1. Top Quark Production at the Large Hadron Collider
Fourth Year Project Seminar
Top Quark Production at the Large Hadron Collider
Jack Goddard
20th March 2009

2. Fourth Year Project Seminar
Outline
Background
The Top Quark
ATLAS
Aim
Method
Backgrounds
Efficiency
Results
Conclusion
20th March 2009
Fourth Year Project Seminar

3. Fourth Year Project Seminar
The Top Quark
Mt = ± 2.1 GeV (PDG)
About the same mass as a gold atom
Point-like to scales ≥10-18m
Discovered at the Tevatron
Expected to couple to electroweak gauge symmetry breaking forces
Free parameter in the standard model
Constrains the mass of the Higgs
arXiv: [hep-ex]
20th March 2009
Fourth Year Project Seminar

4. Fourth Year Project Seminar
Top Pair Production
Fully Hadronic
Leptonic
Semi-leptonic
Fully Hadronic
BR = 44.4%
Semi-leptonic
Leptonic
BR = 4.9%
BR = 29.6%
20th March 2009
Fourth Year Project Seminar

5. Fourth Year Project Seminar
The ATLAS Detector
Multipurpose detector
IP1
J. Pequenao CERN-GE
20th March 2009
Fourth Year Project Seminar

6. Fourth Year Project Seminar
The Inner Detector
3 Layers of Pixel Detectors
4 layers of Semiconductor Tracker (SCT)
Transition Radiation Tracker (TRT)
|η| < 2.5
20th March 2009
Fourth Year Project Seminar

7. The Electromagnetic Calorimeter
Lead Liquid Argon Sampling Calorimeter.
Accordion-shaped kapton electrodes and lead absorber plates.
|η|<2.5 devoted to precision physics.
Segmented into 3 sections.
20th March 2009
Fourth Year Project Seminar

8. Fourth Year Project Seminar
Aim of project
Decay
Branching Ratio
t → bW+
0.998
t → sW+
≈1.9x10-3
t → dW+
≈ 10-4
If:
Dominant H+ decays:
Number of electrons/muons produced in top decay would be reduced, by non SM processes.
Rll/l would be less than SM prediction
Ree/e ≈ 1/19
20th March 2009
Fourth Year Project Seminar

9. Fourth Year Project Seminar
Method
ATHENA
Monte Carlo Samples
Producing Histograms in ROOT
Select top quark decays
Pre-Selection Cuts
Electron Identification Cuts
Jet Cuts
Background Studies
Efficiency Studies
20th March 2009
Fourth Year Project Seminar

10. Fourth Year Project Seminar
Pre-Selection Cuts
For a sample enriched with leptonic and semi-leptonic decays:
ETmiss > 20 GeV
Electron with pT > 20 GeV
At least 2 b-tagged jets with pT > 20 GeV
All in |η| < 2.5
Differentiated by:
A second electron with pT > 20 GeV
With opposite charge to the original electron
20th March 2009
Fourth Year Project Seminar

11. Fourth Year Project Seminar
Pre-Selection Cuts
20th March 2009
Fourth Year Project Seminar

12. Electron Identification Cuts
Efficiency (%)
Loose (had and middle) 88
Medium (Loose + strips + trk qual + trk match)
77.2
Tight (Medium + TRT) 64
20th March 2009
Fourth Year Project Seminar

13. Electron Identification Cuts: Loose
Type
Description
Value
Acceptance of the detector
Pseudorapidity cut
|η| < 2.5
Hadronic Leakage
Ratio of ET in the first sampling of the hadronic to ET of ECal
RET <0.045
Second Layer of the ECal
Ratio of cell energies in 3x7 vs 7x7 cells
Rη > 0.910
Rφ > 0.5
Lateral Width of the shower
wlats <
Simple Electron ID based on information from the calorimeters
Gives good identification efficiency, but low background rejection
20th March 2009
Fourth Year Project Seminar

14. Electron Identification Cuts: Loose
20th March 2009
Fourth Year Project Seminar

15. Electron Identification Cuts: Medium
Strip based cuts effective at rejecting π0 → γγ
Showers studied in a ΔηxΔφ = x 0.2 window
Type
Description
Value
First Layer of the ECal
Difference between energy in the 2nd largest deposit and energy of the minimal value between 1st and 2nd maxima.
ΔES > 0.3
Second largest energy deposit (normalised to the cluster energy)
Rmax2< 0.53
Total Shower Width
wstot < 4.10
Shower width for 3 strips around maximum strip
ws3 < 0.8
Fraction of energy outside core of 3 central strips
Fside < 0.8
Track Quality
Number of hits in the pixel detector
Npix> 0
Number of B-layer hits
Nb-lay > 0
Number of hits in the pixels and SCT
Npix+SCT> 8
Transverse Impact Parameter
A0 < 1mm
20th March 2009
Fourth Year Project Seminar

16. Electron Identification Cuts: Medium
20th March 2009
Fourth Year Project Seminar

17. Electron Identification Cuts: Tight
Makes use of all the electron ID cuts available
Includes the loose and medium cuts
Type
Description
Value
Isolation
Ratio of ET in a cone ΔR<0.2 to the total cluster energy
ET /Etot < 0.8
Vertex-layer
Number of hits in the vertexing layer
Nvert > 0
Track Matching
Δη between the cluster and the track
Δη < 0.005
Δφ between the cluster and the track
Δφ < 0.02
Ratio of cluster energy to the track momentum
0.7 < E/p < 5.0
TRT
Total number of hits in the TRT
NTRT > 15
Ratio of number of high threshold hits to the total number of hits in the TRT
RTRT > 0.08
20th March 2009
Fourth Year Project Seminar

18. Electron Identification Cuts: Tight
20th March 2009
Fourth Year Project Seminar

19. Effect of all the Cuts Missing ET cut Jet cuts
Electrons from semi-leptonic decays
Electron medium cuts
Electron η cut
Electrons from leptonic decays
Electron pT cut
Selected Electrons
Electron loose cuts
20th March 2009
Fourth Year Project Seminar

20. Backgrounds Background from other processes
Background from other top decays
t →bτυτ
Hadronic channel
Decay
Cross-section (pb)
Total top pair production
834
Backgrounds to Semi-leptonic Decays
W + jets → lν + jets
7.8 x 103
W + W → lν + jets
17.1
W + Z → lν + jets
3.4
Backgrounds to both leptonic and Semi-leptonic Decays
Z + jets → ll + jets
1.2 x 103
W + bb → ll + jets
66.7
Z + Z → ll + jets
9.2
20th March 2009
Fourth Year Project Seminar

21. Fourth Year Project Seminar
Tau Background
Under investigation
Either:
Tighten pT cut to 30 GeV or
Account for electrons from taus in systematic errors
20th March 2009
Fourth Year Project Seminar

22. Fourth Year Project Seminar
Efficiency
Look at the MC truth information
Compare to reconstructed data
NID = number of reconstructed electrons
Ntruth = number of true electrons at the generator level
20th March 2009
Fourth Year Project Seminar

23. Fourth Year Project Seminar
Ree/e
From Reconstruction:
Ree/e = 44/839
= ± (stat)
From Truth:
Ree/e = 428/8636
= ± (stat)
R = 1/19 =
20th March 2009
Fourth Year Project Seminar

24. Fourth Year Project Seminar
Conclusion
Leptonic and Semi-leptonic decays selected
Ree/e = ± (stat)
Some systematic errors being investigated
Efficiency and Purity Studies ongoing.
20th March 2009
Fourth Year Project Seminar

25. Fourth Year Project Seminar
References
“Precision Electroweak Measurements and Constraints on the Standard Model” (arXiv: ) : The LEP Collaborations and LEP Electroweak Working Group.
“ATLAS Detector and Physics Performance Technical Design Report” The ATLAS Collaboration.
“Reconstruction and Identification of Electrons” ATLAS CSC Note: D. Zerwas & D. Froidevaux
20th March 2009
Fourth Year Project Seminar

26. Any Questions?

27. Backup Slides

28. Fourth Year Project Seminar
Cross-section
An accurate measurement of BR(W → lv) is of interest for the cross-section calculation:
20th March 2009
Fourth Year Project Seminar

29. Transition Radiation Tracker
Based on use of straw detectors
70% Xe, 20% CO2, 10% CF4
Xenon gas used to detect transition-radiation photons created in a radiator between the straws
Allows a large number of measurements (typically 36) to be made on every track
20th March 2009
Fourth Year Project Seminar

30. Sampling Calorimeters
20th March 2009
Fourth Year Project Seminar

31. Fourth Year Project Seminar
B-tagging
Hadrons with b-quarks have a long lifetime – allows distinct primary and secondary vertices to be seen
Compared to other jets b-jets are:
Higher multiplicities
Higher invariant mass
Contains low energy leptons with momentum perpendicular to the jet
20th March 2009
Fourth Year Project Seminar

32. Fourth Year Project Seminar
The Monte Carlo Sample
Run 5200
Contains only leptonic and semi-leptonic tt(bar) events
Electron, Muon and Tau events
Contains 1861 files of 250 events
465,250 events in total
20th March 2009
Fourth Year Project Seminar

33. Fourth Year Project Seminar
Ree/e = 1/19 ?
Due to a leptonic decays that produce a electron and a muon or tau
20th March 2009
Fourth Year Project Seminar

34. Fourth Year Project Seminar
Error Calculation
20th March 2009
Fourth Year Project Seminar