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Probing the Low-x Structure of the Nucleus with the PHENIX Detector
Mickey Chiu
INT, Seattle, 20 October 2011
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PRL 107.172301
PRL 107.142301
PLB 679 (2009) 321-329
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2
3
1. Di-hadron correlations in d+Au2. J/ Production in d+Au3. UPC (diffractive) J/ in Au+Au
Low-x nucleon/nuclear structure is a very difficult business! We’ll want to test it with as many probes as we can.
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Forward di-Hadron Production in d+Au
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PHENIX Muon Piston Calorimeter
Small cylindrical holes in Muon Magnet Pistons, Radius 22.5 cm and Depth 43.1 cm
SOUTH
PbWO4
North
)( 43 yyTd ee
s
px )( 43 yyT
Au ees
px
•Fwd-Fwd, x~(0.001,0.005)•Mid-Fwd, x~(0.008,0.040)•Mid-Bwd, x~(0.050,0.100)
d(forward) Au(backward)
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MPC Performance
North MPC
Decay photon impact positions for low and high energy 0s. The decay photons from high energy 0s merge into a single cluster
Sometimes use (EM) clusters, but always corrected to 0 energy
Clusters 80% 0 (PYTHIA)
“Trigger”Near
Far
Jet1
Jet2
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•Large suppression in RdA•That increases with centrality•And increases with larger rapidity
•Consistent with previous measurements•However, x covered by single inclusive measurement is over wide range
•Includes shadowing, anti-shadowing, (EMC effect)
RdAu in 2 forward rapidity Bins
Guzey, Strikman, Vogelsang, PLB603, 173
Guzey, Strikman, Vogelsang, PL B603, 173
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CNM effects: dynamical shadowing, Energy Loss, Cronin
RdA Past, di-Hadron Future
Kharzeev, NPA 748, 727 (2005)
•Di-Hadron Correlations allow one to select out the di-jet from the underlying event•Constrains x range (probe one region at a time)•Probe predicted angular decorrelation of di-jets (width broadening)
Kharzeev, Levin, McLerran Nucl. Phys. A748 (2005) 627
Color Glass Condensate
(Qiu, Vitev PLB632:507,2006)
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di-Hadron Signal
Peripheral d+Au Correlation Function
d
dN
NN
NCY
assoc
trigassoctrig
pair 1
CORRELATED
Npair
“Di-Hadron Nuclear Modification factor”
trigdA
trigdAdA RIJ
• Possible indicators of nuclear effects
• JdA < 1, RdA < 1
• Angular decorrelation of widths
pppairpp
dApairdA
colldA N
J
/
/1
ppsglpp
dAsgldA
colldA N
R
/
/1
“Sgl-Hadron Nuclear Modification factor”
“Conditional Yield”
Number of di-jet particle pairs per trigger particle after corrections for efficiencies, combinatoric background, and subtracting off pedestal
Caveats:
1. Low pT (but back-to-back peak is selected)
2. Pedestal Determination (Assumed up to twice the width as a systematic).
3. Di-Hadrons instead of di-jets (but ok if fragmentation unmodified)
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0 (trigger,central)/0 (associate,forward)
<pTa>=0.55 GeV/c
p+p
Cor
rela
tion
Fu
nct
ion
d+Au 0-20%
d+Au 60-88%
<pTa>=0.77 GeV/c <pT
a>=1.00 GeV/c3.0 < pTt < 5.0 GeV/c
for all plots
pTt, 0
pTa, 0 PHENIX Preliminary
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Correlation Widths, d+Au and p+p
• No significant broadening between p+p and d+Au within large experimental uncertainties
Trigger p0: |h| < 0.35, 2.0 < pT < 3.0 GeV Trigger p0: |h| < 0.35, 3.0 < pT < 5.0 GeV
dAu 0-20%
ppdAu 40-88%
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•Widths are consistent between p+p and d+Au (all centralities) within large statistical and systematic errors•No broadening seen (within errors)
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JdA vs Ncoll, pTmid, pT
fwd
MPC 0 pT
•Suppression of di-hadron correlation (relative to p+p binary scaling hypothesis) with
Increasing Centrality
Decreasing pTmid
Decreasing pTfwd
pTt, 0
pTa, 0
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Fwd-Fwd: p+p vs d+Au Peripheral
Peripheral d+Au collisions are similar to p+p collisions
Beam view of d+Au peripheral collisionpT
t, 0
pTa, 0
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Fwd-Fwd: p+p vs d+Au Central
“Monojet” in central d+Au collisions
Beam view of d+Au peripheral collisionpT
t, 0
pTa, 0
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JdA for Fwd-Fwd
fragAuTTfrag
Au zxs
epepx
21
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MPC 0 pT
(assuming LO)•Better way to plot:
•Suppression of JdA gets larger in fwd-fwd correlations•Trend with pT, centrality also consistent with mid-fwd correlations
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xAufrag Dependence
s
epepx TTfrag
Au
21
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Note: points for mid-fwd JdA are offset for visual clarityStatistical and systematic errors are added in quadrature
•Plotting vs suggests that the effect is due to something happening in the nucleus as one probes to lower x•Does it prove CGC?
•Shadowing? Initial state energy loss? Multi-Parton Interactions (MPI)?
60-88%(Peripheral)
0-20%(Central)
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d
cdab
pbpp
ap
d
cdab
Aub
Auda
d
pppairsppcoll
dAupairsdAu
dA
zzDxfxf
zzDxfxf
NJ
c
c
,)()(
,)()(
/
/
Extending the LO picture
b=0-100%Q2 = 4 GeV2
xAu
EPS09 NLO gluonsEskola , Paukkunen, Salgado, JHP04 (2009)065
),(
),(),(
2
22
QxAxG
QxxGQxR
p
AuAuG
Low x, mostly gluons JdA ~ RGAuHigh x, mostly quarks
Weak effects expected
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Where is the Saturation Scale if we are actually seeing the CGC?
Iancu and Venugopalan, hep-ph/0303204
Extended scaling?
H. Kowalski and D. Teaney. Phys. Rev.D, 68:114005, 2003
•We evaluated in PYTHIA the ~ coverage for Q2 and x for the fwd-fwd and cnt-fwd correlations
•No nuclear modifications evaluated yet•Not clear that we are in the saturation region – possibly in extended region?
•Can we explore Qs from the data?•Nuclear Scaling: Look at impact parameter dependence by varying centrality
Fwd-Cnt?
Fwd-Fwd?
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d+Au MC Glauber
d Au
bnucleon
bnucleon
•From Glauber Monte Carlo we can determine the number of nucleons in the path of each nucleon in the deuteron
Centrality0-20%
20-40%40-60%60-88%
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JdA Centrality Dependence
•Fit using EPS09 parametric function:•Evaluate JdA at xfrag = 6x10-4, 6x10-3, 1.5x10-2
])[()( 210axxA
g eexaaaxR
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Can we determine Qs?
)(~
~
xG
RAu
Aug
L~
0
3/122
2 ~),(1
x
xAQxxG
RQ sS
•If we are measuring gluons w/ JdA, then we can perhaps extract length and x dep of Qs, as well as possibly extracting the value of Qs at RHIC????•Eg, are we seeing an approx linear dependence on length????
xfrag ~ 1.5x10-2
xfrag ~ 6x10-3
xfrag ~ 6x10-4
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J/ Production in d+Au
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Traditional shadowing from fits to DIS or from coherence models
high xlow x
D
Dccmoversco-
Absorption (or dissociation) of into two D mesons by nucleus or co-movers
cc
Energy loss of incident gluon shifts effective xF and produces nuclear suppression which increases with xF
R(A/p)
R=1 xF
Gluon saturation from non-linear gluon interactions for the high density at small x - Amplified in a nucleus.
pA
What are the CNM effects that are so strong in Quarkonia production?
shadowing
anti-shadowing
RG in A
u
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What are the CNM effects that are so strong in Quarkonia production?
Reasonable agreement with EPS09 nPDF + br=4 mb for central collisions but not peripheral
EPS09 with linear thickness dependence fails to describe centrality dependence of forward rapidity region.
J/ψ in d+Au – learning about CNM thickness dependence
PHENIXarXiv:1010.1246v1
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Quarkonia Suppression in A+A Collisions – key observations and questions
Overall suppression of J/ψ is very similar between:• SPS (17.2 GeV) • RHIC (200,62,39 GeV)• and LHC (2.76 TeV)
62 GeV 39 GeV
SPS
PHENIX forward
PHENIX y=0
CMS:0 <|y|< 2.4pT > 6.5
(more on LHC in a minute)Npart
Npart
NpartNpart
RA
A
RA
A
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CMS pT > 6.5 GeV/c
• LHC suppressed more than RHIC at y~0(but CMS is pT> 6.5 GeV/c)• LHC suppressed less than RHIC at forward y(here ALICE is pT> 0)Features expected from regeneration, which is concentrated at small pT
High-pT suppressed more than low pT
(but ALICE y~3; ATLAS y~0)
However suppression roughly flat with rapidity for pT>6.5
So may also be consistent with regeneration at small pT
Missing LHC data – y~0, pT> 0 RAA?(where regeneration may be rather large)
Quarkonia Suppression in A+A Collisions – comparing RHIC & LHC
Mid Rapidity
Forward Rapidity
all pT
Npart
y R
cp
CMS
ALICE
ATLAS
caution caution
ALICE, all pT
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Vary the strength of suppression (a) & see what relationship between RdAu and RCP is given strictly by Glauber geometry for different dependences on density-weighted thickness
0
1( ) ( , )T Tr dz z r
Woods-
Saxon
• Break-up has exponential dependence• EPS09 & initial-state dE/dx have unknown dependences
What are the CNM effects that are so strong in Quarkonia production?
J/ψ in d+Au – learning about CNM thickness dependence
The forward rapidity points suggests a quadratic or higher geometrical dependence
)()( :lExponentia TraT erM
)(1)( :Linear TT rarM 2)(1)( :Quadratic TT rarM PHENIX
arXiv:1010.1246v1
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Rapidity or x Coverage
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Does di-hadron data match J/Psi?
•Comparison not so bad, considering many other uncertainties (production model, energy loss, breakup cross-section). Also J/ is generally at higher Q2
•Real way to do this is to try to extract G(x) from di-hadron data, and then predict J/
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Ultraperipheral J/
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Hadronic Interaction:Au-Au --> X ~7 barns
-:AuAu --> AuAu + e+e- ~33 kbarnsAuAu --> AuAu + 2(e+e-) ~680 barnsAuAu --> AuAu + 3(e+e-) ~50 barns
-N: L(-N )=1029 cm-2s-1 2<E<300GeVAuAu --> Au+Au* 92 barns X+neutronsAuAu --> Au*+Au* 3.670.26 barns X+neutrons Y+neutrons
“Hadronic” Collider Processes•You’re probably familiar with the “Hadronic Interactions”•But there are a lot more processes going on at a hadron collider
Hadronic Interaction
Ultra-Peripheral Interaction
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Au J/ Au* measurement in PHENIX
e+
e
• UPC dedicated trigger– Rapidity gap 3<||<4
MB interaction veto (BBC veto)– Large probability to exchange
additional photons by GDR 1 or 2 ZDC trigger
– EmCal trigger (E>0.8GeV)– DiMuon Trigger
Au J/ ( l+l-) Au*– DC & PC tracking detectors– RICH & EmCal electron identification
devices– Muon Tracker
n
l+
l-
||<0.35
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J/ cross section vs theoretical calculations• Compatible with coherent
predictions, • With more statistics,
sensitive to the shadowing parameterizations,
[ 1) P.R.L.89 012301 (2002)…][ 2) P.L.B626 (2005) 72 ]
[ 3) arXiv0706.2810 [hep-ph] ][ 4) arXiv:0706.1532 [hep-ph] ]
[ 1) ][ 2) ]
[ 3) ][ 4) ]
d/dy |y=0 = 76 31 (stat) 15 (syst) b
[Filho et al, PRC78 044904 (2008)]
coherent
incoherent
coherent
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Impact Parameter DependenceHorowitz, INT-PUB-11-005 (arxiv:1102.5058) UPC J/ψ pT
2
distribution(Theoretical)
Coherent(γAu): low pt peakIncoherent(γn): wider pt distribution(Incoherent + neutron tagged :Yellow shadow )
Strikman et al, PLB 626 p. 72-79
EIC Workshop, INT, Seattle 2010
•Fourier transform of t distribution can distinguish the density of the nucleus vs b•However, incoherent contribution is a potentially large source of background
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PT dependence of UPC J/ψ+Xn(N)+Xn(S)
•UPC J/ pT (~t1/2) also confirms existence of incoherent contribution•Strategy: measure at forward rapidities to get incoherent, subtract from total to get remainder
•Major challenges: momentum resolution of 3 MeV! (technical driver for EIC detector)•Statistics (EIC is good, RHIC/LHC is poor)
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+, +p, +A “Applications”•Higgs as well as many SUSY ptcls should be produced at the LHC in + and +p (+A)
•High energy photon interactions at the LHC, de Jeneret et al, arXiv:0908.2020•FP220, FP420
•Observation of exclusive charmonium production and gamma+gamma to mu+mu- in p+pbar collisions at sqrt{s} = 1.96 TeV, CDF, PRL102:242001,2009
x10-3 10-2 10-1 10-3 10-2 10-1 10-3 10-2 10-1
EPS09: A New Generation of NLO and LO Nuclear PDF’s, Eskola,Paukannen,Salgado JHEP 0904:065 2009
•Direct measurement of G(x) at from photoproduction (~g2(x))222/ 105.1/ xWmx AJ
•Possibly study dynamics of J/ propagation through nuclear matter•Feature or Bug?
•Test of QED in strong-coupling regime?: =ZEM~0.6
Nucleus-Nucleus Interactions
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+, +p, +A “Applications”•Higgs as well as many SUSY ptcls should be produced at the LHC in + and +p (+A)
•High energy photon interactions at the LHC, de Jeneret et al, arXiv:0908.2020•FP220, FP420
•Observation of exclusive charmonium production and gamma+gamma to mu+mu- in p+pbar collisions at sqrt{s} = 1.96 TeV, CDF, PRL102:242001,2009
x10-3 10-2 10-1 10-3 10-2 10-1 10-3 10-2 10-1
EPS09: A New Generation of NLO and LO Nuclear PDF’s, Eskola,Paukannen,Salgado JHEP 0904:065 2009
•Direct measurement of G(x) at from photoproduction (~g2(x))222/ 105.1/ xWmx AJ
PHENIX UPC J/Psi
•Possibly study dynamics of J/ propagation through nuclear matter•Feature or Bug?
•Test of QED in strong-coupling regime?: =ZEM~0.6
Nucleus-Nucleus Interactions
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+, +p, +A “Applications”•Higgs as well as many SUSY ptcls should be produced at the LHC in + and +p (+A)
•High energy photon interactions at the LHC, de Jeneret et al, arXiv:0908.2020•FP220, FP420
•Observation of exclusive charmonium production and gamma+gamma to mu+mu- in p+pbar collisions at sqrt{s} = 1.96 TeV, CDF, PRL102:242001,2009
x10-3 10-2 10-1 10-3 10-2 10-1 10-3 10-2 10-1
EPS09: A New Generation of NLO and LO Nuclear PDF’s, Eskola,Paukannen,Salgado JHEP 0904:065 2009
•Direct measurement of G(x) at from photoproduction (~g2(x))222/ 105.1/ xWmx AJ
PHENIX UPC J/Psi
•Possibly study dynamics of J/ propagation through nuclear matter•Feature or Bug?
•Test of QED in strong-coupling regime?: =ZEM~0.6
Nucleus-Nucleus Interactions
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Summary•Three Tests of Saturation in PHENIX (or probes of g(x))•FWD-FWD di-hadron yields in d+Au relative to p+p (JdA)
•Suppression depends strongly on centrality•And gets stronger as both particles go toward more forward rapidities•Nuclear Shadowing? We see extreme Shadowing in most central.•Gluon Saturation/Color Glass Condensate?
•If so, we can extract a wealth of information on Qs from our measurements
•Initial State Energy Loss? MPI?•Angular Broadening of Away Side Jet?
•Mid-Fwd, no increase seen within errors•Mid-MidFwd, also no increase •Fwd-Fwd, currently inconclusive
•J/ Production in d+Au not well understood•Forward rapidities not well described – something extra going on?•Highly important to understand CNM effects for HI interpretation
•Ultraperipheral J/ is a third, very different probe of gluon distribution•Can get a ~10% measurement of G(x) at x~10-2
•Statistics and detector challenged for G(x,b) impact parameter dep measurement
Are these enough to see the elephant in the room?
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Backup Slides
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IdA vs JdA: Can we decouple effects?
d
dN
NN
d
dN
N
N
N
N
NN
d
dN
N
d
dN
NI pp
trigassocpp
evtcoll
dAtrigassoc
dAevt
dAevt
dAtrig
ppevt
pptrigcoll
ppassoc
pptrig
dAassoc
dAtrig
dA)|(
)|(
1
1
1
1
dAtrigdA
trigdA J
RI
1
•IdA is the per trigger comparison of d+Au jet associated counts relative to p+p•JdA is the rate of the associated pairs from a jet (per minbias event)•Can we use this to tell if the jets are modified, or do they disappear?•From the CNT-MPC corrrelations, we get IdA ~ 0.5, and RdA ~ 1.1
•JdA ~ 0.5•The rate of correlated pairs is about half of p+p
•Does this imply that the missing jets have disappeared, and not that they are modified, since IdA ~ JdA?•But not true for STAR FMS triggered-central barrel, where IdA ~ 1 and JdA ~ 0.5
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JdA, RdA vs Ncoll
MPC 0 pT
Qiu-Vitev Shadowing + Energy Loss
(private communication)
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Muon-Central IdA & Widths, 2003 d+Au
Phys.Rev.Lett.96:222301,2006
dAu
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Nuclear Modification in d+Au at Forward(Backward) Rapidity
Phys. Rev. Lett. 94, 082302 (2005)
Punch through hadrons & Hadron decay muons •Forward η suppression •No backward η suppression
•Gluon Saturation?•Cronin, Shadowing, E-loss?
•Look at 2 particle correlations …
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• Rough comparison with HERA e-p data, if coherent incoherent ratio is 50% - 50%
• HERA (H1 & ZEUS) input
• Result: coh = 1.01 0.07
incoh = 0.92 0.08
~ 1, good agreement with HERA data hard probes scaling
UPC Comparison with HERA data
[ZEUS, Eur.Phys.J. C24 (2002) 345]
[H1, Eur.Phys.J. C46 (2006) 585]