6. Physics of W and Z bosons
6.1 W and Z Bosons in the Glashow-Salam-Weinberg theory
6.2 Summary of precision tests at LEP
6.3 W and Z boson production in hadron colliders
6.4 Test of QCD in W/Z (+jet) production
6.5 W mass measurement
W and Z vertex factors
The Z ff vertex factors in the Standard Model
(sin2 W is assumed to be 0.234)
6.2 Summary of electroweak precision tests at LEP
2002
1.0
-0.032
1987
-0.035
gVl
•
Results of 30 years of
experimental
and theoretical progress
• The electroweak theory is
tested at the level of 10-4
–ν μe
mH
mt
-0.038
0.5
-0.041
-0.506
ll
ee
68 CL
-0.503
-0.5
-0.497
gAl
e+ e - →
μ+ μ -
gV 0.0
-0.5
–ν e e
νμ e-
–ν e e
-1.0
-1.0
-0.5
νe e 0.0
gA
0.5
1.0
Cross-section (pb)
Cross sections for W and Z boson production
10 5
Z
10 4
e+ehadrons
10 3
10
2
CESR
DORIS
+
WW
PEP
PETRA
KEKB
PEP-II
TRISTAN
10
0
20
-
40
60
SLC
LEP I
80
100
LEP II
120
140
160
180
200
220
Centre-of-mass energy (GeV)
Precision tests
of the Z sector
Tests of the
W sector
Cross section for e+e- μ+μ- at LEP I
/Z interference
vanishes at s M Z
=(mZ): running el.magnetic coupling [(MZ) = /(1-) mit 0.06]
gV,gA=cV,cA: effective coupling constants (vector and axial vector)
Z
Cross section for e+e- ff at LEP I
/Z interference
Z
vanishes at s M Z
x NC f
number of colour degrees of
freedom for für fermion f
x (I+QCD)
QCD correction term
Cross section for e+e- ff on resonance (s = mZ)
• On resonance, s = mZ:
- */Z interference terms vanishes
- term contributes ~1%
- Z contribution dominates !
• Contribution of the */Z interference term at s = (MZ – 3 GeV)2 : ~0.2%
Total cross section for e+e- μ+μ– (integration over cos )
Peak cross
section
Partial width
Total width
From the energy dependence of the
total cross section (for various fermions f)
the parameters
MZ, Z, f
can be determined.
had nb
Measurement of the Z line-shape
0
40
ALEPH
DELPHI
L3
OPAL
30
Z
20
measurements (error bars
increased by factor 10)
10
from fit
QED corrected
MZ
86
88
90
92
94
Ecm GeV
Radiative corrections (photon radiation)
important
with ISR (initial state radiation)
without ISR
Measurement of the Z line-shape (cont.)
Results on Z line-shape parameters
MZ
Z
had
e
μ
= 91.1876 ± 0.0021 GeV
= 2.4952 ± 0.0023
= 1.7458 ± 0.0027
= 0.08392 ± 0.00012
= 0.08399 ± 0.00018
= 0.08408 ± 0.00022
GeV
GeV
GeV
GeV
GeV
23 ppm (*)
3 lepton flavours
treated independently
Test of lepton
universality
Z
had
e
= 2.4952 ± 0.0023
GeV
= 1.7444 ± 0.0022
GeV
= 0.083985 ± 0.000086 GeV
lepton universality
assumed:
e=μ=
*) Uncertainty on LEP energy measurement: ± 1.7 MeV (19 ppm)
had nb
Number of neutrinos
2
30
20
3
ALEPH
DELPHI
L3
OPAL
4
average measurements,
error bars increased
by factor 10
10
0
86
88
90
92
94
Ecm GeV
N = 2.9840 ± 0.0082
Forward-backward asymmetries
Terms cos in d/dcos
asymmetry
Forward-backward asymmetries
-comparison between ee and μμ final states-
+ + e e e e ()
L3
peak2
peak
1
d / d cos nb
peak+2
0.5
0
-1
-0.5
0
cos 0.5
1
Forward-backward asymmetries and fermion couplings
• Asymmetry at the Z pole
(no interference) is small
-0.032
mt= 178.0 4.3 GeV
mH= 114...1000 GeV
-0.035
mH
since gV is small
(in particular for leptons)
• For off-resonance points, the
interference term dominates and
gives larger contributions
gVl
f
-0.038
mt
-0.041
ll
ee
-0.503
68 CL
-0.502
-0.501
-0.5
gAl
• AFB can be used for the determination
of the fermion couplings
LO Standard Model prediction:
gA = T3
gV = T3 – 2 Q sin2 W
Electroweak radiative corrections
Standard Model relations
(lowest order)
Relations including
radiative corrections
Results of electroweak precision tests at LEP (cont.)
partial decay width versus sin2 W:
0.233
July 2010
mt= 173.3 1.1 GeV
mH= 114...1000 GeV
0.232
sin eff
2 lept
mH
0.231
mt
68 CL
83.6
83.8
ll MeV
84
84.2
Results of electroweak precision tests at LEP (cont.)
Measurement
Summary of results:
• All measurements in agreement
with the Standard Model
(5)
had(mZ)
0.02758 0.00035 0.02768
mZ GeV
91.1875 0.0021
91.1874
Z GeV
2.4952 0.0023
2.4959
0
had
41.540 0.037
41.479
20.767 0.025
20.742
nb
Rl
0,l
Afb
Al(P)
• They can be described with a
limited set of parameters
Fit
Rb
0.1465 0.0032
fit
O |/
1
2
meas
3
0.1481
0.21629 0.00066 0.21579
0.1721 0.0030
0.1723
0,b
Afb
0,c
Afb
0.0992 0.0016
0.1038
0.0707 0.0035
0.0742
Ab
0.923 0.020
0.935
Ac
0.670 0.027
0.668
0.1513 0.0021
0.1481
2 lept
sin eff (Qfb)
0.2324 0.0012
0.2314
mW GeV
80.399 0.023
80.379
W GeV
2.085 0.042
2.092
mt GeV
173.3 1.1
173.4
July 2010
meas
0.01714 0.00095 0.01645
Rc
Al(SLD)
|O
0
0
1
2
3
6.3 W/Z production at hadron colliders
QCD at work
• Important test of NNLO Drell-Yan QCD prediction for the total cross section
• Test of perturbative QCD in high pT region
(jet multiplicities, pT spectra,.)
• Tuning and „calibration“ of Monte Carlos for background predictions in searches
at the LHC
Predictions for the W and Z boson total cross sections at the Tevatron, using
the MRST2004 and CTEQ pdfs, compared with measurements from the CDF
and D0 collaborations. The predictions are shown at LO, NLO, and NNLO. For
the NLO prediction the accompanying pdf uncertainties are shown as band.
W boson production cross sections at the LHC (s = 7 TeV)
Predictions for the W l cross section at NNLO, calculated for the full kinematic
range as well as in the fiducial region (see below).
Major uncertainties: renormalization and factorization scale (~ ±1%)
parton distribution functions (~ ±2)%
uncertainties of s (~ ±1%)
Fiducial region: PT(l) > 20 GeV, < 2.47, excluding 1.37 < < 1.52
ETmiss > 25 GeV
mT > 40 GeV
Z boson production cross sections at the LHC (s = 7 TeV)
Predictions for the Z / * ll cross section at NNLO, calculated for the full kinematic
range as well as in the fiducial region (see below).
Major uncertainties: renormalization and factorization scale (~ ±1%)
parton distribution functions (~ ±1.5)%
uncertainties of s (~ ±1%)
Fiducial region: PT(l) > 20 GeV, < 2.47, excluding 1.37 < < 1.52
66 < mll < 116 GeV
6.4 Test of QCD in W/Z production at hadron colliders
As explained, leptons, photons and missing transverse energy are key
signatures at hadron colliders
Search for leptonic decays: W Z More difficult:
W had (large PT ( ), large PTmiss)
Z e(μ) had Ingredients for a cross-section measurement:
N sel N back
=
L where: Nsel = number of selected events
Nback = number of background events in selected events
L = integrated luminosity (measured from machine, reference process)
= detection efficiency
= acceptance of fiducial cuts (PT(l), ETmiss, MT, mll,.)
How do W and Z events look like ?
A bit of history: one of the first W and Z events seen
(UA2 experiment)
W/Z discovery by the UA1 and UA2 experiments at CERN (1983/84)
Carlo Rubbia (left, UA1) and
Luigi Di Lella (right, UA2)
Transverse momentum of
the electrons
Today’s W / Z e / ee signals
CDF Experiment, Fermilab
CDF
W e
Trigger:
• Electron candidate > 20 GeV/c
Electrons:
• Isolated el.magn. cluster in the calorimeter
• PT> 25 GeV/c
• Shower shape consistent with expectation for electrons
• Matched with tracks
Z ee
• 70 GeV/c2 < mee < 110 GeV/c2
W e
missing transverse momentum PTmiss (GeV/c)
• Missing transverse momentum > 25 GeV/c
Z cross sections
Good agreement with
NNLO QCD calculations,
QCD corrections are large: factor ~ 1.25
C.R.Hamberg et al, Nucl. Phys. B359 (1991) 343.
Precision is limited by systematic effects
(uncertainties on luminosity, parton densities,...)
W Cross Section
(
MWT = 2 PTl PT 1 cos l ,
)
Note: the longitudinal component of the
neutrino cannot be measured
only transverse mass can be reconstructed
Good agreement with
NNLO QCD calculations
C.R.Hamberg et al, Nucl. Phys. B359 (1991) 343.
Precision is limited by systematic effects
(uncertainties on luminosity, parton densities,...)
Comparison between measured W/Z
cross sections and theoretical prediction (QCD)
C. R. Hamberg, W.L. van Neerven and T. Matsuura, Nucl. Phys. B359 (1991) 343
Entries / 5 GeV
First measurements of W/Z production at the LHC
-early ATLAS data: 0.31 pb-1 (Summer 2010)
5
10
ATLAS
104
Data 2010 ( s = 7 TeV)
W L dt = 310 nb
-1
QCD
W Z Distributions of the missing transverse energy, ETmiss,
of muon candidates for data and Monte-Carlo simulation,
broken down into the signal and various background components.
Z 103
tt
102
10
1
0
20
40
60
80
100
Entries / 5 GeV
Emiss
T
105
4
10
ATLAS
120
[GeV]
Data 2010 ( s = 7 TeV )
W e
L dt = 315 nb
-1
QCD
W Z ee
103
Z tt
102
10
1
0
20
40
60
80
100
120
mT [GeV]
Distributions of the transverse mass, mT, of the electron-Etmiss
system without an Etmiss requirement. The data are compared to
Monte-Carlo simulation, broken down into the signal and various
background components.
First measurements of W/Z production at the LHC
-CMS data from 2010: 36 pb-1 -
Distributions of the missing transverse energy, ETmiss,
of electron candidates for data and Monte-Carlo simulation,
broken down into the signal and various background components.
10
CMS preliminary
3
36 pb-1 at
number of events / 1 GeV
1.2
s = 7 TeV
Distributions of the invariant di-electron mass, mee, for events
passing the Z selection. The data are compared to
Monte-Carlo simulation, the background is very small.
1
0.8
data
Z e+e-
0.6
0.4
0.2
0
5
0
-5
60
80
100
M(e+e-) [GeV]
120
W and Z production cross sections at LHC
36 pb-1 at
CMS
s = 7 TeV
36 pb-1 at
CMS
NNLO, FEWZ+MSTW08 prediction
NNLO, FEWZ+MSTW08 prediction, 60-120 GeV
[with PDF4LHC 68% CL uncertainty]
[with PDF4LHC 68% CL uncertainty]
10.44 ± 0.52 nb
0.97 ± 0.04 nb
W e
Z ee
10.48 ± 0.03 stat ± 0.17 syst ± 0.42lumi nb
0.992 ± 0.011stat ± 0.024 syst ± 0.040lumi nb
W μ
Z μμ
10.18 ± 0.03 stat ± 0.16 syst ± 0.41lumi nb
0.968 ± 0.008 stat ± 0.020 syst ± 0.039lumi nb
W l (combined)
Z ll
10.31 ± 0.02 stat ± 0.13 syst ± 0.41lumi nb
0
2
s = 7 TeV
4
6
(combined)
0.975 ± 0.007 stat ± 0.019 syst ± 0.039lumi nb
8
10
12
( pp WX ) B( W l ) [nb]
0
0.2
0.4
0.6
0.8
1
1.2
( pp ZX ) B( Z ll ) [nb]
The measured values of W x BR(W l ) and Z x BR(Z ll) for W and Z production in the CMS experiment,
compared to the theoretical predictions based on NNLO QCD calculations. Results are shown for the electron
and muon final states as well as for their combination. The error bars represent successively the statistical,
the statistical plus systematic and the total uncertainties (statistical, systematic and luminosity).
All uncertainties are added in quadrature.
W Br(W l ) [nb]
W production cross sections at hadron colliders
ATLAS
Data 2010 ( s = 7 TeV)
10
L dt = 310-315 nb
-1
W l
+
+
W l W l
1
NNLO QCD
10
W (pp)
-1
/
/
UA2 W e +
W (pp)
-
1
D0 W (e/ )
UA1 W l W (pp)
W (pp)
CDF W (l/e) /
-
Phenix W (e+/e )
10
s [TeV]
The measured values of W x BR(W l ) for W+, W- and for their sum compared to the theoretical
predictions based on NNLO QCD calculations. Results are shown for the combined electron-muon results.
The predictions are shown for both proton-proton (W+,W- and their sum) and proton-antiproton colliders
(W) as a function of root(s). In addition, previous measurements at proton-antiproton and proton-proton
colliders are shown. The data points at the various energies are staggered to improve readability.
The CDF and D0 measurements are shown for both Tevatron collider energies, s = 1.8 TeV and
s = 1.96 TeV. All data points are displayed with their total uncertainty. The theoretical uncertainties are
not shown.
Z/ * Br(Z/ * ll) [nb]
Z production cross sections at hadron colliders
ATLAS
1
Data 2010 ( s = 7 TeV)
L dt = 316-331 nb
-1
Z/ * ll (66 < m < 116 GeV)
ll
10-1
CDF Z/ * ee (66 < m
ee
D0 Z/ * ee
/
NNLO QCD
Z/ * (pp)
Z/ * (pp)
10-2
(70 < m
ee
< 116 GeV)
< 110 GeV)
CDF Z/ * ee/ (66 < m
ee
D0 Z/ * ee
(75 < m
ee
< 116 GeV)
< 105 GeV)
UA1 Z/ * ee (m > 70 GeV)
ee
UA1 Z/ * (m > 50 GeV)
UA2 Z/ * ee (m > 76 GeV)
ee
1
10
s [TeV]
The measured values of Z x BR(Z ll) where the electron and muon channels have been combined,
compared to the theoretical predictions based on NNLO QCD calculations. The predictions are shown for
both proton-proton and proton-antiproton colliders as a function of s. In addition, previous measurements
at proton-antiproton colliders are shown. The data points at the various energies are staggered to improve
readability. The CDF and D0 measurements are shown for both Tevatron collider energies, s= 1.8 TeV and
s = 1.96 TeV. All data points are displayed with their total uncertainty. The theoretical uncertainties are not
shown.
W cross sections in ATLAS, charge separated
Full ATLAS data set
from 2010
L = 36 pb-1
Distribution of transverse energy (top) and transverse mass mT (bottom) of the electron in the
selected W to electron candidate events after all cuts for positive (left) and negative (right) charge.
The simulated distributions are normalised to the data.
W+ and W- production cross sections at LHC
-
(pp W l )
ATLAS
Theory (NNLO)
L dt = 310-315 nb
+
+
-1
-
(pp W l )
ATLAS
Theory (NNLO)
L dt = 310-315 nb
-1
Data 2010 ( s = 7 TeV)
Data 2010 ( s = 7 TeV)
4.5
5
5.5
6
6.5
7
Electron channel
Electron channel
Muon channel
Muon channel
Combined
Combined
7.5
8
8.5
W+ [nb]
3
3.5
4
4.5
5
5.5
6
W- [nb]
The measured values of sigma_W x BR(W to lnu) for W+ and W- compared to the theoretical predictions
based on NNLO QCD calculations. Results are shown for the electron and muon final states as well as for
their combination. The error bars represent successively the statistical, the statistical plus systematic and
the total uncertainties (statistical, systematic and luminosity). All uncertainties are added in quadrature.
W- [nb]
W cross sections at the LHC, charge separated
ATLAS Preliminary
4.5
Full ATLAS data set
from 2010
L = 36 pb-1
4
Data 2010 ( s = 7 TeV)
MSTW08
HERA
ABKM09
JR09
3.5
5.5
6
total uncertainty
uncorr. exp. + stat.
uncertainty
-1
L dt = 33-36 pb
6.5
7
W+ [nb]
Measured and predicted W- vs. W+ cross sections times leptonic branching ratios. The systematic
uncertainties on the lumninosity, on the acceptance extrapolation and on the missing transverse
energy scale and resolution are treated as fully correlated. The projections of the ellipse to the axes
correspond to one standard deviation uncertainty of the cross sections. The uncertainties of the
predictions are the PDF uncertainties only. There is an additional uncertainty of the theoretical cross
sections due to the uncertainty of the strong coupling constant, at the level of 2% for a 1% error on the
coupling constant itself, which is not included in the theory error bars.
W [nb]
W cross sections at the LHC, charge separated
ATLAS Preliminary
11
Full ATLAS data set
from 2010
L = 36 pb-1
10
9
0.8
Data 2010 ( s = 7 TeV)
MSTW08
HERA
ABKM09
JR09
0.9
total uncertainty
uncorr. exp. + stat.
uncertainty
L dt = 33-36 pb
-1
1
1.1
Z [nb]
Measured and predicted W vs. Z cross sections times leptonic branching ratios. The systematic
uncertainties on the lumninosity, on the acceptance extrapolation and on the missing transverse
energy scale and resolution are treated as fully correlated. The projections of the ellipse to the axes
correspond to one standard deviation uncertainty of the cross sections. The uncertainties of the
predictions are the PDF uncertainties only. There is an additional uncertainty of the theoretical cross
sections due to the uncertainty of the strong coupling constant, at the level of 2% for a 1% error on the
coupling constant itself, which is not included in the theory error bars.
A
W charge asymmetry as a function of pseudorapidity
0.35
0.3
0.25
Data 2010 ( s=7 TeV)
MC@NLO, CTEQ 6.6
MC@NLO, HERA 1.0
MC@NLO, MSTW 2008
W 0.2
ATLAS
0.15
0
0.5
1
-1
L dt = 31 pb
1.5
2
||
The muon charge asymmetry from W-boson decays in bins of absolute pseudorapidity. The kinematic
requirements applied are muon pT > 20 GeV, neutrino pT > 25 GeV and mT > 40 GeV. The data points
(shown with error bars including the statistical and systematic uncertainties) are compared to NLO Monte
Carlo predictions with different PDF sets. The PDF uncertainty bands include experimental uncertainties
as well as model and parametrization uncertainties.
Summary of W/Z cross section results
-comparison between theory and CMS measurements36 pb-1 at
CMS
s = 7 TeV
lumi. uncertainty: ± 4%
B(W)
0.987 ± 0.009 exp. ± 0.051theo.
B ( W+ )
0.982 ± 0.009 exp. ± 0.049 theo.
-
0.6
B(W )
0.993 ± 0.010 exp. ± 0.056 theo.
B(Z)
1.003 ± 0.010 exp. ± 0.047 theo.
R W/Z
0.981 ± 0.010 exp. ± 0.016 theo.
R +/-
0.990 ± 0.011exp. ± 0.037 theo.
0.8
1
1.2
Ratio (CMS/Theory)
1.4
Good agreement between data and NNLO QCD predictions for all
measurements
Test of QCD in W/Z + jet production
- LO predictions fail to describe the data;
- Jet multiplicities and pT spectra in agreement
with NLO predictions within errors;
NLO central value ~10% low
Jet multiplicities in Z+jet production
pT spectrum of leading jet
We + jets
Data 2010, s=7 TeV
We
QCD
W
dibosons
Zee
Z
tt
single top
Ldt=33 pb
-1
105
(W + Njet jets) [pb]
106
103
0
1
2
3
4
5
Inclusive Jet Multiplicity, Njet
We + jets
Data 2010, s=7 TeV
ALPGEN
SHERPA
PYTHIA
BLACKHAT-SHERPA
MCFM
103
ATLAS Preliminary
2
10
-1
ATLAS Preliminary
104
104
Ldt=33 pb
10
Theory/Data
Events
Measurements of W+jets in ATLAS
1
1
0
0
1
2
3
4
5
Inclusive Jet Multiplicity, Njet
The uncorrected inclusive jet multiplicity distribution
for electron channel. The signal and leptonic
backgrounds are normalised to the NNLO cross
sections. The jet background from QCD processes
was determined from data.
W+jets fiducial cross section results as a function of corrected
jet multiplicity for the e channel. The combined statistical
and systematic uncertainties are shown by the black-hashed
regions. Also shown are predictions from ALPGEN, SHERPA,
PYTHIA, MCFM and BLACKHAT-SHERPA, and the ratio of
theoretical predictions to data. The theoretical uncertainties
are shown only for MCFM (NLO prediction for Njet < 2 and
a LO prediction for Njet= 3) and BLACKHAT-SHERPA
(NLO prediction for Njet < 3 and a LO prediction for Njet = 4).
pT spectra of the associated jets
d/dpT [pb/GeV]
103
102
10
1
-1
W+
W+
-1
10
10-2
-3
10
-4
10
We + jets
Data 2010, s=7 TeV
ALPGEN
SHERPA
BLACKHAT-SHERPA
MCFM
Ldt=33 pb
W+
W+
1 je
ts
ATLAS Preliminary
2 je
ts, x
1
0 -1
3 je
ts, x
1
0 -2
4 je
ts, x
1
0 -3
10-5
W+jets fiducial cross section (e channel) as a function of the pT of
the first jet in the event. The pT of the first jet is shown separately
for events with >=1 jet to >=4 jet. The >=2 jet, >=3 jet and >= 4 jet
distributions have been scaled down by factors of 10 and 100,
1000 respectively. For the data, the
combined statistical and systematic uncertainties are shown
by the black-hashed regions. Also shown are predictions
from ALPGEN, SHERPA, MCFM and BLACKHAT-SHERPA,
and the ratio of theoretical predictions to data for >=1 jet to
>=2 jet events. The theoretical uncertainties are shown only
for MCFM (NLO prediction for Njet < 2 and a LO prediction
for Njet= 3) and BLACKHAT-SHERPA (NLO prediction for
Njet < 3 and a LO prediction for Njet = 4).
Theory/Data Theory/Data
10-6
2
W + 1 jet
1.5
1
0.5
2
W + 2 jets
1.5
1
0.5
100
200
300
First Jet pT [GeV]
3
10
102
10
Ldt=33 pb
W+
1
2 je
ts
ATLAS Preliminary
-1
W+
10-2
W+
10
We + jets
Data 2010, s=7 TeV
ALPGEN
SHERPA
BLACKHAT-SHERPA
MCFM
-1
3 je
t
s, x1 -1
0
4 je
t
-3
10
d/dpT [pb/GeV]
d/dpT [pb/GeV]
pT spectra of the associated jets
102
We + jets
Ldt=33 pb
-1
10
W+
1
Data 2010, s=7 TeV
ALPGEN
SHERPA
3 je
ts
BLACKHAT-SHERPA
ATLAS Preliminary
10-1
W+
4 je
t
s, x1
0 -1
10-2
s, x1 -2
0
10-3
10-4
20
40
60
W + 2 jet
1
0.5
50
100
150
Second Jet pT [GeV]
W+jets fiducial cross section (e channel) as a function of the pT of
the second, third and fourth jet in the event.
(further description as above)
Both jet rates and pT spectra are well described by
perturbative QCD calculations
d/dpT [pb/GeV]
Theory/Data
100
120
Third Jet pT [GeV]
10-5
1.5
80
10
-1
Ldt=33 pb
We + jets
Data 2010, s=7 TeV
ALPGEN
1
SHERPA
ATLAS Preliminary
-1
10
10-2
10-3
20
30
40
50
60
70
Fourth Jet pT [GeV]
6.5
W mass measurement
Major contributions: LEP-II, direct mass reconstruction
Hadron collider: Tevatron and LHC (in the future)
Precision measurements of mW and mtop
Motivation:
W mass and top quark mass are fundamental parameters of the Standard Model;
The standard theory provides well defined relations between mW, mtop and mH
Electromagnetic constant
measured in atomic transitions,
e+e- machines, etc.
EM
m W = 2 GF
Fermi constant
measured in muon
decay
1/2
1
sin W 1 r
weak mixing angle
measured at
LEP/SLC
radiative corrections
r ~ f (mtop2, log mH)
r 3%
GF, EM, sin W
are known with high precision
Precise measurements of the
W mass and the top-quark
mass constrain the Higgsboson mass
(and/or the theory,
radiative corrections)
Relation between mW, mt, and mH
W bosons at LEP – II
W mass measurement
Results from W mass measurements at LEP-II
Summer 2006 - LEP Preliminary
1
Entries
0
ALEPH final
80.4400.051
DELPHI
0.75final
80.3360.067
L3 final
80.2700.055
0.5
OPAL final
LEP
0.25
80.4160.053
80.3760.033
2
/dof = 49 / 41
• Results from all four LEP
experiments are consistent
• Statistical error is dominant
• Total precision from LEP-II
LEP EWWG
0
80.0
80 80.2 80.4 80.6 80.8 81.0
81
MWGeV
mW = ± 33 MeV
Results of electroweak precision tests at LEP (cont.)
6
July 2010
mLimit = 158 GeV
July 2010
Theory uncertainty
(5)
had =
5
80.5
0.027580.00035
68 CL
0.027490.00012
2
incl. low Q data
mW GeV
2
4
3
2
1
0
80.4
80.3
Excluded
30
Preliminary
100
mH GeV
LEP2 and Tevatron (prel.)
LEP1 and SLD
300
mH GeV
114
300
150
1000
175
200
mt GeV
• Radiative corrections (loop, quantum corrections) can be used to constrain
yet unobserved particles (however, sensitivity to mH only through log terms)
• Main reason for continued precision improvements in mt , mW
Correlation between mH and mW
mW [GeV]
80.5
July 2010
2
High Q except mW/W
68% CL
mW (LEP2, Tevatron)
80.4
80.3
Excluded
10
10
2
mH [GeV]
10
3
What can hadron collider contribute ?
How can W mass be measured in hadronic collisions ?
Technique used for W mass measurement at hadron colliders:
Event topology:
Observables: PT(e) , PT(had)
PT() = - ( PT(e) + PT(had) )
(
MWT = 2 PTl PT 1 cos l ,
long. component cannot be
)
measured
In general the transverse mass MT is used for the determination of the W mass
(smallest systematic uncertainty).
Shape of the transverse mass distribution is sensitive to mW, the measured
distribution is fitted with Monte Carlo predictions, where mW is a parameter
Main uncertainties:
Ability of the Monte Carlo to reproduce
real life:
• Detector performance
(energy resolution, energy scale,
.)
• Physics: production model
pT(W), W, ......
• Backgrounds
In principle any distribution that is sensitive to mW can be used for the measurement;
Systematic uncertainties are different for the various observables.
What precision can be reached in Run II and at the LHC
Numbers for a
single decay
channel
W e
Int. Luminosity
CDF
0.2 fb-1
DØ
1 fb-1
LHC
10 fb-1
Stat. error
48 MeV
23 MeV
2 MeV
Energy scale, lepton res.
30 MeV
34 MeV
4 MeV
Monte Carlo model
(PTW, structure functions,
photon-radiation.)
16 MeV
12 MeV
7 MeV
8 MeV
2 MeV
2 MeV
Tot. Syst. error
39 MeV
37 MeV
8 MeV
Total error
62 MeV
44 MeV
~10 MeV
Background
?
• Tevatron numbers are based on real data analyses
• LHC numbers should be considered as „ambitious goal“
- Many systematic uncertainties can be controlled in situ, using the large Z sample
(pT(W), recoil model, resolution)
- Lepton energy scale of ± 0.02% has to be achieved to reach the quoted numbers
Combining both experiments (ATLAS + CMS, 10 fb-1), both lepton species and
assuming a scale uncertainty of ± 0.02% a total error in the order of
mW ~ ± 10 - 15 MeV might be reached.
Signature of Z and W decays
Zl+l–
Wl
What precision can be reached in Run II and at the LHC
Numbers for a
single decay
channel
W e
Int. Luminosity
CDF
0.2 fb-1
DØ
1 fb-1
LHC
10 fb-1
Stat. error
48 MeV
23 MeV
2 MeV
Energy scale, lepton res.
30 MeV
34 MeV
4 MeV
Monte Carlo model
(PTW, structure functions,
photon-radiation.)
16 MeV
12 MeV
7 MeV
8 MeV
2 MeV
2 MeV
Tot. Syst. error
39 MeV
37 MeV
8 MeV
Total error
62 MeV
44 MeV
~10 MeV
Background
?
• Tevatron numbers are based on real data analyses
• LHC numbers should be considered as „ambitious goal“
- Many systematic uncertainties can be controlled in situ, using the large Z sample
(PT(W), recoil model, resolution)
- Lepton energy scale of ± 0.02% has to be achieved to reach the quoted numbers
Combining both experiments (ATLAS + CMS, 10 fb-1), both lepton species and
assuming a scale uncertainty of ± 0.02% a total error in the order of
mW ~ ± 10 - 15 MeV might be reached.
Summary of W-mass measurements
W-Boson Mass [GeV]
TEVATRON
80.420 ± 0.031
LEP2
80.376 ± 0.033
Average
80.399 ± 0.023
2/DoF: 0.9 / 1
NuTeV
80.136 ± 0.084
LEP1/SLD
80.363 ± 0.032
LEP1/SLD/mt
80.365 ± 0.020
80
80.2
80.4
mW [GeV]
Precison obtained at the
Tevatron is superior to the
LEP-II precision
Further improvements are
expected from LHC
(may take some time, pile-up
makes the measurement very
challenging)
80.6
July 2010
mW (from LEP2 + Tevatron) = 80.399 ± 0.023 GeV
3•10-4