Transcript Document 7661984

Moriond QCD
Early physics with top quarks at LHC
P.Ferrari
CERN
on behalf of the ATLAS and CMS collaborations
Pamela Ferrari
Rencontres de Moriond 2007 QCD session
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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 !
> 8 millions tt events expected per year
Pamela Ferrari
Rencontres de Moriond 2007 QCD session
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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 σtop ~ 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 (< 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)
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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  lb jjb selection with MC@NLO tt events :
1(e/m) pT>20 GeV, ETmiss>20 GeV, = 4 jets pT>40 GeV (2 b-tagged),
150 GeV< mjjb< 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 limit< 0.5% from
combinatorial backg. and templates shape
a=1
@ 1.3 fb-1
Ej/Eq
« Data »
JES as a function of energy ( n energy
bins and nxn matrix template)
±2%
•b-jets
•Light jets
Best fit
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Rencontres de Moriond 2007 QCD session
Eq
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Calibrating b-tagging
 Using semi-leptonic (and fully leptonic) tt events
 Optimize the jet pairing efficiency via mass
constraints in kinematic fits and likelihoods.
W CANDIDATE
TOP CANDIDATE
 Only one jet is tagged as b-jet (on Whad side)
CMS NOTE 2006-013
Isolate jet samples with a highly enriched b–
jet content, on which the b–jet identification
algorithms can be calibrated.
Main systematics :ISR/FSR
For 1 fb-1 (10fb-1) relative accuracy on the b–jet 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:
Let’s apply a simple selection
W =2 jets maximising pT W in jjj rest frame
 No
No b-tag
b-tag
 relaxing cut on 4th jet: pT>20 GeV:
doubles signal significance!
Hadronic top=3 jets
maximising pT top
4 jets pT> 40 GeV
Isolated lepton
pT> 20 GeV
600 pb-1
|mjj-mW| < 20 GeV
-1-1
100pb
100 pb
Siginficance (s)
ETmiss > 20 GeV
Combinatorial
only with 100 pb-1
W+jets (few days)
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Rencontres de Moriond 2007 QCD session
Luminosity (pb-1)
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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 pT>20 GeV
 ≥4 jets ET>30 GeV |h|<2.4
 2 b-tagged jets
 Coverging kin. fit to mW
stat
total w/o lumi
total w lumi
esel ~ 6.3%
S/B~26.7
@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 =
4
p
jet 1
T
Df(lep,)
 KTmin=min D(h,f) between 2 jets

Pamela Ferrari
Not very useful
to separate
from W+jets
after selection
1fb-1
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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
Dstt/stt
syst (%)
10fb-1
SemiLeptonic
9.7
Dstt/stt
stat (%)
0.4
Dstt/stt
lumi (%)
Main
syst (%)
Eff
(%)
S/B
7
3.4
3.2
tt
W+j
6.3
26.7
3
Btag
PDF
PileUp
5
4
4
ttll with
(Wtt,
tl)
5
5.5
11
10
QCD
1.6
1/9
10fb-1
Dilepton
11
0.9
3
PDF
Btag
JES
1fb-1
hadronic
20
3
5
JES
PileUp
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Main
bkg
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Top mass measurement in lepton + jets channel
1) minimization of c2  reconstruct mW hadronic & jet E rescaling (a1,a2)
χ2
(M




jj
(α1,α2 )  MW )2
Γw2
2
 Ej1 (1  α1 ) 
 E (1  α2 ) 
   j2






σ j1
σ j2




2
keep W’s if |mW - 80.4 GeV| < 2GmW
chose b-jet that maximises top pT
W purity 56.5%, top purity 45%, e=1.1%
10fb-1
MC@NLO
Fullsim
2) Kinematic fit:
CMS NOTE 2006-066
mtop
Kinematic fit to reconstruct entire tt final state:
PYTHIA
Fullsim


•

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(c2 = 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
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
Pamela Ferrari
Top mass measurement (semileptonic channel)
c) Selection of high pT top quarks pT(top) > 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
Source of uncertainty
Had. top
Mtop (GeV/c2)
Kinematic fit
Mtop (GeV/c2)
High PT sample
Mtop (GeV/c2)
Light jet energy scale (1 %)
0.2
0.2
b-jet energy scale (1 %)
0.7
0.7
b-quark fragmentation
0.1
0.1
0.3
ISR
0.1
0.1
0.1
FSR
1.
0.5
0.1
0.1
0.1
Combinatorial background
Mass rescaling
0.9
UE estimate (± 10 %)
1.3
Total
Statistical error @10fb-1
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1.3
0.9
1.6
0.05
0.1
0.2
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Di-lepton channel and Hadronic channels
Dilepton channel: clean channel but need to reconstruct
2 ’s. Reconstruction via 0C fit assuming mW and 2 equal
masses for top mt1=mt2 (6 eq. ,6 unknowns)


S/B = 12
The different  solutions are weighted using the
SM prediction for the  and  E spectra
The neutrino solution with the highest weight is
chosen mtop
PYTHIA
1fb-1
Hadronic channel: full kinematic reconstruction of both
sides but huge QCD multijet background:
6-8 jets, ET>30 GeV
Centrality>0.68,aplanarity>0.024
ETtot- ET of 2 leading j>148 GeV
2 b-tagged jets
Best jet pairing obtained from
likelihood based mainly on
angular distrubution of jets.
Pamela Ferrari
CMS NOTE 2006-077
(stat)
mt (GeV/c2)
(syst)
mt (GeV/c2)
@1fb-1 dilepton
~1.5
~4.2
@1fb-1 hadronic
~0.6
~4.2
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Resonances in Mtt
pp  X  tt with tt decaying semi-leptonically
Technicolor, Strong EW symmetry breaking models, Z’, SUSY:
 usual semi-leptonic events preselection
 use W and top mass constraint:
• neutrino pZ from mW constraint, solution
5fb-1
giving best top mass is retained
• |mjj-mW|  20 GeV
3xZ’ signal
• b-jet associated with hadronic top is the
one maximising pTtop
• |mbjj-mT|  40 GeV
 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/B~0.06-0.08)
Pamela Ferrari
@5 fb-1
MZ’= 1 TeV
MZ’= 1.5 TeV
MZ’= 2 TeV
CL
~2.75s
~2.96s
~3.3s
x-sec (pb)
~4
~3
~3
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Flavour Changing Neutral Currents
u (c,t)
No FCNC at tree level in SM:
SM
10-14-10-12
2HDM
10-7-10-4
MSSM
10-6-10-5
CMS NOTE 2006/093
5σ sensitivity
u
Z/γ
Look for FCNC in top decays:
tqZ
(2jets+3l+missing)
tqg
(+1l+missing)
tqg
(+1l+1g+missing)
SN-ATLAS-2007-059
t
@ 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H+b, spin 0/2 heavy resonance
H/KK gravitons  tt, would move
A away from SM expectation
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
A=0.41 0.014(stat) 0.023(syst)
Pamela Ferrari
A=
s(tLtL) + s(tRtR) - s(tLtR) - s(tRtL)
s(tLtL) + s(tRtR) + s(tLtR) + s(tRtL)
d 2N
1
 (1  A1 2 cos1cos2 )
N dcos1dcos2 4
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
+0.055
(syst)
-0.096
+0.026
0.021(stat) -0.055
(syst)
Abt,lt=0.375 ±0.014(stat)
Aqt,lt=0.346 ±
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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



Early top signal will help



Understand the detectors and control systematics
Complete study using full simulations and NLO generators
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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Rencontres de Moriond 2007 QCD session
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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
Events
t
Miscalibrated detector or
escaping ‘new’ particle
Perfect detector
Missing ET (GeV)
Pamela Ferrari
t
Range: 50 < pT < 200 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 > 2
Top signal
PT cut
40 GeV
Fraction
Jet 4
42.6 %
30 GeV
68.1 %
20 GeV
87.7 %
42.6 %
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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Rencontres de Moriond 2007 QCD session
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CMS NOTE 2006-066
Gaussian Ideogram, full scan ideogram
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 125<mt<225 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
1)
Exclusive (J/ℓℓ) decays: detect clean high mass
products, easy to reconstruct and little background
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 selection+trigger efficiency
CMS NOTE 2006-058
Sensitive
to mt
m(3l)
2) via cross-section: very much sensitive to the top mass: 5% on s gives
mt~2 GeV/c2, error luminosity and the pdfs
Pamela Ferrari
(stat)
mt (GeV/c2)
(syst. instr.)
mt (GeV/c2)
(syst. th.)
mt(GeV/c2)
tot
(GeV/c2)
exclusive
J/ dec
~0.5
~0.5
~1.4
~1.5
via crosssection
~0.1
~0.7
~4.0
~4.1
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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 ET>20 GeV,h<2.5
 Missing ET>40 GeV
 2 jets with ET>20 GeV, h<2.5
 2 b-tagged jets
Main background represented by Z+jets when no b-tagging is
present ( efficiency 15% )
With b-tagging, efficiency about 5% with excellent
background reduction S/B~5 (B mainly from leptonic t 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 s(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(g)+jet)
need excellent energy flow (association tracker+calo+muon system)
 b-tagging efficiency+fake 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
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