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Expectations from LHC for Top Physics. Top Mass Couplings and decays Top spin polarization Single top production. Top quark mass is a fundamental parameter of the EW theory By far has the largest mass of all known fundamental particles In SM: top- and W-mass constrain Higgs mass

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Expectations from lhc for top physics

Expectations from LHC for Top Physics

Top Mass

Couplings and decays

Top spin polarization

Single top production

Motivations for top physics studies

Top quark mass is a fundamental parameter of the EW theory

By far has the largest mass of all known fundamental particles

In SM: top- and W-mass constrain Higgs mass

Sensitivity through radiative corrections

Scrutinize SM by precise determination top mass

Top quark exists and will be produced abundantly !  window for new physics

Many heavy particles decay in tt

Handle on new physics by detailed properties of top

And in addition…

Experiment: Top quark useful to calibrate the detector (LHC)

Beyond Top: Top quarks will be a major source of background for almost every search for physics beyond the SM (LHC)

Motivations for Top Physics studies

Marina Cobal - HCP2004

pp (mainly) at s = 14 TeV

Startup in April 2007

Initial/low lumi L1033 cm-2 s-1

 2 minimum bias/x-ing  “Tevatron-like” environment

10 fb-1 /year

Design/high lumi L=1034 cm-2 s-1

after ~ 3 years

~ 20 minimum bias/x-ing  fast ( 50 ns) radhard detect

100 fb-1 /year

LHC and experiments


27 km ring

1232 dipoles B=8.3 T


pp, general purpose


heavy ions,


LHCb :

pp, B-physics

Marina Cobal - HCP2004

LC machine

  • Maybe some years after the LHC startup

  • ECM of operation 200-500 GeV, possible upgrade to 1 TeV

  • 80% Polarization of e- beam

  • Luminosity: 300 fb-1/year

Marina Cobal - HCP2004

Top production at lhc

Cross section determined to NLO precision

Total NLO(tt) = 834 ± 100 pb

Largest uncertainty from scale variation

Compare to other production processes:

Top production cross section approximately 100x Tevatron

Opposite @ FNAL

Top production at LHC

~90% gg~10% qq

Low lumi

LHC is a top factory!

Marina Cobal - HCP2004

And at lc
..and at LC?

  • The ttbar cross section is ~103 smaller than at LHC, but higher L

  • s ~0,85 pb at max, around 390 GeV, and falls down with energy as 1/s

~200000 tt/year at TESLA parameters

Marina Cobal - HCP2004

Top decay

In the SM the top decays to W+b

All decay channels investigated

Using ‘fast parameterized’ detector response

Checks with detailed simulations

Di-leptons (e/)

BR≈4.9%  0.4x106 ev/y

No top reconstructed

Clean sample

Single Lepton (e/)

BR=29.6%  2.5x106 ev/y

One top reconstructed

Clean sample

Fully Hadronic

BR≈45%  3.5x106 ev/y

Two tops reconstructed

Huge QCD background

Large combinatorial bckgnd

Top decay

Marina Cobal - HCP2004

Top mass where we are
Top mass: Where we are

Marina Cobal - HCP2004

Near future of Mtop

Tevatron only (di-lepton events or lepton+jet ) from W decays

Status of inputs (preliminary):

mt=(178.0  2.7 (stat)  3.3 (syst)) GeV/c2

(latest Tevatron updated combination – RunI data)

mt=(175  17 (stat)  8 (syst)) GeV/c2

(CDF di-leptons – RunII data)

mt=(178+13-9(stat)  7 (syst)) GeV/c2

(CDF lepton+jets – RunII data)

Matter of statistics (also for the main systematics) and optimized use of the available information. Each experiment expects 500 b-tagged tt l+jets events/fb  DMtop ~ 2-3 GeV/c2 for the Tevatron combined (2-4/fb)

  • mt  2.5 GeV ; mW  30 MeV  mH/mH  35%

In 2009 (if upgrade is respected) from Tevatron: DMtop = 1.5 GeV !!

Marina Cobal - HCP2004

What is the top mass

Problem for the top: what is the mass of a colored object?

The top pole mass is not IR safe (affected by large long-distance contributions), cannot be determined to better than O(LQCD)

Measurement of mt:

At Tevatron/LHC: kinematic reconstruction, fit to invariant mass dist.

at the LHC: accuracy  1-2 GeV (limited by FSR)

At the LC, mainly from threshold behavior

Measurement comparison data – Monte Carlo: involves transition from actually measured quantity to suitably defined (short-distance) top mass

‘Threshold mass’ at the LC: accuracy  20 MeV [M. Martinez, R. Miquel ’03]

Transition to MS mass: dm  100 MeV [A. Hoang et al. ’00]

What is the Top Mass?

Marina Cobal - HCP2004

Lhc m top from lepton jet

Br(ttbbjjl)=30%for electron + muon

Golden channel

Clean trigger from isolated lepton

The reconstruction starts with the W mass:

different ways to pair the right jets to form the W

jet energies calibrated using mW

Important to tag the b-jets:

enormously reduces background (physics and combinatorial)

clean up the reconstruction

Lepton side

Hadron side

LHC: MTop from lepton+jet

Typical selection efficiency: ~5-10%:

  • Isolated lepton PT>20 GeV

  • ETmiss>20 GeV

  • 4 jets with ET>40 GeV

  • >1 b-jet (b40%, uds10-3, c10-2)

Background: <2%

W/Z+jets, WW/ZZ/WZ

Marina Cobal - HCP2004

Lhc lepton jet reconstruct top

Hadronic side

W from jet pair with closest invariant mass

to MW

Require |MW-Mjj|<20 GeV

Assign a b-jet to the W to reconstruct Mtop

Kinematic fit

Using remaining l+b-jet, the leptonic part is reconstructed

|mlb -<mjjb>| < 35 GeV

Kinematic fit to the tt hypothesis, using MW constraints





LHC: Lepton + jet, reconstruct top

  • Selection efficiency 5-10%

Marina Cobal - HCP2004

Lhc m top systematics

Method works:

Linear with input Mtop

Largely independent on Top PT

Biggest uncertainties:

Jet energy calibration

FSR: ‘out of cone’ give large variations in mass


Verified with detailed detector simulation and realistic calibration

LHC: MTop systematics


determine the mass of the top around 1 GeV accuracy in one year of LHC

Marina Cobal - HCP2004

Lhc alternative mass determination


LHC: Alternative mass determination

  • Select high PT back-to-back top events:

    • Hemisphere separation (bckgnd reduction, much less combinatorial)

    • Higher probability for jet overlapping

  • Use the events where both W’s decay leptonically (Br~5%)

    • Much cleaner environment

    • Less information available from two ’s

  • Use events where both W’s decay hadronically (Br~45%)

    • Difficult ‘jet’ environment

    • Select PT>200 GeV

Various methods all have different systematics

Marina Cobal - HCP2004

Top mass from j

Use exclusive b-decays with high mass products (J/)

Higher correlation with Mtop

Clean reconstruction (background free)

BR(ttqqb+J/)  5 10-5

 ~ 30%  103 ev./100 fb-1(need high lumi)

Top mass from J/


Different systematics (almost no sensitivity to FSR)

Uncertainty on the b-quark fragmentation function becomes the dominant error




Marina Cobal - HCP2004

M top at lc
Mtop at LC

  • Scan of the threshold for e+e-ttbar

  • Very precise measurements (< 20 MeV)

    • To perform this analysis with small systematic errors need to study:

      beam spread


      initial state radiation

  • Simultaneous measurements of 3 physical observables:

    • stt

    • Pt of top

    • Forward-Backward asimmetry of top AtFB

  • Multiparameter fit up to 4 parameters

    • Mt (1S)

    • as(MZ)

    • Gt

    • gtH

  • Theoretical uncertainties to relate the 1S to the MS mass is ~ 100 MeV


Marina Cobal - HCP2004

Georg Weiglin, LCWS04 Paris

Marina Cobal - HCP2004

Lhc search for resonances

Many theoretical models include the existence of resonances decaying to top-topbar

SM Higgs(but BR smaller with respect to the WW and ZZ decays)

MSSM Higgs(H/A, if mH,mA>2mt, BR(H/A→tt)≈1 for tanβ≈1)

Technicolor Models, strong ElectroWeak Symmetry Breaking, Topcolor, “colorons” production, […]

xBR required for a discovery

σxBR [fb]

30 fb-1

830 fb

300 fb-1

mtt [GeV/c2]

1 TeV

LHC: Search for resonances

  • Study of a resonance Χ once known σΧ, ΓΧ and BR(Χ→tt) at LHC

    • Reconstruction efficiency for semileptonic channel:

      • 20% mtt=400 GeV

      • 15% mtt=2 TeV

1.6 TeV resonance


Marina Cobal - HCP2004

Couplings and decays
Couplings and decays decaying to top-topbar

  • Does the top quark behaves as expected in the SM?

    • Yukawa coupling to Higgs from ttbarH events

    • Electric charge

    • Top spin polarization

    • CP violation

  • At the LHC Yukawa coupling can be measured to < 20% from t-tbar H production

  • @ the LC, the precision of the measurement of the top Yukawa coupling will be better than ~ 10 % if mH ≤ 190 GeV/c2

    • For a light Higgs (mH~120 GeV/c2): Precision ~ 5-6 %

  • Marina Cobal - HCP2004

    Lhc couplings and decays
    LHC: Couplings and decays decaying to top-topbar

    • According to the SM:

      • Br(t Wb)  99.9%, Br(t  Ws)  0.1%, Br(t  Wd)  0.01% (difficult to measure)

    • Can probe t W[non-b] by measuring ratio of double b-tag to single b-tag

      • Statistics more than sufficient to be sensitive to SM expectation for Br(t  W + s/d)

      • need excellent understanding of b-tagging efficiency/purity

    Marina Cobal - HCP2004

    Search for anomalous wtb couplings
    Search for anomalous Wtb couplings decaying to top-topbar

    • The Wtb vertex can be probed and measured using either top pair production or single top production

      • Total tt rate depends weakly from Wtb vertex structure

      • C and P asymmetries, top polarization, spin correlations can provide interesting info

      • The single top production rate is instead directly proportional to the square of the Wtb coupling

    • LHC could rivale the reach of a high L (500 fb-1) 500 GeV LC

    Marina Cobal - HCP2004

    Lhc fcnc rare decays
    LHC: FCNC Rare decays decaying to top-topbar

    • In the SM the FCNC decays are highly suppressed (Br<10-13-10-10)

      • Any observation would be sign of new physics

      • FCNC can be detected through top decay or single top production

    • Sensitivity according to CMS studies (of top decays) :

    • t  Zq(CDF Br<0.137, ALEPH Br<17%, OPAL Br<13.7%)

      • Reconstruct t  Zq  (l+l-)j

      • Sensitivity to Br(t  Zq) = 1,6 X 10-4 (100 fb-1)

    • t  q(CDF Br<0.032)

      • Sensitivity to Br(t  q) = 2,5 X 10-5 (100 fb-1)

    • t  gq

      • Difficult identification because of the huge QCD bakground

      • One looks for “like-sign” top production (ie. tt)

      • Sensitivity to Br(t  gq) = 1,6 X 10-3 (100 fb-1)

    Marina Cobal - HCP2004

    FCNC decaying to top-topbar

    • In general, the LHC will improve by a factor of at least 10 the Tevatron sensitivity to top-quark FCNC couplings

    • LC has smaller statistics but also smaller background

    • For anomalous interactions with a gluon the LHC has an evident advantage

    Marina Cobal - HCP2004

    LHC: t decaying to top-topbarWbZ rare decay

    • Studied at LHC:

      Interesting: BR depends strongly on Mtop

      • Since Mtop~MW+Mb+MZ

      • With present error mt  5 GeV, BR varies over a factor  3

    • B-jet too soft to be efficiently identified 

       “semi-inclusive” study for a WZ near

      threshold, with Z  l+l-and W ->jj

      • Requiring 3 leptons reduces the Z+jets background

    • Sensitivity to Br(t  WbZ)  10-3 for 1 year at low lumi.

      • Even at high L can’t reach SM predictions ( 10-7 -10-6)

    G. Mahlon hep-ph/9810485


    M(top) (GeV)

    Marina Cobal - HCP2004

    Lhc top charge determination

    Radiative top production decaying to top-topbar

    Radiative top decay

    LHC: Top Charge determination

    • Can we establish Qtop=2/3?

      • Currently cannot exclude exotic possibility Qtop=-4/3

        • Assign the ‘wrong’ W to the b-quark in top decays

          • tW-b with Qtop=-4/3 instead of tW+b with Qtop=2/3 ?

    • Technique:

      • Hard  radiation from top quarks

        • Radiative top production, pptt cross section proportional to Q2top

        • Radiative top decay, tWb

          • On-mass approach for decaying top: two processes treated independently

        • Matrix elements havebeen calculated and fed intoPythia MC

    Marina Cobal - HCP2004

    Lhc top charge determination1
    LHC: Top Charge determination decaying to top-topbar

    • Yield of radiative photons allows to distinguish top charge

    • Determine charge of b-jet andcombine with lepton

      • Use di-lepton sample

      • Investigate ‘wrong’ combination b-jet charge and lepton charge

      • Effective separation b and b-bar possible in first year LHC

      • Study systematics in progress

    10 fb-1

    One year low lumi



    Marina Cobal - HCP2004

    Lhc top spin correlations

    e decaying to top-topbar+/+



    LHC: Top spin correlations

    • In SM with Mtop175 GeV, (t)  1.4 GeV » QCD

      • Top decays before hadronization, and so can study the decay of ‘bare quark’

      • Substantial ttbar spin correlations predicted in pair production

    • Can study polarization effects through helicity analysis of daughters

      • Study with di-lepton events

      • Correlation between helicity angles + and -for e+/+ and e-/-

    <CosΘ+ · CosΘ->

    <CosΘ+ · CosΘ->

    No helicity correlation

    With helicity correlation

    Marina Cobal - HCP2004

    Top spin correlations

    Able to observe spin correlations in parameter C decaying to top-topbar

    30 fb-1 of data:

    ± 0,035 statistical error

    ± 0,028 systematic error

    10 statistical significance for a non-zero value with 10 fb-1

    Top spin correlations

    30 fb-1

    <CosΘ+ · CosΘ->

    • For the LC case one can find a top spin quantization axis in which there

    • will be very strong spin correlations . Detailed studies, both at the ttbar

    • threshold and in the continuum region remain to be done.

    Marina Cobal - HCP2004

    Lhc single top production

    Direct determination of the tWb vertex (=V decaying to top-topbartb)


    - Jet multiplicity (higher for Wt)

    More than one b-jet (increase W* signal over W- gluon fusion)

    2-jets mass distribution (mjj ~ mW for the Wt signal only)

    Three production mechanisms at LHC:

    Main Background [xBR(W→ℓ), ℓ=e,μ]:

    tt σ=833 pb [ 246 pb]

    Wbbσ=300 pb [ 66.7 pb]

    Wjjσ=18·103 pb [4·103 pb]

    LHC: Single top production

    1) Determination of Vtb

    2) Independent mass measurement

    3) Opportunity to measure top spin pol.

    4) May probe FCNC


    Wg fusion: 245±27 pb

    S.Willenbrock et al., Phys.Rev.D56, 5919

    Wt: 62.2 pb

    A.Belyaev, E.Boos, Phys.Rev.D63, 034012

    W* 10.2±0.7 pb

    M.Smith et al., Phys.Rev.D54, 6696

    -3. 7

    Wg [54.2 pb]

    Wt [17.8 pb]

    W* [2.2 pb]

    Marina Cobal - HCP2004

    Lhc single top results
    LHC: Single top results decaying to top-topbar

    • Detector performance critical to observe signal

      • Fake lepton rate

      • b and fake rate id 

      • Reconstruction and vetoing of low energy jets

      • Identification of forward jets

    • Each of the processes have different systematic errors for Vtb and are sensitive to different new physics

      • heavy W’  increase in the s-channel W*

      • FCNC gu  t  increase in the W-gluon fusion channel

    • Signal unambiguous, after 30 fb-1:

    • Complementary methods to extract Vtb

    • With 30 fb-1 of data, Vtb can be determined to %-level or better(experimentally)

    Marina Cobal - HCP2004

    Single top at lc

    e decaying to top-topbar+ e-  e+bt, ebt

    Single top at LC

    • Cross section calculated to LO at e+e-, gg and ge collision modes, including various beam polarizations

      • Recently calculated NLO corrections in the ge case, well under control

    • Production in ge collisions is of special interest

      • Rate is smaller than the top pair rate in e+e- only by a factor of 1/8 at 500-800 GeV energies.

      • It becomes the dominant LC process for top

        production at a multi-TeV LC

    • Direct Vtb measurement:

      • Same precision as LHC in the e+e- option.

      • Can arrive to 1% for a polarized ge collider.

    • Improved accuracy in |Vtb| could be input for LHC

      • Measure b-quark distribution function in proton

        (or be used as consistency check for new interactions)

    Marina Cobal - HCP2004

    Few final thoughts
    Few final thoughts… decaying to top-topbar

    • e+-e- collisions at 0.5-1 TeV

    • Kinematics:

    • can use momentum conservation

    • Better defined initial state

    • Background smaller than LHC

    • Not sure yet IF, WHEN, WHERE it will be built

    • LHC pp collision at 14 TeV

    • Kinematics:

    • can use PT conservation

    • Composite nature of protons

       Underlying events, not fixed

    • Strongly interacting particle

      •  Large QCD background

    • Under construction

    Marina Cobal - HCP2004

    Interplay between lhc and lc
    Interplay between LHC and LC decaying to top-topbar

    • Not much interaction between the LHC and LC communities up to short time ago

    • In 2002 a LHC/LC study group was formed first in Europe and then soon it took a worldwide character

    • Working Group contains 116 members from among Theorists, CMS, ATLAS, Members of all the LC study Groups + Tevatron contact persons.

    • Document in preparation: www.ippp.dur.ac.uk/~georg/lhclc

      Electroweak and QCD precision physics

      E. Boos, A deRoeck, S. Heinemeyer, W.J. Stirling.

    Marina Cobal - HCP2004

    ATLAS cavern decaying to top-topbar

    CMS yoke

    LHC is a reality now:

    Huge construction activities going on!!

    Marina Cobal - HCP2004

    What is left before the lhc starts
    What is left before the LHC starts? decaying to top-topbar

    • Cover topics still open: cross section, couplings, exotic, resonances,

    • Define a strategy for validation of the MC input models (e.g: UE modeling and subtraction, jet fragmentation properties, jet energy profiles, b-fragmentation functions..)

      see M. Mangano talk at IFAE 2004

    • Explore the effects of changing detector parameters in evaluating the top mass.

    • Perform commissioning studies with top events

    • Contribute to simulation validation

    Marina Cobal - HCP2004

    Lhc commissioning the detectors

    Determination M decaying to top-topbarTop in initial phase

    Use ‘Golden plated’ lepton+jet


    Isolated lepton with PT>20 GeV

    Exactly 4 jets (R=0.4) with PT>40 GeV


    Select 3 jets with maximal resulting PT

    Signal can be improved by kinematic constrained fit

    Assuming MW1=MW2 and MT1=MT2

    LHC: Commissioning the detectors

    • Calibrating detector in comissioning phase

      • Assume pessimistic scenario:

        • -) No b-tagging

        • -) No jet calibration

        • -) But: Good lepton identification

    No background


    Marina Cobal - HCP2004

    Lhc commissioning the detectors1

    Signal plus background at initial phase of LHC decaying to top-topbar

    Most important background for top: W+4 jets

    Leptonic decay of W, with 4 extra ‘light’ jets

    Alpgen, Monte Carlohas ‘hard’ matrix element for 4 extra jets(not available in Pythia/Herwig)

    LHC: Commissioning the detectors


    W+4 extra light jets

    Jet: PT>10, ||<2.5, R>0.4

    No lepton cuts

    Effective : ~2400 pb

    L = 150 pb-1

    (2/3 days low lumi)

    With extreme simple selection and reconstruction the top-peak should be visible at LHC

    measure top mass (to 5-7 GeV) give feedback on detector performance

    Marina Cobal - HCP2004

    Conclusions decaying to top-topbar

    • Precise determination of Mtop is crucial for EW physics:

      • Precision tests, constraints on Higgs sector, sensitivity to new physics

      • Challenge to get Mtop ~ 1 GeV with LHC, and ~ 100 MeV with LC

    • Confirmation that top-quark is SM particle and search for deviation from SM

      • Measure Vtb, charge, CP, spin, decays

      • Many precision measurements from LC

    • Use top quark for the LHC detectors commissioning

    • Interplay between LC and LHC could be as useful as it was for LEP+SLC+Tevatron

    Marina Cobal - HCP2004

    Backup slides

    BACKUP SLIDES decaying to top-topbar

    What we know

    LEP+SLD: decaying to top-topbar

    VCKM (4)





    GF (1)



    mfermions (9)

    (down to 0.1% level)

    mbosons (2)

    eeqq l.e.:

    What we know..


    No observable directly related to mH. However the dependence can appear through radiative corrections. tree level quantities changed

    , r = f [ln(mH/mW), mt2]

    The uncertainties on mt, mW are the dominating ones in the electroweak fit

    By making precision measurements (already interesting per se):

    • one can get information on the missing parameter mH

    • one can test the validity of the Standard Model

    Marina Cobal - HCP2004

    Lc timeline
    LC Timeline decaying to top-topbar

    • .

    Graphically summarized

    by Jae Yu

    Marina Cobal - HCP2004

    Lc machines
    LC Machines decaying to top-topbar


      • ECM of operation 200-500 GeV

      • Luminosity and reliability for 500 fb-1 in 4 years

      • Energy scan capability with <10% downtime

      • Beam energy precision and stability below about 0.1%

      • Electron polarization of > 80%

      • Two IRs with detectors

      • ECM down to 90Gev for calibration


      • ECM about 1 TeV

      • Allow for ~1 ab-1 in about 3-4 years



    Marina Cobal - HCP2004

    Rare sm top decays
    Rare SM top decays decaying to top-topbar

    • Direct measurement of Vts, Vtdvia decays tsW, tdW

    • Decay tbWZ is near threshold

      (mt~MW+ MZ+mb) 

      BRcut(t bWZ)  610-7

      (cut on m(ee) is 0.8 MW)

    • Decay tcWW suppressed by GIM

      factorBR(t cWW) ~ 110-13

    • If Higgs boson is light: tbWH

    • FCNC decays: tcg, tc, tcZ(BR: 510-11 , 510-13 , 1.310-13 )

    • Semi-exclusive t-decays tbM

      (final state 1 hadron recoiling against a jet:

      BR(t b)  410-8, BR(t bDs)  210-7)

    Marina Cobal - HCP2004

    Top hq

    Signal decaying to top-topbar







    • Various approaches studied

      • Previously: ttbarHq Wb(b-bbar)j(lb) for m(H) = 115 GeV

      • Sensitivity to Br(t  Hq) = 4.5 X 10-3(100 fb-1)

    • New results for:

    • t tbarHq WbWW*q Wb(l lj) (lb)

      • ≥ 3 isolated lepton with pT(lep) > 30 GeV

      • pTmiss > 45 GeV

      • ≥ 2 jets with pT(j) > 30 GeV,

        incl. ≥ 1 jet con b-tag

      • Kinematical cuts making use of

        angular correlations

      • Sensitive to Br(t  Hq) = 2.4 X 10-3

        for m(H) = 160 GeV (100 fb-1)

    Marina Cobal - HCP2004

    Non sm decays of top
    Non-SM Decays of Top decaying to top-topbar

    • 4thfermion family

      Constraints on Vtqrelaxed:

    • Supersymmetry (MSSM)

      • Observed bosons and fermions would have superpartners 

        2-body decays into squarks and gauginos (t  H+ b )

      • Big impact on 1 loop FCNC

    • two Higgs doublets

      • H LEP limit 77.4 GeV (LEP WG 2000)

      • Decay t  H+ b can compete with t  W+ b

      • 5 states (h0,H0,A0,H+,H-) survive after giving W & Z masses

      • H couples to heaviest fermions  detection through breakdown of e / m / t universality in tt production

    Marina Cobal - HCP2004

    Alternative methods

    Continuous jet algorithm decaying to top-topbar

    Reduce dependence on MC

    Reduce jet scale uncertainty

    Repeat analysis for many cone sizes R

    Sum all determined top mass:robust estimator top-mass

    Determining Mtop from (tt)?

    huge statistics, totally different systematics

    But: Theory uncertainty on the pdfs kills the idea

    10% th. uncertainty  mt  4 GeV

    Constraining the pdf would be very precious…

    (up to a few % might not be a dream !!!)

    Alternative methods

    • Luminosity uncertainty then plays the game (5%?)

    Luminosity uncertainty then plays the game (5%?)

    Marina Cobal - HCP2004

    Top mass from di leptons

    Use the events where both W’s decay leptonically (Br~5%) decaying to top-topbar

    Much cleaner environment

    Less information available due to two neutrino’s

    Sophisticated procedure for fitting the whole event, i.e. all kinematical info taken into account (cf D0/CDF)

    Compute mean probability as function of top mass hypothesis

    Maximal probability corresponds to top mass

    Top mass from di-leptons

    80000 events

    (tt) = 20 %

    S/B = 10

    • Selection:

      • 2 isolated opposite sign leptons

        • Pt>35 and Pt>25 GeV

      • 2 b-tagged jets

      • ETmiss>40 GeV

    Mean probability

    Marina Cobal - HCP2004


    Top mass from hadronic decay

    Use events where both W’s decay hadronically (Br~45%) decaying to top-topbar

    Difficult ‘jet’ environment

    (QCD, Pt>100) ~ 1.73 mb

    (signal) ~ 370 pb

    Perform kinematic fit on whole event

    b-jet to W assignment for combination that minimize top mass difference

    Increase S/B:

    Require pT(tops)>200 GeV

    Top mass from hadronic decay

    • Selection

      • 6 jets (R=0.4), Pt>40 GeV

      • 2 b-tagged jets

        • Note: Event shape variables like HT, A, S, C, etc not effective at LHC (contrast to Tevatron)

    3300 events selected:

    (tt) = 0.63 %

    (QCD)= 2·10-5 %

    S/B = 18

    Marina Cobal - HCP2004

    High pt sample

    j decaying to top-topbar2





    High Pt sample

    • The high pT selected sample deserves independent analysis:

      • Hemisphere separation (bckgnd reduction, much less combinatorial)

      • Higher probability for jet overlapping

    • Use all clusters in a large cone R=[0.8-1.2] around the reconstructed top- direction

      • Less prone to QCD, FSR, calibration

      • UE can be subtracted


    Statistics seems OK and syst. under control


    Marina Cobal - HCP2004

    Calibration demands: decaying to top-topbar

    Ultimately jet energy scale calibrated within 1%

    Uncertainty on b-jet scale dominates Mtop: light jet scale constrained by mW

    At startup jet-energy scale known to lesser precision




    Scale light-jet energy

    Scale b-jet energy

    Jet scale calibration


    on light jet scale:Hadronic

    1%  Mt < 0.7 GeV

    10%  Mt = 3 GeV


    On b-jet scale:Hadronic

    1%  Mt = 0.7 GeV

    5%  Mt = 3.5 GeV

    10%  Mt = 7.0 GeV

    Marina Cobal - HCP2004

    Calibration demands: decaying to top-topbar

    Ultimately jet energy scale calibrated within 1%

    Uncertainty on b-jet scale dominates Mtop: light jet scale constrained by mW

    At startup jet-energy scale known to lesser precision




    Scale light-jet energy

    Scale b-jet energy

    LHC: Jet scale calibration


    On b-jet scale:Hadronic

    1%  Mt = 0.7 GeV

    5%  Mt = 3.5 GeV

    10%  Mt = 7.0 GeV


    on light jet scale:Hadronic

    1%  Mt < 0.7 GeV

    10%  Mt = 3 GeV

    Marina Cobal - HCP2004