Top Quark Physics at LHC with ATLAS

1
Top Quark Physics
at LHC
with ATLAS
D. PALLIN Blaise Pascal Univ./ LPC
Clermont-FD Pragua 6/04/09
2
Top quark Discovery

• 11/11/1974 J/? and charm quark c discovery at
  SLAC BNL
• Two families of leptons and de quarks
• Leptons ?e ?? Quarks u c
• e ? d
  s
• 1976-1977 third fermion family is discovered
• ? ? lepton b quark
• Is the b quark belonging to an isospin doublet?
• Quarks u c ?
• d s b
• 1978-1994
• measures suggest an iso-doublet
• Direct Top searches are negatives
• LEP electroweak fit MS M(Top)
• 150-170 GeV
• 1995 Top quark discovery at Tevatron

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The Top quark in the MS

• Complete the 3e quark family
• SU2L weak isospin Partnair of bottom quark
• Spin1/2
• electricCharge2/3
• Color Triplet
• RQ NO direct measurement of quantum numbers of
  theTOP quark, only indirect informations
• The free parameters in the Top sector are
• The Top Mass (free fondamental parameter of the
  MS)
• CKM matrix elements
• unitarity gt Vtb 0.9990 -0.9992 gt t?? Wb
• Coupling fixed by the jauge structure
• Width computable from the SM

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Why the Top quark is so interesting ?

• Large mass
• The only fermion heavier than the W
• Mt MAU 35Mb
• Top-Higgs Yukava coupling ?t ?2 MT /v 1
• Interact heavily with the higgs sector
• gt Suggest that the Top quark play a specific
  role in the electro weak symetry breaking
  (EWSB).
• gt 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
• gt We can study the properties of a  nude 
  quark (Top Mass)

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Shopping list in the Top sector

• Explore properties
• Precise meass measurement gt 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
• Top is a Reference point gt Re- establishment of
  the top
• Tool for Detector commissionning

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Top physics Which measurements ?

• Productions mechanisms
• Production X-sections
• Vtb
• Spin correlations
• Ttbar production by new resonances
• Properties
• Top mass
• Charge
• Decay properties
• Electroweak (V-A) vertex W helicity
• Rare Top decays
• Search for New physics using heavy flavour

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Physique du Top state of Art

• Tevatron CDF D0
• the unique source of Top
• Since 1995 quark Top properties studies
• Run I
• Run 2
• Measurement limited by statistics
• Very good understanding of the detector
• Transition towards précision measurements
• gt4fb-1 ( up to 8fb-1)

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Top and LHC from rare to common

• LHC ?
• Top factory
• Measurement limited by systematics very soon
• New detector generation
• Start-up Phase
• Progressive ramping of the LHC (E, L)
• Detector to be tuned and performances to be
  understood
• But great potential for Top properties

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The Large Hadron Collider

• pp collision cm 14 TeV (x7 Tevatron)
• 25 ns bunch spacing
• 1.1 1011 proton/bunch
• Design luminosity 1034 cm-2s-1
• 100 fb-1 /year ? 20 int./x-ing
• Initial/low lumi L?1033 cm-2 s-1
• 10 fb-1 /year ? ?2 int./x-ing
• 4 interaction regions

ATLAS and CMS pp, general purpose
27 km ring 1232 dipoles B8.3 T
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The Large Hadron Collider

• 1st beam in LHC September 10, 2008
• 1st collision at 450450 GeV expected end
  september
• Incident (dipole connection) on Sept 19.
• No collisions in 2008
• Remember everyone that LHC machine represents a
  challenge
• O(100 pb-1) expected in 2009-2010 _at_ 10 TeV

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Top quark production in hadron colliders
Single Top Production via EW interaction
s channel
Wt channel
t channel
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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
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Production du Top au LHC/tevatron
At 10 TeV, X-sections drop by a factor 2 100pb-1
gt 40000 Top pairs produced At 10 TeV with
about 100 pb-1 the ATLAS top sample has the size
of the complete Tevatron sample
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top decay and tt decay channels

• MS t ? Wb domine

tauX
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mujets
44
ejets
15
ee
15
emu
mumu
all hadronic
1
30 e/? jets 5 ee/e?/??
3
1
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ATLAS detector

• Top quark detection and reconstruction Involves
  many detector properties
• Lepton reconstruction
• and Identification
• Jet reconstruction
• and calibration
• Missing transverse
• Energy evaluation
• b-tagging
• (lower eff at beginning?)
• Complete detector
• capability at play

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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
  
  10pb-1
• Top pair production X-section stat(5)-syst(15-5
  )-lumi(3) 100pb-1
• Top mass measurement (5-2)
  100pb-1,
  1fb-1
• Top as a Tool light jet (2-1) b tag eff 3
  100pb-1, 1fb-1
• Single Top production t channel_at_5?
  1fb-1
• Top properties top charge 5 ?, W pol 5-10,
  FCNC BR 10-3, 1fb-1
• 
  
• BSM
• Search for New physics using Top
  ? 1fb-1

From the updated TDR (CSC BOOK)   Expected
Performance of the ATLAS Experiment
Detector, Trigger and Physics
(arXiv0901.0512 CERN-OPEN-2008-020) Studies
_at_1033 14 TeV, 1fb-1 of data
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tt?(Wb)(Wb) decay

• full hadronic
• tt? (jjb) (jjb)
• large BR 44
• Large multijet BKG
• Lower Trigger efficiency
• Di-leptonic le, µ
• tt? (lvb) (lvb)
• Low BR 4
• Low BKG
• Lepton trigger
• leptonjets le, µ
• tt? (lvb) (jjb)
• BR 30
• Reduced BKG (,Wjets, QCD, single Top, Z-gtll)
• Lepton trigger

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Top pair x-sec measurement with 100pb-1

• Leptonjet evts
• lepton trigger pT leptongt20 Gev
• 4 jets pT gt20 Gev, 3 jets pT gt40 Gev
• ET missgt20 GeV
• Top 3 jets giving Highest Pt sum
• No b tag
• (W constraint Mw- 10 GeV) for 1 jj comb.

Default selection S/B2.3, eff24
With W constraint S/B3.5
Top contribution visible even with 10 pb-1
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Top pair x-sec measurement with 100pb-1

• Likelihood fit
• gaussianchebychev bkg
• extract X-sec by scaling with efficiency
• Counting Method
• sensitive to BKG normalisation, jets, JES,
  less to shape

Muons 508 Top evts 100pb-1
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Top mass measurement with 1 fb-1

• tt???bjjb selection
• avoid contribution from BKG, rely on well
  measured objects
• Select events containing
• at least 1 lepton pTgt20 (25) GeV (trigger)
• at least 4 jets pTgt40 GeV to keep only well
• measured jets
• Missing Et gt20 GeV (for the escaping ?)
• All particles emitted in ?gt2.5 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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tt???bjjb selection

• Physical BKG
• Main background Wn jets
• Others
• QCD bb
• Zjets
• WZ
• tt? jets, tt??X, Single Top
• partially counted as signal
• when only tt? jjb is considered

1 fb-1
Eff 14 (5) Purity75 (91)
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Top Mass measurement strategy

• Top mass estimator built from the invariant mass
  of the hadronic top decay products
• Mjjb provides the most natural way to measure the
  Top mass
• Close to the pole mass O(?QCD) 100MeV
  (fragmentation effects )
• needs to reconstruct the Top decay chain
• Reconstruction of the hadronic top t? Wb ? jjb
• For early data Use simple method and do not rely
  on MC
• Find the jet pair originating from the W ?jj and
  the b jet forming the top
• Wrong association gt combinatorial BKG (reduced
  if jet b-tag used)
• The invariant mass peak should be a gaussian
  distribution centered on the Top mass
• The precision on the mass depend mainly on the
  accuracy to determine the Jet energy scale for
  light jets (JES) and b jets (JESb)

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HadronicTop reconstruction2 b-jet case

• Find first the W jets
• Closest jets
• Min(Mjj-MWpeak)

50 of the events have more than 2 light jets
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HadronicTop reconstruction2 b-jet case

• Find first the W jets
• Closest jets
• Min(Mjj-MWpeak)
• Then the b jet in t?Wb
• Closest b jet from W

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HadronicTop reconstruction2 b-jet case

• Comb bKG is made of
• Wrong association chosen
• One of the jet has not been selected gt the right
  combination cant be selected (main contribution
  to comb BKG) (Wrong W mainly)

gt Purification cuts to remove the comb bkg
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HadronicTop reconstruction2 b-jet case
standard Purification cuts
high Purification cuts
Standard Purification cuts (eff75, 85 of bkg
rejection) Mtop 174.6 0.5 GeV ?14.10.5 GeV
High Purification cuts (eff65, 95 of bkg
rejection) Mtop 175.0 0.4 GeV ?14.30.3 GeV
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HadronicTop reconstruction2 b-jet case

• Mtop? Mjjb-Mjj80.2 is a better estimator of the
  top mass
• Uncetainties on the light jet energies (W jets)
  affect both Mjj Mjjb
• Uncertainties cancel at first order on the mass
  difference Mjjb-Mjj
• Since MW is well known, measure only the mass
  difference
• Impact from the light JES uncertainties is lower
  on the Top mass determination
• The resolution on the Top mass measurement
  improves

Mtop 175.4 0.4 GeV ?10.60.4 GeV
Mtop 175.3 0.3 GeV ?10.60.2 GeV
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Uncertainties on Top mass measurement 2 b-jet
case 1fb-1

• Statistical uncertainty
• Estimated for 1fb-1 using a bootstrap resampling
  technique
• ?(Mtop)stat lt 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 JES accuracy is 1
  (5)

1fb-1
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Top mass measurement with 1 fb-1

• The Top mass is measurable with
• an accuracy of 1 GeV with 1fb-1 of data
• Mainly driven by the reached precision on JES
• For light jets gt JES from Mw
• For b jets gt JES(b)/JES(light) MC modeling at
  start
• Zbjet, (di-jets b / di-jets light),
  when enough stat
• At LHC start
• analyses will try to rely as low as possible on
  MC since not tuned
• Selections should be simple, non biased

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Top as a tool

• Light jet JES
• B jet JES
• B tag eff
• Trigger eff

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Jet reconstruction and calibration
Reconstruction and Calibration Scheme
Detector effects detector response should
reflect the real deposited energy Jet
algorithm effects Energy deposited in
calorimeter cells are grouped in clusters to form
a jet Most of the energy of the jet belong to the
originating parton, but some extra energy comes
from other particles So in principle Eparton
JES Ejet with JES 1 and JES f(Ejet,?jet,)
Calorimeter Cells
clustering
Clusters
EM scale
Global Approach (Default Scheme)
Jet Reco Alg
Uncalibrated Jets
Jet Energy Calib to Particle Level
Calibrated Jets
Particle level
In situ calibration
Physics Jets
Parton level
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JES in-situ determination

• JES determination is the key point of the top
  mass measurement
• AIM Rescale jet to parton energy JES 1/
  (Ejet / Eparton)
• In-situ calibration JES from W?jj in ttbar
  events
• Select an almost pure sample of w?jj candidates
  in tt???bjjb events
• calibration dedicated for TOP?

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JES from W?jj in ttbar events method

• The W mass is a precise reference ?MW?30 MeV
• The W mass depends on
• jet energies and
  opening angle between J1 j2
• Angle (J1,J2) well measured (at the level )

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JES from W?jj in ttbar events method

• The W mass depends mainly on JES
• JES is a simple rescaling to the W PDG mass !
  gtGlobal JES factor
• BUT in general
• JES (E40 GeV) JES (E100 GeV)
• EJ1 EJ2
• gt Simple rescaling to the PDG W mass not
  sufficient
• gt MW Spectra in jet energy, eta,.. windows
• to allow the separation of both jet
• contribution to the JES
• JES(J1) et JES(J2)
• gtJES in function of Jet Energy,?,? ..

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JES from W?jj in ttbar events
1fb-1
10fb-1
JES THEO ATLAS JES found
?jet
JES uncertainty of 1 is achievable with 1 fb-1
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TTbar resonances

• With increasing ttbar mass
• SM BKG decrease
• Comb BKG contribution decrease
• Recons eff drops
• Top decay particles mixed

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Rare Top decays

• FCNC

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Rare Top decays

• FCNC

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conclusion

• But before any measurement
• Detector understanding
• detector performances measurements
• Trigger
• Calibrations
• alignement
• B tagging
• Background studies
• MC tuning on data
• gt Top events serve as a tool for these studies

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BACKUP
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JES from W?jj in ttbar events

• Itérative Method
• From MW spectra in Energy slices
• extraction of MW peak
• R(E) applied to each jet
• Recompute MWgt new Mw spectra
• ? 3 itérations

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JES from W?jj in ttbar events

• JES in function of eta jets

10 fb-1
Expected squares Fitted circles
JES uncertainty of 1 is achievable with 1 fb-1
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