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Forward Physics in d+Au Collisions at PHENIX:Cold nuclear matter probed with J/ production and pion correlations

Richard Seto for the PHENIX CollaborationUniversity of California, Riverside

Rencontres de Moriond QCD and High Energy InteractionsLa Thuile, March 20-27, 2011

Thanks to my colleagues from whomI have shameless stolen slides – Particularly Matt Wysocki, Oleg EyserAnd Beau Meredith

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Why ask about Cold nuclear matter?

• sQGP – How is it born?▫τthermalization<1 fm but RsQGP~10

fm Explaining uniformity?

Early Universe – inflation

▫What sets initial condition of the sQGP? Pre equilibrium interactions ?

Turbulence Strongly coupled (AdS/CFT) Weakly coupled (pQCD)

What does the initial state look like? Structure functions ?

▫BUT in the nucleus they are altered▫ In particular gluons x < 0.01

suppressed

x

xG(x)

τthermalization< 1 fm10 fm

Cold Nuclear Matter is the initial state of interest*

*also interesting in its own right

Cold Nuclear Matter is the initial state of interest*

*also interesting in its own right

Look at 2 models

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Model 1: gluon PDF and nuclear shadowing

b=0-100%”

br2) For the J/ψ include σ to account

for the breakup of the cc pair while

passing through the nucleus

1) Assume linear dependence on density-weighted longitudinal nuclear thickness impact parameter (centrality) dependence

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2

( , )( , )

( , )Pb A

Gp

xG x QR x Q

AxG x Q

Nuclear PDF proton PDF Fit data on nuclei:SLAC, NMC, EMC DIS+DY+PHENIX midrapidty π0

Large uncertaintyAt lox-x

Lack of data large uncertainly in gluon pdf at low-x

We will add two things:

x

gluons

Eskola , Paukkunen, Salgado, JHP04 (2009)065

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Model 2: The Color Glass Condensate (CGC)

• Saturation of low-x gluons▫ high density

Recombination of gluons, hence suppression @ low-x

▫ Characterized by QS

▫ Nuclear Amplification xGA=A1/3xGp

We can exploit this behavior vs centrality

• Region of validity: low-x (forward rapidity)

00,S S

xQ Q

x

Cartoon

Central

Min-bias

2 20, 0

2 20, 0

Central: =.23 Q 2.5 .01 (Kharzeev, Levin private communication)

Min Bias: =.23 Q 0.9 .02 (Alacete,Marquet Phys.Lett.B687:174-179,2010)

S

S

GeV x

GeV x

5

Str

on

g c

ou

pli

ng

Comments:▫plethora of effects e.g. Coherence, Higher twist effects, Initial

state energy loss•The CGC is a full QCD calculation in a particular limit which

should include all such effects•Worry : CGC is a non-perturbative but weakly

coupled theory and requires αS(QS) to be “small”. Much of the bulk (which makes up the sQGP) may be from regions where αS is large▫Saturation calculation at strong coupling using AdS/CFT

Iancu, NPA(2011) 18. (a conformal theory with lots of other stuff – but αS doesn’t change much at the phase transition...)

Confuses experimentalis

ts

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Lets first look at the J/

• g+g J/ψ dominant @RHIC e+

e-

Central Arms

μ+

μ-μ+

μ-

Aud

• e+e- -0.35<<0.35• μ+μ- 1.2<||<2.4

Nice coverage in y or equivalently x(Au) forward y x~0.005mid y x~0.03backward y x~0.1

forwardmid back

J/ dN/dy vs. rapidity7

Aud

d+Au is scaled by 1/Ncoll

Ncoll=number of binary collisions

Suppression clearly visible

Now divide

p+p

d+Au

200 GeVNNS

arXiv:1010.1246

RdAu for minimum bias collisions

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Significant suppression at mid and forward rapidities.

Now compare to the models..

Bars = point-to-point uncorrelated uncertaintiesBoxes = point-to-point correlated uncertainties

y

Rd

Au(0

-100

%)

1 Yield dAu

Yield ppdAucoll

RN

RdAu for minimum bias collisions

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Compare to Model 1:EPS09 nuclear PDF + sbr = 4 mb (red curves).

sbr is the only free parameter.

Reasonable agreement

Dashed lines are the maximum variation included in EPS09.

Note: EPS09, as published, is averaged over all b and we get decent agreement with RdAu(0-100%).

What about the CGC?

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We can break the data down further by dividing events into small and large impact parameter.

Include gluon saturation at low x (affects forward rapidity) Enhancement from double gluon ____exchange with nucleus at midrapidity

Kharzeev and TuchinNPA 770(2006) 40

RdAu central and peripheral

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Model I: EPS09 nuclear PDF + sbr = 4 mb is now deviating from the peripheral data

We can further reduce systematicsby taking the ratio.

Gluon saturation again matches the forward rapidity points relatively well, but not mid-rapidity

peripheral

central

Now with reduced errors Model I with the nuclear PDF and σbreakup=4mb doesnot match the data

The CGC model works at least in the forward region

RCP

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RCP has the advantage of cancelling most of the systematic uncertainties.

( )

( )dAu

CPdAu

R centralR

R peripheral

•Is there something else we can look at which• might be directly related to the condensate?

peripheral

central

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Pion Correlations• Gluons overlap and make a condensate

▫ Incoming quark interacts with condensate coherently

▫pT balanced by condensate leading to “monoJets”▫Look for single “jets” (actually single particles)

with no correlated “jet” on opposite side

Au nucleus

deuteron

Gluon condensate

“monoJet”

Jet

Jetp

p

PHENIXCentral regionSide View

d Au

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MPC

Central Arms

Log(x2)

2

Pythia simulationπ0 MPC (3.2> >3.8) pT>2.25π pT2>1.75

The MPC (Muon Piston Calorimeters)

p0 orclusters

p0 or h+/-

1) Particle into MPC e.g. π0 MPC (3.2> >3.8)

pT>2.252) Choose 2nd particle with

pT2>1.75 azimuthally opposite

3) plot 2 vs x2

2nd Particle in central arm: x2 ~ .03

2nd Particle in MPC: x2 ~ .001

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Kharzeev, Levin, McLerran NPA 748,627(2006)

(rad)0 π 2π

Correlation functionTwo-particle distributionIncluding two-particle acceptance

The Nuclear Modification Factor

Same side peak

will be missing

Npairs

CGC calculation

Qiu,Vitev PLB 692, 507(2006)

Coherent QCD Multiple scattering

1

2

1.5 GeV

1GeVT

T

p

p

1

2

4

0

y

y

Two sides of the same

coin?

Crucial that we haveModels that canDescribe many Aspects of the data

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(rad)0 π 2π

Correlation functionTwo-particle distributionIncluding two-particle acceptance

Conditional yieldNumber particle pairs per trigger particleIncluding acceptance & efficiency

Nuclear modification factorConditional yield ratio d+A/p+p

Indicators of gluon saturationIdA < 1

effect gets stronger with centrality

The Nuclear Modification Factor

Same side peak

will be missing

Npairs

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Corr

ela

tion

Fu

nct

ion

2.0 < pTt < 3.0

GeV/c

for all plots

Df

pp

<pTa>=2.00 GeV/c

dAu 60-88%

dAu 0-20%

0 2

peripheral to central peripheral to central

Central Arm - MPC Correlations

Consistent with CGC

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Both particles in MPC (work in progress)

• Correlation Functions▫Peripheral events

pp and dAu are same

▫Central events dAu looses correlated

peak

Qualitative agreement with a CGC picture

Quantitative Analysis and a publication forthcoming

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Summary• The data

▫ J/psi Unable to reconcile rapidity and centrality dependence with

Shadowing + naïve breakup cross section CGC hypothesis works at forward rapidity

▫ Pion Correlations Suppression with centrality in central-forward correlations (moderate

x) Suppression with centrality in forward-forward correlations (low-x) in

qualitative agreement with CGC model• Closing thoughts

▫ Regime probed in present heavy experiments need new non-pertubative QCD techniques e.g. CGC, AdS/CFT, hydrodynamic codes to explain the data

▫ We must understand Cold Nuclear Matter - the initial condition for the heavy ion reaction – if we are to understand the sQGP