1. The Top Chromo-magnetic Moment
Paul Dervan
The University of Liverpool
ATLAS UK Physics Meeting, Durham 18th-13th September 2006
Overview of the theory
Event Selection
Summary

2. Overview Modified ttg interaction Lagrangian
Top is the most massive particle – a likely place to find new physics
Modified ttg interaction Lagrangian
Kappa, Chromo-magnetic moment (CP conserving)
Kappa tilda, Chromo-electric moment (CP violating), set to 0
References:
SLAC-PUB-7153
Phys.Rev.D52: , 1995
19/ , Durham
Paul Dervan, The University of Liverpool

3. Overview … Solid line is SM.
Top pair invariant mass distribution
Solid line is SM.
Different dot/dash lines are for different values of kappa
19/ , Durham
Paul Dervan, The University of Liverpool

4. Overview … Solid line is SM.
Top pt distribution
Solid line is SM.
Different dot/dash lines are for different values of kappa
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Paul Dervan, The University of Liverpool

5. Event Type
In order to get the top pt distribution and the top pair invariant mass distribution
Need to fully reconstruct the top pair event
Should use the lepton + Jets Channel
tt  WbWb  jjblvb
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Paul Dervan, The University of Liverpool

6. Break down of the sample
Data Sample
Data Sample
CSC Nikhef Ntuple (Thanks to the Nikhef people)
non hadronic events
Break down of the sample
Lepton + Jets (77%)
Leptonic (22%)
Hadronic (0%)
Other (1%)
33% Electrons
33% Muons
33% Tau
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Paul Dervan, The University of Liverpool

7. Event Selection Event selection 4304 events selected (3%)
1 high pt lepton (pt > 20 GeV ||<2.5)
Missing Energy (Et miss > 20 GeV)
At least 4 Jets (pt > 40 GeV ||<2.5) (0.4 Cone)
At least 2 Jets must be tagged as b-jets (weight > 4.5)
4304 events selected (3%)
19/ , Durham
Paul Dervan, The University of Liverpool

8. Event Selection… 4304 events selected 3819 True Lepton + Jets (88.7%)
456 True Leptonic (10.6%)
Other (0.7%)
Of the true Lepton +Jet events
1628 events selected as e events
1360 True e events (83.5%)
True μ events (0.6%)
258 True τ events (15.8%)
2191 events selected as e events
True e events (3.4%)
1794 True μ events (81.9%)
323 True τ events (14.7%)
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Paul Dervan, The University of Liverpool

9. Event Reconstruction: Hadronic Side
Hadronic Side: t  Wb  jjb
Combine all light jets to try and reconstruct the W
Pick the combination with an inv. mass closest to the W
Select the event if the reconstructed inv. mass is within 20 GeV of Mw
Do a chi2 minimisation on the jets constrained to the W mass (JES)
The combine the b jets to the W to form the top
Pick the combination with an inv. mass closest to the top.
19/ , Durham
Paul Dervan, The University of Liverpool

10. jj invariant mass reconstruction
Hadronic side
W from light jet pair with closest invariant mass to MW
Require |MW-Mjj|<20 GeV
(no jet energy scale correction) j1 j2
b-jet t
Red: Correct Jet Assignment
Green: Wrong Jet Assignment
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Paul Dervan, The University of Liverpool

11. j2 j1 b-jet jjb invarient mass Hadronic side
Assign a b-jet to the W to reconstruct Mjjb
Take the b-jet that gives Mjjb closest to Mtop j1 j2
b-jet t
Green: Wrong W
Yellow: Wrong b
19/ , Durham
Paul Dervan, The University of Liverpool

12. Event Reconstruction: Leptonic Side
Leptonic Side: t  Wb  lvb
Assign missing px and py to the neutrino
Constrain Mlv to MW and solve for pz of the neutrino
Combine the reconstructed W with the remaining b jets
Pick the combination with an inv. mass closest to the top.
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Paul Dervan, The University of Liverpool

13. lvb invarient mass Leptonic side Need to reconstruct the neutrino pz
Et miss = Et neutrino
mW = mlv
Get two solutions
Combine with b-jet (best invariant mt)
19/ , Durham
Paul Dervan, The University of Liverpool

14. Top pair invariant mass
Red: Reconstructed
Black: Truth
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Paul Dervan, The University of Liverpool

15. One possible thing to look for is anomalous chromo-magnetic coupling
Summary
Top is the most massive particle – likely place to look for new physics
One possible thing to look for is anomalous chromo-magnetic coupling
Need the pt distribution of the top and the top pair invariant mass dist.
So need to fully reconstruct the top events
Started to reconstruct Lepton + Jets events
Need to look into backgrounds etc….
Need a lot of work on the theory side to get the distributions to fit the distributions too
19/ , Durham
Paul Dervan, The University of Liverpool