Precise measurements of the W mass at the Tevatron
and indirect constraints on the Higgs mass
Rafael Lopes de Sá
for the CDF and DØ Collaborations
March 11, 2012
Rencontres de Moriond
QCD and High Energy Interactions
Precise measurement of the W-boson mass with the CDF II detector arXiv:1203.0275
Measurement of the W Boson Mass with the D0 Detector arXiv:1203.0293
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
1
Motivation
Electroweak theory
The W boson mass is not an input parameter, but can be calculated
πα
M2
= √
MW 1 − W
(1 + ∆r)
2
MZ
2Gµ
H
t
Loop Corrections
∆r(MZ , MH , mt , αs , . . .)
W+
W+
W
W
b̄
Indirect dependence
δMH = 13 GeV [114 → 127]
δmt = 1.8 GeV [172.4 → 174.1]
(5)
δ(∆αHAD ) = 0.0002
Current theoretical uncertainty
R. Lopes de Sá (Stony Brook University)
δMW
−6.2 M eV
10.8 M eV
−3.6 M eV
4 M eV
W Mass at the Tevatron
SM prediction known to complete
2-loop order (and some 3-loop
parts)
Phys.Rev.D69:053006,2004
March 2012
2
Motivation
CDF Run 0/I
80.436 ± 0.081
D0 Run I
80.478 ± 0.083
CDF Run II
80.413 ± 0.048
Tevatron 2007
80.432 ± 0.039
D0 Run II
80.402 ± 0.043
W boson mass (GeV)
Direct Measurements (before February 2012)
CMS excl.
Atlas excl.
80.42
80.38
LEP excl.
80.36
80.420 ± 0.031
LEP2 average
80.376 ± 0.033
80.34
World average
80.399 ± 0.023
80.32
References:
SM prediction: Phys.Rev.D69:053006,2004
July 09
80.2
80.4
mW (GeV)
R. Lopes de Sá (Stony Brook University)
80.6
MH = 122.5
MH = 127.0
80.4
Tevatron 2009
80
68%
Tevatron excl.
Top Mass: 173.2±0.9 GeV (arXiv:1107.5255)
80.3
W Mass at the Tevatron
165
170
175
180
185
Top quark mass (GeV)
March 2012
3
Motivation
Global Electroweak Fit (before February 2012)
Precision EW Measurements
(Tevatron, LEP and SLD data)
W boson mass and width
Z boson mass, total and partial width
Z pole asymmetries and sin θW
Indirect measurement
of the Higgs boson mass
MH = 92+34
−26 GeV
(TEV EWWG and LEP EWWG – July, 2011)
Does not include LHC direct exclusion.
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
4
CDF Detector
General purpose detector. For this
analysis, the important subdetectors
are:
Central Drift Chamber immersed in
a 1.4T solenoid. Provides accurate
lepton momentum measurement
and position measurement.
Electromagnetic Calorimeter.
Lead-aluminium-scintillator
calorimeter. Provides shower
energy measurement as well as
position measurement via wire
chamber embedded at the EM
shower maximum.
Central tracker single muon resolution: 3.2% (for pT = 45 GeV )
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
5
DØ detector
General purpose detector. For this
analysis, the important subdetectors
are:
Central Tracker. Silicon and
scintillating fiber trackers immersed
in a 2T solenoid provide accurate
position measurement.
Electromagnetic Calorimeter.
Highly segmented uranium-liquid
argon calorimeter with good energy
resolution and coverage.
Electromagnetic calorimeter single electron energy resolution (with E = 45 GeV ): 3.33%
at η = 0. Average over central cryostat with W → eν angular spectrum: 4.16%.
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
6
Measurement Strategy
The Tevatron was a pp̄ collider with 1.96 T eV of energy. In a hadron collision, it is
impossible to know the parton system initial longitudinal momentum and, therefore,
to measure the longitudinal momentum of the neutrino from the W boson decay.
The transverse momenta carry part of the mass information. Both CDF and DØ
measurements use binned likelihood fits to extract the value of the W boson mass
from the following kinematical distributions:
Transverse mass mT =
p
2 (pT (`)pT (ν) − p
~T (`) · p
~T (ν))
Lepton transverse momentum pT (`)
Neutrino transverse momentum pT (ν)
el
ec
tr
on
pW
T
peT
/T
E
Underlying
Event
uT
ec
tr
itr
el
R. Lopes de Sá (Stony Brook University)
on
Hadronic Recoil
W Mass at the Tevatron
March 2012
7
Event Selection
CDF analysis
DØ analysis
−1
Analyzed 2.2 f b
Analyzed 4.3 f b−1 (1 f b−1
analyzed before)
.
Uses W → eν and W → µν decay
channels.
Uses W → eν decay channel.
Central leptons |η| < 1 with
30 < pT < 55 GeV
Central electrons |η| < 1.05 with
pT > 25 GeV
Missing transverse energy
/ T < 55 GeV
30 < E
Missing transverse energy
/ T > 25 GeV
E
Transverse mass
60 < mT < 100 GeV
Transverse mass
50 < mT < 200 GeV
Hadronic recoil momentum
uT < 15 GeV
Hadronic recoil momentum
uT < 15 GeV
CDF 2.2 f b−1
DØ 4.3 f b−1
(+1 f b−1 )
W → eν candidates
470, 126
1, 677, 394
R. Lopes de Sá (Stony Brook University)
W → µν candidates
624, 708
–
W Mass at the Tevatron
Total
1, 094, 834
1, 677, 394
2, 177, 224
March 2012
8
Calibration Strategies
Full GEANT detector simulations are not fast nor accurate enough to describe the
kinematical distributions used to measure the W boson mass.
Both CDF and DØ develop parametrized fast simulations of the detector response to
W → `ν events. The parametrizations are calibrated with data, using very different
strategies.
CDF strategy
DØ strategy
Detailed model of lepton
interactions at the central tracker.
Precise alignment using cosmic
rays.
Momentum scale calibrated using
J/ψ → µµ, Υ → µµ and Z → µµ
mass fits.
Use calibrated momentum scale
and E/p distribution in W → eν
events to calibrate the calorimeter
energy scale.
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
Detailed model of the calorimeter
response to electrons and photons.
Detailed model of the underlying
energy flow.
Detailed model of efficiencies.
Calibrate the calorimeter energy
scale using the dielectron invariant
mass and angular distribution in
Z → ee decays (electron energy
scale α and energy offset β).
March 2012
9
Calibration Results
∫ L dt ≈ 2.2 fb
-1
CDF II
∆ p/p
∫ L dt ≈ 2.2 fb
-1
CDF II
events / 0.01
-0.001
20000
χ2/dof = 18 / 22
-0.0015
J/ ψ →µ µ data (stat. only)
10000
Υ →µ µ data (stat. only)
Z→µ µ data (stat. only)
combined ∆ p/p (stat. ⊕ syst.) for W →µ ν events
-0.002
0
0.2
0.4
0.6
µ
<1/p > (GeV-1)
0
Offset, β (GeV)
T
0.3
0.15
0
1
L<0.72
0.72<L<1.4
1.4<L<2.2
L>2.2
1.01
1.02
1.03
1.04
1.05
Scale, α
(L in 1032 cm−2 s−1 )
R. Lopes de Sá (Stony Brook University)
1.2
1.4
1.6
E/p (W→eν)
DØ tests the calibration method doing a closure test
with GEANT simulation treated as data. The
results are consistent with the input value of MW
within statistical uncertainty (≈ 6 M eV ) for a sample
equivalent to 24 f b−1 !
D0 Run II, 4.3 fb-1
0.225
0.075
1
CDF momentum scale and DØ energy scale
precision: ≈ 0.01% (!!!)
W Mass at the Tevatron
March 2012
10
Z Mass Fits
A very strict test of the calibration procedure
∫ L dt ≈ 2.2 fb
-1
events / 0.5 GeV
CDF II preliminary
4000
MZ = (91180 ± 12stat) MeV
All values consistent with the
precisely measured value at LEP.
MZ = 91188 ± 2 M eV
χ2/dof = 30 / 30
2000
0
70
80
90
100
110
mµµ (GeV)
MZ (µµ) = 91180 ± 12(stat) ± 10(syst) M eV
∫ L dt ≈ 2.2 fb
events / 0.5 GeV
Events/0.25 GeV
-1
CDF II preliminary
1000
MZ = (91230 ± 30stat) MeV
χ2/dof = 42 / 38
500
1700
80
90
100
Fast MC
850
0
70
110
mee (GeV)
Fit Region
χ2/d.o.f. = 153/159
75
80
85
90
95
100
105
110
mee, GeV
4
-1
MZ (ee)
± 17(stat) M eV
D0 Run =
II, 4.3 fb91193
W Mass at the Tevatron
χ
MZ (ee) = 91230 ± 30(stat) ± 14(syst) M eV
R. Lopes de Sá (Stony Brook University)
Data
1275
425
0
70
D0 Run II, 4.3 fb-1
2
March 2012
11
Systematic uncertainties
Comparison of systematic uncertainties in the mT (`, ν) measurement
(values in MeV)
Source
CDF mT (µ, ν) CDF mT (e, ν) DØ mT (e, ν)
Experimental – Statistical power of the calibration sample.
Lepton Energy Scale
7
10
16
Lepton Energy Resolution
1
4
2
Lepton Energy Non-Linearity
4
Lepton Energy Loss
4
Recoil Energy Scale
5
5
Recoil Energy Resolution
7
7
Lepton Removal
2
3
Recoil Model
5
Efficiency Model
1
Background
3
4
2
W production and decay model – Not statistically driven.
PDF
10
10
11
QED
4
4
7
Boson pT
3
3
2
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
12
CDF Results
Method (2.2 f b−1 )
mT (µ, ν)
pT (µ)
/ T (µ, ν)
E
Combination
MW (MeV)
80379 ± 16(stat)
80348 ± 18(stat)
80406 ± 22(stat)
(2.2 f b−1 )
R. Lopes de Sá (Stony Brook University)
Method (2.2 f b−1 )
MW (MeV)
mT (e, ν)
80408 ± 19(stat)
pT (e)
80393 ± 21(stat)
/ T (e, ν)
E
80431 ± 25(stat)
80387 ± 19 M eV (syst + stat)
W Mass at the Tevatron
March 2012
13
35000
30000
D0 Run II, 4.3 fb -1
DATA
FAST MC
W->τν
Z->ee
MJ
25000
20000
15000
Fit Region
χ2/dof = 37.4/49
10000
60000
DATA
FAST MC
W->τν
Z->ee
MJ
40000
30000
Fit Region
χ2/dof = 26.7/31
10000
60
70
80
90
0
25
100
mT, GeV
D0 Run II, 4.3 fb -1
30
35
40
45
50
55
60
pe , GeV
T
2
1
4
3
D0 Run II, 4.3 fb-1
2
1
0
-1
0
-1
-2
-3
-2
-3
-4
50
D0 Run II, 4.3 fb -1
50000
χ
χ
4
3
70000
20000
5000
0
50
Events/0.5 GeV
Events/0.5 GeV
DØ Results
60
70
80
90
Method (4.3 f b−1 )
mT (e, ν)
pT (e)
/ T (e, ν)
E
Combination mT ⊕ pT (4.3 f b−1 )
Combination (5.3 f b−1 )
R. Lopes de Sá (Stony Brook University)
-4
25
100
mT, GeV
30
35
40
45
50
55
60
pe , GeV
T
MW (MeV)
80371 ± 13(stat)
80343 ± 14(stat)
80355 ± 15(stat)
80367 ± 26(syst + stat)
80375 ± 23(syst + stat)
W Mass at the Tevatron
March 2012
14
Comparing Results
10
9
CDF (2.2/fb) MET(e,nu)
8
CDF (2.2/fb) pT(e)
7
CDF (2.2/fb) mT(e,nu)
6
CDF (2.2/fb) MET(mu,nu)
5
CDF (2.2/fb) pT(mu)
4
CDF (2.2/fb) mT(mu,nu)
3
D0 (4.3/fb) MET(e,nu)
2
D0 (4.3/fb) pT(e)
1
D0 (4.3/fb) mT(e,nu)
0
CDF 2.2/fb combination (stat+syts)
D0 4.3/fb combination (stat+syts)
D0 MET not included
in the combination
80250
80300
80350
80400
80450
Fitted W boson mass (MeV)
Very consistent results obtained with completely different calibration strategies!
(uncertainties from individual measurements are only statistical)
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
15
Single Experiment Uncertainty
W Mass uncertainty (MeV)
Tevatron Single Experiment Uncertainties
400
DZero (e)
350
300
CDF (e + mu)
250
200
150
100
50
102
103
104
Integrated Luminosity (pb-1)
Both experiments are getting close to the model and theoretical plateau.
Some work need to be done in this front as well!
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
16
Theoretical and modeling issues
Use a wider lepton η–acceptance to be less
sensitive to PDF uncertainties. It has been done
before at the Tevatron (DØ RunI).
Phys.Rev.D62:092006,2000
Asymmetry
Ideas and developments to improve the model
and theoretical uncertainties in the W mass measurement
0.2
-0
DØ, L=0.75 fb-1
ETe>25 GeV
-0.2
EνT>25 GeV
CTEQ6.6 central value
Explore lepton longitudinal momentum to
extract the W mass. Concrete example:
JHEP 1108:023,2011
Study QED uncertianties in the measurement
using NLO QCD ⊕ EW generators. Two recent
implementations in the POWHEG framework.
arXiv:1202.0465, arXiv:1201.4804
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
MRST04NLO central value
-0.4
CTEQ6.6 uncertainty band
-0.6
0
Asymmetry
Use Tevatron W lepton charge asymmetry to
constrain the u/d PDF instead of low energy
experiments. Available: CT10W PDF set.
Phys.Rev.D82:074024,2010
0.5
1
1.5
2
2.5
3
2.5
3
|ηe|
0.2
-0
(a) DØ, L=0.75 fb-1
-0.2
25<EeT<35 GeV
ETν >25 GeV
-0.4
CTEQ6.6 central value
MRST04NLO central value
-0.6
CTEQ6.6 uncertainty band
-0.8
0
0.5
1
1.5
2
|ηe|
March 2012
17
Higgs Constraints
CDF Run I
80.436 ± 0.081
D0 Run I
80.478 ± 0.083
D0 Run II (prel.)
80.376 ± 0.023
CDF II (prel.)
80.387 ± 0.019
W boson mass (GeV)
(Preliminary) New World Average
CMS excl.
MH = 122.5
Atlas excl.
80.42
Tevatron excl.
80.4
68%
MH = 127.0
80.38
LEP excl.
80.36
Tevatron 2012 (prel.)
80.387 ± 0.017
LEP2 average
80.376 ± 0.033
World average (prel.)
80.385 ± 0.015
80.34
80.32
References:
SM prediction: Phys.Rev.D69:053006,2004
Top Mass: 173.2±0.9 GeV (arXiv:1107.5255)
Winter 2012
80
80.2
80.4
mW (GeV)
R. Lopes de Sá (Stony Brook University)
80.6
80.3
W Mass at the Tevatron
165
170
175
180
185
Top quark mass (GeV)
March 2012
18
(Preliminary) Global Electroweak Fit
6
mLimit = 152 GeV
March 2012
Theory uncertainty
∆α(5)
had =
5
0.02750±0.00033
0.02749±0.00010
incl. low Q2 data
∆χ2
4
3
2
1
0
LEP
excluded
40
LHC
excluded
100
200
mH [GeV]
New (preliminary) indirect Higgs mass determination
+34
MH = 94+29
−24 GeV (was MH = 92−26 GeV before)
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
19
Conclusions
CDF and DØ measured the W mass with precision at least as good as the
world average before.
The CDF measurement is now the single most precise measurement of the W
mass.
CDF and DØ measurements in excellent agreement.
Model and theory uncertainties begin to play an important role.
CDF analyzed 2.2 f b−1 . DØ analyzed 4.3 f b−1 of integrated luminosity
collected at high instantaneous luminosity runs of the Tevatron.
The measurements at CDF and DØ can reduce the world average uncertainty
down to 10 M eV when all the rest of the data is analyzed.
The W mass will play an ever increasing role in the determination of the
consistency of the Standard Model.
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
20
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R. Lopes de Sá (Stony Brook University)
3,'420"1$52#,6/+,,7,$7,42#"'8$9"':+$;7&$<.;<
W Mass at the Tevatron
=
March 2012
21
Backup Slides
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
22
CDF Systematic Uncertainties
Source
Uncertainty (MeV)
Experimental – Statistical power of the calibration sample.
Lepton Energy Scale
7
Lepton Energy Resolution
2
Recoil Energy Scale
4
Recoil Energy Resolution
4
Lepton Removal
2
Background
3
Experimental Total
10
W production and decay model – Not statistically driven.
PDF
10
QED
4
Boson pT
5
W model Total
12
Total Systematic Uncertainty
15
W Statistics
12
Total Uncertainty
19
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
23
DØ Systematic Uncertainties
/ T MeV
mT MeV peT MeV
E
Experimental – Z statistics driven!
Electron Energy Scale
16
17
16
Electron Energy Resolution
2
2
3
Electron Energy Nonlinearity
4
6
7
W and Z Electron energy
4
4
4
loss differences
Recoil Model
5
6
14
Electron Efficiencies
1
3
5
Backgrounds
2
2
2
Experimental Total
18
20
24
W production and decay model – Not dependent on Z statistics!
PDF
11
11
14
QED
7
7
9
Boson pT
2
5
2
W model Total
13
14
17
Total Systematic Uncertainty
22
24
29
W Statistics
13
14
15
Total Uncertainty
26
28
33
Source
R. Lopes de Sá (Stony Brook University)
W Mass at the Tevatron
March 2012
24
Soft recoil: Data min-bias (CDF)
or min-bias + zero-bias (DØ)
events.
Width (GeV)
Lepton removal: Hadronic
energy reconstructed as lepton.
η
imb
Hard recoil: Parametrized from
Z → `` events.
6
5.5
χ
D0 Run II, 4.3 fb-1
η
3
D0 Run II, 4.3 fb-1
2
Data
PMCS
1
0
-1
-2
5
10
15
-3
0
20
25
pee
(GeV)
T
D0 Run II, 4.3 fb-1
5
4
10
15
20
25
pee
(GeV)
T
15
20
25
pee
(GeV)
T
D0 Run II, 4.3 fb-1
3
Data
2
PMCS
5
1
imb
Out-of-cone FSR: Photons
reconstructed as recoil.
10
9
8
7
6
5
4
3
2
1
0
0
χ
Mean (GeV)
Recoil Model
4.5
0
-1
4
-2
CDF and DØ: Final tune with Z → ``
5
momentum imbalance.
3.5
3
0
-3
5
10
15
-4
0
20
25
pee
(GeV)
T
5
10
4
pee
T
∫ L dt ≈ 2.2 fb
-1
CDF II preliminary
η̂
· η̂
e+
3
2
χ2 / DoF = 15.6 / 9
1
∫ L dt ≈ 2.2 fb
-1
CDF II preliminary
σ ( 0.65pηZ + uη ) (GeV)
ŷ
0.65pηZ + uη (GeV)
pee
T
6
χ2 / DoF = 15.9 / 9
5.5
5
0
e
−
4.5
-1
x̂
4
-2
ξˆ
uT · η̂
uT
-3
0
5
10
epresentation
of η̂ axisde
andSá
observables
to fit the
recoil model smearing parameters
R. Lopes
(Stonyused
Brook
University)
15
20
25
30
p (Z→ee) (GeV)
T
W Mass at the Tevatron
3.5
0
5
10
15
20
25
30
p (Z→ee) (GeV)
T
March 2012
25
DØ Consistency Check
Instantaneous Luminosity
L<2
2<L<4
mT
pT
MET
4<L<6
L>6
81.6
81.7
81.8
81.9
82
82.1
Blinded W mass (GeV)
R. Lopes de Sá (Stony Brook University)
91 91.05 91.1 91.15 91.2 91.25 91.3 91.35 91.4
Z mass (GeV)
W Mass at the Tevatron
0.895
0.896
0.897
0.898 0.899
0.9
0.901
(Blinded W mass) / (Z mass)
March 2012
26
DØ Consistency Check – Time
Early Run IIb1
Late Run IIb1
mT
pT
MET
Early Run IIb2
Late Run IIb2
81.6
81.7
81.8
81.9
82
82.1
Blinded W mass (GeV)
R. Lopes de Sá (Stony Brook University)
91 91.05 91.1 91.15 91.2 91.25 91.3 91.35 91.4
Z mass (GeV)
W Mass at the Tevatron
0.895
0.896
0.897
0.898 0.899
0.9
0.901
(Blinded W mass) / (Z mass)
March 2012
27
DØ Consistency Check – uk
u|| < 0 GeV
mT
pT
MET
u|| > 0 GeV
81.6
R. Lopes de Sá (Stony Brook University)
81.7
81.8
81.9
82
82.1
Blinded W mass (GeV)
W Mass at the Tevatron
March 2012
28
DØ Consistency Check – Recoil uT
uT < 10 GeV
mT
p
T
MET
uT < 20 GeV
81.6
81.7
81.8
81.9
82
82.1
Blinded W mass (GeV)
R. Lopes de Sá (Stony Brook University)
91 91.05 91.1 91.15 91.2 91.25 91.3 91.35 91.4
Z mass (GeV)
W Mass at the Tevatron
0.895
0.896
0.897
0.898 0.899
0.9
0.901
(Blinded W mass) / (Z mass)
March 2012
29