Tevatron Results on Top Quark Physics - MIT...
Transcript of 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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 3
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 4
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 6
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 7
!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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 8
!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|
7/28/11 Zhenyu Ye, 19th PANIC, MIT 9
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 10
} 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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 11
} 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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 12
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
(gen)labt y (rec) - lab
t y-1 0 1
Even
ts
0
2000
4000
6000 resolutionlab
t yCDF II Preliminary
MC
Forward-Backward Asmmetry Afb
7/28/11 Zhenyu Ye, 19th PANIC, MIT 13
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 14
} 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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 15
)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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 16
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 17
W-helicity in top quark decays Reconstruct angle of lepton in top quark pair events.
W-helicity
7/28/11 Zhenyu Ye, 19th PANIC, MIT 18
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
} 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
Single Top Production
7/28/11 Zhenyu Ye, 19th PANIC, MIT 20
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 21
} 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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 22
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|
7/28/11 Zhenyu Ye, 19th PANIC, MIT 23
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
} 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
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
Zhenyu Ye, 19th PANIC, MIT
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)
Zhenyu Ye, 19th PANIC, MIT
]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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 29
CDF direct top width measurement
0.3 GeV < Γtop< 4.4 GeV @ 68% CL; Γtop< 7.6 GeV @ 95% CL
Top Quark Width
7/28/11 Zhenyu Ye, 19th PANIC, MIT 30
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 31
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 32
• 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
Forward-Backward Asmmetry – L+Jets
7/28/11 Zhenyu Ye, 19th PANIC, MIT 34
} 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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 35
(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
(gen)labt y (rec) - lab
t y-1 0 1
Even
ts
0
2000
4000
6000 resolutionlab
t yCDF II Preliminary
MC
Forward-Backward Asmmetry – L+Jets
7/28/11 Zhenyu Ye, 19th PANIC, MIT 36
!
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
Forward-Backward Asmmetry Afb
7/28/11 Zhenyu Ye, 19th PANIC, MIT 37
+
+
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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 38
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.
Top quark pole mass (GeV)150 160 170 180 190
(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
Approximate NNLO [13] 166.7+5.2!4.5 !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
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.
Top quark pole mass (GeV)150 160 170 180 190
(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
Approximate NNLO [13] 166.7+5.2!4.5 !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
• 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.
Zhenyu Ye, 19th PANIC, MIT
Preliminary
Top-Antitop Quark Mass Difference
7/28/11 Zhenyu Ye, 19th PANIC, MIT 40
} 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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 41
)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
7/28/11 Zhenyu Ye, 19th PANIC, MIT 42
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
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!mt =0.8±1.8(stat)±0.8(syst) GeVD0 3.6 fb-1:
Preliminary Preliminary