Top Physics in ATLAS

M. Cobal, University of UdineM. Cobal, University of Udine

Bologna, 12 Feb 2007Bologna, 12 Feb 2007

What do we know about the top quark?

The top quark completes the three family structure of the SM

Its massive

Spin=1/2,

Charge=+2/3, Isospin=+1/2

tbW

Large =1.42GeV (mb,MW,s,EW corr.)

Short lifetime

had = QCD-1 >> decay

“t-quarks are produced and decay as free particles”

NO top hadrons

m/m <2%

-4/3 excluded @ 94%C.L.(D0)

c<52.5mm @95%C.L.(CDF)

Not directly

The TEVATRON is probing better than ever the top sector…The LHC will allow precision measurements of Top Quark Physics

~100%, FCNC: probed at the 10% level

Not directly

The LHC tunnel at CERN

27 km diameter14 TeVpp collisions

10%

90%

Production: σtt(LHC) ~ 830 ± 100 pb

1 tt-event per second

Top quark production at the LHC

Cross section LHC = 100 x TevatronBackground LHC = 10 x Tevatron

tt

Final states:

t Wb ~ 1 W qq ~ 2/3W lν ~ 1/3

t Wb ~ 1 W qq ~ 2/3W lν ~ 1/3

1) Fully-hadronic (4/9) 6 jets

2) Semi-leptonic (4/9): 1l + 1ν + 4 jets

3) Fully-leptonic (1/9): 2l + 2ν + 2 jetsGolden channel (l=e,μ) 2.5 million events/year

ATLAS Experiment

Toroide+Solenoid (4 magnets) in inner cavityCalorimeters outside field

• Magnets • TrackerSi pixels + stripsTRD alows particle IDSolenoid B=2T/pT ~ 5x10-4 pT 0.01

• Em Calo

Pb-liquid argon/E ~ 10%/E ~1% uniform

• Hadronic Calo

Fe-scint Tile (10 ) /E ~ 50%/E 0.03

• Muon Detectors

/pT < 10 % at 1 TeV

Top physics at the LHC

PRODUCTIONCross sectionSpin-correlationsResonances XttFourth generation t’New physics (SUSY)Flavour physics (FCNC)

PROPERTIESMass (matter vs. anti-matter)ChargeLife-time and widthSpin

DECAYCharged HiggsW helicityAnomalous couplingsCKM matrix elementsCalibration sample !!

kinematic fit (mW)

missing energy

TOP

≈ 0.4 10-24 s

BOTTOM

W

L

jetsb-tagging

trigger

This data will extend the Tevatron precision reach and allow new possible topics.

Top quarks and search for new physics

First year at the LHC:

A new detector AND a new energy regime

2 Understand SM+ATLAS in simple topologies

Understand SM+ATLASin complex topologies

3

Look for new physicsin ATLAS at 14 TeV

4

Process #events 10 fb-1

4 TeV) 1g(m

5GeV) 130h(m

7T

7

-

10 gg

10 h

10 GeV 150P jets QCD

10 bias Min.

10 tt

10 μ/μeeZ

10 eνW

10 bb

~

7

7

8

12

~~

2

3

4

1 Understand ATLASusing cosmics

2008 should look something like…

Hardware commissioning to 7 TeV

Machine Checkout 1 month

Commissioning with beam 2 months

Pilot Physics 1 month

Provisional Reach

1031

Running at 75 ns L~ 1032 cm-2s-1

~ 3 months of running+some optimism ~ 1 fb-1

How many events at the beginning ?

10 pb-1 1 month at1030 and < 2 weeks

at 1031, =50%

100 pb-1 few days

at 1032 , =50%

Assumed selection efficiency:W l, Z ll : 20%tt l+X :1.5% (no b-tag, inside mass bin)

+ lots of minimum-bias and jets (107 events in 2 weeks of data taking if 20% of trigger bandwidth allocated)

1 fb-1

Similar statistics to D0/CDF

Which detector performance on day one ?

Based on detector construction quality, test-beam results, cosmics, simulation

Ultimate statistical precision achievable after few weeks of operation. Then face systematics…. E.g. : tracker alignment : 100 m (1 month) 20m (4 months) 5 m (1 year) ?

Expected performance day 1 Physics samples to improve

ECAL uniformity ~ 1% Minimum-bias, Z eee/ scale ~ 2 % Z ee

HCAL uniformity ~ 3 % Single pions, QCD jetsJet scale < 10% Z ( ll) +1j, W jj in tt events

Tracking alignment 20(100)-200 m in R? Generic tracks, isolated , Z m

Top physics ‘easy’ at the LHC:

Top physics with b-tag information

Selection: Lepton Missing ET 4 (high-PT)-jets (2 b-jets) signal efficiency few % very small SM background

• ‘Standard’ Top physics at the LHC: - b-tag is important in selection - Most measurements limited by systematic uncertainties

• ‘Early’ top physics at the LHC: - Cross-section measurement (~ 20%) - Decay properties

S/B=O(100)Top signal

W+jets background

Top mass (GeV)

Nu

mb

er

of

Even

ts

Top physics without b-tag information

Selection efficiency = 5.3%1 lepton PT > 20 GeV

Missing ET > 20 GeV

4 jets(R=0.4) PT > 40 GeV

TOP CANDIDATE

1) Hadronic top:

Three jets with highest vector-sum pT as the decay products of the top

2) W boson:

Two jets in hadronic top with highest momentum. in reconstructed jjj C.M. frame.

W CANDIDATE

• Assign jets to W-boson and top-quark:

• Robust selection cuts: Still 1500 events/day

Cut on MW

Results for a ‘no-b-tag’ analysis: 100 pb-1

Mjjj (GeV)

electron+muon estimate for L=100 pb-1

Even

ts /

4.1

5 G

eV

ATLAS preliminary

3-jet invariant mass3-jet invariant mass

Mjjj (GeV)

Even

ts /

4.1

5 G

eV

Top-combinatoricsand W+jets background

Top-signal

We can easily see top peak without b-tag requirementWe can easily see top peak without b-tag requirement

100 pb-1 is a few days of nominal low-luminosity LHC operation

What can you do with early tops?

Calibrate light jet energy scale - impose PDG value of the W mass (precision < 1%)

Estimate/calibration b-tagging - From data (precision ~ 5%)

- Study b-tag (performance) in complex events

Study lepton trigger

Calibrate missing transverse energy - use W mass constraint in the event - range 50 GeV < p T < 200 GeV

Estimate (accuracy ~20%) of mt and tt.

Use W boson mass to enhance purity

Missing ET (GeV)

Even

ts

Perfect detector

Miscalibrated detector or

escaping ‘new’ particle

Selected 87000 signal events for L=10fb-1 (S/B~78)

In-situ jet energy calibration (W→jj)

Mass estimator via fit on spectrum

Although errors are dominated by systematics It seems possible to determine mt @ 1GeV level

(with L=10fb-1)

(=3.5%)

Comb.

=10.6GeV

Systematic Errors:Systematic Errors:

ATLAS Eur.Phys.J C39 (2005) 63

Top mass reconstruction

Single top @ LHC

Electroweak top productionThree different Processes (never observed yet)

Powerfull Probe of Vtb ( dVtb/Vtb~few% @ LHC )

t-channel Wt-channel W* (s-channel)

~ 250 pb ~70pb ~ 10 pb

Vtb

Vtb Vtb

Vtb

Probe New Physics Differently: ex. FCNC affects more t-channel ex. W´ affects more s-channel[ PRD63 (2001) 014018]

Single top and new physics

T.Tait, C.-P.Yuan, Phys.Rev. D63 (2001) 0140018

FCNCkZtc=1

4th generation,|Vts|=0.55, |Vtb|=0.835(extreme values allowed w/o the CKM unitarity assumption)

SMTop-flavorMZ’=1 TeVsen2f=0.05

Top-pionMp±=450 GeVtR-cR mixing ~ 20%

s-channel

t-ch

annel

Cross Sections

Theoretical errors at the LHC

Process PDF-scale

(/2-2)

mtop

(at LHC)

s-channel 4% 2% 2%

t-channel <2% 3% 1%

Wt ? <5% 1%

(Z.S

ulli

van

, Phys.

Rev. D

70

(2

00

4)

11

40

12

)

Should be similat to thet-channel and to gg→tt

Less than at Tevatron, since the x-region for the gluon PDFs

is better known.

Single top production

Common feature: 1 lepton, pT>25GeV/cHigh Missing ET

2 jets (at least 1 b-jet)(ATL-COM-PHYS-2006-002)

(ATL-PHYS-PUB-2006-014)

L=30fb-1Separate Channels by (Nj,Nb) in final state:

( <1.5%)

t-channel: Stat: 7000 events (S/B=3)Syst: dominated by Eb-jet and Lum. ErrorBack: tt, Wbb and W+jets

(Nj=2,Nb=1)

Wt-channel: Stat: 4700 events, ~1% (S/B=15%)( / ~ 4%)

(Nj=3,Nb=1)

s-channel:Stat: 1200 events for tb (S/B=10%)Syst: Eb-jet, Lum. Error, back X-sectionBack_t-channel, tt

7-8%)(Nj=2,Nb=2)

Beyond the SM

non-SM production (Xtt) resonances in the tt system MSSM production

unique missing ET signatures from non-SM decay (tXb, Xq)

charged Higgs change in the top BR, can be investigated via direct evidence or via

deviations of R(ℓℓ/ℓ)=BR(Wℓ) from 2/9 (H+,cs). FCNC t decays: tZq tq tgq

highly suppressed in SM, less in MSSM, enhanced in some sector of SEWSB and in theories with new exotic fermions

non-SM loop correction precise measurement of the cross-section

ttNLO-tt

LO/ ttLO <10% (SUSY EW), <4% (SUSY QCD)

typical values, might be much bigger for certain regions of the parameter space

associated production of Higgs ttH

tbttttg ~

, ~ ,~ ~1

02,1

New physics: Resonances in Mtt

• Structure in Mtt

- Interference from MSSM Higgses H,A tt (can be up to 6-7% effect)

Cro

ss s

ecti

on

(a.u

.)

Mtt (GeV)

• Resonances in Mtt

Resonanceat 1600 GeV

# e

ven

ts

ttXpp Z’, ZH, G(1), SUSY, ?

Mtt (GeV)

400 GeV

500 GeV

600 GeV

Gaemers, Hoogeveen (1984)

ATLAS

%6~m

m

s< 10-23 s no ttbar bound states within the SM Many models include the existence of resonances decaying to ttbar

SM Higgs , MSSM Higgs, Technicolor Models, strong ElectroWeak Symmetry Breaking, Topcolor

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

Assume detector resolution > ΓΧ

Excellent experimental resolution in mass, ranging from 3% to 6% !

Reconstruction efficiency for the semileptonic channel:

20% mtt=400 GeV 15% mtt=2 TeV

xBR required for a discovery

1 TeV1 TeV

Shown sensitivity up to a few TeV

Resonances in a tt system

mtt (GeV)

Resonanceat 1600 GeVΔσ/σ ~ 6 %

Resolution m(tt)Study the detector sensitivity in an

inclusive way:

55

fast-sim

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

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

l+,t

lqq

t

Other angular distributions:

AD(LO)

AD(NLO)

-0.217

-0.237

SM:

A(LO)

A(NLO)

0.319

0.326

Although t and t are produced unpolarized their spins are correlated

New Physics affects A

aX=spin analysing power of X

SM:

1 dN 1

N dcos 2( 1 – ADaXaX´cos ) =

Top spin correlation

(Eur.Phys.J.C44S2 2005 13-33)

• Semileptonic + Dileptonic• Syst (Eb-jet,mtop,FSR)• ~4% precision-0.29

0.42

SM

Mtt<550 GeV

0.008 0.010AD

0.014 0.023A

Error (±stat ±syst)

A) Test the tbW decay vertex

Measure W polarization (F0, FL, FR) through

lepton angular distribution in W cm system:

Semilep. +

Dileptonic

• Syst ( Eb-jet,mtop,FSR )• F0/ F0 ~ 2% ; FR ~ 0.01

0.000(mb=0)

0.297

0.703

SM

0.003 0.024FL

0.003 0.012FR

0.004 0.015F0

Error (±stat ±syst) (Mt=175 GeV)

(Eur.Phys.J.C44S2 2005 13-33)

L=10fb-1

Probing the Wtb vertex(1

/)d

/d

cos(

l*)

B) Anomalous Couplings in the tbW decay

Angular Asymmetries: AFB, A+ and A-

AFBA+

A-

cos(cos(ll*)*)

AFB [t=0] A± [t= (22/3-1)]

±

SM(LO):SM(LO):

(PRD67 (2003) 014009, mb≠0)

Probing the Wtb vertex

1 Results:

B) Anomalous Couplings in the tbW decay

SM(LO):

L=0.423R=0.0005 (mb≠0)

L=10fb-1

Probing the Wtb vertex

Top quark FCNC decay

GIM suppressed in the SM Higher BR in some SM extensions (2-Higgs doublet, SUSY, exotic fermions)

3 channels studied:

BR in SM 2HDM MSSM R SUSY QS

tqZ ~10-14 ~10-7 ~10-6 ~10-5 ~10-4

tq ~10-14 ~10-6 ~10-6 ~10-6 ~10-9

tqg ~10-12 ~10-4 ~10-5 ~10-4 ~10-7

Results

BR 5 sensitivity

Expected 95% CL limits on BR (no signal)

Dominant systematics: MT and tag < 20%

tqZ tq tqg

L = 10 fb-1 5.1x10-4 1.2x10-4 4.6x10-3

L = 100 fb-1 1.6x10-4 3.8x10-5 1.4x10-3

tqZ tq tqg

L = 10 fb-1 3.4x10-4 6.6x10-5 1.4x10-3

L = 100 fb-1 6.5x10-5 1.8x10-5 4.3x10-4

Present and future limitsTopological likelihood for three channels

Resulting 95% CL limitst → qZSM bck

signal

t → q

t → qg

With 10 fb-1 already 2 orders of magnitude better than LEP/HERA

Conclusions

DAY-2 top physics: - Single top production - Top charge, spin(-correlations), mass

1) Top quarks are produced by the millions at the LHC: Almost no background: measure top quark properties

2) Top quarks are THE calibration signal for complex topologies: Most complex SM candle at the LHC Vital inputs for detector operation and SUSY background

3) Top quarks pair-like events … window to new physics: FCNC, SUSY, MSSM Higgses, Resonances, …

Backup

b-jet identification efficiency

CMS

Combined b-tagging discriminator

# e

ven

tsNote: Can also use di-lepton events

B-jet sample from top quark pairs:

- Calibrate b-tagging efficiency from data (~ 5%) Dominant systematic uncertainty: ISR/FSR jets

- Study b-tag (performance) in complex events

• A clean sample of b-jets from top events 2 out of 4 jets in event are b-jets (a-priori)

Use W boson mass to enhance purity

B-jet identification efficiency: Important in cross-section determination and many new physics searches (like H, ttH)

light jet energy scale Light jet energy scale calibration (target ~1%)

Precision: < 1% for 0.5 fb-1 Alternative: PT-balance in Z/γ+jet (6% b-jets)

Pro: - Complex topology, hadronic W - Large statisticsCon: - Only light quark jets - Limited PT-range (50-200 GeV)

Pro: - Complex topology, hadronic W - Large statisticsCon: - Only light quark jets - Limited PT-range (50-200 GeV)

Rescale jet energies:Eparton = (1+ ) Ejet, with =(PT,η)

Wjjjjjj MEEM )cos1(2 2121

Invariant mass of jets should add upto well known W mass (80.4 GeV)

Mjj (GeV)#

even

ts σ(Mjj)~ 8 GeV

Purity = 83%Nevt ~ 2400 (1 fb-1)

MW (PDG) = 80.425 GeV

t W jj to calibrate the light JES

all 60 < mjj < 100

Standard selection

1583316.1 ± 0.3 %

4001

56.7 ± 0.8 %

+ only 2 light jets

3558

41.0 ± 0.8 %

1903

69.0 ± 1.1 %

+ mtop

in 150 - 200

1401

73.5 ± 1.2 %

1205

82.6 ± 1.1 %

Number of jj for 491 pb-1:(% purity as fraction of cases with 2 jets at R < 0.25

from 2 W quarks)PT cut = 40 GeV

All jj combinations

Only 2 light jets + 150 < mjjb < 200

Only 2 light jets

mjj (GeV)

Etienvre, Schwindling

Standard tt lb jjb selection cuts Improve W jj purity by requiring:

2 light jets only 150 < mjjb < 200 GeV

Purity ~ 83 %, ~ 1200 W selected for 500 pb-1

(1) Abundant source of W decays into light jets Invariant mass of jets should add

up to well known W mass (80.4 GeV) W-boson decays to light jets only

Light jet energy scale calibration (target precision 1%)

t

t

Jet energy scale (no b-tag analysis) Determine Light-Jet

energy scale

Translate jet 4-vectors to parton 4-vectors

MW = 78.1±0.8 GeV

S/B = 0.5

MW(had)

Even

ts /

5.1

GeV

Search strategies for H±tb

Resolving 3 b-jets: inclusive mode LO production through gb tH±

Large background from tt+jets High combinatorics

Resolving 4 b-jets: exclusive mode LO production through gg tH±b Smaller background (from ttbb and ttjj+ 2 mistags) Even higher combinatorics

Both processes simulated with Pythia; same cross section if calculated at all orders gbtH±: massless b taken from b-pdf gg tH±b: massive b from initial gluon splitting Cross sections for both processes as the NLO gbtH±: cross section

Search for 4 b-jets

Signal properties Exponential decrease with mA

Quadratic increase with tan in interesting region tan > 20 Final state: bbbbqq’ln

Isolated lepton to trigger on Charged Higgs mass can be reconstructed Only final state with muon investigated

Background simulation ttbb ttjj

(large mistag rates, large cross section) b’s from gluon splitting passing theshold of ttbb generation)

Significance and Reach

Kinematic fit in top system Both W mass constraints Both top mass constraints Neutrino taken from fit

Event selection and efficiencies

44

Significance and Reach

Significance as function of cut on signal-background Due to low statistics interpolation of number of background

events as function of number of signal events Optimization performed at each mass point

H±tb

Fast simulation 4 b-jets analysis No systematics (apart uncertainty on background cross sec) Runninng mb

B-tagging static

L = 30 fb-1

ttH

The Yukawa coupling of top to Higgs is the largest.

It is a discovery mode of the Higgs boson for masses less than 130 GeV Measuring the coupling of top to Higgs can test the presence of new physics in the Higgs sector

0.7 pb (NLO)mH=120GeV

Very demanding selection in a high jet multiplicity final state

ttjj: 507 pb ttZ: 0.7 pb ttbb: 3.3 pb

Higgs boson reconstruction Reconstruct ttH(h) WWbbbb (l)(jj)bbbb

Isolated lepton selection using a likelihood method

Jet reconstruction: 6 jets at least, 4 of which b-tagged

Reconstruct missing ET from four-momentum conservation in the event (+W mass constraint in z)

Complete kinematic fit to associate the two bs to the Higgs

(can improve the pairing efficiency to 36%, under investigation)

gttH/gttH~16%for mH=120 GeV hep-ph/0003033

results can be

extrapolated

to MSSM h

Probabilistic approach

Preselection General criteria:

≥ 1 lepton (pT > 25 GeV and || < 2.5) ≥ 2 jets (pT > 20 GeV and || < 2.5) Only 1 b-tagged jet ETmiss > 20 GeV

Events classified into different channels (qZ, q or qg) Specific criteria for each channel

After the preselection,

probabilistic analysis:

N

i

backndiB

N

i

signalis

PL

PL

1

1

tqZ

Specific criteria: ≥ 3 leptons

PTl2,l3 > 10 GeV and |h|<2.5 2 leptons same flavour and

opposite charge PTj1 > 30 GeV

453.8 backgnd evts, x BR = 0.23% L = 10 fb-1

Mjl+l- Mlb

tq Specific criteria:

1 photon PT > 75 GeV and ||<2.5

20 GeV < mj < 270 GeV < 3 leptons

290.7 backgnd evts, x BR = 1,88%

Mj

L = 10 fb-1

PT

tqg Specific criteria:

Only one lepton No with PT > 5 GeV

Evis > 300 GeV

3 jets (PT1 > 40 GeV, PT2,3 > 20 GeV and |h| < 2.5)

PTg > 75 GeV 125 < mgq < 200 GeV

8166.1 backgnd evts, x BR = 0,39%

L = 10 fb-1

MlbMgq