yen-jie lee (mit) for the cms collaboration 3rd heavy ion jet workshop, lisbon 09 july, 2014

13rd Heavy Ion Jet WorkshopYen-Jie Lee (MIT)

Yen-Jie Lee (MIT)

For the CMS collaboration

3rd Heavy Ion Jet Workshop, Lisbon

09 July, 2014

nPDF sensitive observables in CMSnPDF sensitive observables in CMS


Introduction

J.Phys. G39 (2012) 015010

W/Z bosons

LHC energies allow measurements of photons, W, Z bosons and high pT jets in heavy ion collisions

Jets and photons: sensitive to quark and gluon PDF

W and Z bosons: sensitive to (anti-)quark PDF


Nuclear Parton Distribution Function

Gluon nPDF/PDF comparison between EPS09, EKS98, nDS and FGS10

PDF

nPDFR

François Arleo and Jean-Philippe Guillet http://lapth.cnrs.fr/npdfgenerator/

p Pb

PDF nPDF


Expected modification in PbPb collisions• Good theoretical control on high pT photons, Z and W bosons• Expected modification of high pT photon and Z production at the LHC energy

due to nPDF: 0-20% level

ArXiv:1103.1471PbPb 5.5 TeV

ArXiv:1010.5392Photons

Z bosons


W and Z boson production in PbPb

PLB 715 (2012) 66CMS-PAS-HIN-13-004

~10%precision

Z

W

Confirmed binary scaling in PbPb collisions


Photon and Z boson RAA in PbPbPhoton RAA Z boson RAA vs. yZ

Results are consistent with theoretical expectation Limited by statistical and systematical uncertainties

PLB 710 (2012) 256

CMS-PAS-HIN-13-004


Electroweak probes in pPb collisions

Asymmetric system:

Motivates measurements of forward-backward asymmetry and rapidity distributions

J.Phys. G39 (2012) 015010


Z bosons in pPb @ 5.02 TeV



Dimuon invariant mass in [60, 120] GeV/c2

Muon selection– p

Tμ > 20 GeV/c

– |ημlab

| < 2.4

(muon detector coverage) Asymmetric acceptance in

rapidity in the center-of-mass frame due to the boost

2183 Z candidatesCMS-PAS-HIN-14-003

pPb→Z→μμ



dσ/dy shifted to center-of-mass frame Uncertainties dominated by statistical uncertainties Consistent with scaled pp predictions CMS-PAS-HIN-14-003



Nuclear effects expected in the forward and backward regions CMS-PAS-HIN-14-003



Forward-backward ratio expected

to be more sensitive to nuclear

effects

Without nuclear effects should be 1

Hint of nuclear effects visible

Large uncertainties both on data

and nuclear PDF central

predictionsCMS-PAS-HIN-14-003



Compared to POWHEG+PYTHIA

– Agreement seen in high pT

CMS-PAS-HIN-14-003

High pT





– Deviations seen at low pT, consistent with 7 & 8 TeV pp results

CMS-PAS-HIN-14-003

High pTpT=0 Low pT



CMS-PAS-HIN-14-003



– Deviations seen at low pT, consistent with 7 & 8 TeV pp results

– Compared to PYTHIA6 Z2

– Better agreement at low pT

(similar to 7 & 8 TeV pp results)

High pTpT=0 Low pT


W bosons in pPb @ 5.02 TeV

W → eν

W → μν

~20 000 candidatesin both electron and muon channels

More difficult to reconstruct but ten times more statistics than Z



Event selection

– pT

μ / e > 25 GeV/c

– |η μ (e)lab

| < 2.4 (2.5)

– Quality and isolation requirements

– Veto on dilepton events

Fitting the missing transverse energy

for both charges and every η bin

Templates for QCD from data-based

method and electroweak (Z→μμ /

ee, W→τν) from simulation

CMS-PAS-HIN-13-007



Combined cross section compared to NLO theory predictions with and without nuclear effects from EPS09

New data to constrain the nPDFsPredictions from H. Paukkunen, C. A. Salgado, JHEP 1103 (2011) 071 CMS-PAS-HIN-13-007


W bosons in pPb @ 5.02 TeV Luminosity (experimental) and scale (theory)

uncertain-ties cancel in asymmetries Charge asymmetry

Sensitive probe of the up to down quark PDF ratio

Isospin symmetry (Ru =

Rd) assumed in nPDF fits

Assuming Rd < R

u would

decrease the charge asymmetry in negative η

Pb fragmentation p fragmentation

CMS-PAS-HIN-13-007


W bosons in pPb @ 5.02 TeV Forward-backward asymmetry More sensitive to nuclear modifications Different behavior of different charges

→hint of Ru ≠ R

d ? CMS-PAS-HIN-13-007



Forward-backward

asymmetry of the charge-

summed W bosons

Good sensitivity for nuclear

modification of parton

distributions

Probes the shadowing over

anti-shadowing and/or EMC

effect

χ2 probabilities of 10% with

CT10 and 27% with

including EPS09 CMS-PAS-HIN-13-007


Charged particles and jets in pPb collisions

Expected size of the effects ~ 0-20%

~10%

~20%

JHEP 1310 (2013) 213

Int.J.Mod.Phys. E22 (2013) 1330007


Charged particle RpPb compared to EPS09

• Measured RpPb

for charged particles in |ηCM

|<1 compared to

y=0 EPS09 prediction for π0

Enhancement of high pT charged particles with respect to the interpolated pp reference can not be explained by nPDF

CMS-PAS-HIN-12-017


Charged particle Yasym in the CM frame in pPb

• Yield asymmetries in the CM frame: Pb-going/p-going side

• Low-pT yields are significantly larger on the Pb-going side

high-pT yields are similar (for DS events)

Measurement without pp reference

CMS-PAS-HIN-12-017



CMS-PAS-HIN-12-017 Int.J.Mod.Phys. E22 (2013) 1330007


Nuclear Modification Factor (Jets)

CMS-PAS-HIN-14-001


Jet RpPb and forward backward ratio

Sensitivity limited by systematical uncertainty

Consistent with no modification

CMS-PAS-HIN-14-001


Dijet pseudorapidity distribution in pPb collisionsJet quenching from dijet analysis in pPb: dijet asymmetry <2%

No significant jet quenching seen in γ-jet analysis in pPbNo sizable deviation from unity found in jet RpPb

Jets for nPDF studies(pp PDF)

Anti-shadowingAnti-shadowing

ShadowingShadowing

EMCEMC

Anti-shadowingAnti-shadowing

ShadowingShadowing

EMCEMC

Boosted PYTHIA6 Z2@ 5.02 TeV

= (η1+η2)/2

ArXiv 1401.4433


Compare to CT10 and CT10+EPS09ArXiv 1401.4433

Hannu’s QM14 talk


Dijet η vs. forward calorimeter energy in pPb

The large modification can not be explained by nPDF“Color fluctuation”? “Fluctuating proton size”? “Energy-momentum conservation”?

= (η1+η2)/2

p p+π p+2π

…?

ArXiv 1401.4433

(See Doga’s talk)


Summary

• Confirmed Binary scaling in pPb and PbPb collisions

• Z and W production: modification of quark parton

distribution function

• Inclusive charged particle RpPb:

– Enhancement at high pT to be understood

• Inclusive jet measurement RpPb:

– No indication of jet quenching

– Consistent with no modification within large uncertainty

• Dijet eta: consistent with EPS09, suggest gluon EMC effect


Backup slides


Electroweak boson and dijet production in pPb collisions

Forward/Backward

(in lab)

Forward/Backward

(in CM) Z boson W boson

nPDF (pPb)

PDF (pp)

PDF (pp)

Dijet η

nPDF(pPb)

PDF (pp)

Measurement of W / Z / dijet rapidity distributions sensitive to nPDF

nPDF (pPb)


Electroweak boson and dijet production in pPb collisions

Forward/Backward

(in lab)

Forward/Backward

(in CM)Z boson W boson

pPb data: consistent NLO with nPDF, inconsistent NLO with pp PDF

nPDF (pPb)

PDF (pp)

PDF (pp)

Dijet η

nPDF(pPb)

PDF (pp)

Measurement of W / Z / dijet rapidity distributions sensitive to nPDF

nPDF (pPb)

PDF (pp)

ηdijet= (η1+η2)/2


Photon-jet: fraction of photons with a jet partner

pPb PbPb 30-100% PbPb 0-30%

No significant modification in pPb collisionsIsolated photons lost the away-side jet partner in central (head-on) PbPb collisions

Jet pT>30 GeVJet |η|<1.6

?

Fra

ctio

n

Photon pT Photon pT Photon pT



Inclusive Z production cross section in pPb collisions Compared to NLO POWHEG calculation scaled by

A=208

σ(pPb→Z→μμ)Measured

σ ± stat. ± syst. ± lumi.σNLO POWHEG × A

Full phase space 134.4 ± 2.9 ± 7.1 ± 4.7 nb 134 ± 7 nb

-2.5 < yc.m.

< 1.5 94.1 ± 2.1 ± 2.4 ± 3.3 nb 94.0 ± 4.7 nb

Restricted rapidity range to reduce uncertainties on the acceptance correction

Results consistent with σpp × A



Cross section as a function of η in laboratory frame Electron and muon results agree → combination is

performed CMS-PAS-HIN-13-007


7TeV


pPb and PbPb collisions:

Electroweak probes: Isolated photons, W and Z bosons

pPb collisions:

Charged particle RpPb

Jet RpPb

Dijet


Introduction LHC energies allow first measurements of Z and W bosons in

heavy ion collisions

Electroweak bosons are essentially not perturbed by the QCD medium → observed binary scaling in PbPb collisions

Constrain nuclear PDFs with pPb data– CMS-PAS-HIN-14-003→ Z in pPb 2013– CMS-PAS-HIN-13-007→ W in pPb 2013

new

PLB 715 (2012) 66CMS-PAS-HIN-13-004

~10%precision

Z

W


Nuclear parton distribution functions (nPDFs)

Nuclear PDF parametrizations

Z and W bosons are

sensitive to (valence) quark

and (sea) antiquark content

of the nucleus

Dominant processes (LO)

J.Phys. G39 (2012) 015010


Isolated high pT photons

Solution: measurement of the isolated photons Decay photons from hadrons in jets such as π0, η γ γ are largely suppressed UE subtracted isolation variables are developed

γ

Isolated Isolated Non-isolated

LO NLO

γ

γ

same object to the detector

Isolated Non-isolated

yen-jie lee (mit) for the cms collaboration 3rd heavy ion jet workshop, lisbon 09 july, 2014

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