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W bosons in pPb collisions in the CMS experiment
Emilien Chapon
Laboratoire Leprince-Ringuet, Ecole Polytechnique, Palaiseau
QCD dans les collisions pA et AA:GDR-PH-QCD meeting, WG 3LPSC Grenoble, Nov. 5-7, 2014
Motivation
Electroweak bosons are produced anddecay very early in the collision.
They are not affected by the medium.
Isospin effect for W (different between pp,pn and nn binary collisions).
However they are sensitive to nuclearmodifications of the parton distributionfunctions.
Good probes for nuclear effects in PDFs(shadowing / anti-shadowing).
0.2
0.6
1.0
1.5
10-3
10-2
10-1
1
ya
ye
xa xe
y0 shadowing
antishadowing
EMC-effect
Fermi-motion
JHEP 0904 (2009) 065
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 2 / 16
The CMS experiment
Muon reconstruction: silicon tracker + muon sub-detectorsTracker pT resolution: 1-2% up to pT ∼ 100 GeV/c:
Electron reconstruction: tracks associated with an ECAL cluster.h/e discrimination: shower shape + fraction of energy in ECAL.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 3 / 16
W production
Leading order
ud →W+, du →W−p p
u d
Yields
Expect 2× more W+ than W− in pp.
Expect more balanced production in pPb.
Rapidity
W boosted towards the valence quark.
Spin conservation + parity violation: µ+ (µ−) boostedback to (away from) midrapidity.
⇒ different rapidity distributions between µ+ and µ−.
W+
μ+
ν
W-
μ-
ν_
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 4 / 16
W production
Leading order
ud →W+, du →W−p p
u d
Yields
Expect 2× more W+ than W− in pp.
Expect more balanced production in pPb.
Rapidity
W boosted towards the valence quark.
Spin conservation + parity violation: µ+ (µ−) boostedback to (away from) midrapidity.
⇒ different rapidity distributions between µ+ and µ−.
W+
μ+
ν
W-
μ-
ν_
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 4 / 16
Kinematics
Remember that we don’t reconstruct the full kinematics of the W boson(because of the escaping undetected neutrino)
µη-2 -1 0 1 2
Wy
-4
-3
-2
-1
0
1
2
3
0
5
1
1
2
2µ+
µη-2 -1 0 1 2
Wy-3
-2
-1
0
1
2
3
0
2
4
6
8
1
1
1
1µ−
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 5 / 16
W → `ν in pPb
W bosons in pPb collisions
W → µν W → eν
Asymmetric collisions: new observables compared to PbPb(e.g. forward / backward asymmetries)
Better sensibility to nPDF
higher cross section as compared to PbPb, because of√sNN
(√sNN = 2.76 TeV in PbPb → √sNN = 5.02 TeV in pPb)
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 6 / 16
W → `ν in pPb
Selection
0 20 40 60 80 100 120 140
Num
ber
of e
vent
s / 4
GeV
1
10
210 +µ < 0.0
µη -0.5 <
CMS Preliminary = 5.02 TeV NNs pPb
-1 L=34.6 nb
Dataν+µ → +W
µµ →Z ν+τ → +W
QCDFit
[GeV]TE0 20 40 60 80 100 120 140
N(D
ata)
/N(F
it)
-1
0
1
2
30 20 40 60 80 100 120 140
Num
ber
of e
vent
s / 4
GeV
1
10
210 +e < 0.0
eη -0.5 <
CMS Preliminary = 5.02 TeV NNs pPb
-1 L=34.6 nb
Dataν+ e→ +W
ee→Z ν+τ → +W
QCDFit
[GeV]TE0 20 40 60 80 100 120 140
N(D
ata)
/N(F
it)
-1
0
1
2
3
Electron and muon channels.
pT > 25 GeV, |ηµ| < 2.4, |ηe | < 2.5, no /ET cut.
Requiring isolated lepton (to reject the HF and jet backgrounds)
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 7 / 16
CMS-HIN-13-007
W → `ν in pPb
Cross section
labη
-2 -1 0 1 2
[nb]
lab
η)
/ dν+ l
→+ (
Wσd
0
20
40
60
80
100
120
140
CMS Preliminary
= 5.02 TeVNNspPb -1L = 34.6 nb
ν + +µ → +W
ν + + e→ +W
Luminosity uncertainty: 3.5%
> 25 GeV/clT
p
ν + + l→ +W
labη
-2 -1 0 1 2 [n
b]la
bη
) / d
ν- l→-
(W
σd0
20
40
60
80
100
120
140
CMS Preliminary
= 5.02 TeVNNspPb -1L = 34.6 nb
ν + -µ → -W
ν + - e→ -W
Luminosity uncertainty: 3.5%
> 25 GeV/clT
p
ν + -
l→ -W
Good agreement between the electron and muon channels.
Combine the two channels for a better precision.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 8 / 16
CMS-HIN-13-007
W → `ν in pPb
Cross section
labη
-2 -1 0 1 2
[nb]
lab
η)
/ dν+ l
→+ (
Wσd
0
20
40
60
80
100
120
140
CMS Preliminary = 5.02 TeVNNspPb
-1L = 34.6 nbDataCT10EPS09
Luminosity uncertainty: 3.5%
> 25 GeV/clT
p
ν + + l→ +W
P(χ2) = 11%CT10 (17%EPS09)
labη
-2 -1 0 1 2 [n
b]la
bη
) / d
ν- l→-
(W
σd
0
20
40
60
80
100
120
140
CMS Preliminary = 5.02 TeVNNspPb
-1L = 34.6 nbDataCT10EPS09
Luminosity uncertainty: 3.5%
> 25 GeV/clT
p
ν + -
l→ -W
P(χ2) = 41% (80%)
NLO theory predictions with or without nuclear effects from EPS091.
Good agreement with prediction.
Poor discrimination between CT10 and CT10+EPS09: build asymmetries.
1Predictions from H. Paukkunen and C. A. Salgado, JHEP 1103 (2011) 071
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 9 / 16
CMS-HIN-13-007
W → `ν in pPb
Charge asymmetry (N+ − N−)/(N+ + N−)
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 10 / 16
CMS-HIN-13-007
W → `ν in pPb
Charge asymmetry (N+ − N−)/(N+ + N−)
labη
-2 -1 0 1 2
)-+
N+
)/(N
--N+
(N
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
CMS Preliminary = 5.02 TeVNNspPb
-1L = 34.6 nbDataCT10EPS09
P(χ2) = 6% (24%)
Deviation at large negative η: different u vs. d quark modification?
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 10 / 16
CMS-HIN-13-007
Charge asymmetry doesn’t seem to bring anything
change in the sea
black: original EPS09red: re-weighted
Monday, September 22, 2014
Pıa Zurita, HQ 2014
W → `ν in pPb
Forward-backward asymmetry N±(+ηlab)/N±(−ηlab)
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 12 / 16
W → `ν in pPb
Forward-backward asymmetry N±(+ηlab)/N±(−ηlab)
labη
0 0.5 1 1.5 2 2.5
)la
bη
(--)/
Nla
bη
(+-N
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4CMS Preliminary
= 5.02 TeVNNspPb -1L = 34.6 nb
DataCT10EPS09
ν + -
l→ -W
P(χ2) = 16% (72%)
labη
0 0.5 1 1.5 2 2.5)
lab
η(-+
)/N
lab
η(++
N
1
1.5
2
2.5
3 CMS Preliminary = 5.02 TeVNNspPb
-1L = 34.6 nbDataCT10EPS09
ν + + l→ +W
P(χ2) = 19% (13%)
F/B asymmetries are more sensitive to nuclear modifications.
Negative leptons favor EPS09.
Unclear conclusion for positive leptons.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 13 / 16
CMS-HIN-13-007
W → `ν in pPb
A sensitive but difficult asymmetry
A2
A2 =N+(+η)− N+(−η)
N−(+η)− N−(−η)
-1.5
-1.0
-0.5
0.0
0.5
1.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
No nuclear effectsEPS09 nuclear effects
yR
Another Asymmetry, pPb at = 8.8TeV
R
R= (W+,yR, M=MW) - (W+,-yR, M=MW)(W-,yR, M=MW) - (W-,-yR, M=MW)
R ( +,yR, pT=MW/2) - ( +,-yR ,pT=MW/2)( -,yR, pT=MW/2) - ( -,-yR, pT=MW/2)
√s
Proposed by Hannu and Carlos (JHEP 1103 (2011) 071).
Sensitive to nuclear effects.
Potentially small number at the denominator: very non Gaussian statisticalbehavior, difficult to make sense of the measured value.
Propose the forward-backward asymmetry for combined charges:N+(+η)+N−(+η)
N+(−η)+N−(−η), with similar sensitivity but better statistical behavior.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 14 / 16
W → `ν in pPb
A sensitive but difficult asymmetry
A2
A2 =N+(+η)− N+(−η)
N−(+η)− N−(−η)
2A-60 -40 -20 0 20 40 60
Num
ber
of p
seud
o-ex
perim
ents
1
10
210
310
410
Pseudo-experiments
68% confidence
95% confidence
Proposed by Hannu and Carlos (JHEP 1103 (2011) 071).
Sensitive to nuclear effects.
Potentially small number at the denominator: very non Gaussian statisticalbehavior, difficult to make sense of the measured value.
Propose the forward-backward asymmetry for combined charges:N+(+η)+N−(+η)
N+(−η)+N−(−η), with similar sensitivity but better statistical behavior.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 14 / 16
W → `ν in pPb
Forward-backward asymmetry N(+ηlab)/N(−ηlab)
labη
0 0.5 1 1.5 2 2.5
)la
bη
)/N
(-la
bη
N(+
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8CMS Preliminary
= 5.02 TeVNNspPb -1L = 34.6 nb
DataCT10EPS09
ν l + →W
P(χ2) = 17% (31%)
Disfavoring the absence of nuclear modifications of PDFs.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 15 / 16
CMS-HIN-13-007
Conclusion
Sensitivity to nuclear modifications of the PDFs.What additional constrains do this data bring to current nPDF sets?
Some tension between data and theory in the charge asymmetry (differentu and d PDF modifications?).
How easy / hard is it for nPDF to accomodate for this?
Additional material
Systematics (pPb)
Sources W → µν (%) W → eν (%)
EWK background normalization 1.1− 2.0 1.1− 2.0QCD background template 0.1− 2.0 0.5− 3.8Data / MC efficiencies 2.2− 7.5 3.4− 12.7Electron energy scale − 0.1− 2.0Pile-up rejection filter 0.5 0.5Luminosity 3.5 3.5
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 2 / 7
W → µν in PbPb
W → µν: event display
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 3 / 7
W → µν in PbPb
W → µν: event kinematics
[GeV/c]T
p0 20 40 60 80 100 120
Num
ber
of E
vent
s / 1
GeV
/c
-110
1
10
210
310
410
510 CMS = 2.76 TeVNNsPbPb
-1bµ = 7.3 intL
| < 2.1µη|
(a)
Data
νµ → W →Pythia pp
Background
νµ →Fit: Background + W
Centrality [%]100 80 60 40 20 0
[G
eV/c
]⟩
Tp ⟨
0
10
20
30
40
50
60
70CMS
= 2.76 TeVNNsPbPb
-1bµ = 7.3 int L
(b)
PbPb data (muon triggers)
| < 2.1)µη > 25 GeV/c, |µ
TPbPb data (p
PYTHIA + HYDJET
Signal visible in pT spectrum ofgood quality muons.
Background concentrated at lowpT .
Missing momentum from tracks.
Signal: no dependence of /pTwith
centrality.
Balanced energy in backgroundevents.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 4 / 7
Phys. Lett. B 715 (2012) 66
W → µν in PbPb
W → µν: transverse mass
Selection of events
One good quality muon (|ηµ| < 2.1, pµT > 25 GeV/c) and significant
momentum imbalance (/pT> 20 GeV/c).
] 2 [GeV/cTm0 50 100 150 200 250
2N
umbe
r of
Eve
nts/
8 G
eV/c
0
20
40
60
80
100
120
140
160 CMS
-1bµ (PbPb) = 7.3 int L-1 (pp) = 231 nbint L
> 25 GeV/cµT
p
| < 2.1µη|
(c)
> 20 GeV/cT
p
= 2.76 TeV )spp data (
= 2.76 TeV )NN
sPbPb data (
PYTHIA + HYDJET
mT =√
2pµT /pT (1− cosφ)
Almost background-freesample.
Good agreement betweenPbPb data and simulation.
Consistent shapes between ppand PbPb data.
Sightly worse resolution inPbPb.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 5 / 7
Phys. Lett. B 715 (2012) 66
W → µν in PbPb
W → µν: nuclear modification factor
RAA(W ) = 1.04± 0.07± 0.12
RAA(W+) = 0.82± 0.07± 0.09
RAA(W−) = 1.46± 0.14± 0.16
partN0 50 100 150 200 250 300 350
[pb]
AA
/ Tη∆/
WN
0
100
200
300
400
500
600
700
800
900
1000
[50-100]% [30-50]% [20-30]% [10-20]% [0-10]%
[0-100]%
CMS = 2.76 TeVs
-1bµL (PbPb) = 7.3
-1L (pp) = 231 nb
pp
>25 GeV/cµT
p|<2.1µη|
W+W-W
MB PbPb pp
No centrality dependence.
Expect ∼ 2 times more W+
than W− in pp (isospin).
More balanced production inPbPb (mixture of protons andneutrons).
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 6 / 7
Phys. Lett. B 715 (2012) 66
W → µν in PbPb
W → µν: charge asymmetry
Charge asymmetry =dN(W+)− dN(W−)
dN(W+) + dN(W−)
|µη|0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Cha
rge
Asy
mm
etry
-0.4
-0.2
0
0.2
0.4
0.6
0.8CMS -1bµ (PbPb) = 7.3 int L
-1 (pp) = 231 nbint L > 25 GeV/cµ
Tp
| < 2.1µη|
= 2.76 TeV )spp data (
= 2.76 TeV )NNsPbPb data (
MCFM+MSTW08NNLO (pp)
MCFM+MSTW08NNLO+EPS09 (PbPb)
pp: small dependence on η,asymmetry ∼ 1/3.
PbPb: asymmetry closer to 0,larger dependence with η.
E. Chapon LLR W bosons in pPb collisions in the CMS experiment GDR-PH-QCD WG3 7 / 7
Phys. Lett. B 715 (2012) 66
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