0.01
  • 26. Page 26Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Fermilab and CDF Fermilab and CDF...
  • 27. Page 27Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Fermilab  Tevatron ~4 miles around  1.96 TeV Collisions  Over 6 fb-1 data integrated luminosity  We're still the most energetic and highest luminosity hadron collider on Earth!
  • 28. Page 28Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Fermilab  Tevatron ~4 miles around  1.96 TeV Collisions  Over 6 fb-1 data integrated luminosity  We're still the most energetic and highest luminosity hadron collider on Earth!
  • 29. Page 29Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Fermilab
  • 30. Page 30Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Event Reconstruction
  • 31. Page 31Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Event Reconstruction
  • 32. Page 32Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Event Reconstruction
  • 33. Page 33Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Event Reconstruction
  • 34. Page 34Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Event Reconstruction Reconstruction  Match jets to b, b, up, down  Constrain fit W mass to 80.4 GeV/c2  Constrain fit for a 175.0 GeV/c2 Top  Float momenta of jets within known resolution  Use combination with lowest χ2 ? ? 2 2 2 2 2 2 2 2 2 2,, 2 2,, )()()()( , )( , )(2 top topbl top topbjj w wl w wjj j fitUE j measUE j i fiti t measi t MMMMMMMM yxj pp jetslep pp                
  • 35. Page 35Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Top Pair Production and Decay Top signals isolated using secondary vertex = “b-tag” Lepton + Jets mode: qq → g → tt → (W+b)(W-b) → (l+νb)(qqb) → l+ + Et + 4j + ≥ 1 “tagged” jet Vertex for b-quarks is roughly ½ mm Charm jets are only tagged ~1/10 the time
  • 36. Page 36Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Selection  Measurement is performed using semi-leptonic top pair decays  3.2 fb-1 of CDF data  ≥ 4 jets (Et > 20 GeV and |η| < 2)  ≥ 1 b-tagged jet  1 electron (Et > 20 GeV and |η| < 1) –or– 1 muon (Pt > 20 GeV/c and |η| <1)  Missing Transverse Energy > 20 GeV 776 Candidate Events Data Sample
  • 37. Page 37Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Selection  Measurement is performed using semi-leptonic top pair decays  3.2 fb-1 of CDF data  ≥ 4 jets (Et > 20 GeV and |η| < 2)  ≥ 1 b-tagged jet  1 electron (Et > 20 GeV and |η| < 1) –or– 1 muon (Pt > 20 GeV/c and |η| <1)  Missing Transverse Energy > 20 GeV Data Sample 776 Candidate Events
  • 38. Page 38Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24  Hadronic top decay is more accurately reconstructed, so we will measure yhad  Get hadronic top charge from lepton  Invoke CP invariance and multiply hadronic-only distribution by -1 * lepton charge to find equivalent top rapidity in each event  Measure Afb using -Qlep• yhad in the lab frame, count forward and backward events  Correct Afb back to parton level backward forward → W– b → l ν b → W+ b → j j b t t Measurement Techniques
  • 39. Page 39Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Selection  Measurement is performed using semi-leptonic top pair decays  3.2 fb-1 of CDF data  ≥ 4 jets (Et > 20 GeV and |η| < 2)  ≥ 1 b-tagged jet  1 electron (Et > 20 GeV and |η| < 1) –or– 1 muon (Pt > 20 GeV/c and |η| <1)  Missing Transverse Energy > 20 GeV Backgrounds  Use W+Jets tagged backgrounds to model the ttbar tagged backgrounds present in the semi-leptonic sample Data Sample 167 ± 34 Background Events
  • 40. Page 40Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Data Sample Say number of events and number of events in backgrounds. Process W+HF Jets 86.56 ± 27.40 Mistags (W+LF) 27.43 ± 7.70 Non-W (QCD) 33.44 ± 28.06 Single Top 7.82 ± 0.50 WW/WZ/ZZ 7.57 ± 0.74 Z+Jets 4.78 ± 0.59 Top 569.08 ± 78.81 Total Prediction 736.64 ± 89.22 776 Candidate Events 167 ± 34 Predicted Background Selection  Measurement is performed using semi-leptonic top pair decays  3.2 fb-1 of CDF data  ≥ 4 jets (Et > 20 GeV and |η| < 2)  ≥ 1 b-tagged jet  1 electron (Et > 20 GeV and |η| < 1) –or– 1 muon (Pt > 20 GeV/c and |η| <1)  Missing Transverse Energy > 20 GeV Backgrounds  Use W+Jets tagged backgrounds to model the ttbar tagged backgrounds present in the semi-leptonic sample
  • 41. Page 41Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Raw Asymmetry Signal MC is MC@NLO, normalized so signal+background = number of data events
  • 42. Page 42Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Raw Asymmetry Signal MC is MC@NLO, normalized so signal+background = number of data events We wish to correct for this shape
  • 43. Page 43Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Background Subtraction  Background has a known asymmetry that modifies the top Afb measurement  Check anti-tagged data and MC for consistency and cross-section  Subtract backgrounds from our data Backgrounds in W+Jets
  • 44. Page 44Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Background Subtraction  Background has a known asymmetry that modifies the top Afb measurement  Check anti-tagged data and MC for consistency and cross-section  Subtract backgrounds from our data
  • 45. Page 45Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Background Subtraction  Background has a known asymmetry that modifies the top Afb measurement  Check anti-tagged data and MC for consistency and cross-section  Subtract backgrounds from our data
  • 46. Page 46Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Background Subtraction • We subtract off this total background from the data shape, 167 events out of 776 data events, which has Afb = -0.059 ± 0.0079 • Negative values mostly due to electroweak processes (W+HF)  Background has a known asymmetry that modifies the top Afb measurement  Check anti-tagged data and MC for consistency and cross-section  Subtract backgrounds from our data
  • 47. Page 47Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Background Afbs
  • 48. Page 48Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetry Correction Technique  We optimize for number of bins and bin spacing  Reconstruction of the event causes smearing between bins. We use a matrix unfolding technique to correct for this effect.  Acceptance efficiencies also bias our sample, so an analogous correction is made using an acceptance matrix Sij = Nij recon/Ni truth Aii = Ni sel/Ni gen Ncorrected = A-1•S-1•Nbkg-sub Bin smearing
  • 49. Page 49Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Sij = Nij recon/Ni truth Aii = Ni sel/Ni gen Ncorrected = A-1•S-1•Nbkg-sub Corrected Asymmetry Measurement
  • 50. Page 50Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Test of Method  We make control samples with known asymmetry by reweighting Pythia Cos(θ) signal in the ttbar frame by 1+A•Cos(θtt)  Propagate these changes to obtain appropriate factors for reweighting Ypp  Check corrected measurement vs known Afb for Cos(θtt) signal
  • 51. Page 51Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Corrected Asymmetry Measurement Raw Afb = 9.8 ± 3.6% Bkg-sub Afb = 14.1 ± 4.6% Final Corrected Afb = 19.3 ± 6.5%
  • 52. Page 52Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Corrected Asymmetry Measurement Raw Afb = 9.8 ± 3.6% Bkg-sub Afb = 14.1 ± 4.6% Final Corrected Afb = 19.3 ± 6.5% ~3σ significance from LO QCD (A=0%)
  • 53. Page 53Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Corrected Asymmetry Measurement Raw Afb = 9.8 ± 3.6% Bkg-sub Afb = 14.1 ± 4.6% Final Corrected Afb = 19.3 ± 6.5% >2σ significance from NLO (A~5%)
  • 54. Page 54Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 nominal bkg QCD difference Wbb difference new uncertainty 0.187 0.200 0.013 0.178 -0.009 0.013 Sigma is the max abs value of these two differences Background subtraction is our largest component of the systematic error. We have two main sources: background shape and size. For the shape uncertainty:  Take the QCD (or WHF) contribution of the background and rescale it to have the method II number of events.  Make a distribution using “Reweighted Signal + Modified Bkg”  Subtract off the original Method II background, unfold, and measure an Afb.  Compare with nominal value Background Shape Systematic Uncertainty
  • 55. Page 55Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Calc. Uncert. Background Shape 0.011 Background Size 0.018 ISR/FSR 0.008 JES 0.002 PDF 0.001 MC Generator 0.003 Shape / Unfolding 0.006 Total Uncertainty 0.024 Vary simulated data by ±σ and calculate Afb difference with known sample Systematic Uncertainties and Final Result
  • 56. Page 56Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Systematic Errors and Result Calc. Uncert. Background Shape 0.011 Background Size 0.018 ISR/FSR 0.008 JES 0.002 PDF 0.001 MC Generator 0.003 Shape / Unfolding 0.006 Total Uncertainty 0.024 Final Corrected Afb = 19.3 ± 6.5 ± 2.4% Vary simulated data by ±σ and calculate Afb difference with known sample
  • 57. Page 57Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Calc. Uncert. Background Shape 0.011 Background Size 0.018 ISR/FSR 0.008 JES 0.002 PDF 0.001 MC Generator 0.003 Shape / Unfolding 0.006 Total Uncertainty 0.024 Final Corrected Afb = 19.3 ± 6.5 ± 2.4% Vary simulated data by ±σ and calculate Afb difference with known sample Statistical error dominates total error Systematic Uncertainties and Final Result
  • 58. Page 58Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Corrected Asymmetry Measurement Raw Afb = 9.8 ± 3.6% Bkg-sub Afb = 14.1 ± 4.6% Final Corrected Afb = 19.3 ± 6.5% ± 2.4%
  • 59. Page 59Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 q q t t F F C 2 G 2 2 2 2 G G G 2 2 2 2 2 G G G T C πβ2 2 2dσ S Ndcos θ 2s 2s (s-m ) q t 2 2 V V(s-m ) +m Γ q t s A A (s-m ) +m Γ q 2 q 2 t 2 2 2 V A V t 2 2 2 q q t t A V A V A = α 1+c + 4m + [g g (1+c + 4m ) +2g g c]+ ×[((g ) +(g ) )((g ) (1+c + 4m ) +(g ) (1+c + 4m ))+8g g g g c] { } Back to theory… Theoretical Predictions Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
  • 60. Page 60Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions 2 2 2 2 2 2 2 2 2 2 2 2 2 cos 2 2 ( ) 2 2 ( ) ( ) 2 2 2 2 2 2 2 2 1 4 [ (1 4 ) 2 ] [(( ) ( ) )(( ) (1 4 ) ( ) (1 4 )) 8 ] { } q q t t F F C G G G G G G G T Cd S Nd s s s m q t V Vs m m q t s A A s m m q q t V A V t q q t t A V A V A c m g g c m g g c g g g c m g c m g g g g c                            Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
  • 61. Page 61Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions 2 2 2 2 2 2 2 2 2 2 2 2 2 cos 2 2 ( ) 2 2 ( ) ( ) 2 2 2 2 2 2 2 2 1 4 [ (1 4 ) 2 ] [(( ) ( ) )(( ) (1 4 ) ( ) (1 4 )) 8 ] { } q q t t F F C G G G G G G G T Cd S Nd s s s m q t V Vs m m q t s A A s m m q q t V A V t q q t t A V A V A c m g g c m g g c g g g c m g c m g g g g c                            Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
  • 62. Page 62Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions 2 2 2 2 2 2 2 2 2 2 2 2 2 cos 2 2 ( ) 2 2 ( ) ( ) 2 2 2 2 2 2 2 2 1 4 [ (1 4 ) 2 ] [(( ) ( ) )(( ) (1 4 ) ( ) (1 4 )) 8 ] { } q q t t F F C G G G G G G G T Cd S Nd s s s m q t V Vs m m q t s A A s m m q q t V A V t q q t t A V A V A c m g g c m g g c g g g c m g c m g g g g c                            Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
  • 63. Page 63Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions 2 2 2 2 2 2 2 2 2 2 2 2 2 cos 2 2 ( ) 2 2 ( ) ( ) 2 2 2 2 2 2 2 2 1 4 [ (1 4 ) 2 ] [(( ) ( ) )(( ) (1 4 ) ( ) (1 4 )) 8 ] { } q q t t F F C G G G G G G G T Cd S Nd s s s m q t V Vs m m q t s A A s m m q q t V A V t q q t t A V A V A c m g g c m g g c g g g c m g c m g g g g c                            Ferrario and Rodrigo – Physical Review D 80, 051701(R) (2009)
  • 64. Page 64Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions 2 2 2 2 2 2 2 2 2 2 2 2 2 cos 2 2 ( ) 2 2 ( ) ( ) 2 2 2 2 2 2 2 2 1 4 [ (1 4 ) 2 ] [(( ) ( ) )(( ) (1 4 ) ( ) (1 4 )) 8 ] { } q q t t F F C G G G G G G G T Cd S Nd s s s m q t V Vs m m q t s A A s m m q q t V A V t q q t t A V A V A c m g g c m g g c g g g c m g c m g g g g c                            Frampton, Shu, and Wang – [hep-ph] arxiv:0911.2955 IPMU-09-0135
  • 65. Page 65Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Future Plans • dA/dM (Afb vs Mtt) • dA/dy (Afb as a function of y) • increase data set • understand the systematic uncertainties better • submit thesis!
  • 66. Page 66Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Conclusions  Previous results (1.9 fb-1): Afb cos(θ) method: Afb = 17 ± 8%  Current results (3.2 fb-1): Afb -Q*yhadtop method: Afb = 19.3 ± 6.5 ± 2.4%  Are measured values are higher than the 5% SM prediction, but consistent within ~2 sigma lab lab
  • 67. Page 67Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 and while our current asymmetry measurement is quite exciting…
  • 68. Page 68Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Stay tuned for more exciting asymmetries!
  • 69. Page 69Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 The End
  • 70. Page 70Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Backup Slides Backup Slides
  • 71. Page 71Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Measurement of Forward-Backward Asymmetry in Top Quark Pair Production in ppbar Collisions at 1.96 TeV Glenn Strycker University of Michigan On behalf of the CDF Collaboration 1999-2003 2003-2010
  • 72. Page 72Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Previous Methods  Earlier Michigan/Davis measurements counted forward and background events from a Cos(θ) distribution in the lab frame.  Karlsruhe group investigated ∆y, the difference in rapidity between the top and antitop. This quantity is Lorentz-invariant, so the corresponding Afb is a measurement in the ttbar frame.
  • 73. Page 73Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Fermilab
  • 74. Page 74Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 CDF The CDF Detector
  • 75. Page 75Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 CDF Here are several important components of CDF
  • 76. Page 76Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 CDF Construction of CDF
  • 77. Page 77Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 CDF Geometry y=0 y=2 Rapidity Geometry of CDF y=1 1 ln 2 L L E p y E p       
  • 78. Page 78Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Various Plots of Observables
  • 79. Page 79Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Jet Energies
  • 80. Page 80Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Jet Rapidities
  • 81. Page 81Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Validation Plots for Fitter Variables Overflow bin
  • 82. Page 82Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Validation Plots for Fitter Variables
  • 83. Page 83Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Reconstructed W Quantities
  • 84. Page 84Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Reconstructed B Quantities
  • 85. Page 85Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Background Systematics – Size nominal afb less less difference more more difference new uncertainty 0.187 0.176 -0.011 0.200 0.013 0.012 What if our Method II numbers are off? Then we will be subtracting off the wrong amount of background. Use our reweighted dataset for three cases: 1) subtract off background + find Afb 2) subtract off 0.75x background 3) subtract off 1.25x background  Use |more-less|/2 if the more/less values are on either side of the nominal value  Use |max difference/2| if values are on the same side
  • 86. Page 86Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 “Reweight Function” Systematic nominal Cos+Cos^6 diff Cos+Sin^2 diff Cos^5 diff Cos^3 diff new uncertainty 0.187 0.185 -0.002 0.188 0.001 0.195 0.007 0.193 0.006 0.004 What if our reweighted shape does not match the data? 1+A*Cos() Cos^6 Sin^2 Cos^5 Cos^3 Cos() Rapidity Y Uncertainty (for now) is the max abs value of these differences divided by 2 A = 0.20A = 0.20 If we are reweighting central or outer bins more heavily than we should, our unfolding technique will have error. To find the uncertainty...  Use several functions, each with an “A” parameter  Adjust A to get a given true asymmetry in the lab frame  Compare the unfolded Afb for each function with our nominal 1+A*cos(θtt) reweighted MC.
  • 87. Page 87Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Signal Systematics What if our MC parameters are incorrect, introducing error into our unfolding matrices? To measure this uncertainty we use the following method:  generate MC with more/less of various components  reweight by same A parameter as used earlier  use our original unfolding matrices to find a corrected Afb  compare to our original nominal value. nominal afb less more less difference more difference new uncertainty ISR 0.187 0.191 0.186 0.004 -0.001 0.003 FSR 0.187 0.172 0.178 -0.015 -0.009 0.007 JES 0.187 0.181 0.189 -0.006 0.002 0.004 PDF 0.187 0.183 0.189 -0.004 0.002 0.002 TopMC 0.187 0.175 -0.012 0.012
  • 88. Page 88Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Verify Correction Procedure Now that we've subtracted background we can calculate our smear and acceptance matrices from truth info in MC. How do we know that this unfold method is returning reasonable values? We test our method on MC reweighted to have an asymmetry. We will calculate the corrected measurement and compare with the truth asymmetry of the signal MC
  • 89. Page 89Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Reweighted Shapes To test unfolding method, we use “fake” data samples with different forms of known asymmetry AFB. Simplest case: add linear (1+AFBcos(θtt)) term to Pythia MC true cos(θ) distribution in ttbar frame.  Define weights:  Ni is the content of bin i-th  ni is the content of ith bin assuming a AFBcos(θi) distribution  Reweight events in reconstructed cos(θ) using wi i ii N nN iw  
  • 90. Page 90Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Test of method Propagation of Afb from ttbar frame to lab frame rapidity  We make control samples with known asymmetry by reweighting Pythia Cos(θ) signal in the ttbar frame by 1+A•Cos(θtt)  Propagate these changes to obtain appropriate factors for reweighting Ypp  Check corrected measurement vs known Afb for Cos(θtt) signal
  • 91. Page 91Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Correction Bias Check  First plot (truth info precut) shows the dilution of true Afb caused by changing reference frames (center of momentum → lab frame)  Second plot shows the statistical fluctuations across subsets of the MC sample – MC is consistent with itself
  • 92. Page 92Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Nominal Comparison – Reweighted MC We use the standard technique of simulating the entire analysis and varying the model assumptions to understand their effect and calculate the systematic uncertainty. In this analysis, in order to find the correct uncertainties we must first generate a sample having a known asymmetry. black = ttbar MC blue = 0.20 reweight Rapidity Y  reweight cos(θtt) distribution → rapidity distribution as explained on previous slides  choose our “A” parameter such that our final corrected rapidity Afb in the reweighted sample is closest to that observed in the corrected data measurement.
  • 93. Page 93Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Test of 1+A•Cos(θ) Hypothesis The simplest form of an asymmetry is 1+A•cos(θtt)  Use Pythia MC (initial Afb=0) and reweight by 1+A•cos(θtt) in the rest frame  Propagate reweights to lab frame and rapidity variable to make a template  Set up binned likelihood fit to data rapidity distribution using signal (templates) plus background contribution (Gaussian-constrained to background parameters)  The good fit to the data supports the linear hypothesis
  • 94. Page 94Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Rapidity Template Fit We can test our 1+Acos(θtt) linear hypothesis by comparing template models with the data  Set up binned likelihood fit to data rapidity distribution using signal (templates) plus background contribution (Gaussian-constrained to M24U parameters)  Find minimum of -log(likelihood) vs Afb tt  From template generation, we found the dilution factor to be Afb pp/Afb tt = 0.5593 ± 0.0023, which we use to convert results to ppbar frame
  • 95. Page 95Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Rapidity Template Fit -log(likelihood) fit is tested with PE's and is found to have good coverage Here is the best fit – 30% asymmetry in the ttbar frame
  • 96. Page 96Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetry Analogy  Who has the better football team?  Which metric should we use? (overall record, scores, Heisman winners?)  Are the differences (asymmetries) statistically significant? -VS- Afb = Asymmetry“Forward-Backward” → A“Football”
  • 97. Page 97Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetry Analogy -VS- 21 Wins 1887 1888(2) 1898 1899 1900 1902 1908 1942 1978 1981 1985 1986 1991 1994 1997 1999 2003 2006 2007 2009 15 Wins 1909 1943 1979 1980 1982 1987 1988 1989 1990 1993 1998 2002 2004 2005 2008 1 Tie 1992 Asymmetry is A = 16.2% ± 15.8% LARGE Asymmetry, but *NOT Statistically Significant! *In fact, A>1910 = 3.6% ± 18% Afootball = WinsMichigan − WinsND Total GamesPlayed Information from Wikipedia and http://grfx.cstv.com/photos/schools/nd/sports/m-footbl/auto_pdf/07fbguidehistory.pdf
  • 98. Page 98Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetry Analogy Information from Wikipedia and http://grfx.cstv.com/photos/schools/nd/sports/m-footbl/auto_pdf/07fbguidehistory.pdf “Score Asymmetry” is Ascore = 11.6% ± 2.7% only a medium-sized asymmetry, but a 4.4 sigma significance A= Points Michigan − PointsND Total Points Scored – To be unbiased, we should look at MC and choose our “best” variable before using it to measure data. Notre Dame won by... Michigan won by...
  • 99. Page 99Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Asymmetry Analogy  Choosing the “correct” variable isn't trivial  There are additional convolutions, smearing, acceptance effects, and systematic errors to take into account  Maybe physics is easier than football! (At least we have MC...) Afb = Asymmetry“Forward-Backward” → A“Football”
  • 100. Page 100Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Vary simulated data by ±σ and calculate Afb difference with known sample Calc. Uncert. Background Shape 0.011 Background Size 0.018 ISR/FSR 0.008 JES 0.002 PDF 0.001 MC Generator 0.003 Shape / Unfolding 0.006 Total Uncertainty 0.024 Systematic Errors and Result Final Corrected Afb = 19.3 ± 6.5 ± 2.4% Hmm... statistical error dominates total error
  • 101. Page 101Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Simple Asymmetries in the tt System t p p t ( 0) ( 0) ( 0) ( 0) t t t t N y N y fb N y N yA       Forward-Backward Asymmetry: ( 0) ( 0) ( 0) ( 0) t t t t N y N y C N y N yA       Charge Asymmetry: ( 0) ( 0)tt N y N y   C fbA A Assuming CP parity holds,  
  • 102. Page 102Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Fermilab This slide should have last weeks data, total data, estimates of future integrated luminosity, etc.
  • 103. Page 103Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Publications Referencing Our Results ArXiv:0906.5541 – Ferrario and Rodrigo
  • 104. Page 104Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24  Forward-backward Afb – does the top preferentially go one way or the other?  Count events from rapidity distribution in lab frame and calculate: Afb = (forward-backward)/total  QCD at lowest order predicts 0 asymmetry  NLO processes predicted an asymmetry Afb = 5±1.5% lab  Unexpected new top production processes with chiral or axial couplings may cause an additional asymmetry + + t p p t Asymmetries in tt Production
  • 105. Page 105Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Theoretical Predictions 2 2 2 2 2 2 2 2 2 2 2 2 2 cos 2 2 ( ) 2 2 ( ) ( ) 2 2 2 2 2 2 2 2 1 4 [ (1 4 ) 2 ] [(( ) ( ) )(( ) (1 4 ) ( ) (1 4 )) 8 ] { } q q t t F F C G G G G G G G T Cd S Nd s s s m q t V Vs m m q t s A A s m m q q t V A V t q q t t A V A V A c m g g c m g g c g g g c m g c m g g g g c                           
  • 106. Page 106Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24 Event Reconstruction
  • 107. Page 107Glenn Strycker – University of Michigan Notre Dame HEP Seminar – 2009-11-24  Are more top quarks produced in the direction of the proton?  First order measurement – simply count numbers produced forward and backward  To increase sensitivity, account for acceptance and reconstruction effects t p p t Asymmetries in tt Production