Moriond QCD Early physics with top quarks at LHC P.Ferrari CERN

Rencontres de Moriond 2007 QCD sessionPamela Ferrari 1Pamela Ferrari 1

Moriond QCDMoriond QCD

Early physics with top quarks at Early physics with top quarks at LHCLHC

P.FerrariCERN

on behalf of the ATLAS and CMS collaborations


LHC is a tt factoryLHC is a tt factory

tt production cross section at LHC:

~833 pbtt production cross-section

at Tevatron:6.7 pb

Total production cross section

+

(90%)

(10%)

2 tt events per second !2 tt events per second !> 8 millions tt events expected per > 8 millions tt events expected per

yearyear

2 tt events per second !2 tt events per second !> 8 millions tt events expected per > 8 millions tt events expected per

yearyear


Top Physics day oneTop Physics day oneIn 2008 ECM = 14 TeV In 2008 ECM = 14 TeV few fbfew fb-1-1 already already negligible statistical err negligible statistical err

1)1) Top properties and basic SM physics at Top properties and basic SM physics at s = 14 TeV :s = 14 TeV : Estimate of σEstimate of σtoptop ~ 20% accuracy ~ 20% accuracy Start to tune Monte CarloStart to tune Monte Carlo Measure top mass Measure top mass feedback on detector performance feedback on detector performance

2)2) Understand/calibrate detector and trigger: tt Understand/calibrate detector and trigger: tt bl bl bjj bjj Light jet energy scale selecting a pure sample of W Light jet energy scale selecting a pure sample of W jj in tt events (< jj in tt events (<

1%)1%) b-tag efficiency (~ 5%)b-tag efficiency (~ 5%) Missing energy calibrationMissing energy calibration

3)3) Prepare for new physics: Prepare for new physics: Resonances, MSSM higgses, SUSY, FCNCResonances, MSSM higgses, SUSY, FCNC Measure differential cross sections (dMeasure differential cross sections (d/dp/dpTT,d,d/dM/dMtttt) sensitive to new ) sensitive to new

physicsphysics

(provides also an accurate test of SM predictions)(provides also an accurate test of SM predictions)


Light jet energy Light jet energy calibrationcalibration

Template histograms with different E scales Template histograms with different E scales and relative E and relative E resolutions resolutions

W W qq in ~10 qq in ~1066 PYTHIA tt events PYTHIA tt events

Simple tt Simple tt l lb jjb selection with MC@NLO tt events :b jjb selection with MC@NLO tt events :1(e/1(e/) p) pTT>20 GeV, E>20 GeV, ETmissTmiss>20 GeV, = 4 jets p>20 GeV, = 4 jets pTT>40 GeV (2 b-tagged), >40 GeV (2 b-tagged),

150 GeV< m150 GeV< mjjbjjb< 200 GeV< 200 GeV W purity ~83% W purity ~83%

Fit each template histogram to mFit each template histogram to mjjjj in the « data », find best in the « data », find best 22

= 0.937 = 0.937 ± 0.004, ± 0.004, = 1.47 ± 0.05 = 1.47 ± 0.05

= 1« Data »

Best fit

Statistics limited. Unknown syst limit< 0.5% Statistics limited. Unknown syst limit< 0.5% from combinatorial backg. and templates from combinatorial backg. and templates shapeshape

Eq

±2%

JES as a function of energy ( n JES as a function of energy ( n energy bins and nxn matrix energy bins and nxn matrix template)template)

@ 1.3 fb@ 1.3 fb-1-1@ 1.3 fb@ 1.3 fb-1-1E

j/Eq

•b-jetsb-jets•Light jetsLight jets


Calibrating b-taggingCalibrating b-tagging

For 1 fbFor 1 fb-1-1 (10fb (10fb-1-1) ) relative accuracy on the b–jet identification relative accuracy on the b–jet identification efficiency is ~ 6% (4%) in barrel region and about 10% (5%) in the efficiency is ~ 6% (4%) in barrel region and about 10% (5%) in the endcapsendcaps..

CMS NOTE 2006-013

TOP CANDIDATE

W CANDIDATE

Isolate jet samples with a highly Isolate jet samples with a highly

enriched b–jet content, on which the b–enriched b–jet content, on which the b–

jet identification algorithms can be jet identification algorithms can be

calibrated.calibrated.

Main systematics :ISR/FSRMain systematics :ISR/FSR

Using semi-leptonic (and fully leptonic) tt eventsUsing semi-leptonic (and fully leptonic) tt events Optimize the jet pairing efficiency via mass Optimize the jet pairing efficiency via mass constraints in kinematic fits and likelihoods.constraints in kinematic fits and likelihoods.

Only one jet is tagged as b-jet (on WOnly one jet is tagged as b-jet (on Whad had side)side)


No b-tag No b-tag

|m|mjjjj-m-mWW| < 20 GeV| < 20 GeV

No b-tagNo b-tag relaxing cut on 4relaxing cut on 4thth jet: p jet: pTT>20 >20 GeV: GeV: doubles signal significance!doubles signal significance!

Day one: can we see the top? Day one: can we see the top?

600 pb600 pb-1-1600 pb600 pb-1-1

Isolated lepton Isolated lepton p pTT> 20 GeV > 20 GeV

EETTmissmiss > 20 GeV > 20 GeV

4 jets p4 jets pTT> 40 GeV> 40 GeV

We will have a non perfect detector:We will have a non perfect detector:Let’s apply a simple selectionLet’s apply a simple selection

100pb-

1

W+jets

CombinatorialS

igin

fican

ce (

s)

Sig

infi

can

ce (

s)

only with 100 pbonly with 100 pb--

11 (few days) (few days)

100 pb100 pb-1-1100 pb100 pb-1-1

Luminosity (pbLuminosity (pb-1-1))

Hadronic top=3 jetsHadronic top=3 jets

maximising pmaximising pTT top top

W =2 jets maximising pW =2 jets maximising pTT W in jjj rest frame W in jjj rest frame


More refined selection studied with the aim of applying it More refined selection studied with the aim of applying it to to

x-section, mass, polarization studies.. x-section, mass, polarization studies..

@5 fb@5 fb-1-1 tttt))==0.6% (stat)± 9.2% (syst)0.6% (stat)± 9.2% (syst)±±5.0%(lumi)5.0%(lumi)

selsel ~ 6.3% ~ 6.3%

S/B~26.7S/B~26.7

Exploiting new topological variables from D0?

Sphericity S and Aplanarity A Sphericity S and Aplanarity A Centrality CCentrality C

HHTT = =

(lep,(lep,)) KKTminTmin=min =min between 2 jets between 2 jets

Refining the selections: semi-leptonic Refining the selections: semi-leptonic casecase

4

1jetTp

Example: Example: CMS NOTE 2006/064CMS NOTE 2006/064

1 isolated lepton p1 isolated lepton pTT>20 GeV >20 GeV

≥ ≥4 jets E4 jets ETT>30 GeV |>30 GeV ||<2.4|<2.4

2 b-tagged jets2 b-tagged jets Coverging kin. fit to mCoverging kin. fit to mWW

1fb1fb-1-11fb1fb-1-1

Not very usefulNot very usefulto separateto separatefrom W+jets from W+jets after selection after selection

statstattotal w/o lumitotal w/o lumitotal w lumitotal w lumi


Summary of cross-sectionSummary 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/ 2006-077CMS NOTES 2006-064/ 2006-077

tttt//tttt

syst (%)syst (%)

tttt//tttt stat stat (%)(%)

tttt//tttt lumi lumi (%)(%)

MainMain

syst (%)syst (%)MainMain

bkgbkgEffEff

(%)(%)S/BS/B

10fb10fb-1-1

Semi-Semi-LeptonicLeptonic

9.79.7 0.40.4 33

Btag 7Btag 7PDF 3.4 PDF 3.4 PileUp PileUp 3.2 3.2

tttt

W+jW+j6.36.3 26.726.7

10fb10fb-1-1

DileptonDilepton1111 0.90.9 33

PDF 5PDF 5

Btag 4Btag 4

JES 4JES 4

ttttll ll withwith

(W(W, , l)l)

55 5.55.5

1fb1fb-1-1

hadronic hadronic 2020 33 55

JES 11 JES 11 PileUp 10PileUp 10 QCDQCD 1.61.6 1/91/9


1) minimization of 1) minimization of 22 reconstruct m reconstruct mWW hadronic & jet E rescaling hadronic & jet E rescaling ((11,,22))

keep W’s if |mkeep W’s if |mWW - 80.4 GeV| < 2 - 80.4 GeV| < 2mWmW

chose b-jet that maximises chose b-jet that maximises top ptop pTT W purity 56.5%, top purity 45%, W purity 56.5%, top purity 45%, =1.1%=1.1%

2) Kinematic fit:2) Kinematic fit:

•

Top mass measurement in lepton + jets Top mass measurement in lepton + jets channelchannel

PYTHIAPYTHIAFullsimFullsim

PYTHIAPYTHIAFullsimFullsim

Kinematic fit to reconstruct entire tt final state: Kinematic fit to reconstruct entire tt final state: 22 based on kinematic constraints (E based on kinematic constraints (El,j l,j & directions vary & directions vary

within resolution) within resolution) 22 minimisation, event by event minimisation, event by event MMtop top fitted in slices offitted in slices of 22 Estrapolation from linear fit: mEstrapolation from linear fit: mtoptop = m = mtoptop((22 = 0) = 0)

Gaussian/Full Scan Ideogram estimator for mGaussian/Full Scan Ideogram estimator for mtt:: Event by event Event by event likelihood method convoluting the event likelihood method convoluting the event

resolution function with expected theoretical template. resolution function with expected theoretical template. mmtoptop obtained from maximum likelihood method obtained from maximum likelihood method

2

j2

2j22

j1

1j1

2w

2W21jj2

σ

α1E

σ

α1E

Γ

Mα,αMχ

mtop

10fb-1

MC@NLO Fullsim

10fb-1

MC@NLO Fullsim

CMS NOTE 2006-066CMS NOTE 2006-066


Top mass measurement (semileptonic Top mass measurement (semileptonic channel)channel)

c) Selection of high pSelection of high pTT top quarks p top quarks pTT(top) > 200 GeV/c:(top) > 200 GeV/c:

t and t tend to be back-to-back t and t tend to be back-to-back used as constraint to reduce bkg used as constraint to reduce bkg

3 jets in 1 hemisphere tend to overlap: 3 jets in 1 hemisphere tend to overlap: collect E in a cone around collect E in a cone around

candidate topcandidate top

less sensitive to jet calibrationless sensitive to jet calibration. Mass scale recalibration based on . Mass scale recalibration based on

hadronic W, hadronic W,

independent systematic errors independent systematic errors gain in combination gain in combinationSource of uncertaintySource of uncertainty

Had. top Had. top MMtop top (GeV/c(GeV/c22))

Kinematic fitKinematic fitMMtoptop (GeV/c (GeV/c22))

High PHigh PTT sample sample

MMtoptop (GeV/c (GeV/c22))

Light jet energy scale (1 %)Light jet energy scale (1 %) 0.20.2 0.20.2

b-jet energy scale (1 %)b-jet energy scale (1 %) 0.70.7 0.70.7

b-quark fragmentationb-quark fragmentation 0.10.1 0.10.1 0.30.3

ISRISR 0.10.1 0.10.1 0.10.1

FSRFSR 1.1. 0.50.5 0.10.1

Combinatorial backgroundCombinatorial background 0.10.1 0.10.1

Mass rescalingMass rescaling 0.9 0.9

UE estimate (± 10 %)UE estimate (± 10 %) 1.31.3

Total Total 1.3 1.3 0.90.9 1.61.6

Statistical error @10fb-1Statistical error @10fb-1 0.050.05 0.10.1 0.20.2

Com

pari

ng

th

e 3

meth

od

sC

om

pari

ng

th

e 3

meth

od

s



PYTHIAPYTHIA

HHadronic channel:adronic channel: full kinematic reconstruction of full kinematic reconstruction of both both sides but huge QCD multijet background:sides but huge QCD multijet background:

6-8 jets, ET>30 GeV6-8 jets, ET>30 GeVCentrality>0.68,aplanarity>0.024Centrality>0.68,aplanarity>0.024EETtotTtot- E- ETT of 2 leading j>148 GeV of 2 leading j>148 GeV2 b-tagged jets2 b-tagged jetsBest jet pairing obtained from Best jet pairing obtained from likelihood based mainly on likelihood based mainly on angular distrubution of jets.angular distrubution of jets.

Di-lepton channel and Hadronic Di-lepton channel and Hadronic channelschannels

Dilepton channel:Dilepton channel: clean channel but need to clean channel but need to reconstruct reconstruct

2 2 ’s. Reconstruction via 0C fit assuming m’s. Reconstruction via 0C fit assuming mWW and 2 and 2 equal equal

masses for top mmasses for top mt1t1=m=mt2t2 (6 eq. ,6 unknowns) (6 eq. ,6 unknowns)

The different The different solutions are weighted using the solutions are weighted using the SM prediction for the SM prediction for the andand E spectra E spectra The neutrino solution with the highest weight is The neutrino solution with the highest weight is

chosenchosen m mtoptop

CMS NOTE 2006-077

(stat)(stat)

mmt t (GeV/c(GeV/c22))(syst) (syst)

mmt t (GeV/c(GeV/c22))

@1fb@1fb-1-1 dilepton dilepton ~1.5~1.5 ~4.2~4.2

@1@1fbfb-1 -1 hadronic hadronic ~0.6~0.6 ~4.2~4.2

--

S/B = 12S/B = 12


usual semi-leptonic events preselection usual semi-leptonic events preselection

use W and top mass constraint:use W and top mass constraint:

• neutrino pneutrino pZZ from m from mWW constraint, solution constraint, solution

giving best top mass is retainedgiving best top mass is retained

• |m|mjjjj-m-mWW| | 20 GeV 20 GeV

• b-jet associated with hadronic top is theb-jet associated with hadronic top is the

one maximising pone maximising pTtop Ttop

• |m|mbjjbjj-m-mTT| | 40 GeV 40 GeV

Resonances in MResonances in Mtttt pp pp X X tt with tt decaying semi-leptonically tt with tt decaying semi-leptonically

Technicolor, Strong EW symmetry breaking models, Z’, SUSY:Technicolor, Strong EW symmetry breaking models, Z’, SUSY:

@5 fb@5 fb-1-1 MMZ’Z’= 1 TeV= 1 TeV MMZ’Z’= 1.5 = 1.5 TeVTeV

MMZ’Z’= 2 = 2 TeVTeV

CLCL ~2.75~2.75 ~2.96~2.96 ~3.3~3.3

x-sec x-sec (pb)(pb)

~4~4 ~3~3 ~3~3

3xZ’ signal

Since pSince pTT of top from resonance of top from resonance decay is decay is

larger than in direct productionlarger than in direct production

Add lower cut on top pAdd lower cut on top pT T 370,390,500 370,390,500 GeV/c for mGeV/c for mZ’Z’ =1,1.5,2 GeV/c =1,1.5,2 GeV/c22 to to increase purity (s/B~0.06-0.08)increase purity (s/B~0.06-0.08)



Flavour Changing Neutral CurrentsFlavour Changing Neutral Currents

u (c,t)

Z/γ

t

No FCNC at tree level in No FCNC at tree level in SM:SM:

Look for FCNC in top Look for FCNC in top decaysdecays::

uSMSM 1010-14-14-10-10-12-12

2HDM2HDM 1010-7-7-10-10-4-4

MSSMMSSM 1010-6-6-10-10-5-5

55σσ sensitivitysensitivity

55σσ sensitivitysensitivity

CMS NOTE 2006/093CMS NOTE 2006/093

@ 10 fb@ 10 fb-1-1 2 orders of magnitude better than Tevatron/LEP/HERA 2 orders of magnitude better than Tevatron/LEP/HERA

ttqZqZ(2jets+3l+missing)(2jets+3l+missing)

ttqgqg(+1l+missing(+1l+missing

))

ttqq(+1l+1(+1l+1+missi+missi

ng)ng)

SN-ATLAS-2007-059

Rencontres de Moriond 2007 QCD sessionPamela FerrariPamela Ferrari 14

t and t are produced unpolarized, but spins are correlatedt and t are produced unpolarized, but spins are correlated

Top spin correlationsTop spin correlations

anomalous coupling anomalous coupling (technicolor), t(technicolor), tH+b, spin H+b, spin 0/2 heavy resonance H/KK 0/2 heavy resonance H/KK gravitons gravitons tt, would move A tt, would move A away from SM expectationaway from SM expectation

(tLtL) + (tRtR) - (tLtR) - (tRtL)

(tLtL) + (tRtR) + (tLtR) + (tRtL)A=

Eur.Phys.J.C44S2 2005 13-33Eur.Phys.J.C44S2 2005 13-33Semilep. + dilep. (10 fbSemilep. + dilep. (10 fb-1-1))Fitting to distribution ofFitting to distribution of

• angles bewteen top spin angles bewteen top spin analyser in top rest frame versus analyser in top rest frame versus angle of t spin angle of t spin analyser in antitop rest frame analyser in antitop rest frame

• Syst. dominated by b-JES, top Syst. dominated by b-JES, top mass and FSRmass and FSR

A=0.41 A=0.41 0.014(stat) 0.014(stat) 0.023(syst)0.023(syst)

CMS NOTE 2006/111CMS NOTE 2006/111 Semilep. (10 fbSemilep. (10 fb-1-1))Fitting to distribution of Fitting to distribution of

• lepton angle vs b-quark lepton angle vs b-quark angleangle in in the tt rest framethe tt rest frame

• lepton anglelepton angle vs lower energy vs lower energy quark angle from the W-decay in quark angle from the W-decay in the tt rest framethe tt rest frame

AAbt,ltbt,lt=0.375 =0.375 ±±0.014(stat) 0.014(stat) (syst)(syst)

AAqt,ltqt,lt=0.346 =0.346 ±± 0.021(stat) 0.021(stat) (syst)(syst)

+0.055+0.055-0.096-0.096+0.026+0.026-0.055-0.055

Fit to double differential Fit to double differential distributiondistribution

212121

2

coscos141

dcosdcos NNd

A


ConclusionsConclusions LHC startup will require a long period of development and LHC startup will require a long period of development and

understanding understanding

LHC is a top factory, but before performing LHC is a top factory, but before performing precision precision measurements, a huge effort is needed in order tomeasurements, a huge effort is needed in order to

Understand the detectors and control systematicsUnderstand the detectors and control systematics Complete study using full simulations and NLO generators Complete study using full simulations and NLO generators

Early top signal will helpEarly top signal will help

We could get top signal with ~ 100 pbWe could get top signal with ~ 100 pb-1-1

(tt) to ~13% and M(tt) to ~13% and Mtoptop to 1% with 1fb to 1% with 1fb-1-1

In addition our aim is, as soon as we get a large statistics In addition our aim is, as soon as we get a large statistics (few fb(few fb-1-1), to be ready for early discovery of new physics! ), to be ready for early discovery of new physics!


BACK-UP TRANSPARENCIES


t

t

Calibrating the missing energyCalibrating the missing energy

Missing ET (GeV)

Even

ts

Perfect detector

Miscalibrated detector or escaping ‘new’ particle

Miscalibrated detector or escaping ‘new’ particle

We can calibrate the missing Energy since in semileptonic tt We can calibrate the missing Energy since in semileptonic tt events the momentum of the neutrino is constrained from events the momentum of the neutrino is constrained from kinematics : Mkinematics : MWW

known amount of missing energy per eventknown amount of missing energy per event

Calibration of missing energy vital for all Calibration of missing energy vital for all

(R-parity conserving)(R-parity conserving) SUSY and most exoticsSUSY and most exotics!

Range: 50 < pRange: 50 < pTT < 200 GeV < 200 GeV

Example from SUSY analysis


Effect of cut on PEffect of cut on PTT of 4 of 4thth jet jet

Lowering PT requirement on 4th jet to 20 GeVLowering PT requirement on 4th jet to 20 GeV

increase in significance top signal by factor > 2increase in significance top signal by factor > 2

42.6 %

40 GeV 42.6 %40 GeV 42.6 %

30 GeV 68.1 %30 GeV 68.1 %

20 GeV 87.7 %20 GeV 87.7 %

PPTT cut cut FractionFraction

Top signal

Cut on 4Cut on 4thth jet does not bring any advantage jet does not bring any advantage

Actually … it cuts away important information:Actually … it cuts away important information: large fraction of jets associated to quarks from W large fraction of jets associated to quarks from W decaydecay

Jet 4

42.6 %


Gaussian Ideogram, full scan Gaussian Ideogram, full scan ideogramideogram

(1.5%)

(2%)

(5% On-Off)

Build event by event from the covariance matrices of the kinematics of the 3 fitted Build event by event from the covariance matrices of the kinematics of the 3 fitted jets,jets,the relative compatibility of the reconstructed kinematics of the event with the the relative compatibility of the reconstructed kinematics of the event with the hypothesis of the a heavy object of mass mt decays into 3 jetshypothesis 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 eventThis is usually called the ideogram of the event or resolution function of the eventIn the full scan the top mass is not fixed but varies 125<mIn the full scan the top mass is not fixed but varies 125<mtt<225 GeV<225 GeV

To estimate the top mass, the ideogram is convoluted with the Theorectical expected 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 probability density function and after combining all likelihoods from all events a maximum likelihood method is applied to get mmaximum likelihood method is applied to get mtt..

SourcesSources

CMS NOTE 2006-066


Mtop other 2 methods

1)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 mt~2 GeV/c2, error luminosity and the pdfs

(stat)(stat)

mmt t

(GeV/c(GeV/c22))

(syst. instr.) (syst. instr.)

mmt t (GeV/c(GeV/c22) ) (syst. th.)(syst. th.)

mmtt(GeV/(GeV/cc22) )

tot tot

(GeV/(GeV/cc22))

exclusive exclusive J/J/ dec dec

~0.5~0.5 ~0.5~0.5 ~1.4~1.4 ~1.5~1.5

via cross-via cross-sectionsection

~0.1~0.1 ~0.7~0.7 ~4.0~4.0 ~4.1~4.1

different systematic contributionsdifferent systematic contributions: top mass determined top mass determined via m(3l):via m(3l):

without using the b-tagging !without using the b-tagging ! almost ignoring jet reconstruction !almost ignoring jet reconstruction !

large improvement in combination with direct reconstruction Challenging because of the extremely low branching ratio: BR = 0.810-4 after selection+trigger efficiency

m(3lm(3l))

Sensitiveto mt

CMS NOTE 2006-058


Di-lepton event selectionDi-lepton event selection

Cut based selectionCut based selection:: Single or di-lepton triggerSingle or di-lepton trigger

Two isolated oppositely charged Two isolated oppositely charged leptons with Eleptons with ETT>20 GeV,>20 GeV,<2.5<2.5

Missing EMissing ETT>40 GeV>40 GeV

2 jets with E2 jets with ETT>20 GeV, >20 GeV, <2.5<2.5

2 b-tagged jets2 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 decays)

CMS NOTE 2006-077


Systematic effects for top Systematic effects for top physicsphysics Almost all SM measurements at LHC dominated by Almost all SM measurements at LHC dominated by

systematic errors.systematic errors. Can be divided into instrumental and from theory/modelingCan be divided into instrumental and from theory/modeling Dominant instrumental uncertainties for top physics:Dominant instrumental uncertainties for top physics:

Luminosity:Luminosity: Reasonable goal is 3-5%Reasonable goal is 3-5%

measure number of interactions/bunch crossing (HF) and measure number of interactions/bunch crossing (HF) and (pp) (pp) (TOTEM)(TOTEM)

Reconstruction related:Reconstruction related: Jet energy scaleJet energy scale

need calibrated calorimetry (beam tests, MB, single particles, Z, W…)need calibrated calorimetry (beam tests, MB, single particles, Z, W…)need jet energy calibration to a few % (with Z(need jet energy calibration to a few % (with Z()+jet))+jet)

need excellent energy flow (association tracker+calo+muon system)need excellent energy flow (association tracker+calo+muon system) b-tagging efficiency+fake rateb-tagging efficiency+fake rate

use tt for calibration: to 4-5% with 10fbuse tt for calibration: to 4-5% with 10fb-1-1

Lepton identification and energy scaleLepton identification and energy scale use Z, other mesons. Less crucial than for the W mass measurementuse Z, other mesons. Less crucial than for the W mass measurement

Theory related systematics are as important as instrumental ones !Theory related systematics are as important as instrumental ones !


Theoretical systematic Theoretical systematic uncertaintiesuncertainties

ISR/FSR:vary QCD and Q2

max 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

Documents

Moriond QCD Early physics with top quarks at LHC P.Ferrari CERN