forward physics in d+au collisions at phenix: cold nuclear matter probed with j/ production and...
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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