The top quark at LHC: status and prospects

1

The top quark at LHC: status and prospects

Marina Cobal-Grassmann“Journee ATLAS France”Londe Les Maure, 3-5 May, 2004

Marina Cobal - Londe Les Maure 2004

P 2 Motivations for Top Physics studies

Top quark exists and will be produced abundantly! In SM: top- and W-mass constrain Higgs mass

Sensitivity through radiative corrections Scrutinize SM by precise determination top mass

Beyond SM: New Physics? Many heavy particles decay in tt Handle on new physics by detailed

properties of top

Experiment: Top quark useful to calibrate the detector

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

direct

indirectEXCLU

DED

Summer 2003 result


P 3

1cos22

2

WZ

W

mm

No observable directly related to mNo observable directly related to mHH. However the dependence can . However the dependence can appear through radiative correctionsappear through radiative corrections. tree level quantities changedtree level quantities changed

mH

, , r r = f [ln(m= f [ln(mHH/m/mWW), m), mtt22]]

By making precision measurements (already interesting per se):By making precision measurements (already interesting per se):• • one can get information on the missing parameter mone can get information on the missing parameter mHH• • one can test the validity of the Standard Modelone can test the validity of the Standard Model

SMmfermions (9)mbosons (2)

VCKM (4)

GF (1)s(1)

)1(sin2 2

2 rG

mFW

W

The uncertainties on mThe uncertainties on mtt, m, mWW are the dominating ones in the electroweak fit are the dominating ones in the electroweak fit

predictions

(down to 0.1% level)

lepteffcbl

bclFBcblhWZWZ AARm 2

,,,,,00

,,0

,, sin,,,,,,

tWW mm ,,

had

W2sin

WW CsQ 2sin)(

LEP+SLD:LEP+SLD:UA2+Tevatron:UA2+Tevatron:

NuTeV:NuTeV:

APV:APV:eeeeqq l.e.:qq l.e.:

What we know..


P 4

Top mass: Where we are


P 5

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

Status of inputs (preliminary):mmtt=(178.0 =(178.0 2.7 2.7 (stat) (stat) 3.3 3.3 (syst)(syst)) GeV/c) GeV/c22

(latest Tevatron updated combination – RunI data)(latest Tevatron updated combination – RunI data) mmtt=(175 =(175 17 17 (stat) (stat) 8 8 (syst)(syst)) GeV/c) GeV/c22

(CDF di-leptons – RunII data)(CDF di-leptons – RunII data) mmtt=(178=(178+13+13

-9-9 (stat) (stat) 7 7 (syst)(syst)) GeV/c) GeV/c22

(CDF lepton+jets – RunII data)(CDF lepton+jets – RunII data)

Matter of statistics (also for the main systematics) and optimized use of the 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 available information. Each experiment expects 500 b-tagged tt l+jets events/fb Mtop ~ 2-3 GeV/cMtop ~ 2-3 GeV/c22 for the Tevatron combined (2-4/fb) for the Tevatron combined (2-4/fb)

mmt t 2.5 GeV ; 2.5 GeV ; mmW W 30 MeV 30 MeV mmHH/m/mH H 35% 35%

Near future of Mtop

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


P 6

What can we do at LHC?

LHCTeVatron1034

1033

<1032

Luminosity[cm-2s-1]

100 10

0.3

∫L[fb-1/y]

14 LHC (high lum)14 LHC (low lum)

2 TeVatron

√s[TeV]

processprocess (pb)(pb) Events/sEvents/s Events/yEvents/y

bbbb 55101088 101066 10101313

ZZeeee 1.51.5101033 ~3~3 101077

WWℓℓℓ=e,μℓ=e,μ 33101044 ~60~60 101088

WWWWeeXX 66 1010-2-2 101055

tttt 830830 ~1.7~1.7 101077

HH(700 GeV/c(700 GeV/c22)) 11 221010-3-3 101044

--

--


P 7

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

32121 10~ ; ˆ xxxsxs

~90% gg~10% qqProcess N/s N/year Total collected

before start LHC

W e 15 108 104 LEP / 107 FNAL

Z ee 1.5 107 107 LEP

tt 1 107 104 Tevatron

bb 106 1012-13 109 Belle/BaBar ?

H (130) 0.02 105 ?

LHC is a top factory!

Low lumi


P 8

Top decay In the SM the top decays to W+b

All decay channels investigated Using ‘fast parameterized’ detector

response Checks with detailed simulations

1. Di-leptons (e/) BR≈4.9% 0.4x106 ev/y No top reconstructed Clean sample

2. Single Lepton (e/) BR=29.6% 2.5x106 ev/y One top reconstructed Clean sample

3. Fully Hadronic BR≈45% 3.5x106 ev/y Two tops reconstructed Huge QCD background Large combinatorial bckgnd


P 9

MTop from lepton+jet

Br(ttbbjjl)=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

Typical selection efficiency: ~5-10%:•Isolated lepton PT>20 GeV•ET

miss>20 GeV•4 jets with ET>40 GeV•>1 b-jet (b40%, uds10-3, c10-2)

Background: <2%W/Z+jets, WW/ZZ/WZ


P 10

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 |mlb -<mjjb>| < 35 GeV Kinematic fit to the tt hypothesis,

using MW constraints

j1

j2

b-jet

t

Selection efficiency 5-10%


P 11

Top mass 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 B-fragmentation

Verified with detailed detector simulation and realistic calibration

Source of uncertainty

Hadronic

Mtop

(GeV)

Fitted Mtop (GeV)

Light jet scale

0.9 0.2

b-jet scale 0.7 0.7

b-quark fragm

0.1 0.1

ISR 0.1 0.1

FSR 1.9 0.5

Comb bkg 0.4 0.1

Total 2.3 0.9

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


P 12 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

Mtop

Various methods all have different systematics


P 13

Uncertainty On b-jet scale: Hadronic 1% Mt = 0.7 GeV5% Mt = 3.5 GeV10% Mt = 7.0 GeV

Uncertainty on light jet scale: Hadronic 1% Mt < 0.7 GeV10% Mt = 3 GeV

Jet scale calibration Calibration demands:

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 b-jet energyScale light-jet energy

MTop MTop

±10%


P 14

Alternative methods

Continuous jet algorithm 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 !!!)

Luminosity uncertainty then plays the game (5%?)

Luminosity uncertainty then plays the game (5%?)


P 15

Use exclusive b-decays with high mass products (J/) Higher correlation with Mtop Clean reconstruction (background free) BR(ttqqb+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

M(J/+l) Pttop

MlJ/

M(J/+l)


P 16

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, […] Study of a resonance Χ once known σΧ, ΓΧ and BR(Χ→tt)

Reconstruction efficiency for semileptonic channel: 20% mtt=400 GeV 15% mtt=2 TeV

1.6 TeV resonance

Mtt

xBR required for a discovery

mtt [GeV/c2]

σxB

R [f

b]

30 fb-1

300 fb-1

1 TeV

830 fb


P 17

Couplings and decays Does the top quark behaves as expected in the SM?

Yukawa coupling to Higgs from ttbarH events Electric charge Top spin polarization CP violation

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


P 18

In the SM the FCNC decays are highly suppressed (Br<10-13-10-10) Any observation would be sign of new physics

Sensitivity according to ATLAS and CMS studies :

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 X 10-4 (100 fb-1)

t q (CDF Br<0.032) Sensitivity to Br(t q) = 1 X 10-4 (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) = 7 X 10-3 (100 fb-1)

Rare decays: FCNC


P 19

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

tW-b with Qtop=-4/3 instead of tW+b with Qtop=2/3 ?

Technique: Hard radiation from top quarks

Radiative top production, pptt cross section proportional to Q2top

Radiative top decay, tWb

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

Matrix elements havebeen calculated and fed intoPythia MC

Radiative top production

Radiative top decay


P 20

Top Charge determination Yield of radiative photons allows

to distinguish top charge Q=2/3 Q=-4/3

pptt 101 ± 10 295 ± 17

pptt ; tWb 6.2 ± 2.5 2.4 ± 1.5

Total background 38 ± 6

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

i

κ

i

i

κ

iibjet

pj

pjqq

pT()

events

10 fb-1One year low lumi


P 21

Top spin correlations In SM with Mtop175 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-/-

e+/+

top+

With helicity correlationNo helicity correlation

<CosΘ+ · CosΘ-> <CosΘ+ · CosΘ->

leptons)for 1 :qualityanalyser (spin base)helicity in n correlatiospin of (degree 34.0

4coscos1

coscos1 2

C

Cdd

d


P 22

Top spin correlations Also study spin correlations in

hadronic decays (single lepton events) Least energetic jet from W

decay: ~ 0.5

Ratio between ‘with’ and ‘without’ correlations

Able to observe spin correlations in asymmetry C 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

30 fb-1<CosΘ+ · CosΘ->


P 23

Single top production

Direct determination of the tWb vertex (=V tb)

Discriminants:- Jet multiplicity (higher for Wt)

- More than one b-jet (increase W* signal over W- gluon fusion)- 2-jets mass distribution (m jj ~ mW for the Wt signal only)

Three production mechanisms:

Main Background [xBR(W→ℓ), ℓ=e,μ]: tt σ=833 pb [ 246 pb] Wbb σ=300 pb [ 66.7 pb] Wjj σ=18·103 pb [4·103 pb]

Wg fusion: 245±27 pbS.Willenbrock et al., Phys.Rev.D56, 5919

Wt: 62.2 pbA.Belyaev, E.Boos, Phys.Rev.D63, 034012-3. 7

+16.6 W* 10.2±0.7 pbM.Smith et al., Phys.Rev.D54, 6696

Wg [54.2 pb]Wt [17.8 pb]W* [2.2 pb]

1) Determination of Vtb

2) Independent mass measurement


P 24

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)

Single top results 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

ProcessVtb

(stat)Vtb

(theory)

Wg fusion 0.4% 6%

Wt 1.4% 6%

W* 2.7% 5%

Process Signal Bckgnd S/B

Wg fusion 27k 8.5k 3.1Wt 6.8k 30k 0.22W* 1.1k 2.4k 0.46


P 25

Undergoing analyses CP violation in top events (K. Martens, University of Toronto ) Top spin polarization in di-lepton events (V. Simak et al., Prague) Top spin polarization in single lepton events (E. Monnier, P. Pralavorio,

F. Hubaut, CPPM) Single top studies (M. Barisonzi, NIKHEF) Optimization of kinematic reconstruction in the single lepton channel (V.

Kostioukhine, University of Genova)

Commissioning studies (S. Bentvelsen, NIKHEF) New MC validation (S. Bentvelsen, E. Monnier, P. Pralavorio) Full simulation studies of detector effects (A. Etienvre, J. Schwindling,

JP Meyer, Saclay) Full simulation studies of b-tagging (S. Moed, University of Geneva) Top mass and calibration studies (D. Pallin, F. Binet, Clermont-

Ferrand) Ttbar resonances (E. Cogneras, Clermont-Ferrand)


P 26 What is left before the LHC starts?

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 …


P 27

Commissioning the detectors Determination MTop in initial

phase Use ‘Golden plated’ lepton+jet

Selection: Isolated lepton with PT>20 GeV Exactly 4 jets (R=0.4) with

PT>40 GeV Reconstruction:

Select 3 jets with maximal resulting PT

Signal can be improved by kinematic constrained fit Assuming MW1=MW2 and MT1=MT2

PeriodStat Mtop (GeV)

Stat /

1 year 0.1 0.2%1 month 0.2 0.4%1 week 0.4 2.5%

No background included

Calibrating detector in comissioning phaseAssume pessimistic scenario:

-) No b-tagging-) No jet calibration-) But: Good lepton identification


P 28

Commissioning the detectors

Signal plus background at initial phase of LHC

Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets

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

ALPGEN:W+4 extra light jetsJet: PT>10, ||<2.5, R>0.4

No lepton cuts Effective : ~2400 pb

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

L = 150 pb-1

(2/3 days low lumi)

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


P 29

Top in DC2 Tier test The 10M tier1 events in light of top:

Generation/simulation of 106 top events, inclusive decays, using MC@NLO Using Herwig for MC + UE Simulation with full geometry

Simulation 500K top events with displaced ID Same truth generated top events as above 1 cm displacement of ID – check tracking performance

Simulation of 106 W+jet events MC@NLO For W+2 jet background

Simulation of 250K W+4jet events with AlpGen pT>15 GeV approximately


P 30

What is still missing? Top production is ‘over-weighted’ in 10M sample

Unrealistic to ask for more in this sample

One of priorities in ‘post-production’: Regenerate half of the top MC@NLO sample Now using Jimmy UE (much more activity), after tuning

Still on the wish-list: Top events with spin correlations

TopRex available, perhaps AcerMC? Single top events

also TopRex Dedicated samples single- and dilepton top events Top events with PYTHIA (cross check with DC1)


P 31

Conclusions

Precise determination of Mtop is waiting… Challenge to get Mtop ~ 1 GeV

Confirmation that top-quark is SM particle Measure Vtb, charge, CP, spin, decays

Top quarks for commissioning the detectors Top peak should be visible with eyes closed

Today’s signal, tomorrow’s background Top quarks as main background

for many new physics channels

LHC is top factory(tt)~830 pb-1 107 events in first year


P 32

Rare SM top decays Direct measurement of Vts, Vtd via decays tsW, tdW

Decay tbWZ is near threshold (mt~MW+ MZ+mb)

BRcut(t bWZ) 610-7

(cut on m(ee) is 0.8 MW) Decay tcWW suppressed by GIM factor BR(t cWW) ~ 110-13

If Higgs boson is light: tbWH FCNC decays: tcg, tc, tcZ (BR: 510-11 , 510-13 , 1.310-13 ) Semi-exclusive t-decays tbM (final state 1 hadron recoiling against a jet: BR(t b) 410-8, BR(t bDs) 210-7)

2 2b Wm M


P 33

Top mass from di-leptons Use the events where both W’s decay leptonically (Br~5%)

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 massSource of uncertainty

Di-lepton Mtop (GeV)

statistics 0.3

b-jet scale 0.6

b-quark fragm 0.7

ISR 0.4

FSR 0.6

pdf 1.2

Total 1.7

80000 events (tt) = 20 %S/B = 10

Mea

n pr

obab

ility

mass

Selection:2 isolated opposite sign leptons

Pt>35 and Pt>25 GeV

2 b-tagged jetsETmiss>40 GeV


P 34

Top mass from hadronic decay Use events where both W’s decay hadronically (Br~45%) 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 GeVSource of uncertainty

Hadronic Mtop

(GeV)

Statistics 0.2

Light jet scale 0.8

b-jet scale 0.7

b-quark fragm 0.3

ISR 0.4

FSR 2.8

Total 3.0

3300 events selected:

(tt) = 0.63 %(QCD)= 2·10-5 %S/B = 18

Selection6 jets (R=0.4), Pt>40 GeV2 b-tagged jets

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


P 35

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

j1

j2

b-jet

t

Statistics seems OK and syst. under control

R

Mtop Mtop

P 36

Uncertainty On b-jet scale: Hadronic 1% Mt = 0.7 GeV5% Mt = 3.5 GeV10% Mt = 7.0 GeV

Uncertainty on light jet scale: Hadronic 1% Mt < 0.7 GeV10% Mt = 3 GeV

Jet scale calibration Calibration demands:

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 b-jet energyScale light-jet energy

MTop MTop

±10%


P 37

Interesting: branching ratio 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)

(t

WbZ

)/(t

Wb)

Rare decays: topWbZ


P 38

Various approaches studied Previously: ttbarHq Wb(b-bbar)j(lb)

for m(H) = 115 GeV Sensitivity to Br(t Hq) = 4.5 X 10-3

(100 fb-1)

New results for: t tbarHq WbWW*q Wb(l lj) (lb)

≥ 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)

SignalttH

tt

SignalttH

tt

topHq


P 39

Non-SM Decays of Top 4thfermion family Constraints on Vtqrelaxed: 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 tt production

at ( ( )) ~ 3bBR t W b W c 10 m 100GeV

, , 01 1 1 1 1t t g t b t t

Documents

The top quark at LHC: status and prospects