CERN EP Seminar October 16, 2006

Measurement of the top quark B(t→Wb) and electric chargewith the D detectorChristophe Clément (CERN)

CERN EP SeminarOctober 16, 2006

October 16, 2006 C.Clément CERN EP seminar October 16, 2006 C.Clément CERN EP seminar 2/39 2/39

Top quarkTop quark

e

u c

d s b

e

t•Kinematics in -lepton data in Mark I•Discovery of the Υ at FNAL

GIM mechanism b-quark in isodoublet! t-quark must exist

•What Mass for top? [b/c/s = 4.5/1.5/0.5 Mtop = 15 GeV?]

•Discovered at ~175 GeV (!) in 1995 by CDF and D0

Is there anything else special about the top quark?

Does it have predicted properties, standard model Does it have predicted properties, standard model quantum numbers?quantum numbers?

October 16, 2006 C.Clément CERN EP seminar 3/39October 16, 2006 C.Clément CERN EP seminar 3/39

Is there anything special about the top quark?Is there anything special about the top quark?

High mass potentially important role for top quark Yukawa coupling to Higgs:

New physics in EW symmetry breaking sector could be reflected in top quark properties.

Top decays in ~0.5×10-24 s, before hadronizes

Decays as a free quark: spin correlation, W-helicity in top decays...

Heavy enough to decay to exotic particles (H+, W’…)

Same experimental signature as some exotic particles, background to supersymmetry

12

top t

(2 2 )

178GeV (strong)1.02

f

f f F

H

m G

M

171 GeV

0.98


Top at TevatronTop at Tevatron

Tevatron p-pbarTevatron p-pbar√√s=1.96 TeV, L~10s=1.96 TeV, L~103232cm-2s-1cm-2s-1

Run I (√s=1.8 TeV L~1030cm-2s-1)

1995 CDF and D discover top quarkFull Run I sample ~120pb-1 / experiment

A few dozens of events (l + jets), dilepton channels ≤10 events

Mass known ~3%Cross section ~25%B(t->Wb) ~30%

Run II (√s=1.96 TeV L~1032cm-2s-1)

Already >1 fb-1/experiment on tapeMass known to 1.2% (hep-ex/060832)Cross section ~10%-final states observed Precise tests of production and decay

mechanismsDoes it have SM quantum numbers?

So far only observed in pairs produced via QCDSo far only observed in pairs produced via QCD Cross section derived from pQCD (from mCross section derived from pQCD (from mtoptop and and √√s)s)

Strong top quark productionStrong top quark production

σtt=6.8±0.4pb (theory) for p-pbar @ √s = 1.96 TeV√s = 1.96 TeV

t t


Strong production pairs

Electroweak production single top

t t

LHC: p-p collisions @ √s = 14TeV√s = 14TeV dominated by gg processes σtt~ 833 pb (theory)

Phys. Rev. D68, 114014 (2003)

s-channel, ”tb” σs~0.9 pb

t-channel, ”tqb” σt~2.0 pb

• Allows to measure directly |VAllows to measure directly |Vtbtb|, |, σs, σt ∝∝|V|Vtbtb||22

• Experimentally challengingExperimentally challenging because not too different from because not too different from W+2 jetW+2 jet background background

Use events with Use events with WW→→ev or Wev or W→→ vv Signature: 1 isolated lepton, MET, 2 or more jetsSignature: 1 isolated lepton, MET, 2 or more jets ss-channel: 1,2 b-tagged jets -channel: 1,2 b-tagged jets tt-channel: 1 -channel: 1 bb-tagged jet + 1 light -tagged jet + 1 light

jetjet Major backgrounds: W+jets, , fake leptonsMajor backgrounds: W+jets, , fake leptons

• Not yet observed!Not yet observed!

Electroweak Top ProductionElectroweak Top Production


t t

Top Quark DecayTop Quark Decay


3 quark generations + direct measurements of Vub and Vcb predict Vtb,~1 B(t→Wb)~1

SM predicts FCNC decays are tiny, t→Wq is dominant

In SM top decays via V-A charged current

⇒ Mostly left handed b-quarks in the decay

ig

2 2b 1 5 V tb tW

How top quark was discovered!How top quark was discovered!

Used for most measured top Used for most measured top quark properties so farquark properties so far

tt W

W

Dileptonb

b

ll

vv

tt W

W

Lepton+jetsb

b

lq

vq’

tt W

W

All hadronic

b

b

q

q’q

q’

Top Pair Final StatesTop Pair Final States


44%

ee+e+ ~5%

”golden channel”e+jets & +jets ~32%

B(t→Wb)

• B(t→Wb)=1 usually assumed by CDF and D analyses

• B(t→Wb) might deviate from unity:– Additional quark singlets or doublets– ”Pollution” of top sample by non-top

process!– Non-SM processes in the production– Non-SM in the decay (H+,...)

• Experimentally B(t→Wb) affects number of b-jets need to experimentally discriminate b/w t→Wb and t→Wqlight

Vtb unconstrained without 3x3 unitarity constraint


• Select a top-enriched sample– e+jets and +jets channel– larges statistics, good S/B

• # events with 0, 1 and ≥2 b-jets– B(t→Wb)– b-tagging efficiency– Jet identification efficiency– Probability to tag background

Deriving B(t→Wb) experimentally....


t

t

W

Wb-jets?light jets?

Missing transverse

energyOne high pT

isolated lepton

light, c-jets

• .

• W+jets

• Z+jets

• WW, WZ, ZZ

• single top

• multijet

Lepton + jets sample composition


t t

True isolated lepton processes

Ntrue

fake isolated lepton processes

Nfake

Fake isolated electron Jets with leading /πo , convertions, γwith random tracks,...

Fake isolated inside jets from heavy flavor or in flight decaysDetermine Ntrue , Nfake on a

statistical basis-Two lepton ID criteria-loose ⊃ tight lepton-P(tight | loose) for fake and true leptons

• .

• W+jets

• Z+jets

• WW, WZ, ZZ

• single top

• multijet

Lepton + jets sample composition


t t

Ntrue

Nfake

•Small•derived from MC•Lepton, jet efficiencies calibrated on data• σfrom data or NLO

Nother

Nbefore tag = Ntt + NWj + Nfake +

Nother

Nn-tags = Pntt (B(t→Wb)) Nn

tt + PnWj Nn

wj + N’nfake + Pn

other Nn

other

n-tags = 0, 1, 2Fit B(t→Wb), Ntt, Nwj to the Nn-tags

Tagging probability

Explicitely reconstruct displaced secondary vertices: Secondary Vertex Tagger

1. Taggable jets are: Calorimeter jets with pT>15 GeV, ||<2.5 ΔR(calo jet, track jet)=0.5, ≥ 2 tracks in ΔR=0.5, Δz<2cm ≥ 1 hit in the innermost tracking detector, pT>0.5 GeV ≥ 1 track with pT>1 GeV

Decouple b-tagging from experimental issues

2. Tagged jets: Are taggable jets Contain a SV, χ2

Lxy > n σLxy

Similar algorithm used by CDF

How to identify b-quark jets...


Lxy=

Taggability,tagger independent

B-tagging efficiency

b-tagging efficiency

From di-jet data: extract b-tagging efficiency for muonic b-jets

We need the b-tagging efficiency for ”all kinds of b-jets”

PPbbtagtag(E(ETT,,) =) =

b,MCb,MC ------------------------------------ bb→→, data, data Taggability Taggability C Ctaggabilitytaggability(b) (b) bb→→,MC,MC

b→b→, data, data

bb

Transform semi-muonic b-tag efficiency into inclusive one


See for examplePhys. Rev. D71, 052003 (2005)

Pntt versus B(t→Wb)

• Probability to see n-tags in events (Pntt )

depends on the number of b-jets

• Pn tt = R2 Pn tag(tt→bb) + 2 R(1-R)Pn tag(tt→bql) + (1-R)2Pn

tag(tt→qlql)

t t


ttbar → l+jets

Take into account ”contamination” by → llt t

• Fit B(t→Wb) and Ntt simultaneously to 12 bins: e/+ 3/≥4 jets, 0/1/≥2 tags

• B(t→Wb) is constrained by relative 0/1/≥2 tags populations

• Very poor S/B in 0-tag sample, Nttbar~√Nobs


Prediction 7pb & B(t→Wb)=1

Single Tag l+jets

Double Tag l+jets

Jet multiplicity

Jet multiplicity


R=1.0, σ=7pb R=0.5, σ=7pbIllustration...

The 0-tag sample

• Ntt ~ √N(0-tag)

• Without further information on the 0-tag events low B(t→Wb) + large Ntt (σtt) can still be consistent with data

• Use topological properties of events in 0-tag sample for addititional constraint of Ntt in the 0-tag sample.

Cro

ss

Sect

ion

RPreliminary result from summer 2004 (170pb-1) No 0-tag sample used


Low B(t→Wb)Large σtt

1. SphericityS = 3(λ2+λ3)/2 , λ’s smallest eigenvalues of

momentum tensor M (ttbar S~1)

2. K’Tmin

K’Tmin = Rminjj/EW

T with EWT = El

T + MET

3. CentralityC=HT/H , HT is scalar sum of jets ET and

H is the sum of the jet energies.

H’T2

H’T2 = HT2/Hz, HT2 : pT sum of all jets but leading jet,

Hz is the scalar sum of all jets |Ez| plus |Ez| of the neutrino (W-assumption)

Likelihood discriminant in 0-tag samplel + 4 jets before tagging


Likelihood discriminant in l+4jets 0-tag sample

Data and prediction for

7pb and R=1

Data


Likelihood discriminant output

Nevents

Systematics on template shapes

Some systematic uncertainties can affect the template shapes...

Systematics on template shapes on ttbar→ l+jet

JESJetIDJet energy resolutionW-modeling (W+jets)TaggabilityTagging probabilities for b, c and light jets


Results (230 pb-1)

October 16, 2006 C.Clément CERN EP seminar October 16, 2006 C.Clément CERN EP seminar 22/39 22/39

3 jets ≥4 jets

≥4 jets, 0 tag


Phys. Lett. B 639 (2006)

No 0

-tag

B(t→Wb) = 1.03+0.19-0.17

Confidence contour plots in R, Ntt

B(t→Wb) B(t→Wb)

Lower limit on B(t→Wb) and |Vtb|

• Prior π(B(t→Wb))=0 outside [0,1]

• Monte Carlo integration over 191 nuisance parameters associated to systematic errors

• Provides a 2D p.d.f. for B(t→Wb) and Ntt

• Limit on |Vtb| can be derived using |Vtb|=√B(t→Wb) (SM)

68% CL : B(t→Wb)>0.78 |Vtb|>0.8895% CL : B(t→Wb)>0.61 |Vtb|>0.78


OROR

??

Top Quark Charge


Lift ambiguity present in all top analyses!t→W+b or ”t”→W-b

Test exotic models...t

bQ1

Q4

+2/3

-1/3

-4/3mixing

M(Q4)~175GeV

Mtop~270GeV

Phys.Rev. D65 (2002) 053002

OROR ??

Q jettrack i

qi . pTi0.6

track ipTi

0.6 kinematic fit++ == Qtop

Ingredients

What is the expected shape of Qtop for SM top”top” with 4e/3 charge?


Analysis strategy Discriminate between |Qtop| = 2e/3 and |Q”top”|=4e/3

Two |Qtop| per event

Use the pure sample-- lepton+4≥jets events with 2 SVT

Compute the jet charge of the 2 b-tagged jets

Associate the b-jets to correct W boson (charged lepton)

Combine the 2 jet charges and the lepton charge to derive the 2 |Qtop|

Compare the observed |Qtop| with expected SM and exotic distributions

Double tagged events


t t

S/B~10

Jet Charge Algorithm

Compute jet charge only for b-tagged jets - (2 per events)

Jet charge =

Optimizaton on MC gives a=0.6Sum over tracks with

pT>0.5GeV, ΔR(track, jet) <0.5 of the jet axis

Algorithm:

i

aT

i

aTi

jet

i

i

p

pqQ


Derive expected shape of Qjet from data

with minimal input from simulation

pTi,qi

Why does it work?The charge of the quark is correlated with the charge ofthe highest pT hadron resulting of the hadronization

Simple study carried out with Pythia:Generated QCD 2→2 process, pT>15GeV

And look at hight pT b quarks produced in the process.

Usually large number of hadrons Produced, most quite modest pT

Thís is then smeared by detector effects...

The original b-quark not always In highest pT hadron

Charge of b-quark

Charge of highest p T hadron


Jet Charge Performance in Data

Tag and probe method in ”pure” events

In reality: Is it pure ? ? flavor excitation? g→ ? B→ B →D → → B→ light hadrons → Charge misidentification

Tight di-jet sample

>3.0


_bb

Ideal case: sign of q = sign of qb

_bb

_cc

_bb

´ _Bo→ Bo

Charge flipping processes

Data Calibration Corrections

Discriminant Power

bb

bb

VV

aaD

||

Tag and probe

Method: Z→bbOctober 16, 2006 C.Clément CERN EP seminar 31/39October 16, 2006 C.Clément CERN EP seminar 31/39

Tag and probe

Method: data

MC truth on tag side

Is the triple tag sample pure ”bb”? The fraction of c-jets in the triple tag sample is determined by pTrel fit of the

order of a few percents,

Flavor excitation/ splitting?

>3.02 b-jets back to back dominate

Phys. Rev. D 65, 094006 (2002)

-


P+ (Qjet) = (1-xc) (1-xflip)Pb (Qjet)+ (1-xc)xflip Pb + xc Pc

- -

Fraction of derived from pT

rel spectrum of (1+2

-1%)

_ccFraction of charge

flipping processes30±1% from MC, Cross checked on data

Similar equation for P- (Qjet)

4 Unknown p.d.f’sPb, Pb, Pc, Pc

Tight di-jet sample

+

p.d.f of Qjet in probe jet

- -

p.d.f.’s Qb, Qb, Qc, Qc from data...--


p.d.f.’s Qb, Qb, Qc, Qc from data...--


P+ (Qjet) = 0.69 Pb (Qjet) + 0.30 Pb + 0.01 Pc

P- (Qjet) = 0.30 Pb (Qjet) + 0.69 Pb + 0.01 Pc

P´+(Qjet) = 0.567 Pb (Qjet) + 0.243 Pb + 0.19 Pc

P´- (Qjet) = 0.243 Pb (Qjet) + 0.243 Pb + 0.19 Pc

- -

-

-

-

-

Tight di-jet sample

loose di-jet sample

-Correct for different

t→Wb and bb kinematics

SM Top Charge Observables We need an observable and an expectation for the ”2e/3” and ”4e/3” scenarios Consider only lepton+jets channel (e/µ + 4 jets) double-tagged events

Two top quarks in the event measure the charge ”twice”

Use kinematic fit to assign b-jets to correct W-bosons in MC

qb and qB are taken from the data derived jet charge templates

BlSM

blSM

qqQ

qqQ

2

1 qb = b lept. side

qB = b hadr. side

qB

qb

qB

qb


t t

Exotic Top Charge Observables

qb and qB are taken from the data derived jet charge templates

The exotic scenario is obtained by permuting the charge of the SVT tagged jets

blEX

BlEX

qqQ

qqQ

2

1

qb = b lept. side

qB = b hadr. side

qBqB

qbqb


Admixture of Q4 and t-quark - not excluded by - fQ4 <0.52 at 68% C.L.

σtt

Result on ~0.4 fb-1 of D data 21 l + jets double tagged events 16 events with converged kinematic fit 32 measured top charges

Perform likelihood ratio test b/w 4e/3 and 2e/3 hypothesis

Exclude 4e/3 at 92%C.L. (91 % expected)


hep-ex/060844hep-ex/060844

Systematic uncertainties


Kinematic fit

Jet charge specific

Conclusions

• Acknowledge collaborators:

J. Strandberg and P. Hansson

• Large data sets from CDF and D allow new fundamental measurements of the top quark properties

• Development of analysis techniques:

often analyses outperform expectation

• First ”measurement of the top electric charge”...


Backup slides

Top pair final states

Ensemble tests

Fitted versus true B(t→Wb)

Coverage for B(t→Wb)

Expected statistical errors

Statistical error on R vs R

Statistical error on σ vs σ

b-tagging efficiency from data

”n-sample”:1 jet with a

Not -taggedn

-taggedn

Not -tagged, SVT taggednSVT

-tagged, SVT taggedn ,SVT

”p-sample”:2 b-2-b jets 1 jet with a

Not -taggedp

-taggedp

Not -tagged, SVT taggedpSVT

-tagged, SVT taggedp,SVT

Documents

CERN EP Seminar October 16, 2006