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LPNHE - PARIS
Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 1/40
Top Physics at DØ
Dr. JeanRoch VlimantNuclear and High Energy Physics Laboratory (LPNHE)
University of Paris VI France
Hosted by Dr. Tyce DeYoung
HEP seminar at PSU Friday November 11th
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 2/40
Outline
● Fermilab and The Tevatron
● The DØ detector
– Calorimetery
● The Top quark
– Production and decay
– Cross section measurements analysis
– Properties
● Summary
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 3/40
Fermilab
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The Tevatron●Cesium gun
➢ Source of H ions●Cockcroft Walton
➢ Continuous stream➢ 750 KeV
●Linac➢ 100 Mhz, 6.3E12 ions bunches➢ 400 MeV
●Carbon foil : H to proton●Booster
➢ 12 bunches ➢ 8 GeV
●Main injector➢ 150 Gev➢ Nickel target : antiproton source
●Tevatron➢ 3 x 12 bunches ➢ 980 GeV➢ Bunch spacing 396 ns
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 5/40
The Tevatron
Peak luminosity record ~ 1.6E32 cm2s1 = 160 ∝ b1s1
Delivered luminosity nearly 1.4 fb1
~5 fb1 by 2009
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The DØ Run II detector
Coarse hadronic
Liquid argon sampling Calorimeter
End cap calorimeter
ElectromagneticFine hadronic
Coarse hadornicElectromagnetic
Fine Hadronic
Microstrip tracker and Fiber trackerSolenoid BField 2 Tesla
Muon systemToroidal BField 1.8 Tesla
5
5
10 5 1050
0
[m]
Beam axis (z)
3 level trigger system
➔Collision rate 2.5 MHz➔L1. Calo., track and muon 2 kHz➔L2. Calo., track, muon and vertex 800 Hz➔L3. online reconstruction 50 Hz
Central calorimeter
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The DØ detector
~1 fb1 recorded~480 pb1 current analysis
~88% data taking efficiency
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The DØ calorimeter
Beam axe
Online reconstruction
3 sectionsElectromagnetic (Ur/Ar)
Fine hadronic (CuNo/Ar)coarse hadronic (Fe/Ar)
Readout electronics➔ Preamplification➔ Shaping➔ Level 1 trigger signal➔ Gain selection➔ Analog memory (SCA L1)➔ Baseline subtraction (BLS)➔ Analog memory (SCA L2)➔ Digitisation (ADC)➔ Online reconstruction➔ Tape recording
Gain selector
ADC boardBLS boardPreamp.Gains
Shaper
Level 1 trigger signal
x8
x1 x8
L2L1
L3
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 9/40
The DØ calorimeter
Improvements●Readout corrections●Noise suppression●Non linearity correction●Energy calibration
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 10/40
Readout corrections
●Slight problem in synchronisation between BLS and ADC boards●Selected gain information was sometimes lost (hardware fixed)●Factor 8,1/8 in cell energy●Bad estimation of object energy
electron e
lec
tro
n
Online reconstruction
ADC boardBLS boardPreamp.Gains
Shaper
Level 1 trigger signal
x8
x1 x8
Gain selector
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 11/40
Readout corrections
●Algorithm for correction uses the L1 trigger information that is redundant●Modify (*8,*1/8) cells energy when large L3L1 transverse energy●Test the algorithm on Z→ e+e data.It recovers correct electron energy.
e+e mass [GeV]
ET L3L1 [GeV]
BeforeAfter
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 12/40
Noise suppression
●Online suppression at 1.5 ped
➔ Limits data files size●Offline suppression at 2.5
ped
➔ Decreases the level of noise
Neighboring cells
Gaussian electronics noise with width
ped➔Elec. ~50 MeV➔Fine Had. ~90 MeV➔Coarse Had. ~300 MeV
●Implementation of the T42 algorithm➔ Keep cells with signal greater than 4
ped
and neighboring cells with signal greater than 2.5
ped
➔ Dynamic noise suppression➔ Enhance cluster like energy deposition➔ Reject isolated energy deposition
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 13/40
Noise suppression
PT sans T42 [GeV]
(PT s
ans
T42
P
T ave
c T
42)
[Ge
V]
Remove noisy jets
Remove uniforme energy density
METx [GeV] METx [GeV]
14%
centered
7%
MET/SET [GeV½]
Missing transverse energy decreases
Significance improves
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 14/40
DAC
AD
C
DAC
AD
C
DA
C
Nonlinearity
Gain 8Gain 1
Electronics calibrationSCA ship (analogical memory) have
non linear functionning regions
Calorimeter pulser system : ● Artificial signal injected before preamps.● Digital respons as a function of pulse strength● Calorimeter readout calibration
➔ NLC corrections, ...
e+e mass (GeV]
28%Effect on electron energy resolution
● 28% on J/mass● 6% on Z mass
e+e mass (GeV]
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 15/40
The Top quark
●Discovered in 1995 at Fermilab➢ Electroweck partner of the B quark (1977 Fermilab)
●Short lifetime E25 s (hadronisation E23 s)➢ Decay as a « free » quark➢ Helicity, spin, mass propagated to decay products
●Large mass 174 GeV➢ Yukawa coupling ~1➢ EW constraint on the Higgs mass➢ Constraint on physics behond the Standard Model
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 16/40
Top quark production
➔Strong force vertex : pair productionCacciari et al., JHEP, 404, 68 (2004)Kidonakis and Vogt, Phys. Rev. D 68 (2003) 114014
6.7 pb ±6%
85% 15%
➔Vtb
electroweak vertex : single top productionHarriset et al., Phys.Rev. D66 (2002) 054024
0.88 pb ±8%
1.98 pb ±11%
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 17/40
Top quark decayWithin SM, top quarks EW decays electroweakly before hadronisationCKM matrix element V
tb~1 ( Unitarity with 3 generations )
Top quark predominantly decays into Wb
bjet, may be identified with btagging
W decay channels Quark pair (67% incl.) : mostly two jets Lepton/neutrino (11% each flavour) : high pT
lepton and missing transverse energy (MET)
➔Top decay channels classified by W decay channels➔Exotic decays far below current exp. precision
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 18/40
Single Top productionSM cross section of the order 3 pb (~half of pair production)But overhelming background from any lepton+2 jets+MET events
➢ Top pair production➢ W+jets, dibosons production
➔Need 1.5 fb1 for observation (by 2006)➔Need 4.5 fb1 for discovery (by 2009)
DØ preliminary result with 370 pb1 ~370 1btag events (370±27 back. 19±2 exp. signal)Signal expectation is consistent with background uncertaintyNo observation yetLimits on cross section are set
➢ schannel < 5.0 pb➢ tchannel < 4.4 pb
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 19/40
Top pair production
Three main channels➔All jets :
Both W into quark pair, BR~46% Dominant QCD background
➔Dilepton : Both W into lepton/neutrino, BR(ll)~1.2, BR(ll')~2.4 % Pure but low statistics
➔Lepton+jets : One W into lepton/neutrino, the other into quark pair, BR~15% Compromise between background and statistics
Common to all channels
At least 2 bjets
ex. W → e ν decay
ex. W →∝ ν decay
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 20/40
➢Bhadrons (lifetime ~1.5 ps) don't decay at PV➢SV significantly displaced [500m to few mm] from PV➢B decay tracks with large PV impact parameter (d0)
d0 significance
bjet identificationSVT (secondary vertex tagger)
➔ signal btagging efficiencies✔ 45% with 1 tag✔ 15% with 2 tags
jetBtagging
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 21/40
Top pair productionAlljet channel
ttalljets=5.2−2.5
2.6stat −1.01.5syst ±0.3lumi pb
●At least 6 jets➢ 0.5 cone jets➢ p
T > 15 GeV,| η |<2.8
●At least one btagged jet●Neural Net. on 6 Kinematics variables
➢ NN>0.9
●Main systematics➔ Jet Energie calibration➔ Jet reconstruction➔ Tagging efficiency
350 pb1, Preliminary result
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 22/40
Top pair production Dilepton channel
●At least 2 jets➢ 0.5 cone jets➢ p
T > 35 GeV,| η |<2.5
●2 leptons (ee, e ∝ , ∝ ∝ )➢ p
T > 15 GeV
➢ Electron | η |<1.1 or 1.5<|η |< 2.5
➢ Muon | η |< 2.0●At least 2 neutrinos
➢ ETmiss > 25 GeV
ttdilepton=8.6−2.0
2.2stat −1.01.2syst ±0.6lumi pb
370 pb1, Preliminary result
●Main systematics➔ Jet Energie calibration➔ Jet reconstruction➔ Lepton identification
●Event counting analysis●Backgrounds
➔ QCD (fake leptons)➔ Dibosons production➔ Z/ → dilepton
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 23/40
Top pair production Lepton+jets channel
●At least 4 jets➢ 0.5 cone jets➢ p
T > 15,20 GeV,| η |<2.5
●Only 1 lepton (e, ∝ )➢ p
T > 20 GeV
➢ Electron | η |<1.1 ➢ Muon | η |< 2.0
●At least 1 neutrinos➢ E
Tmiss > 20 GeV
Main backgrounds➢ W+jets➢ Multijets
Secondary backgrounds➢ Dilepton channel➢ Single top➢ Diboson production
Two methods➔Counting events after btagging of 1 or 2 jets➔Use event kinematics discrimination wihtout btagging
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 24/40
Analysis using btagging
l+jets (2 tags)l+jets (1 tag)
Lepton+jet trigger,W(→ℓ)+jets (p
T>15 GeV) selection
Likelihood optimisation⊗Estimation of number of events with 1 and 2 jets tagged
Determine the signal content in 8 independent channels
⊗Nuisance of sytematic sourcesAllow for shift of Xsec
➢ Third and fourth inclusive jet multiplicity➢ Estimation of QCD background purely from data➢ W+jets normalisation from data
W+jets Flavor composition from MC simulation➢ Other backgrounds from NLO cross sections
single top, diboson
AnalysisControl bins
Analysis
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 25/40
Analysis using btagging
Systematics : 11%➔W+jets flavor 5%➔btag efficiency 5%
Top enriched sample
Presence of W boson Large transverse energy
ttbtag. lepton jets=8.1−1.2
1.3statsyst ±0.5lumi pb
370 pb1, Preliminary result
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 26/40
Analysis using event kinematics
●W boson from top quark are more transverse than in the W+jets background●QCD and W+jets topology are very similar●
●Build a likelihood function out of six variables that optimize the expected stat+syst error
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 27/40
Analysis using event kinematics
●Signal peaks at 1●W+jets peaks at 0●QCD and Z+jets backgrounds have same shape as W+jets●Top pair to dilepton contribution is taken into account
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 28/40
Analysis using event kinematicsLepton+jet trigger,
W(→ℓ) + jets (pT>20 GeV) selection
Likelihood optimisation⊗Kinematics discriminant distributiondiscriminate signal from background
⊗QCD background estimationdiscriminate QCD from W+jets
e+(4jets) +(4jets)
➢ Fourth inclusive jet multiplicity➢ Estimation of QCD background purely from data➢ Dilepton channel contamination relative to signal estimated
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 29/40
Analysis using event kinematics
ttKine. lepton jets=6.7−1.3
1.4stat −1.11.6syst ±0.4 lumi pb
230 pb1, Published result
Excesses modeled by signal simulation
Sample with large transverse energy
Systematics : 22%➔Jet Energy calibration 18%
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 30/40
Analysis using event kinematics
Improvements forseen➔Using event with at least 3 jets to double the signal statistics : preliminary results not approuved yet because of trouble in mu+jets channel➔4 times more statistics data sample➔Improved systematics treatement➔Improved fitting method
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 31/40
Summary➔DØ has precise measurements of Top pair production cross section ➔Results are in agreement with SM expectation➔Best single Top production cross section limit
➔Even more precise measurement foreseen with 1fb1 recorded
11% stat. → 7% Work on the main sources of
systematic uncertainty➔Single top observation by 2006
➔Controled samples for Top properties measurements
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 32/40
Top quark properties
✔ Top quark massYukawa couling ~1
✔ Top quark and W helicitytest VA theory, find exotic Top decay
✔ t → Wb branching ratiotest of SM, V
tb measurement
From top enriched sample with little, controlled background, Top quark properties can be study
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 33/40
Top quark mass
●Top quark mass enters the EW constraint on the Higgs mass. Even constraint of physics beyond SM●Need a sample as pure as possible in top quark pairs
➔ Lepton+jets : have to control the backgrounds➔ Dilepton : pure, but very few events
●Need to reconstruct the event kinematics➔ Constraint fit with W boson mass and 2 equal Wb
masses : low bias template method➔ Use kinematics directly from matrix elements : matrix
elements method
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 34/40
Top quark massLepton+jets channel
low bias template method
Most probable Top mass
●Increase purity➔ Use kinematics discriminant➔ btagging (shown below)
●Reconstruct event kinematics fit constraint fit●Fit top mass distribution to MC with different assumption of Top mass
230 pb1230 pb1
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 35/40
Top quark mass
Dilepton channel
Lepton+jets channel
Most probable Top mass
●Calculate probability to have the observed final state from matrix element and PDFs.
➔ Peak mass in dilepton channel. Fit peak mass disstribution with MC
➔ Top mass in lepton+jet channel. Most probable value.
Most probable Top mass
Most probable JES
Matrix element method
320 pb1
230 pb1
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 36/40
Top quark mass●DØ contributes to Top quark mass world average●Same order of precision than RunI results
➢ Different dectector (calorimeter elec.)➢ Still working on main systematics (JES)
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 37/40
Top quark and W helicity
right handed fraction
t
b
W
%0≈+f
left handed fraction
t
b
W
%30≈−f
Longitudinally polarized fraction
t
b
W
)%6.11.70(
2 222
2
0
±=++
=btW
t
mmM
mf
Top quark decays before hadronizationMassless bquark has negative helicityRight handed W boson is rare
+½
+½
0 +½ +½½
+1
+1
½
(Forbiden if mb=0)
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 38/40
Top quark and W helicity●Study electron angular distribution in W( → eν ) rest frame with respect to Top quark momentum●Consistent with no right handed W boson
WL
WR
W0
Theory Experiment230 pb1
btagged
f+=0.00±0.13 stat ±0.07 syst , f+0.25@95% C.L.230 pb1, Published result
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 39/40
Top → Wb branching ratio●Lepton+jets final state●Events with 3 or more than 4 jets●Breakdown into 0tag, 1tag, 2tag
R=1.03−0.170.19statsyst , R0.64, ∣V tb∣0.80@95% C.L.
230 pb1, preliminary result
Pn−tag=R2Pn−tagbb 2R 1−R Pn−tagbq 1−R 2Pn−tagqq
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Dr. JeanRoch Vlimant, 11 Nov 2005Top Physics at DØ 40/40
Summary● The Tevatron is the only current experiment for Top quark
analysis, even for years to come, before LHC warms up
– Production and properties
– Electroweak production obersvation soon
● DØ data quality has been significantly improved over last years.
– Decreased systematics and energy resolution
● More than 1fb1 recorded for analysis in the pipeline
● Analysis are constantly improving
● Far into precision era at the Tevatron : CDF and DØ.
%d0%a4%d0%93%d0%9e%d0%a1%203 %20%d0%9b%d0%b5%d1%87%d0%b5%d0%b1%d0%bd%d0%be%d0%b5%20%d0%b4%d0%b5%d0%b
%d0%9d%d0%b0%d0%b2%d1%87%d0%b0%d0%bb%d1%8c%d0%bd%d0%be %d0%bc%d0%b5%d1%82%d0%be%d0%b4%d0%b8%d1%87%d0