tevatron results on top quark physics - mit...

Tevatron Results on Top Quark Physics

Zhenyu Ye / Fermilab on behalf of the CDF and DØ Collaborations

19th Particles and Nuclei International Conference

Massachusetts Institute of Technology, 7/24-29/2011

Top Quark

7/28/11 Zhenyu Ye, 19th PANIC, MIT 2

}  Top quark (discovered at the Tevatron in 1995) }  isospin partner of b quark, heaviest fundamental particle }  Yukawa coupling ~1, special role in EW symmetry breaking? }  lifetime << ΛQCD, decay before hadronization }  important background for Higgs and new physics search

}  Window to new physics }  deviation from SM prediction in

precise measurements }  direct search in specific models

that involve top quarks (Z’èttbar)

Top Quark Physics


t

t

b

b

W+

W-

q

v q’

l-

q

v q’

l+ branching ratio CKM |Vtb|

rare decay (FCNC) anamolus couplings

mass, width, charge mass difference

production cross section forward-backward asymmetry

spin correlation CP violation

resonant production Z’ new particle t’

W-helicity new particle W’

p p

Fermilab Tevatron Collider


Birthplace of the top quark: Proton-antiproton collider at √s=1.96 TeV Delivered L>11 fb-1, data taking eff.>90%

Results presented here ~ 4-6 fb-1

Selected Results

7/28/11 5

}  Top Pair Production and Decay }  production cross-section }  branching ratio }  forward-backward asymmtry Afb

}  spin correlation }  W-helicity

}  Single Top Production }  t-channel cross section }  s+t-channel and |Vtb|2

}  Top Quark Properties }  mass, width, charge

Zhenyu Ye, 19th PANIC, MIT

Top Pair Production and Decay


Tevatron: 85% qqbar, 15% gg

production decay

lepton+jets

dilepton

all-hadronic

SM: t->W+b ~100% ΓSM≈1.4 GeV

Event selection: high pT lepton and jets, large ET, b-quark jets. Dominant background: Z+jets, W+jets, multi-jets.

b-jet ID

Top-Pair Production Cross Section


!t t

l+jets=7.8 +0.8-0.6

pb

!t t

dilepton=7.4 +0.9-0.8

pb

!t t

l+jets=7.8±0.6 pb

!t t

dilepton=7.4±1.0 pb

l+jets dilepton

! =Ndata -Nbkg

" !L

Top-Pair Production Cross Section


!t t

CDF =7.5±0.31(stat)±0.34(syst)±0.15(theory) pb

CDF August 2009

~6% precision ~8% precision (6% lumi)

!t t

D0 =7.56+0.63(stat+syst+lumi) pb -0.56

Branching Ratio and |Vtb|


l+jets

dilepton

B(t->Wb)/B(t->Wq) = 0.90+0.04 (stat+syst)

|Vtb| = 0.95 ± 0.02 (assume unitary CKM)

l+jets

SM: B(t->Wb)/B(t->Wq) = |Vtb|2 ÷Σq|Vtq|2~100%

Forward-Backward Asmmetry Afb


}  In early 80s asymmetry observed in e+e-èμ+μ- at √s=34.6 GeV << M(Z) was used to verify the validity of EW theory PRL 48 (1982) 1701.

}  Similarly, asymmetry in ttbar production might

give information about new physics }  mediator with axial coupling in s-channel }  abnormally enhanced t-channel production

!y = yt " yt

!

A =N("y > 0) # N("y < 0)N("y > 0) + N("y < 0)

θ

µ+

µ-

e+ e-

p _ p

t

_ t

A = N(cos! > 0)! N(cos! < 0)N(cos! > 0)+ N(cos! < 0)

y = 12ln Ez + pzEz ! pz

y

+ ? SM

Forward-Backward Asmmetry – L+Jets


}  Kinematic fitter (constraints from mt, mW with known detector responses) to reconstruct top’s

}  Build a MV discriminant and fit events with Δy>0 and Δy<0. !y = yt " yt = ql (yleptonic " yhadronic )

Forward-Backward Asmmetry - Dilepton


labt y

-2 -1 0 1 2labt y

-2 -1 0 1 2

Even

ts

0

20

40

60 in Labtbar-ytopy

Datatt

error 1 ±

FakeDYZWW/WZ/ZZ

CDF II Preliminary-1 L dt = 5.1 fb

labt y

-2 -1 0 1 2labt y

-2 -1 0 1 2

Even

ts

0

20

40

60 in Lab (best fit)tbar-ytopy

Datatt

error 1 ±

FakeDYZWW/WZ/ZZ

CDF II Preliminary-1 L dt = 5.1 fb

Afb (unfolded) = 0.42 ± 0.15(stat) ± 0.05(syst) Afb (theo.) = 0.06 ± 0.01.

(gen)labt y

-1 0 1

(rec

) la

bt

y

-1

0

1

labt y

CDF II PreliminaryMC

(gen)labt y (rec) - lab

t y-1 0 1


t y-1 0 1

Even

ts

0

2000

4000

6000 resolutionlab

t yCDF II Preliminary

MC



CDF and D0 inclusive results are consistent, and both deviate from predictions. Will need more data to reach a consistent conclusion on the Mttbar-dependence.

(l+jets)

(l+jets) (l+jets)

(l+jets)

(l+jets)

(l+jets)

(l+jets)

(l+jets) (combined) 20.1±6.7

Spin Correlation


}  Even though top quarks are not produced in a polarized state, their spins are correlated. The correlation strength is defined as:

}  Correlation strength at Tevatron at NLO using beam basis 0.777±0.042. }  A depend on production, i.e quark-antiquark annihilation or gluon fusion,

thus different at Tevatron and LHC. Measuring spin correlation allows a test of the SM from strong production to EW decay.

A =N!!+ N"" # N!" # N"!

N!!+ N""+ N!"+ N"!

Spin Correlation – L+Jets


)d) cos(lcos(-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

0

0.05

0.1

0.15

0.2

0.25 Helicity TemplatesUnpolarized sample

OH basis template

SH basis template

)d)*cos(lcos(-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Even

ts

020406080

100120140160180200220240

)d)*cos(lcos(-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Even

ts

020406080

100120140160180200220240

)d)*Cos(lBeam Basis Bilinear Cos(

Opposite Spin

Same Spin

Backgrounds

Data : 0.72 +/- 0.64 +/- 0.26

-1CDF Run II preliminary L=5.3 fb

C=0.72±0.64(stat)±0.26(syst)

θl

d, s

u, c

θd

Spin Correlation – Dilepton


R=Psgn (c=1)

Psgn (c=0)+Psgn (c=1)

Psgn (x;H) =1

! obs (mtop )! dq1 dq2fPDF (q1)fPDF (q2 )

(2!)4 M(y;H) 2

4 (q1 "q2 #m1m2

d$6W(y;x)%

C=0.57±0.31

fPDF PDFs

M y;H( ) matrix element

W y;x( ) transfer function

y partonic kine.

x measured kine.

H hypothesis

W-helicity


W-helicity in top quark decays Reconstruct angle of lepton in top quark pair events.

W-helicity


f+ f0

CDF l+jets -0.20±0.11±0.06 0.90±0.11±0.06

CDF dilepton -0.09±0.09±0.03 0.72±0.18±0.07

D0 l+jets & dilepton 0.02±0.04±0.03 0.67±0.08±0.07

CDF+D0 combined -0.04±0.03±0.03 0.73±0.06±0.05

SM value 4x10-4 0.698

Measurements consistent with SM

W-helicity in top quark decays

Selected Results

7/28/11 19

}  Top Pair Production and Decay }  production cross-section }  branching ratio and |Vtb| }  forward-backward asymmtry Afb





Single Top Production


s-channel t-channel tW production

σ ~1 pb σ ~ 2pb σ ~0.25 pb

Direct access to the Wtb coupling - overall rate and ratio between s- and t-channels are sensitive to NP

Event selection: high pT lepton and jets, large ET, b-quark jets. Dominant background: W+2jets S/B~1/200 pre-btag

t-channel Cross Section


}  Optimize for t-channel production (s-channel as background) }  Combine three multivariate methods and combine their final

decriminanats: Boosted Decision Trees, Bayesian Neural Networks, Neuroevolution of Augmented Topologies.

}  Check discriminant performance using data control samples. }  Use discriminant output to measure cross section.

t-channel Cross Section


t-channel s-channel

2.90±0.59 pb 0.98±0.63 pb

Observed significance > 5 SD

20% total uncertainty 11% systematic: JES, jet energy resolution, b-jet ID

s+t-channel Cross Section and |Vtb|


Expected Observed

s+t-channel 3.49±0.77 pb 3.43±0.74 pb

•  cross section proportional to |Vtb|2 •  allow direct determination of |Vtb|2

w/o assumption on unitary CKM:

Selected Results

7/28/11 24

}  Top Pair Production and Decay }  production cross-section }  branching ratio }  forward-backward asymmtry Afb





80.3

80.4

80.5

155 175 195

mH [GeV]114 300 1000

mt [GeV]

mW

[G

eV]

68% CLLEP1 and SLDLEP2 and Tevatron

July 2011

Top-Quark Mass – L+Jets

7/28/11 25

GeVtm170 172 174 176 178 180 182

JES

k

0.980.99

11.011.021.031.041.051.06 -1DØ, 2.6 fb

1sd

2sd

3sd


mt(3.6 fb!1)=174.9±0.8(stat)±1.3(syst+JES)

signal modeling 0.7 GeV

jet energy resolution 0.3 GeV

data-MC jet response 0.3 GeV

jet ID efficiency 0.3 GeV

Dominant systematic uncertainties

Psig (x;mtop,kJES) =1

! obs (mtop )! dq1 dq2f(q1)f(q2 )

(2!)4 M(y,mtop )2

4 (q1 "q2 #m1m2

d$6W(y;x,kJES)%

L(!x;mtop,kJES)= Pevt (x;mtop,kJES)!

kJES global JES factor, constrained by hadronic mW

Top-Quark Mass – All Hadronic

7/28/11 26

}  Build MC templates for quantities sensitive to top quark mass and JES. }  Fit data to MC templates with different generated top masses or JES.

]2 [GeV/crecWm

20 40 60 80 100 120 140 160 180 200

)2Fr

actio

n of

Eve

nts/

(2.5

GeV

/c

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

]2 [GeV/crecWm

20 40 60 80 100 120 140 160 180 200

)2Fr

actio

n of

Eve

nts/

(2.5

GeV

/c

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

JES = -2.0

JES = 0.0

JES = 2.0

JES),top

| MrecW

(msP

= 172.5)top

templates, 1 tag events (MrecW mtt

]2 [GeV/ctopM166 168 170 172 174 176 178 180

]JE

SJE

S [

-1.5

-1

-0.5

0

0.5

1

1.5

Fitted Values

) = 4.5max -Ln(L/L

) = 2.0max

-Ln(L/L

) = 0.5max -Ln(L/L

2-tag events) Contours, 1 + max

-Ln(L/L

)-1CDF Run II Preliminary (5.8 fb

mt=172.5±1.4(stat)±1.5(syst)


]2 [GeV/crecWm

20 40 60 80 100 120 140 160 180 200

]2Ev

ents

/[5.0

GeV

/c

0

50

100

150

200

250

300

350

400

]2 [GeV/crecWm

20 40 60 80 100 120 140 160 180 200

]2Ev

ents

/[5.0

GeV

/c

0

50

100

150

200

250

300

350

400 1-tag events

Data

t Fitted t Fitted Bkg

/Ndof = 33.1 / 402

Prob = 0.772

)-1CDF Run II Preliminary (5.8 fb]2 [GeV/crec

tm100 150 200 250 300

)2Fr

actio

n of

Eve

nts/

(2.5

GeV

/c

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

]2 [GeV/crectm

100 150 200 250 300

)2Fr

actio

n of

Eve

nts/

(2.5

GeV

/c

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

= 160.0top M

= 172.5top M

= 185.0top M

JES),top

| Mrect

(msP

JES = 0.0) templates, 1 tag events (rect mtt

]2 [GeV/crectm

100 150 200 250 300

]2Ev

ents

/[5.0

GeV

/c

0

20

40

60

80

100

120

140

160

180

200

220

]2 [GeV/crectm

100 150 200 250 300

]2Ev

ents

/[5.0

GeV

/c

0

20

40

60

80

100

120

140

160

180

200

220 1-tag events

Data

t Fitted t Fitted Bkg

/Ndof = 29.4 / 322

Prob = 0.601

)-1CDF Run II Preliminary (5.8 fb

Top-Quark Mass – ET+Jets

3/26/11 27

}  Signal is W->τν(hadrnoic τ decay) or missing lepton+jets }  Template fit to 3 jet invariant mass from the hadronic top decay.

mt=172.3±2.4(stat+JES)±1.0(syst)

Zhenyu Ye, Moriond-QCD 2011

Top Quark Mass - Combination

7/28/11 28 Zhenyu Ye, 19th PANIC, MIT

Tevatron Combination July 2011: mt=173.2±0.6(stat)±0.8(syst)

Prepare the Top Legacy Measurements

7F. Déliot, general top meeting, 11-FEB-11

• legacy:- top mass • competitive with LHC:

- s-channel single top

• complementary with LHC:- ttbar spin correlation- top forward-backward asymmetry

Top Quark Width


CDF direct top width measurement

0.3 GeV < Γtop< 4.4 GeV @ 68% CL; Γtop< 7.6 GeV @ 95% CL

Top Quark Width


NP? ! t"Wb( )! t-channel( )

=!SM t"Wb( )! SM t-channel( )

width is proportional to cross section for any coupling, including new physics

! t =! t"Wb( )B t"Wb( )

=! t-channel( )B t"Wb( )

#!SM t"Wb( )! SM t-channel( )

=0.962+0.068(stat) +0.064(syst) -0.066 -0.052 B t!Wb( )

=3.14+0.094 pb -0.080 ! t-channel( )B t!Wb( )=1.26 GeV !SM

NLO t"Wb( )

=2.14±0.18 pb ! SMNLO t-channel( )

will be updated soon with new t-channel cross section & B(t->Wb)

Γt=1.99+0.69 GeV -0.55

Top-Quark Charge


f+-1 -0.5 0 0.5 1 1.5 2

-2ln

[L]

-7680

-7675

-7670

-7665

-7660

-7655

-7650

-7645

-7640

f+-1 -0.5 0 0.5 1 1.5 2

-2ln

[L]

-7680

-7675

-7670

-7665

-7660

-7655

-7650

-7645

-7640

-1CDF Run II preliminary L = 5.6 fb

f+=0.83

Q(W) * Q(b-jet)-1.0 -0.5 0.0 0.5 1.0

Even

ts

0

20

40

60

80

100

120

140

160

180

-1.0 -0.5 0.0 0.5 1.00

20

40

60

80

100

120

140

160

180

-1CDF Run II preliminary L = 5.6 fb

W+HFMistagSingle TopDibosonQCD

eventsttData

XM likeSM like

f+-0.5 0 0.5 1 1.5

Num

ber o

f pse

udoe

xper

imen

ts

1

10

210

310

410

510

-1CDF Run II preliminary L=5.6 fb

XM SM

pXM=1.4x10-4 pSM =0.13

Qb-jet = qi pTi( )!

wpTi( )!

w

SM W+

b

XM ? W+

b Kinematic fitter to reconstruct top’s and pair W and b-jets. Estimate b-jet charge from the tracks of the b-jet:

Exclude XM at 95% C.L.

FSM+FXM=1 f+=FSM=+0.83

XM: t->W+b Q(t)=+4/3

SM: t->W+b Q(t)=+2/3

Summary and Outlook


•  Very rich top quark physics program at the Tevatron. Unique/complementary opportunities to test SM and look for new physics.

}  Results are consistent among different final states and between CDF and D0, and agree with SM (with the exception of Afb?) }  CDF http://www-cdf.fnal.gov/physics/new/top/top.html

}  DØ http://www-d0.fnal.gov/Run2Physics/top/top_public_web_pages/top_public.html

}  With the final data set of ~2 times the statistics shown

here, the expected final will be improved. Stay tuned!!!

Challenges and Solutions

3/26/11 33

}  Jet-parton match: njet! Permutations }  b-jet ID helps reducing the number of permutations. }  kinemiatc fitter to pick up the permutation(s) with best χ2.

Zhenyu Ye, Moriond-QCD 2011



}  Using kinematic variables of l+jets events construct a discriminant and fit events with Δy>0 and Δy<0 for top fraction.

!y = yt " yt = ql (yleptonic " yhadronic )

Forward-Backward Asmmetry - Dilepton


(gen)labt y

-1 0 1

(rec

) la

bt

y

-1

0

1

labt y

CDF II PreliminaryMC


t y-1 0 1


t y-1 0 1

Even

ts

0

2000

4000

6000 resolutionlab

t yCDF II Preliminary

MC



!

Octet A A = 0.024+/-0.0072 + Bkgtt

A = -0.012+/-0.0071

!

Octet A A = 0.18+/-0.0097

+ Bkgtt A = -0.016+/-0.01

QCD NLO tt

ttA

2GeV/c 450 ttM

0.0

2.0

4.0

2.0

-1fb 5.3 data CDFlevel-parton tt

Massive gluon model (Octet A) with gV=0, gA(t)=-3/2, MG=2 TeV



+

+

A(l + 4 jets) =12.2± 4.2%A(MC@NLO) = 3.9± 0.3%

A(l+ ! 5 jets) = "3.0± 7.8%A(MC@NLO) = "2.9± 0.7%

We choose one particular generator: MC@NLO Will future MC generators predict other Afb? Low pttbar less gluon radiation =? larger predicted Afb

Spin Correlation - Template


d 2!cos"1 cos"2

!1"Ccos"1 cos"2

θ

C=0.10±0.45

Top Quark Mass From Cross Section

7/28/11 39

5

function of mpolet and consequently, comparing the ex-266

perimental !tt̄ as a function of mMCt to these theoretical267

predictions provides a value of mpolet . The relation be-268

tween mMCt and mpole

t or mMSt is still under investigation.269

Arguments have been made that the MC mass should270

be close to the pole mass [5]. Therefore, we (i) extract271

mpolet assuming that the definition of mMC

t is equivalent272

to mpolet , and (ii) we take mMC

t equal to mMSt to esti-273

mate the maximum e!ect of interpreting mMCt as any274

other mass definition. The di!erence between the two275

results is included into the systematic uncertainties.276

For case (i), the mass in the MC simulations equals277

mpolet , and Fig. 1 shows the parameterization of the mea-278

sured and the predicted tt̄ cross sections [11–13] as a279

function of mpolet . The results for the determination of280

mpolet are given in the left column of Table II. All values281

are consistent within 2 sd with the Tevatron average top282

quark mass of mt = 173.3± 1.1 GeV [1].283

Top quark pole mass (GeV)150 160 170 180 190

(pb)

tt!

2

4

6

8

10

12

14

+X)t t"p(p!Measured !Measured dependence of

NNLO approx KidonakisNNLO approx Moch and UwerNLO+NNLL Ahrens et al.


(pb)

tt!

2

4

6

8

10

12

14-1DØ, L=5.3 fb

FIG. 1: (Color online) Experimental and theoretical [11–13]values of !tt̄ as functions of m

polet , assuming that mMC

t can beequated to the pole mass. The colored dashed lines representthe uncertainties for all three theoretical calculations from thechoice of the PDF and the renormalization and factorizationscales (added quadratically). The point shows the measured!tt̄ for mMC

t =172.5 GeV, the black curve is the fit to Eq. (1),and the gray band corresponds to the total experimental un-certainty.

TABLE II: Values of mpolet , with their 68% C.L. uncertainties,

extracted for di!erent predictions of !tt̄. The results assumethat mMC

t corresponds to mpolet (left column). The right col-

umn shows the di!erence " to these results if it is assumedthat mMC

t corresponds to mMSt . The combined experimental

and theoretical uncertainties are shown.

Theoretical prediction mpolet (GeV)

MC mass assumption mMCt = m

polet "(mMC

t = mMSt )

NLO [9] 164.8+5.7!5.4 !2.8

NLO+NLL [10] 166.5+5.5!4.8 !2.6

NLO+NNLL [11] 163.0+5.1!4.6 !3.3

Approximate NNLO [12] 167.5+5.2!4.7 !2.6


To quantify the maximum impact of alternative inter-284

pretations of mMCt , we now assume in case (ii) that mMC

t285

is interpreted as mMSt . However, because the cross sec-286

tion predictions use the pole-mass convention, the value287

mMCt = mMS

t must be converted to mpolet using the fol-288

lowing relationship at the two-loop level [19, 20]:289

mpolet = mMS

t (mMSt )

!

1 +4

3

"s(mMSt )

#(3)

+ 8.28

"

"s(mMSt )

#

#2

+ ...$

+ O("QCD) ,

where "s is the strong coupling in the MS mass scheme,290

and "QCD is the scale of the strong interaction. The last291

term in Eq. (3) indicates that the pole mass has an un-292

avoidable ambiguity of order "QCD [19]. For a top quark293

pole mass of mpolet = 173.3 GeV, the respective mass294

mMSt (mMS

t ) is lower by 9.7 GeV. With this change of the295

mMCt interpretation in Eq. (1) we form a new likelihood296

fexp(!|mt) and extract mpolet using Eq. (2). The di!er-297

ence between assuming that mMCt is equal to mpole

t and298

assuming that mMCt is equal to mMS

t is given in the right299

column of Table II. This shows that, given the uncertain-300

ties, interpreting the MC mass as either the pole mass or301

as the MS mass has no significant bearing on the value302

of the extracted mass. We include half of this di!erence303

symmetrically in the systematic uncertainties. As a re-304

sult we extract a top quark pole mass of 163.0+5.4!4.9 using305

the calculation of [11] and 167.5+5.4!4.9 using the calculation306

of [12].307

Calculations of the tt̄ cross section [11, 12] have also308

been performed as a function of the MS mass. Compar-309

ing the dependence of the experimental cross section to310

theory as a function of mt provides an estimate of mMSt .311

We note that a previous extraction of mMSt [12] ignored312

the mass dependence of the measured !tt̄.313

We extract the value of mMSt , again, for two cases: (i)314

5

function of mpolet and consequently, comparing the ex-266

perimental !tt̄ as a function of mMCt to these theoretical267

predictions provides a value of mpolet . The relation be-268

tween mMCt and mpole

t or mMSt is still under investigation.269

Arguments have been made that the MC mass should270

be close to the pole mass [5]. Therefore, we (i) extract271

mpolet assuming that the definition of mMC

t is equivalent272

to mpolet , and (ii) we take mMC

t equal to mMSt to esti-273

mate the maximum e!ect of interpreting mMCt as any274

other mass definition. The di!erence between the two275

results is included into the systematic uncertainties.276

For case (i), the mass in the MC simulations equals277

mpolet , and Fig. 1 shows the parameterization of the mea-278

sured and the predicted tt̄ cross sections [11–13] as a279

function of mpolet . The results for the determination of280

mpolet are given in the left column of Table II. All values281

are consistent within 2 sd with the Tevatron average top282

quark mass of mt = 173.3± 1.1 GeV [1].283


(pb)

tt!

2

4

6

8

10

12

14

+X)t t"p(p!Measured !Measured dependence of

NNLO approx KidonakisNNLO approx Moch and UwerNLO+NNLL Ahrens et al.


(pb)

tt!

2

4

6

8

10

12

14-1DØ, L=5.3 fb

FIG. 1: (Color online) Experimental and theoretical [11–13]values of !tt̄ as functions of m

polet , assuming that mMC

t can beequated to the pole mass. The colored dashed lines representthe uncertainties for all three theoretical calculations from thechoice of the PDF and the renormalization and factorizationscales (added quadratically). The point shows the measured!tt̄ for mMC

t =172.5 GeV, the black curve is the fit to Eq. (1),and the gray band corresponds to the total experimental un-certainty.

TABLE II: Values of mpolet , with their 68% C.L. uncertainties,

extracted for di!erent predictions of !tt̄. The results assumethat mMC

t corresponds to mpolet (left column). The right col-

umn shows the di!erence " to these results if it is assumedthat mMC

t corresponds to mMSt . The combined experimental

and theoretical uncertainties are shown.

Theoretical prediction mpolet (GeV)

MC mass assumption mMCt = m

polet "(mMC

t = mMSt )

NLO [9] 164.8+5.7!5.4 !2.8

NLO+NLL [10] 166.5+5.5!4.8 !2.6

NLO+NNLL [11] 163.0+5.1!4.6 !3.3



To quantify the maximum impact of alternative inter-284

pretations of mMCt , we now assume in case (ii) that mMC

t285

is interpreted as mMSt . However, because the cross sec-286

tion predictions use the pole-mass convention, the value287

mMCt = mMS

t must be converted to mpolet using the fol-288

lowing relationship at the two-loop level [19, 20]:289

mpolet = mMS

t (mMSt )

!

1 +4

3

"s(mMSt )

#(3)

+ 8.28

"

"s(mMSt )

#

#2

+ ...$

+ O("QCD) ,

where "s is the strong coupling in the MS mass scheme,290

and "QCD is the scale of the strong interaction. The last291

term in Eq. (3) indicates that the pole mass has an un-292

avoidable ambiguity of order "QCD [19]. For a top quark293

pole mass of mpolet = 173.3 GeV, the respective mass294

mMSt (mMS

t ) is lower by 9.7 GeV. With this change of the295

mMCt interpretation in Eq. (1) we form a new likelihood296

fexp(!|mt) and extract mpolet using Eq. (2). The di!er-297

ence between assuming that mMCt is equal to mpole

t and298

assuming that mMCt is equal to mMS

t is given in the right299

column of Table II. This shows that, given the uncertain-300

ties, interpreting the MC mass as either the pole mass or301

as the MS mass has no significant bearing on the value302

of the extracted mass. We include half of this di!erence303

symmetrically in the systematic uncertainties. As a re-304

sult we extract a top quark pole mass of 163.0+5.4!4.9 using305

the calculation of [11] and 167.5+5.4!4.9 using the calculation306

of [12].307

Calculations of the tt̄ cross section [11, 12] have also308

been performed as a function of the MS mass. Compar-309

ing the dependence of the experimental cross section to310

theory as a function of mt provides an estimate of mMSt .311

We note that a previous extraction of mMSt [12] ignored312

the mass dependence of the measured !tt̄.313

We extract the value of mMSt , again, for two cases: (i)314

•  Measured cross section where MC is used to estimate the acceptance is less dependent on the top quark mass in MC.

•  A constraint on the top quark pole mass can be obtained by combining the experimental and theoretical inputs.

•  The result is insensitive to the interpretation of the top quark mass in MC.


Preliminary

Top-Antitop Quark Mass Difference


}  Because of the very short life time, the top (and antitop) quark decays before hadronizing.

}  This allows direct measurements of top and antitop masses and to examine the CPT invariance theorem.

}  The first result from DØ (1 fb-1) in 2009:

}  The first result from CDF (5.6 fb-1) in 2010:

!mt =3.8±3.4(stat)±1.2(syst) GeV PRL 103, 132001 (2009)

!mt =-3.3±1.4(stat)±1.0(syst) GeV arxiv: 1103.2782 Submitted to PRL

2σeffect ?!

Mass Difference from Template Fitting


)2 (GeV/creco m-100 0 100

)2Ev

ents

/(15

GeV

/c

020406080

100120140160

taggedreco m

!mt =-3.3±1.4(stat)±1.0(syst) GeVCDF 5.6 fb-1

)2 (GeV/creco m-100 0 100

)2Ev

ents

/(15

GeV

/c

0

50

100

150 )-1CDF Data (5.6 fbBackground

)2 ( 0 GeV/ctt )2 (-4 GeV/ctt

nontaggedreco m

arxiv: 1103.2782 Submitted to PRL

Mass Difference from ME Method


17

(GeV)tM170 175 180

(GeV

)t

M

170

175

180 (a) -1DØ 3.6 fbe+jets

(GeV)tM170 175 180

(GeV

)t

M

170

175

180 (b) -1DØ 3.6 fb+jetsµ

FIG. 11: Combined likelihoods of the 2.6 fb−1 and 1 fb−1 measure-ments as functions of Mt and Mt̄ in data for the (a) e+ jets and(b) µ +jets channel. The solid, dashed, and dash-dotted lines rep-resent the 1, 2, and 3 sd contours of two-dimensional Gaussian fitsdefined in Eq. 21 to the distributions, respectively. No pull correc-tions have been applied, and therefore the figures are for illustrativepurposes only.

multiple hadron interactions, which is assumed to be uncor-921

related, since the reweighting according to the instantaneous922

luminosity profile is performed independently for each analy-923

sis.924

The 1 fb−1 analysis used a data-driven method to esti-925

mate systematic uncertainties from modeling of signal pro-926

cesses which did not distinguish between different sources927

such as: (i) higher-order corrections, (ii) initial and final state928

radiation, (iii) hadronization and the underlying event, and929

(iv) color reconnection. We therefore replace the correspond-930

ing systematic uncertainties estimated in the 1 fb−1 analysis931

by the uncertainties (i)–(iv) determined above, in accordance932

with Ref. [11].933

The following uncertainties from modeling of detector pe-934

formance (Table 8) are taken to be uncorrelated between the935

two measurements: (i) JES, (ii) remaining JES, and (iii) trig-936

ger efficiency. The rest are taken to be correlated.937

In the 1 fb−1 analysis, a systematic uncertainty of 0.4 GeV938

from the difference in calorimeter response to b and b̄ quarks939

was estimated using MC studies and checks in data. This940

systematic uncertainty has been re-evaluated using an entirely941

data-driven approach (item (iv) in Sec. VII B), and we there-942

fore use this new result for the analysis based on the 1 fb−1943

data.944

All other systematic uncertainties not explicitly mentioned945

above are taken as uncorrelated.946

The combined result for ∆M corresponding to 3.6 fb−1 of947

data is948

∆M = 0.84±1.81 (stat.)±0.76 (syst.) GeV . (23)

In this combination, BLUE determines a relative weight of949

73.0% (27.0%) for the 2.6 fb−1 (1 fb−1) measurement. The950

χ2/NDOF of the combination is 0.95. The combined likeli-951

hood densities for the two analyses are presented in Fig. 11 as952

functions of Mt and Mt̄ , separately for the e+jets and µ+jets953

channels.954

IX. CONCLUSION955

We have applied the matrix element method to the mea-956

surement of the mass difference ∆M between the top and957

antitop quarks using tt̄ candidate events in the lepton+jets958

channel in data corresponding to an integrated luminosity of959

about 3.6 fb−1. We find960

∆M = 0.8±1.8 (stat.)±0.8 (syst.) GeV ,

which is compatible with no mass difference at the level of961

≈1% of the mass of the top quark.962

The probability of measuring a larger absolute mass differ-963

ence in the SM is approximately 67%.964

Acknowledments965

We thank the staffs at Fermilab and collaborating insti-966

tutions, and acknowledge support from the DOE and NSF967

(USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom and968

RFBR (Russia); CNPq, FAPERJ, FAPESP and FUNDUNESP969

(Brazil); DAE and DST (India); Colciencias (Colombia);970

CONACyT (Mexico); KRF and KOSEF (Korea); CONICET971

and UBACyT (Argentina); FOM (The Netherlands); STFC972

and the Royal Society (United Kingdom); MSMT and GACR973

(Czech Republic); CRC Program and NSERC (Canada);974

BMBF and DFG (Germany); SFI (Ireland); The Swedish Re-975

search Council (Sweden); and CAS and CNSF (China).976

[1] J. Schwinger, Phys. Rev. 82, 914 (1951).977

[2] G. Luders, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 28, 5978

(1954).979

[3] Niels Bohr and the Development of Physics, ed. W. Pauli980

(McGraw-Hill, New York, 1955), p30.981

[4] J. S. Bell, Proc. R. Soc. A 231, 479 (1955).982

[5] D. Colladay and V. A. Kostelecky, Phys. Rev. D 55, 6760983

(1997).984

[6] O. W. Greenberg, Phys. Rev. Lett. 89, 231602 (2002).985

[7] K. Nakamura et al. (Particle Data Group), J. Phys. G 37, 075021986

(2010).987

[8] J. A. R. Cembranos, A. Rajaraman, and F. Takayama, Europhys.988

Lett. 82, 21001 (2008).989

[9] F. Abe et al. (CDF Collaboration), Phys. Rev. Lett. 74, 2626990

(1995).991

[10] S. Abachi et al. (D0 Collaboration), Phys. Rev. Lett. 74, 2632992

(1995).993

[11] Tevatron Electroweak Working Group, arXiv:1007.3178,994

(2010).995

[12] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 103,996

132001 (2009).997

[13] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. D 74,998

092005 (2006).999

[14] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 101,1000

!mt =0.8±1.8(stat)±0.8(syst) GeVD0 3.6 fb-1:

Preliminary Preliminary

tevatron results on top quark physics - mit...

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