Moriond QCD

1

• Moriond QCD
• Early physics with top quarks at LHC
• P.Ferrari
• CERN
• on behalf of the ATLAS and CMS collaborations

2
LHC is a tt factory
Total production cross section
(90)

(10)
tt production cross section at LHC 833 pb
tt production cross-section at Tevatron 6.7 pb
2 tt events per second ! gt 8 millions tt events
expected per year
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Top Physics day one

• In 2008 ECM 14 TeV few fb-1 ? already
  negligible statistical err
• 1) Top properties and basic SM physics at ?s 14
  TeV
• Estimate of stop 20 accuracy
• Start to tune Monte Carlo
• Measure top mass ? feedback on detector
  performance
• 2) Understand/calibrate detector and trigger tt
  ? bl? bjj
• Light jet energy scale selecting a pure sample of
  W ?jj in tt events (lt 1)
• b-tag efficiency ( 5)
• Missing energy calibration
• 3) Prepare for new physics
• Resonances, MSSM higgses, SUSY, FCNC
• Measure differential cross sections
  (ds/dpT,ds/dMtt) sensitive to new physics
• (provides also an accurate test of SM
  predictions)

4
Light jet energy calibration

• Template histograms with different E scales a and
  relative E resolutions b
• W ? qq in 106 PYTHIA tt events
• Simple tt ? lnb jjb selection with MC_at_NLO tt
  events
• 1(e/m) pTgt20 GeV, ETmissgt20 GeV, 4 jets pTgt40
  GeV (2 b-tagged),
• 150 GeVlt mjjblt 200 GeV ? W purity 83
• Fit each template histogram to mjj in the
   data , find best c2
• a 0.937 0.004, b 1.47 0.05

• Statistics limited. Unknown syst limitlt 0.5 from
  combinatorial backg. and templates shape

JES as a function of energy ( n energy bins and
nxn matrix template)
Ej/Eq
2
_at_ 1.3 fb-1

• b-jets
• Light jets

Eq
5
Calibrating b-tagging

• Using semi-leptonic (and fully leptonic) tt
  events
• Optimize the jet pairing efficiency via mass
• constraints in kinematic fits and likelihoods.
• Only one jet is tagged as b-jet (on Whad side)

CMS NOTE 2006-013
Isolate jet samples with a highly enriched bjet
content, on which the bjet identification
algorithms can be calibrated. Main systematics
ISR/FSR
For 1 fb-1 (10fb-1) relative accuracy on the
bjet identification efficiency is 6 (4) in
barrel region and about 10 (5) in the endcaps.
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Day one can we see the top?
We will have a non perfect detector Lets apply
a simple selection

• No b-tag
• relaxing cut on 4th jet pTgt20 GeV
• doubles signal significance!

4 jets pTgt 40 GeV
600 pb-1
Isolated lepton pTgt 20 GeV
ETmiss gt 20 GeV
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Refining the selections semi-leptonic case

• More refined selection studied with the aim of
  applying it to
• x-section, mass, polarization studies..

• Example CMS NOTE 2006/064
• 1 isolated lepton pTgt20 GeV
• 4 jets ETgt30 GeV hlt2.4
• 2 b-tagged jets
• Coverging kin. fit to mW

stat
total w/o lumi total w lumi
esel 6.3 S/B26.7
_at_5 fb-1 stt(m)0.6 (stat) 9.2
(syst)5.0(lumi)

• Exploiting new topological variables from D0?
• Sphericity S and Aplanarity A
• Centrality C
• HT
• Df(lep,n)
• KTminmin D(h,f) between 2 jets

1fb-1
Not very useful to separate from Wjets after
selection
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Summary of cross-section

• The cross-section has also been extracted from in
  the di-leptonic and fully hadronic channels here
  examples from
• CMS NOTES 2006-064/ 2006-077

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Top mass measurement in lepton jets channel

• 1) minimization of c2 ? reconstruct mW hadronic
  jet E rescaling (a1,a2)
• keep Ws if mW - 80.4 GeV lt 2GmW
• chose b-jet that maximises top pT
• W purity 56.5, top purity 45, e1.1
• 2) Kinematic fit

10fb-1 MC_at_NLO Fullsim
CMS NOTE 2006-066
mtop

• Kinematic fit to reconstruct entire tt final
  state
• c2 based on kinematic constraints (El,j
  directions vary within resolution) c2
  minimisation, event by event
• Mtop fitted in slices of c2
• Estrapolation from linear fit mtop mtop(?2
  0)
• Gaussian/Full Scan Ideogram estimator for mt
• Event by event likelihood method convoluting the
  event resolution function with expected
  theoretical template. mtop obtained from maximum
  likelihood method

PYTHIA Fullsim
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Top mass measurement (semileptonic channel)

• c) Selection of high pT top quarks pT(top) gt 200
  GeV/c
• t and t tend to be back-to-back ? used as
  constraint to reduce bkg
• 3 jets in 1 hemisphere tend to overlap collect E
  in a cone around candidate top
• less sensitive to jet calibration. Mass scale
  recalibration based on hadronic W,
• independent systematic errors ? gain in
  combination

Comparing the 3 methods
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Di-lepton channel and Hadronic channels

• Dilepton channel clean channel but need to
  reconstruct
• 2 ns. Reconstruction via 0C fit assuming mW and
  2 equal
• masses for top mt1mt2 (6 eq. ,6 unknowns)
• The different n solutions are weighted using the
• SM prediction for the n and n E spectra
• The neutrino solution with the highest weight is
  chosen? mtop

S/B 12
-

• Hadronic channel full kinematic reconstruction
  of both
• sides but huge QCD multijet background
• 6-8 jets, ETgt30 GeV
• Centralitygt0.68,aplanaritygt0.024
• ETtot- ET of 2 leading jgt148 GeV
• 2 b-tagged jets
• Best jet pairing obtained from
• likelihood based mainly on
• angular distrubution of jets.

CMS NOTE 2006-077
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Resonances in Mtt
pp ? X ? tt with tt decaying semi-leptonically Tec
hnicolor, Strong EW symmetry breaking models,
Z, SUSY

• usual semi-leptonic events preselection
• use W and top mass constraint
• neutrino pZ from mW constraint, solution
• giving best top mass is retained
• mjj-mW ? 20 GeV
• b-jet associated with hadronic top is the
• one maximising pTtop
• mbjj-mT ? 40 GeV

5fb-1
3xZ signal

• Since pT of top from resonance decay is
• larger than in direct production
• Add lower cut on top pT 370,390,500 GeV/c for mZ
  1,1.5,2 GeV/c2 to increase purity
  (s/B0.06-0.08)

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Flavour Changing Neutral Currents
CMS NOTE 2006/093
u (c,t)
No FCNC at tree level in SM
5s sensitivity
u
Z/?
Look for FCNC in top decays
SN-ATLAS-2007-059
t?qg (1l1gmissing)
t?qg (1lmissing)
t?qZ (2jets3lmissing)
t
_at_ 10 fb-1 2 orders of magnitude better than
Tevatron/LEP/HERA
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Top spin correlations
t and t are produced unpolarized, but spins are
correlated
anomalous coupling (technicolor), t?Hb, spin 0/2
heavy resonance H/KK gravitons ? tt, would move A
away from SM expectation
s(tLtL) s(tRtR) - s(tLtR) - s(tRtL)
A
s(tLtL) s(tRtR) s(tLtR) s(tRtL)
Fit to double differential distribution

• Eur.Phys.J.C44S2 2005 13-33
• Semilep. dilep. (10 fb-1)
• Fitting to distribution of
• angles bewteen top spin analyser in top rest
  frame versus angle of t spin
• analyser in antitop rest frame
• Syst. dominated by b-JES, top mass and FSR
• A0.41 ?0.014(stat) ?0.023(syst)

• CMS NOTE 2006/111
• Semilep. (10 fb-1)
• Fitting to distribution of
• lepton angle vs b-quark angle in the tt rest
  frame
• lepton angle vs lower energy quark angle from
  the W-decay in the tt rest frame

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Conclusions

• LHC startup will require a long period of
  development and understanding
• LHC is a top factory, but before performing
  precision measurements, a huge effort is needed
  in order to
• Understand the detectors and control systematics
• Complete study using full simulations and NLO
  generators
• Early top signal will help
• We could get top signal with 100 pb-1
• s(tt) to 13 and Mtop to 1 with 1fb-1
• In addition our aim is, as soon as we get a large
  statistics (few fb-1), to be ready for early
  discovery of new physics!

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• BACK-UP TRANSPARENCIES

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Calibrating the missing energy
We can calibrate the missing Energy since in
semileptonic tt events the momentum of the
neutrino is constrained from kinematics MW
known amount of missing energy per
event Calibration of missing energy vital for
all (R-parity conserving) SUSY and most exotics!
Example from SUSY analysis
Miscalibrated detector or escaping new particle
Events
Perfect detector
Range 50 lt pT lt 200 GeV
Missing ET (GeV)
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Effect of cut on PT of 4th jet
Lowering PT requirement on 4th jet to 20
GeV increase in significance top signal by factor
gt 2
42.6
Cut on 4th jet does not bring any
advantage Actually it cuts away important
information large fraction of
jets associated to quarks from W decay
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Gaussian Ideogram, full scan ideogram
CMS NOTE 2006-066

• Build event by event from the covariance matrices
  of the kinematics of the 3 fitted jets,
• the relative compatibility of the reconstructed
  kinematics of the event with the hypothesis of
  the a heavy object of mass mt decays into 3 jets.
• This is usually called the ideogram of the event
  or resolution function of the event
• In the full scan the top mass is not fixed but
  varies 125ltmtlt225 GeV
• To estimate the top mass, the ideogram is
  convoluted with the Theorectical expected
  probability density function and after combining
  all likelihoods from all events a maximum
  likelihood method is applied to get mt.

Sources
(5 On-Off)
(1.5)
(2)
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Mtop other 2 methods

• Exclusive (J/??ll) decays detect clean high mass
  
• products, easy to reconstruct and little
  background
• 2) via cross-section very much sensitive to the
  top mass 5 on ? gives ?mt2 GeV/c2, error
  luminosity and the pdfs

• different systematic contributions top mass
  determined
• via m(3l)
• without using the b-tagging !
• almost ignoring jet reconstruction !
• ? large improvement in combination with direct
  reconstruction
• Challenging because of the extremely low
  branching ratio
• BR 0.8?10-4 after selectiontrigger
  efficiency

CMS NOTE 2006-058
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Di-lepton event selection
CMS NOTE 2006-077

• Cut based selection
• Single or di-lepton trigger
• Two isolated oppositely charged
• leptons with ETgt20 GeV,???lt2.5
• Missing ETgt40 GeV
• ?2 jets with ETgt20 GeV, ???lt2.5
• 2 b-tagged jets

Main background represented by Zjets when no
b-tagging is present ( efficiency 15 ) With
b-tagging, efficiency about 5 with excellent
background reduction S/B5 (B mainly from
leptonic ? decays)
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Systematic effects for top physics

• Almost all SM measurements at LHC dominated by
  systematic errors.
• Can be divided into instrumental and from
  theory/modeling
• Dominant instrumental uncertainties for top
  physics
• Luminosity
• Reasonable goal is 3-5
• ? measure number of
  interactions/bunch crossing (HF) and ?(pp)
  (TOTEM)
• Reconstruction related
• Jet energy scale
• need calibrated calorimetry (beam tests, MB,
  single particles, Z, W)
• need jet energy calibration to a few (with
  Z(?)jet)
• need excellent energy flow (association
  trackercalomuon system)
• b-tagging efficiencyfake rate
• use tt for calibration to 4-5 with 10fb-1
• Lepton identification and energy scale
• use Z, other mesons. Less crucial than for the W
  mass measurement
• Theory related systematics are as important as
  instrumental ones !

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Theoretical systematic uncertainties

• ISR/FSR
• vary ?QCD and Q2max from low to high radiation
• light jets Fragmentation vary only the
  fragmentation parameters within
• errors from LEP/SLD tunings.
• b jets Fragmentation
• error estimated changing the Peterson
  parameter (-0.006 ) within its
• theoretical uncertainty (0.0025)
• Minimum bias and underlying event
• extrapolate from low energy UA5/Tevatron and
  change main pT cut-off
• parameter
• PDF parametrization CTEQ6M, contrain using
  LHC data
• Hard process scale switch among reasonable
  definition of the Q2 scale