Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://nuclphys.sinp.msu.ru/conf/lpp14/240809/PallinDominique.pdf Äàòà èçìåíåíèÿ: Wed Sep 16 16:32:48 2009 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:47:00 2012 Êîäèðîâêà: ISO8859-5 Ïîèñêîâûå ñëîâà: jet

Top Physics
with the ATLAS detector at LHC

D. PALLIN

on behalf of the ATLAS collaboration

CNRS-Blaise Pascal Univ./ LPC Clermont-FD D v in Mos , Augu t 9 14th LomonosoPallConfcow Augusts2002009 Moscow 1


Why the Top quark is so interesting ?
Properties
Large mass Mt = 173.1Á1.2 GeV [arXiv:0903.2503 (CDF+D0)] Top-Higgs Yukava coupling : t = 2 MT /v ~1 Interact heavily with the higgs sector => Suggest that the Top quark play a specific role in the electro weak symmetry breaking (EWSB). => All New Physics in connection with EWSB should couple preferentially to the Top quark : Top sector is an ideal laboratory to search for `New Physics' Short lifetime => The Top Quark decays before hadronisation => study the properties of a ? bare Ë quark (Top Mass)

Shopping list
Explore properties
Production mechanisms (X-sec, search for resonances), top properties ( mass, charge,decays...) top&W polarisations,...

Precise meass measurement => consistency test of the SM, and constraint for the Higgs boson Search for new physics Top is a BKG for New Physics searches, need to be understood ( X-sections) In addition at LHC 1. Top is a Reference point => Re- establishment of the top 2. Tool for Detector commissionning :
JES determination, b-tag and trigger efficiency measurement D Pallin Moscow August 2009 2


Top and LHC : from rare to common
LHC
Top factory

Measurement limited by systematics very soon
New generation of detectors Start-up Phase

Progressive ramping of the LHC (E, L) to reach 14 TeV, 1033 cm-2sO(100 pb-1) expected in 2009-2010 @ 7-10 TeV

1

Detector to be tuned and performance to be understood => But great potential for Top properties Projections from ATLAS @14TeV and 10 TeV shown here 14 TeV 10 TeV
`expected Performance of the ATLAS Experiment ATLAS-PHYS-PUB-2009-086 Detector, Trigger and Physics ' ATLAS-PHYS-PUB-2009-087

CERN-OPEN-2008-020

and

ATL-PHYS-PUB-2009-081
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Top Production at LHC/Tevatron
Pair production Single Top production

qq t t

gg t t

~10%
Opposite to Tevatron

~90%
6,7pb 833pb(Á
11%)

t channel
Tev LHC 1.47 pb 250 pb
(Á4%)

Wt channel
0.15 pb 60 pb
(Á8%)

qq tt

s channel
0.75 pb 10 pb
(Á8%)

LHC, 14 TeV At 7 TeV, X-sections drop by a factor 5 vs 14 TeV 100pb-1 @ 7TeV => ~16000 Top pairs produced

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top decay and tt decay channels
MS: BR (t Wb) 1 t t W+ b Wb
Decay Mode :

l+ b l- b
Di lepton

qq b l- b
21 %

l+ b qq b

qq b qq b
full hadronic

semi-leptonic or Lepton+jets

44 % 15 % 15 %

tau+X mu+jets e+jets e+e e+mu mu+mu all hadronic

1%

3%1%

D Pallin Moscow August 2009

30% e/Å + jets 5% ee/eÅ/ÅÅ

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Top Physics & the ATLAS detector
l

v b-jet

Top quark detection and reconstruction involve many detector properties :
Lepton reconstruction and Identification +trigger Jet reconstruction and calibration Missing transverse Energy evaluation Tracking b-tagging (lower eff at beginning?)

l b-jet jet jet

hadronic Calorimeter EM Calorimeter

Vertex detector Muon chambers Complete detector capability at play

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Top pair x-sec measurement with 200pb-1 @ 10 TeV
Lepton+jet evts
W CANDIDATE

ÇBaseline analysis
Ç Ç Ç Ç Ç Ç Ç lepton trigger pT >15 GeV 1 lepton pT >20 GeV 4 jets pT >20 GeV, 3 jets pT >40 GeV ET miss>20 GeV No b tag Had Top recons= 3 jets giving Highest Pt sum (W constraint Mw+- 10 GeV) for 1 jj comb.
T OP CANDIDATE

ÇAlternative method with no ET miss cut
Default selection S/B~1.5, eff~24%, 2600 evts With W constraint S/B~3.5 eff~ 12%, 1600 evts

200pb-1 @ 10 TeV Baseline analysis

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Top pair x-sec measurement with 200pb-1 @ 10 TeV
Ç

Counting Method (Baseline analysis)

Ç

BKG estim from MC+data driven method

ÇSignal Efficiency from MC Ç sensitive to BKG normalisation, #jets, JES, less to shape Muons 396 Á59 Top evts in gaussian peak

ÇLikelihood fit (Baseline analysis)
Ç gaussian+chebychev bkg Çextract X-sec by scaling with efficiency

e
Cut & count

/ = 3( stat)

Å
e
Fit

( syst) Á 22(lumi) - 15 + 12 / = 3( stat) ( syst) Á 22(lumi) - 15 +6

+ 14

% % % %

/ = 14( stat)

Å

( syst) Á 20(lumi) - 15 +6 / = 15( stat) ( syst) Á 20(lumi) - 15

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Top pair x-sec measurement with 200pb-1 @ 10 TeV
Di-Lepton evts
Ç Selection
Ç Ç Ç Ç Ç Ç lepton trigger pT >15 GeV 2 opp charge leptons (ee,eÅ,ÅÅ) pT >20-35 GeV 2 jets pT >20 GeV ET miss>20 GeV No b tag BKG rejection |M(Z)-m(ll)| >5 GeV ee channel

Ç Data driven evaluation of
ÇDY evts Ç jets miss-identified as leptons in QCD & W+jets evts ÅÅ channel

Ç other BKG from MC
Ç Di-boson, single Top Wt, Z-> , Wbb

X-sec from counting method
/ = 3.1( stat) + 9.6 - 8.7 + 26.2 - 17.4
eÅ channel

( syst)

(lumi)

%

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Top mass measurement with 1 fb-1 @ 14 TeV
tt lb+jjb
Golden plated channel avoid contribution from BKG, rely on well measured objects Top mass estimator built from the invariant mass of the hadronic top decay products The precision on the mass depends mainly on the accuracy to determine the Jet energy scale for light jets (JES) and b jets (JESb)

Selection at least 1 lepton pT>20 (25) GeV (trigger) at least 4 jets pT>40 GeV to keep only well measured jets Missing Et >20 GeV (for the escaping ) All particles emitted in ||<2.5 to keep only well measured & Identified particles Select sub-samples with
0, 1 or 2 identified b-jets among all selected jets
eff(b) =60% ; light jet rejection factor ~ 130

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Top mass measurement with 1 fb-1 (2 b-tag)
Mjjb =10.6Á0.4 GeV (Mtop)stat < 0.4 GeV Systematics uncertainties
Dominant uncertainty after a few fb-1 of data Main contribution to syst are JES & JESb (Mtop)syst ~ 1 (3.5) GeV if b-JES accuracy is 1 (5)% standard Purification cuts
Source of systematics Light JES b jet scale (1%) ISR/FSR b quark fragmentation background method TOTAL Top mass shift (GeV/cÂ) 0.2 /% 0.7 /% 0 .3 0 .1 negligible 0;1-0.2 0 .8

high Purification cuts

1 fb - 1 @ 14 TeV

ATLAS will measure the Top mass with a precision of 1 (3.5) GeV if b-JES controlled at 1% (5%)
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TTbar resonances
Top sector is an ideal laboratory to search for `New Physics'. Models provide candidates for tt resonances
Z' topcolor, kk excited states,...

Could be revealed in the tt mas spectrum (distortion or resonance)
Model independant search for a generic resonance

Study with standard ATLAS Top reconstruction
full reconstruction of tt lb+jjb with 2 b-tagged jets

1fb-1 @14 TeV : ATLAS able to discover tt res 700 GeV if çBR>11pb
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TTbar resonances
ATLAS study dedicated to high masses
At increasing tt masses
SM `BKG' decrease Combinatorial BKG contribution decrease But Reconstruction efficiency drops Top decay particles mixed => monojet =>Lepton non isolated

Look at tt lb+jjb evts with 2 monojets pt>300 geV
1 bjet merged with a lepton from leptonic Top
1 cluster of 3 jets merged from hadronic Top

Log likelihood variable (yL) using jet mass and k splitting scales
to cut QCD multijet evts

ATL-PHYS-PUB-2009-081

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Top Quark Charge with 1 fb-1 @ 14 TeV
4/3 excluded at 87% (92%) CL by CDF (D0) tt lb+jjb; lb+ lb with 2 b-tagged jets
Lepton+ b-jet pairing (purity 86%) using Ml-b cuts W->l charge from lepton b -quark charge from
b-quark charge from charge weighting technique Lepton charge in semi-leptonic b-decay l

Q

b-jet

weighting technique

b quark

b quark

= -0.094Á0.004s

tat

= xxx Á0.06s

tat

+0.08s

yst

Qt = Ql + Q

b - jet

çC

b

Cb b-jet charge calibration coefficient

ATLAS is likely to distinguish between SM and exotic charge hypotheses with a significance well above 5 for 1fb-1 of data
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Rare Top decays
FCNC
Current exp. limits
BR( t-> FCNC) in several models

Study Atlas reach for tt->(blv)(qX) , X=g,,Z-> l+lFull event reconstruction (no b-tag)

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conclusion
LHC will be a Top factory offering a great potential for Top precision studies
Hope to see first Tops in Europe in 2010

SM tests with Top (from ATLAS studies @14TeV)
Establish Top signal Top pair production X-section stat(5%)-syst(15-5%)-lumi(3%) Top mass measurement (5%-2%) Top as a Tool light jet (2-1%) b tag eff 3% Single Top production t channel@5 Top properties top charge 5 , W pol 5-10%, FCNC BR 10-3, ~10pb-1 ~100pb-1 ~100pb-1, 1fb-1 ~100pb-1, 1fb-1 ~1fb-1 ~ 1fb-1 1fb-1

BSM
Search for New physics using Top

But before any measurement
Detector understanding Measurement of detector performance Trigger, Calibrations, alignement, b tagging Background studies MC tuning on data

=>Top events serve as a tool for these studies
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Polarisations in tt events
Test of the top quark production and decay mechanisms
W boson or top spin information inferred from angular distribution of daugther particles in the parent rest frame

W-boson polarization
W produced with different helicities

BSM : different helicity fractions possible





Measurement of W helicities in tt lb+jjb channel: Determination of the Cos*l distribution Correct distribution distorded mainly by event selection, quark fragmentation and particle radiation Extraction of F0 FL FR
xxx xxx xxx

1 fb-1 : F0 FL FR measured with a precision of 0.04, 0.04 and 0.03 respectively
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Polarisations in tt events
Top spin correlations
Top quark decays before hadronisation => spin information conserved
SM : top unpolarised but top spins correlated. production asymmetry BSM : different correlation allowed

two angular distributions can be used to probe the top spins correlation

Measurement of the spins correlation in tt lb+jjb channel:
from 1,2 (A) and (AD) angular distributions (corrected from phase space) assuming =0.51 => two unbiased estimators of A and AD are built C=-9 x cos1 x cos2 and D=-3 x cos
xxx xxx

1 fb-1 : A and AD measured with a precision of 50% and 34% respectively

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anomalous couplings at the Wtb vertex
The W-boson polarisation is sensitive to new anomalous couplings associated wtih the Wtb vertex
General Wtb vertex

L = FL / F0 R = FR / F0 A+ = 3 [ F0 + (1 + ) FR ] A- = -3 [ F0 + (1 + ) FL ]

fit

V g g

R L R

SM : VL = Vtb and other term vanish ATLAS limits on anomalous couplings (1fb-1) :
red : analysis with b-tagging Yellow : analysis without b-tagging

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Single Top production
The single Top X-sections measurement
lead to direct measurement of Wtb Constitute a probe for new physics (H+, t channel -> FCNC, s channel -> W' bosons,.. )

Atlas measurement
W+jets,Top pair channel brings large BKG => complex analyses
use of multivariate tools

Common preselection for all channels; dedicated MVA for each channel

1fb-1 : ATLAS results @ 14 TeV
t channel : / =5.7(stat)Á22(syst) % Wt channel : / =20.6(stat)Á48(syst) % s channel : / =60(stat)Á90(syst) %
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=> |Vtb| / |Vtb| = 12%


BACKUP

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Top physics: broad physics content
Productions mechanisms
Production X-sections Vtb Spin correlations Ttbar production by new resonances
Production cross-section Resonance production Production kinematics Top Spin Polarization Top Mass Top Width Top Spin Top Charge W helicity

l W+

+

Anomalous Couplings CP violation

Properties
Top mass Charge Decay properties
Electroweak (V-A) vertex: W helicity Rare Top decays

t
_

b Y X

t

Search for New physics using heavy flavour

Rare/non SM Decays Branching Ratios

|Vtb|

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The Large Hadron Collider
pp 25 1.1 De ATLAS and CMS : pp, general purpose collision cm : 14 TeV (x7 Tevatron) ns bunch spacing 1011 proton/bunch sign luminosity 1034 cm-2s-1 100 fb-1 /year; 20 int./x-ing Initial/low lumi L1033 cm-2 s-1 10 fb-1 /year ; 2 int./x-ing 4 interaction regions

27 km ring 1232 dipoles B=8.3 T
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Top Production at LHC
At low Luminosity (1033), 14 TeV ~ one top pair produced per second LHC is a Top factory But 108 evts /s are produced

LH C

tt ~830 pb X100

Tevatron tt ~ 6,7 pb LHC Low L 1033 cm-2s-1 X10 Tevatron 1032 Prod Rate X1000
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Which detector performance on day one ?

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Some examples of studies
SM tests with Top
Establish Top signal Top pair production X-section stat(5%)-syst(15-5%)-lumi(3%) Top mass measurement (5%-2%) Top as a Tool light jet (2-1%) b tag eff 3% Single Top production t channel@5 Top properties top charge 5 , W pol 5-10%, FCNC BR 10-3, ~10pb-1 ~100pb-1 ~100pb-1, 1fb-1 ~100pb-1, 1fb-1 ~1fb-1 ~ 1fb-1 1fb-1

BSM
Search for New physics using Top

From the updated TDR (CSC BOOK)
Expected Performance of the ATLAS Experiment : Detector, Trigger and Physics' (arXiv:0901.0512 ; CERN-OPEN-2008-020) Studies @1033 14 TeV, 1fb-1 of data
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tt Åb+jjb selection
Physical BKG
Main background: W+n jets Others
QCD bb Z+jets WZ tt jets, tt+X, Single Top
partially counted as signal when only tt jjb is considered

1 fb-1
Eff= 14% (5%) Purity=75% (91%)

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HadronicTop reconstruction 2 b-jet case
Comb bKG is made of
Wrong association chosen One of the jet has not been selected => the right combination cant be selected (main contribution to comb BKG) (Wrong W mainly)

=> Purification cuts to remove the comb bkg

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HadronicTop reconstruction 2 b-jet case
standard Purification cuts high Purification cuts

Standard Purification cuts (eff=75%, 85% of bkg rejection) Mtop= 174.6 Á0.5 GeV =14.1Á0.5 GeV

High Purification cuts (eff=65%, 95% of bkg rejection) Mtop= 175.0 Á0.4 GeV =14.3Á0.3 GeV

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TTbar resonances
ATLAS study dedicated to high masses
At increasing tt masses
SM `BKG' decrease Combinatorial BKG contribution decrease But Reconstruction efficiency drops Top decay particles mixed => monojet =>Lepton non isolated

Look at tt lb+jjb evts with

1

non isolated lepton in a jet pT>200 GeV (b jet)

Fake lepton rejection Missing Et associated to v to reconstruct leptonic W 1 monojet candidate pT>300GeV (bqq) Log likelihood variable (yL) using jet mass and k splitting scales

ATL-PHYS-PUB-2009-081

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