Transcript Probing the Low-x Structure of the Nucleus with the PHENIX Detector Mickey Chiu INT, Seattle, 20 October 2011

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
1
2
PRL 107.142301
Low-x nucleon/nuclear structure is a
very difficult business! We’ll want to
test it with as many probes as we
can.
1. Di-hadron correlations in d+Au
2. J/ Production in d+Au
3. UPC (diffractive) J/ in Au+Au
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PLB 679 (2009) 321-329
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Forward di-Hadron Production in d+Au
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PHENIX Muon Piston Calorimeter
xd 
pT y3
p
(e  e y4 ) x Au  T (e  y3  e  y4 )
s
s
d(forward)
Au(backward)
SOUTH
North
•Fwd-Fwd, x~(0.001,0.005)
•Mid-Fwd, x~(0.008,0.040)
•Mid-Bwd, x~(0.050,0.100)
PbWO4
Small cylindrical holes in Muon Magnet Pistons, Radius 22.5 cm and Depth 43.1 cm
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4
MPC Performance
North MPC
“Trigger”
Near
Far
Jet1
Jet2
Decay photon impact positions for
low and high energy p0s. The
decay photons from high energy
p0s merge into a single cluster
Sometimes use (EM) clusters,
but always corrected to p0 energy
Clusters  80% p0 (PYTHIA)
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RdAu in 2 forward rapidity Bins
Guzey, Strikman, Vogelsang, PL B603, 173
•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)
Guzey, Strikman, Vogelsang, PLB603, 173
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RdA Past, di-Hadron Future
Color Glass Condensate
Kharzeev, NPA 748, 727 (2005)
CNM effects: dynamical shadowing,
Energy Loss, Cronin
(Qiu, Vitev PLB632:507,2006)
Kharzeev, Levin, McLerran
Nucl. Phys. A748 (2005) 627
•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)
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di-Hadron Signal
Peripheral d+Au
Correlation Function
“Conditional Yield”
CY 
N pair
Ntrig assoc
1 dN assoc

Ntrig dDf
Number of di-jet particle pairs per trigger
particle after corrections for efficiencies, CORRELATED
combinatoric background, and subtracting
Npair
off pedestal
“Di-Hadron Nuclear
Modification factor”
J dA
trig
trig
J dA  I dA
 RdA
pair
/  dA
1  dA

pair
N coll  pp
/  pp
RdA
Df
“Sgl-Hadron Nuclear
Modification factor”
sgl
/  dA
1  dA

sgl
N coll  pp
/  pp
• Possible indicators of nuclear effects
Caveats:
JdA <
RdA <
1
1.• Low
pT1, (but
back-to-back
peak is selected)
Angular decorrelation
widths up to twice the width as a systematic).
2.• Pedestal
Determinationof(Assumed
3. Di-Hadrons instead of di-jets (but ok if fragmentation unmodified)
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p0 (trigger,central)/p0 (associate,forward)
t
3.0 < pT < 5.0 GeV/c
<pTa>=0.55 GeV/c
<pTa>=0.77 GeV/c
<pTa>=1.00 GeV/c
for all plots
d+Au 60-88%
d+Au 0-20%
Correlation Function
p+p
pTt, p0
pTa, p0
Df
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%
pp
dAu 40-88%
•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, pTfwd
MPC p0 pT
•Suppression of di-hadron
correlation (relative to p+p
binary scaling hypothesis)
with
Increasing Centrality
Decreasing pTmid
Decreasing pTfwd
pTt, p0
pTa, p0
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Fwd-Fwd: p+p vs d+Au Peripheral
Peripheral d+Au collisions
are similar to p+p collisions
t,
pT
pTa, p0
p0
Beam view of d+Au
peripheral collision
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Fwd-Fwd: p+p vs d+Au Central
“Monojet” in central d+Au
collisions
t,
pT
pTa, p0
p0
Beam view of d+Au
peripheral collision
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MPC p0 pT
JdA for Fwd-Fwd
•Suppression of JdA gets larger in fwd-fwd correlations
•Trend with pT, centrality also consistent with mid-fwd correlations
frag
•Better way to plot: x Au

pT 1 e
 1
 pT 2 e
s
 2
 x Au z frag
(assuming LO)
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xAufrag Dependence
60-88%
(Peripheral)
0-20%
(Central)
pT 1 e
 1
 pT 2 e
 2
frag
x

Au
•Plotting vs
suggests that the effect is due to
s
something happening in the nucleus as one probes to lower x
•Does it prove CGC?
•Shadowing? Initial state energy loss? Multi-Parton Interactions (MPI)?
Note: points for mid-fwd JdA are offset for visual clarity
Statistical and systematic errors are added in quadrature
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Extending the LO picture
xGAu ( x, Q 2 )
R ( x, Q ) 
AxGp ( x, Q 2 )
Au
G
2
Eskola , Paukkunen, Salgado, JHP04 (2009)065
EPS09 NLO gluons
b=0-100%
Q2 = 4 GeV2
xAu
 ab cd
pairs
b
 dAu
/  dAu
f da ( xd )  f Au
( x Au )  
 D z , z d 
J dA 

pairs
 ab cd
N coll  pp /  pp
f pa ( x p )  f pb ( x p )  
 D z , z d 
c
c
High x, mostly quarks
Weak effects expected
Low x, mostly gluons
 JdA ~ RGAu
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Where is the Saturation Scale if we are actually seeing the CGC?
Extended scaling?
Fwd-Cnt?
Fwd-Fwd?
H. Kowalski and D. Teaney. Phys. Rev.D, 68:114005, 2003
Iancu and Venugopalan, hep-ph/0303204
•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
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d+Au MC Glauber
d
Au
bnucleon
bnucleon
Centrality
0-20%
20-40%
40-60%
60-88%
•From Glauber Monte Carlo we can
determine the number of nucleons in the
path of each nucleon in the deuteron
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JdA Centrality Dependence
•Fit using EPS09 parametric function: Rg ( x)  a0  (a1  a2 x)[e
•Evaluate JdA at xfrag = 6x10-4, 6x10-3, 1.5x10-2
A
x
 e xa ]
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Can we determine Qs?
~ RgAu
xfrag ~ 1.5x10-2
Au
~ G ( x)
xfrag ~ 6x10-3
xfrag ~ 6x10-4
1
2
1/ 3  x 
Q  s
xG( x, Q ) ~ A  
2
pR
 x0 

~L
2
S
•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????
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J/ Production in d+Au
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What are the CNM effects that are so strong in Quarkonia production?
RG in Au
Traditional shadowing from fits to
DIS or from coherence models
anti-shadowing
D
co-movers
D
shadowing
Absorption (or dissociation) of cc
into two D mesons by nucleus or comovers
Gluon saturation from non-linear gluon
interactions for the high density at
small x - Amplified in a nucleus.
low x
cc
high x
A
Energy loss of incident
gluon shifts effective xF
and produces nuclear
suppression which
R=1
increases with xF
p
R(A/p)
xF
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What are the CNM effects that are so strong in Quarkonia production?
J/ψ in d+Au – learning about CNM thickness dependence
PHENIX
arXiv:1010.1246v1
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.
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SPS
Overall suppression of J/ψ is very
similar between:
• SPS (17.2 GeV)
• RHIC (200,62,39 GeV)
• and LHC (2.76 TeV)
RAA
RAA
Quarkonia Suppression in A+A Collisions – key observations and questions
Npart
62 GeV
39 GeV
CMS:
0 <|y|< 2.4
pT > 6.5
Npart
(more on LHC in a minute)
Npart
Npart
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Quarkonia Suppression in A+A Collisions – comparing RHIC & LHC
caution
caution
Mid Rapidity
all pT
CMS pT > 6.5 GeV/c
Forward Rapidity
• 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)
ATLAS
Rcp
ALICE, all pT
ALICE
Npart
However suppression
roughly flat with rapidity
for pT>6.5
CMS
So may also be consistent
with regeneration at
Missing LHC data – y~0, pT> 0 RAA?
(where regeneration may be rather large) small pT
y
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What are the CNM effects that are so strong in Quarkonia production?
J/ψ in d+Au – learning about CNM thickness dependence
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
(rT ) 
1
0
 dz  ( z, rT )
Exponential : M (rT )  e
WoodsSaxon
 a ( rT )
Linear : M (rT )  1  a(rT )
PHENIX
arXiv:1010.1246v1
Quadratic : M (rT )  1  a(rT )2
• Break-up has exponential dependence
• EPS09 & initial-state dE/dx have
unknown dependences
The forward rapidity points suggests a quadratic
or higher geometrical dependence
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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” 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:
Au-Au --> X
~7 barns
-:
AuAu --> AuAu + e+e~33 kbarns
AuAu --> AuAu + 2(e+e-) ~680 barns
AuAu --> AuAu + 3(e+e-) ~50 barns
-N: L(-N )=1029 cm-2s-1 2<E<300GeV
AuAu --> Au+Au*
92 barns
X+neutrons
AuAu --> Au*+Au*
3.670.26 barns
X+neutrons
Y+neutrons
Hadronic Interaction
Ultra-Peripheral Interaction
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 Au  J/ Au* measurement in PHENIX
n
l+
l-
•
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
||<0.35
p
e+
e

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J/ cross section vs theoretical calculations
 d/dy |y=0 = 76  31 (stat) 15 (syst) b
[ 1) ]
[ 2) ]
[ 3) ]
[ 4) ]
coherent
• Compatible with
coherent predictions,
• With more statistics,
sensitive to the
shadowing
parameterizations,
incoherent
[ 1) P.R.L.89 012301 (2002)…]
[ 2) P.L.B626 (2005) 72 ]
[ 3) arXiv0706.2810 [hep-ph] ]
[ 4) arXiv:0706.1532 [hep-ph] ]
coherent
[Filho et al, PRC78 044904 (2008)]
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Impact Parameter Dependence
Strikman et al, PLB 626 p. 72-79
Horowitz, INT-PUB-11-005 (arxiv:1102.5058)
UPC J/ψ pT2
distribution
(Theoretical)
Coherent(γAu):
low pt peak
Incoherent(γn):
wider pt distribution
(Incoherent + neutron
tagged :
Yellow shadow )
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
Nucleus-Nucleus Interactions
•Direct measurement of G(x) at
x  mJ2 /  / WA2  1.5x102 from photoproduction (~g2(x))
10-3 10-2 10-1
10-3 10-2 10-1
10-3 10-2 10-1
x
EPS09: A New Generation of NLO and LO Nuclear PDF’s, Eskola,Paukannen,Salgado JHEP 0904:065 2009
•Possibly study dynamics of J/ propagation through nuclear matter
•Feature or Bug?
•Test of QED in strong-coupling regime?: =ZEM~0.6
35
+, +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
Nucleus-Nucleus Interactions
•Direct measurement of G(x) at
x  mJ2 /  / WA2  1.5x102 from photoproduction (~g2(x))
PHENIX UPC J/Psi
10-3 10-2 10-1
10-3 10-2 10-1
10-3 10-2 10-1
x
EPS09: A New Generation of NLO and LO Nuclear PDF’s, Eskola,Paukannen,Salgado JHEP 0904:065 2009
•Possibly study dynamics of J/ propagation through nuclear matter
•Feature or Bug?
•Test of QED in strong-coupling regime?: =ZEM~0.6
36
+, +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
Nucleus-Nucleus Interactions
•Direct measurement of G(x) at
x  mJ2 /  / WA2  1.5x102 from photoproduction (~g2(x))
PHENIX UPC J/Psi
10-3 10-2 10-1
10-3 10-2 10-1
10-3 10-2 10-1
x
EPS09: A New Generation of NLO and LO Nuclear PDF’s, Eskola,Paukannen,Salgado JHEP 0904:065 2009
•Possibly study dynamics of J/ propagation through nuclear matter
•Feature or Bug?
•Test of QED in strong-coupling regime?: =ZEM~0.6
37
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
toenough
understand
effects inforthe
HIroom?
interpretation
Are these
to seeCNM
the elephant
•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
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Backup Slides
39
IdA vs JdA: Can we decouple effects?
I
I dA 
dA
1 dNassoc
dA
N trig
dDf
pp
assoc
1 dN
pp
N trig
dDf
trig
dA
1
 trig  J dA
RdA
pp
N coll N trig

pp
N evt
dA
N trig
N
dA
evt

dA
1 dN( assoc |trig )
dA
N evt
dDf
pp
1 dN( assoc |trig )
N coll
pp
N evt
dDf
•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
40
JdA, RdA vs Ncoll
MPC p0 pT
Qiu-Vitev Shadowing +
Energy Loss
(private communication)
41
Muon-Central IdA & Widths, 2003 d+Au
Au
d
Phys.Rev.Lett.96:222301,2006
42
Nuclear Modification in d+Au at
Forward(Backward) Rapidity
Punch through hadrons & Hadron decay muons
•Forward η suppression
•No backward η
suppression
•Gluon Saturation?
•Cronin, Shadowing, Eloss?
•Look at 2 particle
correlations …
Phys. Rev. Lett. 94, 082302 (2005)
43
43
UPC Comparison with HERA data
• 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
[ZEUS, Eur.Phys.J. C24 (2002) 345]
[H1, Eur.Phys.J. C46 (2006) 585]
  ~ 1, good agreement with HERA data hard probes scaling
44