Transcript PowerPoint 演示文稿 - Shandong University
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
Tevatron Collider in Run II The Tevatron is a proton-antiproton Collider with 980 GeV/beam
s
=1.96TeV in RunII (1.8TeV in RunI) 36 P and Pbar bunches
a
396 ns between bunch crossing Increased from 6X6 bunches with 3.
5m
s in Run I Increased instantaneous luminosity Run II goal Current: ~
Run II D0 Data Taking Status 85~90%
D0 Detector (Run II)
Silicon Microstrip Detector (SMT)
PVrt/IP~ 15 m m
Vertex resolution: ~10
m
m (design) Primary Vertex vs. Impact parameter
Center Fiber Tracker (CFT) SMT combines vertex and tracking capabilities and provides good primary and secondary vertex resolutions.
The Calorimeter y
q j
x Z Resolution: s/E ~ 15%/√E(GeV) “fine” EM 50%/√E(GeV) “coarse” jet sMET ~ a + b*ST + c*ST2 ST scalar sum of ET a ~1.89GeV, (run1) b ~6.7E-3, c ~9.9E-6/GeV
Muon Detector
J/Psi: Local / Global
D0 Detector Performance
Motivation for the Top Quark Studies (I)
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Top quark has been discovered by CDF and D0 in 1995;
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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.
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Tevatron is the only palce to study top quark properties before LHC operation.
Motivation for the Top Quark Studies (II) Top Mass, W Mass Measurement
Top Physics Understanding Program Top production & decay Tools Cross section Mass Single top Spin correlation W helicity
Top Quark Production at Tevatron Top-antitop quark Pair Production (mainly)
s
(pb) RunI RunII 4.87(10%) 90% 6.70(10%) 85% 10% 15% Single top quark production (not yet observed) RunII
s
(pb) 0.9(10%) 2.0(10%)
Top Quark Decay In the standard model, the top quark is short lived and decay almost exclusively to W and b quark
Methodology& tools Full characterization of the chosen final state signature in term of SM background processes (control region)
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Optimize signal for best measurement precision How to separate signal from background:
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Top events have very distinctive signatures
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Decay products (leptons, neutrinos, jets) have large PT
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Event topology: central and spherical
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Heavy flavor content: always 2 b jets in the final state Tools (need multipurpose detectors)
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Lepton ID: detector coverage and robust tracking
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Calorimetry: hermetic and well calibrated
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B identification: algorithms pure and efficient
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Simulation: essential to reach precision goals
Production cross section
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t t
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5 1 1 .
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pb
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7 1 .
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pb
RunI~100 events
Top cross section: dilepton channels
CDF & D0: dilepton channels
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Top cross section: lepton+jets “Golden” mode for top studies: ~ 30% yield and relatively clean
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 (
p o
and
g
)
m+jets : due to heavy flavor decays Estimate real W+4 jets contribution with scaling law Additional topological cuts: ≥ H Aplanarity> H 4 jets T T > 180 GeV (e) (jets,p T 0.06
(W))> 220GeV (μ) “Matrix” method N loose N tight = N W =
sig
+ N QCD N W +
qcd
N QCD
D0: b tagging Soft lepton tag b tagging efficiency
Lepton+jets: topological cuts and SLT
Cross section from topological analyses
D0: lepton+jets channels with b-tagging
CDF: lepton+jets channels with b-tagging
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 signal and background; use likelihood function to extract the signal event fraction of the total pre-selected events.
simulation result:
Discriminator:
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x i
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P s
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x i P s
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x i P b
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x i
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P s
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signal probability
P b
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background probability
W
background
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e
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jets
signal D(x)
Run II cross section summary
Cross section √s dependence
First Run II look at all jets channel Challenging signature:
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Very low S/B !
cross section & mass measured in Run I (CDF, D0) Tools needs: kinematical quantities, neural networks, b-tagging … D0 Run I all hardonic channel
Top mass measurement
Lepton + Jets mass method Additional complications from background events detector effect (mismeasurement + resolution) initial and final state radiations
Lepton + Jets mass method
Mass from lepton + jets (Run I)
Mass from alljets (Run I)
Dilepton mass method The final state momentum and angular information is sensitive to the top quark mass.
Dilepton mass method D0: Run I CDF: Run I
First look at top mass in Run II (CDF)
Single top physics Run I results:
Search for single top in Run II
Spin correlation
Spin correlation
Spin correlation D0 Run I Result:
W boson helicity if b quark mass=0, W polarizations can be analyzed from the angular or PT distributions of the charged leptons.
W boson helicity
Summary 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!