BAG TECHNIQUE Bag technique-a tool making use of public health bag through wh...
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
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 ggtt 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
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