nuclear matter effects in charmonium production in proton-nucleus collisions

Nuclear matter effects

in charmonium production

in proton-nucleus collisions

Hermine K. WöhriLIP – Lisbon, PortugalQWG 2008, Nara, Japan, December 2008

• Initial state versus final state “cold nuclear effects”

• Energy dependence of the J/ “absorption cross section”

• abs values estimated for the SPS heavy-ion conditions

• A joint analysis of the J/ and ’ abs values

Work done in collaboration withCarlos Lourenço, Ramona Vogt,Pietro Faccioli and João Seixas

• 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

“Anomalous suppression” vs. “normal nuclear absorption”

Hermine Wöhri — QWG 2008 — Nara, Japan, December 2008 2

J/ normal nuclear

absorption curve

heavy-ion

’

J/

proton-nucleusBe

AlCu

AgW Pb

1. Collect the J/ 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 ’ decays should be taken into account. We first present results effectively convoluting all J/ sources:

Strategy to evaluate the “normal nuclear absorption”


Within the Glauber model the J/ “normal nuclear absorption” can be described with one single parameter : the “absorption cross section”, abs

From 400 GeV J/ and ’ cross sections in p-A collisions (6 nuclei), NA50 obtained

abs(J/ = 4.6 ± 0.6 mb

abs(’) = 10.1 ± 1.6 mb

using free proton PDFs, i.e. notconsidering nuclear effects on the PDFs

Such effects can be modelled by usingEKS 98 or other parameterisations, which indicate sizeable anti-shadowingat x values around 0.1–0.4

→ It is important to separate thisinitial state nuclear matter effect fromfinal state absorption

Initial state nuclear matter effects: modification of PDFs

gluon density function in Pb

gluon density function in p


Nuclear effects on the PDFs vs. final state absorption

At SPS energies, the gluon anti-shadowing makes the J/ 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

The abs values quoted byPHENIX (obtained with EKS 98)cannot be directly compared tothe NA50 (“NONE”) numbers…

When comparing different datasets, we must consistently use the same shadowing model


0 < ycms < 1

EKS 98abs = 0 mb

abs = 4.5 mbno n.e.

EKS 98abs = 7.0 mb

J/

NONE

BeAl

CuW

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


• The J/was measured in p-A collisions at different energies (Elab = 200920 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)

Available measurements : a global summary


• 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)

abs is insensitive to the PDFs

(GRV94, GRV98, CTEQ6L, MRST2001)but depends on the nuclear effects(EKS98, EPS08, nDS, nDSg)

Extracting abs from cross-section ratios

GRV94-NONECTEQ6L-NONE

GRV94-EKS98CTEQ6L-EKS98

0.1 < xF < 0.14

abs [mb]

4.20.5 EKS982.40.5 NONE


J/ ’

The fits are equally good using EKS98 or “NONE”, but give very different abs

values

A simultaneous fit of the six p-A cross sections measured by NA50 at 400 GeV

(using “NONE”) gives abs

(J/) = 4.8 ± 0.6 mb and abs

(') = 10.8 ± 1.8 mb, in

good agreement with the values published by NA50

0.425 < ycms < 0.575


Extracted abs from each data set, xF dependent


E866

Another influence of nuclear PDFs on abs

• 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

pA = 0 A


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)


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

Rapidity dependence of abs


NONEEKS98

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

nDSg EPS08


EKS98

abs

(ycms

=0) decreases with increasing NN collision energy

NONE

•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


power lawcurves

• 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

power lawcurves

nDSg EPS08


abs

extrapolated to the SPS heavy-ion kinematical conditions

Remark: the absorption cross section used in the SPS heavy-ion analyses, with free proton PDFs, has been 4.2 ± 0.5 mb

• Using EKS98 nuclear PDFs, the absorption cross section integrated in the NA50/60 rapidity window (0<ycms<1) is 8.2 ± 0.5 mb.


at ycms = 0 (0<ycms<1)NONE 5.9±0.8 5.9±0.8nDSg 9.8±0.5 7.0±0.3EKS98 9.9±0.6 8.2±0.5EPS08 15.6±0.8 9.3±0.5

at ycms = 0 (0<ycms<1)NONE 5.4±0.8 5.4±0.8nDSg 7.7±0.4 5.5±0.3EKS98 8.7±0.7 7.2±0.6EPS08 12.5±0.8 8.1±0.5

first NA3 point excluded

first NA3 point included

abs 158 GeV

N-N

J/-NIf the absorption process depends on the J/energy (rather than on the proton beam energy), we should study the J/ nuclear absorption as a function of the J/nucleon c.m.s. energy...

abs vs. J/ kinematics: an alternative scenario

ψ

NNψψN m

s+m=s 1 for x

F = p

T = 0

Also in terms of this alternative variableno 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”)


What can we learn from a joint J/ and ’ analysis?

• The observed J/ yield is affected by c and ’ feed-down decays and these two states should have different (larger) break-up cross sections

• The ’ component is directly measurable:

• Can both measurements, J/ and ’, 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 Wednesday2. 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/ and ’ data sets in a global fit, as well as obtaining two independent estimates

2Sabs2

1Pabs1

1Sabs21

/abs 1 SF+SF+SFF=S SPSPψJ absabs exp σzb,ρdz=S A

2Sabs

'abs S=Sψ


• NA50 published J/ and ' 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/) and 2.74±0.31 mb (from ’)

• E866 reported J/ and ’ 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/) and 1.46±0.27 mb (from ’)

• Both NA50 and E866 indicate a ~30% larger abs(1S) value from the J/ data than from the ’ data but with a large uncertainty

• Side-remark: also the ’ 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


Summary

The nuclear dependence of the J/ 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 energy2. 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/ suppression analyses :

abs(J/) = 8.2 ± 0.5 mb with EKS98 and 5.9 ± 0.8 mb with free proton PDFs

The J/ and ’ 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...


Backup Slides

“450-LI” “450-HI”“400”

“450-LI” “450-HI”“400”

NA50 data sets

J/

’


The J/ (and ’) 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 atmid-rapidity values

xF dependence of charmonium absorption

Note: is “inversely proportional” to abs

pA = 0 A = 0 A exp(L abs)

J/

xF

E866

xF


nuclear matter effects in charmonium production in proton-nucleus collisions

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