Top Physics at DØlpnhe-d0.in2p3.fr/d0-paris/seminaires/PSU05.pdf · Top Physics at DØ Dr. JeanRoch Vlimant, 11 Nov 2005 12/40 Noise suppression Online suppression at 1.5 ped Limits

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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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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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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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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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The DØ calorimeter

Improvements●Readout corrections●Noise suppression●Non linearity correction●Energy calibration

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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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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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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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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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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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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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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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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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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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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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➢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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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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Top pair production Dilepton channel


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


●Main systematics➔ Jet Energie calibration➔ Jet reconstruction➔ Lepton identification

●Event counting analysis●Backgrounds

➔ QCD (fake leptons)➔ Dibosons production➔ Z/ → dilepton

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Top pair production Lepton+jets channel


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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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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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


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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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●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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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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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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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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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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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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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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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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Top quark mass

Dilepton channel

Lepton+jets channel


●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 JES

Matrix element method

320 pb1

230 pb1

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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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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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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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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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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.

Documents

Top Physics at DØlpnhe-d0.in2p3.fr/d0-paris/seminaires/PSU05.pdf · Top Physics at DØ Dr. JeanRoch Vlimant, 11 Nov 2005 12/40 Noise suppression Online suppression at 1.5 ped Limits