Determining Top Couplings

ATLAS Overview Week, 2ATLAS Overview Week, 2th-th-66th th October, CERNOctober, CERN

António OnofreAntónio Onofre ([email protected])([email protected])

ATLAS Collaboration

Determining Top Determining Top CouplingsCouplings


OutlineOutline

2) Couplings in single top 2) Couplings in single top events events

3) Conclusions3) Conclusions

Cross section and VCross section and Vtbtb Beyond SM at productionBeyond SM at production

1) Studies with tt events1) Studies with tt events Probing the Wtb vertexProbing the Wtb vertex FCNC top decaysFCNC top decays


What do we know about the top What do we know about the top quark?quark?

The top quark completes the three family structure of The top quark completes the three family structure of the SMthe SM

It´s massive It´s massive Spin=1/2Spin=1/2 Charge=+2/3Charge=+2/3 Isospin=+1/2Isospin=+1/2 ttbWbW

Large Large =1.42GeV (m=1.42GeV (mbb,M,MWW,,ss,EW corr.),EW corr.)

Short lifetimeShort lifetimehad QCD-1 >> decay

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

NO top hadrons

m/m ~1%

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

c<52.5m @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%

Not directly


~ 833pb @ 14TeV

Production X-section:Production X-section:

2 b-jets

pT>20 GeV

pT>20 GeV

≥4 jets,R=0.4 pT>40 GeV t(had)

t(lep)

W(had)

Used for several studies

Semileptonic Topology:Semileptonic Topology:

Golden Channel

1) Studies with tt events1) Studies with tt events

Clean Topology: tt back to back

dominated by gg fusion (90% of total X-section)


With only L=100pb-1 (few days of nominal low-lumi LHC operation)

1) Studies with tt events1) Studies with tt eventsNews from Commissioning Analysis

Ivo van Vulpen (Top meeting Sep.27th)

Wouter VerkerkeNo b-tag and MW cut

Relax cut on 4th jet (pT>20GeV)

(increases signal significance)

(ATLAS note available soon)

100pb-1

W+jets

Combinatorial

3-jet invariant mass


Probing the Wtb vertexProbing the Wtb vertex1) Studies with tt events1) Studies with tt events

t

Longitudinal fraction F0

W

b

+1/2

+1/2

0

Left-Handed fraction

FL

tb

W

+1/2

-1/2

+1

Right-Handed fraction FR

tW

b

+1/2+1

-1/2V-A

SUPPRESSED

W boson has three helicity states: “Left-handed”, “Longitudinal”, “Right-

handed” Top quark decay is the most significant

source of Longitudinal Ws.


Probing the Wtb vertexProbing the Wtb vertex


Cos(

(1/

)d

/dcos(

)

Measure W polarization (F0, FL, FR) through:

Using lepton angular distribution (with respect to top direction) in W cm system:

SM(LO): F0=0.703 , FL=0.297 , FR=3.6x10-4

(NLO): F0=0.695 , FL=0.304 , FR=0.001


FR ~ 0.01F0 F0~ 0.02

L=10fb-1



Most Important Systematic Errors:Most Important Systematic Errors: mmtt,E,Eb-jetb-jet, FSR, Hadronization and , FSR, Hadronization and bbtagtag

ATLFASTATLFAST

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

+e,

e,

b

b

WW

Semileptonicchannel

Dileptonicchannel

TOPREXTOPREX(Assuming SM)(Assuming SM)

Corr.functionCorr.function

Correct for Detector Effects




Comparison Fast and Full Simulation: SM results (with MC@NLO)

MC@NLOMC@NLO

Good agreement observed

Full Simulation Studies with “Rome” sample (TopRex,MC@NLO,ALPGEN):

ATL-PHYS-PUB-2006-022

(SEMILEPTONIC ONLY)

MC@NLO

ATLFAST

FULL SIMFULL SIM

Statistical Errors Only


(PRD67 (2003) 014009, mb≠0)


Anomalous Couplings in the tbW decay:

How to test these new couplings ? (They affect the F0,FL and FR and the angular distribution used before)

Several methods were used to test the sensitivity @ ATLAS:

» Extract limits from previously obtained F0,FL and FR ;

» Fit the angular distr. with new observables R=FR/F0 and L=FL/F0;

» Fit with the known dependence with VR, gL and gR (mb≠0);

» Use new Asymmetries and study their dependence with VR, gL and gR.


New Angular Asymmetries: AFB, A+ and A-

SM(LO):

AFB [t=0] A± [t= (22/3-1)]±±

A+

AFB

A-

Anomalous Couplings in the tbW decay


(NLO): AFB=-0.2269 , A+=0.5429 , A-=-0.8402

SM(LO): L = 0.423R = 0.0005 (mb≠0)

(NLO): L=0.438 R=0.002

Use tbW decay in Semileptonic tt Events

New observables R=FR/F0 and L=FL/F0


New Discriminant Analysis for Semileptonic Channel:


Preselection: 1 lepton ( pT>25GeV, ||<2.5 )

≥4 jets ( pT>20GeV, ||<2.5 )

2 b-jets and pT>20GeV

Discriminant Variables: mt(lep)mW(had)

Back.

Signal

Likelihood Ratio:

mW,mt(had),mt(lep),pT-jets

Statistics (10fb-1):∆ Signal: =9% (220k)∆ Back: 36k events

ATLFASTATLFAST



1 Results:

L=10fb-1Anomalous Couplings in the tbW decay


AFB

/AFB ~ 6.6%

A+/A+ ~ 1.9%

A-/A- ~ 0.4%

ATLFASTATLFAST1) Studies with tt events1) Studies with tt events


Anomalous Couplings in the tbW decay


Correlations Taken into Account:

Two Dimension Regions: (ex. gR versus gL)

Studies with Full simulation under way (CSC):

gL=-0.016 ± 0.067

A+,A-,R and L


1 Limits: (only 1 at a time ≠ 0 )

(2) lim: gR >3 times better then current ind. limits


The top quark The top quark almostalmost always decays to a b quark, always decays to a b quark, B(tB(tWb)~1Wb)~1

B(tB(tWs)<0.18%, B(tWs)<0.18%, B(tWd)<0.02% (other decays are really Wd)<0.02% (other decays are really rare) rare) FCNC decays are highly suppressed in the SM:

At LHC Look for:

hep-ph/0409342APPB35(2004)2695

FCNC top decaysFCNC top decays1) Studies with tt events1) Studies with tt events

q,q,


FCNC top decaysFCNC top decaysSeq. and Disc. Type of analysisSeq. and Disc. Type of analysis

1) Studies with tt events1) Studies with tt eventsATLFASTATLFAST

(p.d.f.)

ATL-COM-PHYS-2006-049


B) In the absence of signal:B) In the absence of signal:

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

A) 5A) 5 Sensitivity:Sensitivity:

ATLAS: Br(tqg@95%CL)<0.14%Tevatron (L=2fb-1): level of few %

FCNC top decaysFCNC top decays

Dominant Systematic Dominant Systematic Errors: mErrors: mtt and and b-tagb-tag < <

20%20%

Br @ 95%CL (absence of signal)

1) Studies with tt events1) Studies with tt eventsATLFASTATLFAST


Three different Processes (never observed yet)

[ PRD 70 (2004) 114012, PL B524 2002 283-288 ]

• Quark-gluon luminosity inside b-quark (PDF)• Renormalization scale ()• top mass (mtop=4.3GeV W*) changed by 3%)

Theoretical uncertainties:

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

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

~ 250 pb ~70pb ~ 10 pb

VtbVtb Vtb

Vtb

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

2) Couplings in single top 2) Couplings in single top events events Cross section and VCross section and Vtbtb


Common feature: 1 lepton, pT>25GeV/cHigh Missing ET (>25GeV)2 jets, pT>25GeV, (at least 1 b)

(ATL-COM-PHYS-2006-057)

L=30fb-1

Separate Channels by (Nj,Nb) in final state:

Cross section and VCross section and Vtbtb

Errors Dominated by Systematics

( ~ 12%)

t-channel: Pre-Sel+ HT,mT: 2710 events (S/B=3.)

Syst: (as above)Back: tt and W+jetsNj=2,Nb=1 (L=10fb-1)

s-channel:

Pre-Sel.: 1200(840) events for tb (tb),S/B=0.11( 0.09)HT,mT : 385(275)

S/B=0.14(0.12)Syst: Eb-jet, btag, ISR+FSR, Lum. Error, back X-section

( ~ 18%)Nj=2(3),Nb=2

(L=30fb-1)

Wt-channel:

Stat: =4.6% (S/B=0.09)Syst: (as above) ( ~

12%) Nj=3,Nb=1

(L=30fb-1)



t-channel/ ~12%(tot)s-channel/ ~18%(tot)

L=30fb-1

Tait,Yuan PRD63 (2001) 014018

Cross section and VCross section and Vtbtb

(ATL-COM-PHYS-2006-057)

Illustration of the Physics Potential:Illustration of the Physics Potential:



Beyond SM at productionBeyond SM at production

Results (10fbResults (10fb-1-1):):

Effective Lagrangian:Effective Lagrangian:

In progress…

kkqg qg = anomalous = anomalous couplingcoupling= scale of new = scale of new physicsphysics

Analysis:Analysis: 1 lep ( p1 lep ( pTT>20GeV, >20GeV, <2.5 )<2.5 )

1 b-jet ( p1 b-jet ( pTT>30GeV, >30GeV, <2.5 )<2.5 )

ppT T > 20GeV> 20GeV

( W and t p( W and t pTT, m, mblbl, H, HTT, m, mtt ) )

Systematics:Systematics:

signal signal (16%)(16%)bbtagtag (13%) (13%)ISR,FSR (6%)ISR,FSR (6%)PileUp (8%)PileUp (8%)

Tot ~20-25%Tot ~20-25%


PreliminaryPreliminary


3) Conclusions3) Conclusions Several analysis using ATLFAST have

already tested the ATLAS potential to probe top couplings. News are encouraging.

New full simulation tests under way (CSC Notes in preparation)

Other couplings are challenging (ttH, ttZ, tt, ttg …); many studies under way

There is a lot of free room for improvements from the experimental point of view: several orders of magnitude can be gained with respect to present experimental limits on some couplings

Determining Top Couplings

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