1. Top quark production cross section Top quark mass measurement
Top Quark Physics at D0
Yi Jiang
University of Science & Technology of China
Introduction
Top quark production cross section
Top quark mass measurement
Single top physics
Spin correlation
Summary

2. Tevatron Collider in Run II
The Tevatron is a proton-antiproton
Collider with 980 GeV/beam
=1.96TeV in RunII (1.8TeV in RunI)
36 P and Pbar bunches a396 ns
between bunch crossing
Increased from 6X6 bunches
with 3.5ms in Run I
Increased instantaneous luminosity
Run II goal
Current: ~

3. Run II D0 Data Taking Status
85~90%

4. D0 Detector (Run II)

5. Silicon Microstrip Detector (SMT)
PVrt/IP~15mm
Vertex resolution: ~10mm (design)
Primary Vertex vs. Impact parameter

6. Center Fiber Tracker (CFT)
SMT combines vertex and tracking capabilities and provides good primary and secondary vertex resolutions.

7. The Calorimeter Resolution: s/E ~ 15%/√E(GeV) “fine” EMZ y x q j
Resolution:
s/E ~ 15%/√E(GeV) “fine” EM
50%/√E(GeV) “coarse” jet
sMET ~ a + b*ST + c*ST (run1)
ST scalar sum of ET
a ~1.89GeV,
b ~6.7E-3,
c ~9.9E-6/GeV

8. Muon Detector
J/Psi:
Local/Global

9. D0 Detector Performance

10. Motivation for the Top Quark Studies (I)
Top quark has been discovered by CDF and D0 in 1995;
Top quark mass ~175GeV and strong Yukawa coupling ~1;
- Study of the top quark provides an excellent probe
of the electroweak symmetry breaking mechanism;
- New physics may be discovered in either its
production or decays;
- Top quark spin can be directly observed.
Tevatron is the only palce to study top quark properties
before LHC operation.

11. Motivation for the Top Quark Studies (II)
Top Mass, W Mass
Measurement

12. Top Physics Understanding
Program
Top production & decay
Tools
Cross section
Mass
Single top
Spin correlation
W helicity

13. Top Quark Production at Tevatron
Top-antitop quark Pair Production (mainly)
s(pb)
RunI
4.87(10%)
90%
10%
RunII
6.70(10%)
85%
15%
Single top quark production (not yet observed)
RunII
s(pb)
0.9(10%)
2.0(10%)

14. Top Quark Decay In the standard model, the top quark is short lived
and decay almost exclusively to W and b quark

15. Methodology& tools
Full characterization of the chosen final state signature in term
of SM background processes (control region)
[ Optimize signal for best measurement precision
How to separate signal from background:
a Top events have very distinctive signatures
8 Decay products (leptons, neutrinos, jets) have large PT
8 Event topology: central and spherical
8 Heavy flavor content: always 2 b jets in the final state
Tools (need multipurpose detectors)
8 Lepton ID: detector coverage and robust tracking
8 Calorimetry: hermetic and well calibrated
8 B identification: algorithms pure and efficient
8 Simulation: essential to reach precision goals

16. Production cross section
RunI~100 events

17. Top cross section: dilepton channels

18. ------------------------------------------------------
CDF & D0: dilepton channels

19. Top cross section: lepton+jets
“Golden” mode for top studies: ~ 30% yield and relatively clean

20. Lepton+jets channel: topological analysis
Preselect a sample enriched in W events
Evaluate QCD multijet background
from data for each jet multiplicity
bin using “matrix” method
e+jets: due to fake jets (po and g)
m+jets: due to heavy flavor decays
Estimate real W+4 jets contribution
with scaling law
Additional topological cuts:
≥ 4 jets
HT>180 GeV (e)
Aplanarity>0.06
HT(jets,pT(W))>220GeV (μ)
“Matrix” method
Nloose = NW + NQCD
Ntight = sig  NW + qcd  NQCD

21. D0: b tagging
Soft lepton tag
b tagging efficiency

22. Lepton+jets: topological cuts and SLT

23. Cross section from topological analyses

24. D0: lepton+jets channels with b-tagging

25. CDF: lepton+jets channels with b-tagging

26. D0: e+jets channels with matrix element method
use the signal and background process matrix elements to calculate the
observation probability function;
for each pre-selected event(e+X), calculate the probability of being the
signal and background;
fit the data with the discriminator plot to extract the probability of
use likelihood function to extract the signal event fraction of the total
pre-selected events.
simulation result:
Discriminator:
background
signal
events
signal probability
background probability
D(x)

27. Run II cross section summary

28. Cross section √s dependence

29. First Run II look at all jets channel
Challenging signature: Very low S/B !
9 cross section & mass measured in Run I (CDF, D0)
Tools needs:
kinematical quantities, neural networks, b-tagging …
D0 Run I all hardonic channel

30. Top mass measurement

31. Lepton + Jets mass method
Additional complications from
background events
detector effect (mismeasurement + resolution)
initial and final state radiations

32. Lepton + Jets mass method

33. Mass from lepton + jets (Run I)

34. Mass from alljets (Run I)

35. Dilepton mass method The final state momentum and angular information
is sensitive to the top quark mass.

36. Dilepton mass method
D0: Run I
CDF: Run I

37. First look at top mass in Run II (CDF)

38. Single top physics
Run I results:

39. Search for single top in Run II

40. Spin correlation

41. Spin correlation

42. Spin correlation
D0 Run I Result:

43. W boson helicity if b quark mass=0, W polarizations can be
analyzed from the angular
or PT distributions of the
charged leptons.

44. W boson helicity

45. Summary A very rich top physics program is underway:
The Tevatron is the top quark factory until LHC:
First Run II results cover a variety of channels and topics
CDF and D0 are exploiting their upgraded detector features
Several top properties studied using Run I data (limited statistic)
There is a big potential to improve crucial aspects of physics
analyses (tracking in jets, physics object identification,
b-tagging optimization and many others).
A very rich top physics program is underway:
let’s see what the top quark can do for us!