W bosons in pPb collisions in the CMS experiment · 2016-12-09 · W bosons in pPb collisions in...

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.


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_


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−


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)


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)


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


ν + -µ → -W

ν + - e→ -W


> 25 GeV/clT

p

ν + -

l→ -W

Good agreement between the electron and muon channels.

Combine the two channels for a better precision.


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


> 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




> 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


CMS-HIN-13-007

W → `ν in pPb

Charge asymmetry (N+ − N−)/(N+ + N−)


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



P(χ2) = 6% (24%)

Deviation at large negative η: different u vs. d quark modification?


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)


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


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


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.


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.


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.


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


DataCT10EPS09

ν l + →W

P(χ2) = 17% (31%)

Disfavoring the absence of nuclear modifications of PDFs.


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

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


W → µν in PbPb

W → µν: event display


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.


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.


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).


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.


Phys. Lett. B 715 (2012) 66

W bosons in pPb collisions in the CMS experiment · 2016-12-09 · W bosons in pPb collisions in...

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