Nuclear matter effects
in charmonium production
in proton-nucleus collisions
• Initial state versus final state “cold nuclear effects”
• Energy dependence of the J/y “absorption cross section”
• abs values estimated for the SPS heavy-ion conditions
• A joint analysis of the J/y and y’ abs values
Hermine K. Wöhri
LIP – Lisbon, Portugal
QWG 2008, Nara, Japan, December 2008
Work done in collaboration with
Carlos Lourenço, Ramona Vogt,
Pietro Faccioli and João Seixas
“Anomalous suppression” vs. “normal nuclear absorption”
• The charmonium production yields should be considerably suppressed if a medium
of deconfined quarks and gluons is formed in high-energy heavy-ion collisions
• But already in p-nucleus collisions the charmonium states are absorbed
• This “normal absorption” must be well understood before high-density QCD signals
can be extracted from the nucleus-nucleus data
proton-nucleus
Be
Al
J/y
Cu
Ag
W Pb
heavy-ion
y’
J/y normal nuclear
absorption curve
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Strategy to evaluate the “normal nuclear absorption”
1. Collect the J/y production cross sections measured in proton-nucleus collisions by
NA3, NA50, E866 and HERA-B (plus the PHENIX d-Au values)
2. Compare the measured “heavy / light” cross-section ratios to those calculated with
the Glauber model, to get a abs value from each data set (energy, rapidity window)
3. Derive the abs value suitable for the SPS heavy-ion kinematical conditions
Remark :
• Feed-down contributions from c and y’ decays should be taken into account.
We first present results effectively convoluting all J/y sources:
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Initial state nuclear matter effects: modification of PDFs
Within the Glauber model the J/y “normal nuclear absorption” can be described with
one single parameter : the “absorption cross section”, abs
From 400 GeV J/y and y’ cross sections in p-A collisions (6 nuclei), NA50 obtained
abs(J/y = 4.6 ± 0.6 mb
abs(y’) = 10.1 ± 1.6 mb
using free proton PDFs, i.e. not
considering nuclear effects on the PDFs
Such effects can be modelled by using
EKS 98 or other parameterisations,
which indicate sizeable anti-shadowing
at x values around 0.1–0.4
→ It is important to separate this
initial state nuclear matter effect from
final state absorption
gluon density function in Pb
gluon density function in p
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Nuclear effects on the PDFs vs. final state absorption
At SPS energies, the gluon anti-shadowing makes the J/y production cross section
per nucleon increase from pp to p-Pb (before final state absorption)
 abs = 4.5 mb assuming
no nuclear effects on the PDFs
(“NONE”) is equivalent to
abs = 7.0 mb with EKS 98
W
0 < ycms < 1
Al
Be
Cu
EKS 98
abs = 0 mb
J/y
 The abs values quoted by
PHENIX (obtained with EKS 98)
cannot be directly compared to
the NA50 (“NONE”) numbers…
abs = 4.5 mb
no
n.e.
NONE
EKS 98
abs = 7.0 mb
 When comparing different data
sets, we must consistently use
the same shadowing model
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Nuclear effects on the PDFs vs. xF: E866
• The nuclear effects on the PDFs are a function of Bjorken-x
 they are energy and xF (or y) dependent
• At xF < 0.2 : strong anti-shadowing in EKS98 and EPS08 :
 In the absence of other effects, the E866 data should be higher than unity
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Available measurements : a global summary
• The J/y was measured in p-A collisions at different energies (Elab = 200920 GeV)
and using several target nuclei
(the s = 200 GeV PHENIX d-Au / pp y-dependent ratios were also
analysed but will not be shown today because of their large errors)
• NA50 provided absolute production cross sections for each of the 5 (6) targets,
while the others provided “heavy / light” cross-section ratios
• NA3 is different in several respects: secondary proton beam; long liquid target;
only two target nuclei and one of them being a proton (not exactly a “nucleus”);
took data 25 years ago (and was not yet cross-checked by a newer experiment)
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Extracting abs from cross-section ratios
• For each energy and kinematical window, the relevant cross-section ratios were
calculated as a function of a running abs value, with several (nuclear) PDF sets
• Comparing calculations to data gives the corresponding abs (and error)
0.1 < xF < 0.14
GRV94-EKS98
CTEQ6L-EKS98
 abs is insensitive to the PDFs
(GRV94, GRV98, CTEQ6L, MRST2001)
but depends on the nuclear effects
(EKS98, EPS08, nDS, nDSg)
GRV94-NONE
CTEQ6L-NONE
abs [mb]
4.20.5 EKS98
2.40.5 NONE
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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A simultaneous fit of the six p-A cross sections measured by NA50 at 400 GeV
(using “NONE”) gives abs(J/ y) = 4.8 ± 0.6 mb and abs(y') = 10.8 ± 1.8 mb, in
good agreement with the values published by NA50
0.425 < ycms < 0.575
J/y
y’
The fits are equally good using EKS98 or “NONE”, but give very different abs values
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Extracted abs from each data set, xF dependent
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Another influence of nuclear PDFs on abs
pA = 0 A
E866
• If no nuclear effects are considered, we see a flat nuclear dependence at xF ~ 0,
both in  and in abs
• For nDSg, EKS98 or EPS08 the xF ~ 0 values require an xF dependent study,
with abs values decreasing as xF increases
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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abs in the forward hemisphere
• At xF ~ 0, the E866 and HERA-B differential abs values, extracted with nDSg,
EKS98 or EPS08, can be parameterised by an asymmetric Gaussian
• At forward xF other effects than “absorption” play a leading role
 Glauber-like absorption must be studied at xF ~ 0 (i.e. |ycms| < 1.0)
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Rapidity dependence of abs
EKS98
NONE
The E866 and HERA-B patterns set the shape of the rapidity dependence of abs :
an asymmetric Gaussian if EKS98 is used;
flat if free proton PDFs are used
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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nDSg
EPS08
The nDSg and EPS08 cases are similar to the EKS98 case
The leftmost NA3 point is consistently below the trend suggested by other points
We can now see how the ycms=0 value of abs evolves with collision energy, s
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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abs(ycms=0) decreases with increasing NN collision energy
NONE
EKS98
power law
curves
• Using EKS98, abs(ycms=0) extrapolated to 158 GeV is 9.9±0.6 mb excluding the
lowest xF NA3 point and 8.7±0.7 mb otherwise
• Using free proton PDFs, the respective values are: 5.9±0.8 mb and 5.4±0.8 mb
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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nDSg
EPS08
power law
curves
• Also with nDSg and EPS08, we see that abs significantly drops with increasing
NN collision energy
• The impact of the first NA3 point is even more visible with these nuclear PDFs
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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abs extrapolated to the SPS heavy-ion kinematical conditions
• Using EKS98 nuclear PDFs, the
absorption cross section integrated in
the NA50/60 rapidity window (0<ycms<1)
is 8.2 ± 0.5 mb.
NONE
nDSg
EKS98
EPS08
Remark: the absorption cross section used
in the SPS heavy-ion analyses, with free
proton PDFs, has been 4.2 ± 0.5 mb
NONE
nDSg
EKS98
EPS08
abs 158 GeV
first NA3 point excluded
at ycms = 0 (0<ycms<1)
5.9±0.8
5.9±0.8
9.8±0.5
7.0±0.3
9.9±0.6
8.2±0.5
15.6±0.8
9.3±0.5
first NA3 point included
at ycms = 0 (0<ycms<1)
5.4±0.8
5.4±0.8
7.7±0.4
5.5±0.3
8.7±0.7
7.2±0.6
12.5±0.8
8.1±0.5
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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abs vs. J/y kinematics: an alternative scenario
If the absorption process depends on the J/y energy (rather than
on the proton beam energy), we should study the J/y nuclear
absorption as a function of the J/ynucleon c.m.s. energy...
J/y-N
N-N
Also in terms of this alternative variable
no scaling is seen: the magnitude of abs
depends on the data set
Similar results are obtained with the other
sets of nuclear PDFs (including “NONE”)
sψ N = mψ
s NN
1+
mψ
for xF = pT = 0
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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What can we learn from a joint J/y and y’ analysis?
• The observed J/y yield is affected by c and y’ feed-down decays and these two
states should have different (larger) break-up cross sections
J /ψ
1S
1P
2S
S abs
= 1  F1P  F2 S   S abs
+ F1P  S abs
+ F2 S  S abs
S abs = exp   dz  ρ A b, z   σ abs
ψ'
2S
• The y’ component is directly measurable: S abs
= S abs
• Can both measurements, J/y and y’, be described in a global and simple way ?
Our assumptions :
1. Feed-down fractions: F1P = 25 ± 5% and F2S = 8.1 ± 0.3%
see JHEP 10 (2008) 004 and talk of P. Faccioli on Wednesday
2. abs values are geometrically related, with r1P / r1S = 1.44 and r2S / r1S = 1.80
see H. Satz, NPA 783 (2007) 249c.
• With these values, we can evaluate abs(1S) from the J/y and y’ data sets in a
global fit, as well as obtaining two independent estimates
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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• NA50 published J/y and y' production cross sections at 400 at 450 GeV (at |y|<0.5)
A single abs(1S) value (3.05±0.25 mb) describes both data sets with a good quality
Independent abs(1S) values: 3.94±0.40 mb (from J/y) and 2.74±0.31 mb (from y’)
• E866 reported J/y and y’ p-W / p-Be cross-section ratios at 800 GeV vs. xF
We use the |xF|<0.1 window, analogous to the NA50 rapidity window.
Again, a single abs(1S) value (1.60±0.24 mb) describes very well both data sets
Independent abs(1S) values: 2.04±0.50 mb (from J/y) and 1.46±0.27 mb (from y’)
• Both NA50 and E866 indicate a ~30% larger abs(1S) value from the J/y data
than from the y’ data but with a large uncertainty
• Side-remark: also the y’ data show that abs decreases with collision energy:
8.9±1.0 mb (400/450 GeV)  4.7±0.9 mb (800 GeV) with free proton PDFs
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Summary
 The nuclear dependence of the J/y production cross section, in p-A collisions,
cannot be described by the Glauber model with a “universal” abs value :
1. abs changes significantly with collision energy
2. and with rapidity (even at mid-rapidity, when using n-PDFs)
 A power-law extrapolation gives abs values in the SPS heavy-ion kinematical
conditions larger than currently used in the J/y suppression analyses :
abs(J/y) = 8.2 ± 0.5 mb with EKS98 and 5.9 ± 0.8 mb with free proton PDFs
 The J/y and y’ data sets can be simultaneously described assuming geometrical
scaling of the charmonium break-up cross sections.
Including a pre-resonant state improves the global description...
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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Backup Slides
“400”
NA50 data sets
“450-LI”
“450-HI”
J/y
“400”
“450-LI”
“450-HI”
Y’
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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xF dependence of charmonium absorption
The J/y (and y’) absorption is much stronger at forward xF than at mid-rapidity…
clearly indicating that extra absorption mechanisms come into play at high xF
To stay away from such extra issues,
we start our study by only looking at
mid-rapidity values
J/y
E866



xF
xF
Note:  is “inversely proportional” to abs
pA = 0 A = 0 A exp(L abs)
Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008
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