Electroweak Physics
at the Tevatron
Aidan Robson
University of Glasgow
for the CDF and D0 Collaborations
Aspen, 13 February 2011
CDF Zee
(from Stirling, ICHEP04)
2004, using < 100 pb–1
Electroweak Physics at the Tevatron
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Susy
Higgs
dibosons
top
quark
W/Z
bottom
quark
Jets
Electroweak Physics at the Tevatron
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 Motivation
 High-statistics precision measurements
 Diboson physics
 pT(Z) x3
 G(W)
 Zg
 WZ
 ZZ
 WW/WZ -> lnjj
 Outlook
Electroweak Physics at the Tevatron
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Tevatron
h = 0.6
h = 1.0
h = 2.0
muon
chambers

D0
CDF
pre-radiator
2
shower max
hadronic cal
had
cal
EM cal
solenoid
1
0
tracker
0
silicon
1
h=1
E
M
cal
2
had cal
h=2
h=3
3 m
Fibre tracker to |h|<1.8
Calorimeter to |h|<4
Muon system to |h|<2
Drift chamber to |h|<1
Further tracking from Si
Calorimeter to |h|<3
Muon system to |h|<1.5
Electroweak Physics at the Tevatron
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W and Z selection
Electrons:
good EM shower shape
small hadronic energy
isolated in calorimeter
well-matching good track
(except far forward)
Muons:
MIP in calorimeter
isolated
hits in muon chamber
well-matching good track
Z selection:
2 oppositely-charged electrons or muons
invariant mass consistent with mZ
W selection:
exactly one electron or muon
energy imbalance in reconstructed
event, associated with neutrino
Electroweak Physics at the Tevatron
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pT
E+pz
y1/2 ln E–p
z
[~angular variable]
pZ
antiproton
l–
proton
CDF
2
Z
resummation / parton shower
with non-perturbative model
Z
pQCD reliable
resummation
required
2
Z
0
pT(Z)
multiple
soft gluon
radiation
Z
0
30
0<|y|<1
30 p (Z) 0
1<|y|<2
d/dpT
pT(Z)
d/dpT
Z/g*
d/dpT
q
pT(Z)
l+
q
p
30 p (Z) 0
2<|y|<3
30 p (Z)
distribution
different for
different y?
7
Earlier pT(Z)
Electron channel:
PRL 100 102002 (2008)
Compare 4 models:
Resbos with default parameters
Resbos with additional NLO–NNLO K-factor
NNLO (Melnikov and Petriello)
NNLO rescaled at to data at 30GeV/c
Electroweak Physics at the Tevatron
RESBOS event generator
implements NLO QCD and
CSS resummation
8
pT(Z)
However for comparison with
previous measurement, correct
to 4p and for mass window:
New measurement in muon channel
Presented at the level of particles entering the detector
to avoid model-dependent corrections
Phys. Lett. B 693 522
Electroweak Physics at the Tevatron
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At particle level:
pT(Z)
Phys. Lett. B 693 522
Electroweak Physics at the Tevatron
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f*h
aT : component of pT(ll) transverse to dilepton thrust axis.
Less susceptible than pT(ll) to detector effects
Best variable:
fh  tan( facop /2)sin( h )
*
*
– highly correlated with aT/mll
( h* measures scattering angle of leptons wrt beam, in rest frame of dilepton system)
Electroweak Physics at the Tevatron
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f*h
m
m
e
e
Electroweak Physics at the Tevatron
e
arXiv:1010.0262
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f*h
arXiv:1010.0262
Electroweak Physics at the Tevatron
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Drell-Yan angular coefficients
Rest frame of dilepton system
LO term
cos2θ :
higher order term
(θ, φ) terms
LO term : determine Afb
Integrate over all
φ,
very small terms
Integrate over all cosθ ,
=0
Electroweak Physics at the Tevatron
=0
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Drell-Yan angular coefficients
A2=A0 at LO
‘Lam-Tung’ relation
True only for spin-1 gluons,
strongly broken for scalar gluons
Electroweak Physics at the Tevatron
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Drell-Yan angular coefficients
A4 sensitive to Weinberg angle
A4 using 2.1 fb-1 data = 0.1098 ± 0.0079
Translated to sin2θW in FEWZ :
sin2θW = 0.2331±0.0008
Translated sin2θW in POWHEG :
sin2θW = 0.2328±0.0008
Electroweak Physics at the Tevatron
CDF Run II Preliminary
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W charge asymmetry
d
u
u
p
u
u
d
p
W±
l±
n
d (W+)/dy – d (W–)/dy
AW(y) 
d (W+)/dy + d (W–)/dy
Al(h) 
d (l+)/dh – d (l–)/dh
d (l+)/dh + d (l–)/dh
= A(yW)  (V–A) ~
d(x)
u(x)
Run 1 measurement resulted in d quark
increased by 30% at Q2=(20GeV)2
Electroweak Physics at the Tevatron
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W charge asymmetry
Electroweak Physics at the Tevatron
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mW
m W:
D0: mW = 80402 ± 43 MeV/c2
CDF: mW = 80413 ± 48 MeV/c2
Tev: mW = 80420 ± 31
MeV/c2 (includes Run 1)
LEP: mW = 80376 ± 33 MeV/c2
CDF
DmZ (stat)
published (200/pb)
43 MeV
expected (2.3/fb)
13 MeV
Heading to CDF 25MeV/c2 measurement
Electroweak Physics at the Tevatron
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GW
GW predicted in Standard Model:
GWSM = 2091±2 MeV (PDG)
Tev error improves from 62 to 49 MeV
Electroweak Physics at the Tevatron
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Dibosons
W/Z
q
W/Z/g
q’
W/Z/g
Wg Zg WW tt WZ t ZZ
Electroweak Physics at the Tevatron
H→
WW
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Zg
g
Using (Z→ll)+g
and (Z→nn)+g
g
Z
Z
non-SM
g
non-SM
h3, ZZg
events
Z
SM
|h3| < 0.037, |h4| < 0.0017
@95%CL (L=1.2TeV)
photon ET (GeV)
h , Zgg
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WZ
q
W
W
q’
Z/g
σ(pp → WZ) / σ(pp → Z)
= (5.5 ± 0.9) x 10-4
σ(pp → WZ)
= (4.1 ± 0.7) pb
23
WZ
σ(pp → WZ)
= (3.9 +1.01
(stat+sys) ± 0.31 (lumi)) pb
–0.85
arXiv:1006.0671
Electroweak Physics at the Tevatron
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WZ
0.075  Z  0.093
0.027  D Z  0.080
arXiv:1006.0671
for L=2TeV
Electroweak Physics at the Tevatron
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ZZ4l
q
q’
Z
Z
ZZ seen in 4 lepton at 5.7σ
All now observed!
Wg Zg WW tt WZ t ZZ
H→
WW
σ(pp → ZZ) / σ(pp → Z)
= (2.3+1.5-0.9 (stat) ± 0.3 (syst)) x 10-4
σ(pp → ZZ )
= (1.7 +1.2-0.7 (stat)
± 0.2 (syst)) pb
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ZZllnn
Electroweak Physics at the Tevatron
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WW/WZ  lnjj
Similar final state
to low-mass Higgs:
Electrons
Electroweak Physics at the Tevatron
Muons
28
WW/WZ  lnjj
σ(WW+WZ )
= (18.1 ± 3.3(stat)
5.4 ± 2.5(sys) )pb
5.2 significance
Electroweak Physics at the Tevatron
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WW/WZ  lnjj
Use matrix
element
techniques
σ(WW+WZ )
= (16.5 +3.35.4
-3.0) pb
5.4 significance
Electroweak Physics at the Tevatron
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Tevatron outlook
Integrated luminosity (pb–1)
End : Sep 2011(?)
On tape: ~ 8.5 fb-1 per experiment
Results shown today : 1-7 fb-1
2002
now
Electroweak Physics at the Tevatron
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Outlook
♦ Completing strong electroweak physics programme
♦ Focusing on high-statistics Tevatron legacy measurements
and diboson physics underpinning symmetry-breaking searches
Electroweak Physics at the Tevatron
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33
34
WW/WZ  lnjj
differences q.g jets
Electroweak Physics at the Tevatron
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WW scattering
W+
W+
Z/g
W–
W–
W+
H
W+
W+
W+
W+
W–
W–
W–
W+
required to
cancel highenergy
behaviour
Z/g
W–
W+
W+
H
W–
W–
W–
W–
36
W/Z primitive objects
for non-collider physicists
Electroweak Physics at the Tevatron
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p
g
H
p
g
Electroweak Physics at the Tevatron
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PDFs
p
g
H
p
g
pp→H = gg→H fg/p(x1,Q=MH) fg/p(x2,Q=MH) + …
Tevatron
y=
2
0
2
LHC
Higgs Physics at the Tevatron
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Matrix element method
 Use LO matrix element (MCFM) to compute event probability
xobs:
LO |M|2 :
px
px
py
py
lep1
pz
pz
Ex , E y
lep2
(with true values y)
HWWlnln
WWlnln
ZZllnn
W+partonln+jet
Wgln+g
parton lepton fake rate
g conversion rate
ET model
lepton energy resn
 Compute likelihood ratio discriminator
R=
Ps
Ps + SkbiPbi
kb is relative fraction of expected background contrib.
Ps computed for each mH
i
 Fit templates (separately for high S/B and low S/B dilepton types)
Higgs Physics at the Tevatron
40/54
Neural network method
 Various versions. Current:
 Apply preselection (eg ET to remove Drell-Yan)
 Train on {all backgrounds / WW} against Higgs
mH=110,120…160…200 { possibly separate ee,em,mm}
score
var1
ET
SET
mll
Elep1
Elep2
ETsig
ETjet1
DRleptons
Dfleptons
Df ET lep or jet
ETjet2
Njets
Most recent CDF
“combined ME/NN”
analysis also uses
ME LRs as NN input
variables
NN
var2
var n
x10
0
1
Background Higgs
 Pass signal/all backgrounds through net
 Form templates
Data
HWW
WW
DY
Wg
WZ
ZZ
tt
fakes
NN
0
1
 Pass templates and data to fitter
Higgs Physics at the Tevatron
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mt
xobs:
px
py
pz
px
py
pz
lep1
Matrix element-based top mass measurement
Lepton+jets with 4.8fb-1
NN for background discrimination
Likelihood fit over variables sensitive to top mass
Simultaneous constraint of jet energy scale using W in lepton+jets
More precise than CDF 2009!
Expect 1GeV precision achievable
Higgs Physics at the Tevatron
Ex , E y
etc.
jet1 (true values y)
ET model
lepton energy resn
mt =172.8 ± 1.3total GeV
(0.7stat 0.6JES 0.8sys)
42
Single top
d
u
l
l
W
W
b
t
g
W
Single top observed 2009.
b
b
b
t
n
W
d
s-channel
b
t-channel cross section [pb]
t-channel
n
u
s-channel cross sectionHiggs
[pb]
Physics at the Tevatron
43
Limit setting
Higgs signal x 10
H1=SM+Higgs (of mass mH)
H0=SM only
X
X = some observable
 Construct test statistic Q = P(data|H1)/P(data|H0)
–2lnQ = c2(data|H1) – c2(data|H0) ,
marginalized over nuisance params except  H
0
 Find 95th percentile of resulting  H distribution
– this is 95% CL upper limit.
 Repeat for pseudoexperiments drawn from
expected distributions to build up expected
outcomes
 Median of expected outcomes is “expected limit”
95%
rescale
1
2
H (pb)
0
2
H/SM
Median = expected limit
Expected outcomes
 When computed with collider data this is the
“observed limit”
95%
PDF
signal
separation
Background
events
background
suppression
95% CL Limit/SM
Higgs Physics at the Tevatron
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Indirect constraints
e+
e–
Z
Z
H
mH>114GeV
b
b
mH<154GeV
estimated
final precision
45
Integrated luminosity (fb–1)
Tevatron projection
End : Sep 2011?
On tape: ~ 6 fb-1 per experiment
Results shown today : 3-5 fb-1
Higgs Physics at the Tevatron
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W charge asymmetry
PRD 71 052002
First Run 2 charge asymmetry measurement: similar approach to Run 1
measurement relies on calorimeterseeded silicon tracking
experimental challenges:
alignment; charge misidentification
|he|
unknown neutrino pZ is a smaller effect
for higher ET electrons
measurement divided into two ET
regions
|he|
Aidan Robson
for given he, ET regions probe different
yW and therefore different x
Glasgow University
47/22
W charge asym. – new method
Instead: probe the W rapidity directly
MW constraint  two kinematic solutions for pz of n.
Ambiguity can be resolved statistically from known
centre-of-mass * distribution for V-A decay
weight solutions according to (cos*, y, pTW )
d/dy is an input; iterate to remove dependence.
Relies on Si-only tracking
Uncertainties:
Charge mis-ID rate
Energy scale and mismeasurement
Background/trigger/electron ID
cos*
Aidan Robson
Glasgow University
cos*
48/22
W charge asym. – new method
Under improvement using better
forward tracking and higher stats
Aidan Robson
Glasgow University
49/22
 Generator:
LO MC
matched with Resbos (QCD ISR)
and Berends/Kleiss (QED FSR)
W width
c2/dof=27.1/22
 Fast simulation for templates:
electron conversions + showering
muon energy loss
parametric model of recoil energy
(QCD, underlying event + brem)
 Tracking scale/resn
DG 17 MeV, 26 MeV
mmm (GeV)
DG 54 MeV (ele), 49 MeV (mu)
 Backgrounds
c2/dof=18/22
 Calorimeter scale/resn
DG 21 MeV, 31 MeV
DG 32 MeV
mT (GeV)
DG 33 MeV
mT (GeV)
mee (GeV)
W width
PRL 100 071801 (2008)
GW = 2032 ± 73 (stat+sys) MeV
Compare to CDF indirect measurement:
 ( pp  W )
G(Z)
G(W  l n )
R


 ( pp  Z) G(Z  ll )
G(W )
NNLO calc
From LEP
World most precise
single measurement
(GWSM = 2091 ± 2 MeV)
SM value
GW (indirect) = 2092 ± 42 MeV
J Phys G 34 2457