Weak Boson Results
from CMS
Probing the initial stage in nuclear collisions
using electroweak measurements
Initial Stages in High-Energy Nuclear Collisions
Napa, CA
5/Dec/2014
Manuel Calderón de la Barca Sánchez
for the CMS Collaboration
Z, W bosons: well calibrated
probe in pp
LHC energies: access to Z, W in
pA and AA
Electroweak bosons in dilepton
channel:
No final state modification by
QCD medium
JHEP 0904 (2009) 065
EPS09 NLO
valence quarks
Test Binary scaling hypothesis
`Standard candle’ for initial state
Sensitive to initial state
Constrain nuclear PDFs
IS2014
Manuel Calderón de la Barca Sánchez
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Muons: Si tracker + muon detectors
pT resolution: 1-2% up to pT ~ 100 GeV/c
Electrons: Si tracker + ECAL cluster
h/e discrimination: shower shape + EECAL/ptrack
pT resolution: 1-2% for pT ~ 45 GeV/c
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60
50
40
30
20
10
0
60
300
250
200
150
100
50
Same charge
Opposite charge
Lint = 150 mb-1
sNN = 2.76 TeV
CMS PbPb
110
CMS PbPb
sNN = 2.76 TeV
m
Lint = 150 mb-1
pT > 20 GeV/c
110
|hm| < 2.4
mm
100
120
M (GeV/c2)
120
Mee (GeV/c2)
100
|he| < 1.44
T
PYTHIA+HYDJET
90
NN ® Z ® ee
80
90
pe > 20 GeV/c
70
Opposite charge
Same charge
80
PYTHIA+HYDJET
NN ® Z ® mm
70
100
80
60
40
20
0
60
250
200
150
100
50
0
60
s = 2.76 TeV
CMS pp
100
110
|he| < 1.44
T
pe > 20 GeV/c
Lint = 5.4 pb-1
Opposite charge
90
Same charge
80
PYTHIA
pp ® Z ® ee
70
CMS pp
110
120
120
Mee (GeV/c2)
Opposite charge
mm
M (GeV/c2)
100
|hm| < 2.4
pT > 20 GeV/c
m
Same charge
90
s = 2.76 TeV
Lint = 5.4 pb-1
80
PYTHIA
pp ® Z ® mm
70
6
IS2014
Manuel Calderón de la Barca Sánchez
CMS, arXiv:1410.4825
Submitted to JHEP
Excellent S/B
Signal shape agrees
with simulation in
both channels and
both collision
systems!
0
60
PbPb data, 2011 run
pp data, 2013 run
Muon and electron
channels.
Lepton selection
Events / (2 GeV/c2)
Events / (2 GeV/c2)
pT > 20 GeV/c
|hm(e)|<2.4 (1.44)
Events / (2 GeV/c2)
Events / (2 GeV/c2)
PLB 715 (2012) 66
W signal in muon spectrum at high pT
Require good quality muons + Z veto
Missing pT : obtained from tracking
All muon triggers: momentum is balanced
Some centrality dependence: resolution for missing pT
Signal events with high pT muon: large imbalance
No centrality dependence, agrees with simulation
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PLB 715 (2012) 66
Excellent S/B
Good agreement
between data and
simulation
Shape is consistent
between pp and PbPb
modulo resolution
difference
IS2014
mT = 2 pTm p/ T (1- cosq )
Manuel Calderón de la Barca Sánchez
8
PLB 715 (2012) 66
RAA
2.5
2
1.5
1
0.5
0
0
CMS s = 2.76 TeV
NN
Z ® mm, |y| < 2.0, p > 0 GeV/c
T
Z ® mm, 0-100% centrality
Z ® ee, |y| < 1.44, p T > 0 GeV/c
Z ® ee, 0-100% centrality
pp luminosity uncertainty
50 100 150 200 250 300 350 400
Npart
CMS, arXiv:1410.4825
RAA(Z) = 1.10 ± 0.05 ± 0.09
RAA(W) = 1.04 ± 0.07 ± 0.12
RAA ~ 1, no centrality dependence: Standard candles!
Best test of binary-collision scaling hypothesis
Can study finer initial-state nuclear effects via kinematics
IS2014
Manuel Calderón de la Barca Sánchez
9
-6
CT10
CT10 + EPS09
Z ® ee, pT > 0 GeV/c, 0-100% centrality
Lint = 150 mb-1
CMS PbPb sNN = 2.76 TeV
0.7 ´10
0.6
0.5
0.4
0.3
0.2
0.1
|y|
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
-6
CT10
CT10 + EPS09
T
Z ® mm, p > 0 GeV/c, 0-100% centrality
Lint = 150 mb-1
CMS PbPb sNN = 2.76 TeV
0.7 ´10
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
|y|
10
Differences between
models are small in PbPb
Manuel Calderón de la Barca Sánchez
IS2014
dN/dy
dN/dy
Comparison to POWHEG
No nuclear effects: CT10
With nuclear effects: CT10 + EPS09
pp:
little dependence of
asymmetry on h.
PbPb:
asymmetry is smaller
larger dependence on
h.
Comparison between
pdfs:
Clear preference for
nuclear pdf in PbPb
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Higher cross sections in pPb: √s=5.02 TeV
Asymmetric collisions: new observables
forward/backward asymmetries
better sensitivity to nPDF
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Manuel Calderón de la Barca Sánchez
12
CMS-HIN-14-003
Events / [2 GeV/c2]
CMS-HIN-13-007
700
600
500
400
300
200
100
0
60
CMS Preliminary
90
pPb sNN = 5.02 TeV
Lint = 34.6 nb-1
80
Opposite charge
Same charge
Pythia + Hijing
70
100
110
120
mm invariant mass [GeV/c 2]
W selection: pT > 25 GeV/c, |hmlab|<2.4, |helab|<2.5
Z selection: pT > 20 GeV/c, |hmlab|<2.4
Asymmetric acceptance in rapidity
CM Frame is boosted
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13
1.3
1.2
1.1
1
0.9
0.8
0.7
30
25
20
15
10
5
0
Theory
Data
/
Theory
-2
-1
0
y
c.m.
1
2
yc.m.
208 ´ (MCFM+MSTW08NLO+DSSZ pN ®Z®mm)
208 ´ (MCFM+MSTW08NLO+EPS09 pN®Z®mm)
208 ´ (MCFM+MSTW08NLO pp ®Z®mm)
Data
pPb sNN = 5.02 TeV Lint = 34.6 nb -1
CMS Preliminary
y
/
c.m.
Data
yc.m.
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1.3
1.3
MCFM+MSTW08NLO
MCFM+MSTW08NLO+EPS09
MCFM+MSTW08NLO+DSSZ
1.2
1.2
1.1
1.1
1
1
0.9
0.9
0.8
0.8
0.7
0.7
-2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2
ds/dy [nb]
Theory
/
Data
Distribution shifted to c.m. frame
Scaling spPb = A spp works well: scaled pp
predictions consistent with data
P(c2) = 11%CT10
(17%EPS09)
ds (W+®l+n) / dh
140
120
lab
100
80
60
40
20
0
140
120
80
60
40
20
0
100
lab
CT10
EPS09)
P(c2) = 41%
ds (W ®(80%
l n) / dh [nb]
[nb]
T
T
-1
-1
-
W ®l +n
1
-
W+ ® l+ + n
2
2
lab
h
CMS Preliminary
pPb sNN = 5.02 TeV
L = 34.6 nb-1
Data
CT10
EPS09
1
CMS Preliminary
pPb sNN = 5.02 TeV
L = 34.6 nb-1
Data
CT10
EPS09
0
0
Luminosity uncertainty: 3.5%
pl > 25 GeV/c
-2
Luminosity uncertainty: 3.5%
pl > 25 GeV/c
-2
lab
h
Distribution displayed in lab frame
pp predictions in good agreement with data
Nuclear effects are too small for discriminating: use asymmetries.
IS2014
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15
N(+hlab ) / N(-hlab )
EPS09)
P(c2) = 19%CT10 (13%
-
EPS09)
P(c2) = 16%CT10 (72%
+
+
N (+h )/N (-h )
N (+h )/N (-h )
lab
lab
lab
lab
3
2.5
2
1.5
1
0
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0
1
1.5
2
2
2.5
lab
2.5
h
CMS Preliminary
pPb sNN = 5.02 TeV
L = 34.6 nb-1
Data
CT10
EPS09
1.5
CMS Preliminary
pPb sNN = 5.02 TeV
L = 34.6 nb-1
Data
CT10
EPS09
-
1
W+ ® l+ + n
-
0.5
W ®l +n
0.5
lab
h
Better sensitivity to nuclear modification.
W- data show preference for nuclear effect
W+ data: no pdf is preferred
IS2014
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16
For Z boson, F/B ratio
should be 1 if no
nuclear effects present
Data: hint of nuclear
effects.
Forward / Backward
1.6
1.4
1.2
1
0.8
0.6
0.4
CMS Preliminary
pPb sNN = 5.02 TeV Lint = 34.6 nb -1
Data
MCFM+MSTW08NLO pp®Z®mm
1.5
MCFM+MSTW08NLO+EPS09 pN®Z®mm
1
MCFM+MSTW08NLO+DSSZ pN ®Z®mm
0.5
2
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|yc.m.|
Data: statistical
uncertainty dominates
nPDF: uncertainty
band not shown
0.2
0
0
Uncertainties are still
large
Manuel Calderón de la Barca Sánchez
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We are beginning to explore nuclear modification in
heavy-ion collisions using electroweak probes!
To first order: Z and W production scales with
binary collisions in pPb and PbPb
Initial-state nuclear effects on quark PDFs O(10%)
Hints of nuclear effects in W & Z data
Asymmetries vs. rapidity show promise for discriminating
power
More luminosity would help distinguish between nPDFs
Z,W Cross sections and asymmetries can be used as
part of global fits of nPDFs
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Manuel Calderón de la Barca Sánchez
18