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Probing deconfinement with quarkonia : new answers to old questions

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Probing deconfinement with quarkonia :new answers to old questions

Outline:

What are the “cold nuclear matter effects” in charmonium absorption ?

How do they affect the SPS J/ and ’ suppression patterns ?

What gets you into trouble is not what you don’t knowbut what you think you know

Mark Twain

(Larry at QM08)

[ work being done in collaboration withRamona Vogt and Hermine K. Wöhri ]

Carlos Lourenço, CERN

Workshop on Hot and Dense Matter, Mumbai, Feb. 2008

“Seeing” the QCD matter formed in heavy-ion collisions

We study the bulk QCD matter produced in HI collisions by seeing how it affectswell understood probes

as a function of the temperature of the system (centrality of the collisions)

Matter under study

QGP ?

Probe

Calibrated

“probe source”

Calibrated

“probe meter”

Calibrated

heat source

Challenge: find the good probes of QCD matter

vacuum

The good QCD matter probes should be:

Well understood in “pp collisions”

hadronicmatter

Slightly affected by the hadronic matter, in a well understood way, which can be accounted for

QGP

Strongly affected by the deconfined QCD medium...

Heavyquarkonia (J/, ’, , ’, etc) are very good QCD matter probes !

p-Be

p-Pb

centralPb-Pb

reference process

reference data

S-U

p-A

Pb-Pb

J/ normal nuclear absorption curve

NA38 / NA51 / NA50

J/y suppression: the NA38/50/51 picture

The yield of J/ mesons (per DY dimuon) is “slightly smaller” in p-Pb collisions than inp-Be collisions; and is strongly suppressedin central Pb-Pb collisions

Drell-Yan dimuons are not affected by the dense medium they cross

Interpretation:strongly bound c-cbar pairs are “dissolved” by the QCD medium created in central Pb-Pb collisions at SPS energies

Quarkonium studies in proton-nucleus collisions: why?

We must have a robust and well understood reference baseline, in A-A collisions, with respect to which we can clearly and unambiguously identify patterns specific to the high-density medium produced in high-energy nuclear collisions

What should we really expect in the absence of a deconfined QCD medium but accounting for all the other “standard” aspects of nuclear collisions?

This requires :

→ Understanding the basic properties of quarkonium production in pp and p-A

→ A robust model to turn the p-A patterns into reliableA-A expectations

NA50 p-A 400 GeV

J/y

y’

Charmonium absorption in p-nucleus collisions

The J/y and y’ production cross sections scale less than linearly with the number of target nucleons (unlike high-mass Drell-Yan dimuons)

p-Pb @ 400 GeV

sJ/y ~ 105 MeV

The Glauber model describes the J/ and ’ “normal nuclear absorption” with a single parameter: the absorption cross section

From a global fit to the 400 and 450 GeV p-A data (16 independent measurements), NA50 determined the following absorption cross sections (with GRV94LO PDFs):

sabs(J/y) = 4.5 ± 0.5 mb ; sabs(y’) = 8.3 ± 0.9 mb from production cross sections

sabs(J/y) = 4.2 ± 0.5 mb ; sabs(y’) = 7.7 ± 0.9 mb from cross-section ratios (y/DY)

abs = 4.5 ± 0.5 mb

abs = 8.3 ± 0.9 mb

c2/ndf = 0.7

c2/ndf = 1.4

These calculations assume that the reduction of the production cross section per target nucleon is exclusively due to charmonium final-state absorption

PHENIX

J/y

Low x2 ~ 0.003

(shadowing region)

0 mb

3 mb

J/y absorption in p-A collisions vs. collision energy

It seems that the J/y absorption, at mid-rapidity, becomes weaker with increasing collision energy, at least between SPS and RHIC energies

The 158 GeV p-A data of NA60 will clarify if the trend continues to lower energies

Slide shown at HP06

Without nuclear effects on the PDFs

With nuclear effects on the PDFs

J/y

J/y absorption in p-A collisions vs. pT and xF

E866

NA50

Slide shown at HP06

The increase of a with pT is identical at 400, 800 and 920 GeV (at mid-rapidity)

Maybe the increase of a from NA50 to E866 to HERA-B to PHENIX is due to the increase of the average pT of the J/y when s increases...

And astrongly decreases at high xF where the J/ and ’ have similar absorptions

1.0

a

0.9

0.8

Models (with variants):

- - R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539
- K.G. Boreskov & A.B. Kaidalov, JETPL 77 (2003) 599

xF

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

B&K

1.0

Vogt: final state absorption

0.9

HERA-Bpreliminary

E86638.8 GeVBe/Fe/W

E78938.8 GeVBe/C/Cu/W

E77238.8 GeVH2/C/Ca/Fe/W

NA5029.1 GeVBe/Al/Cu/Ag/W

NA322.9 GeVH2/Pt

0.8

xF

0.7

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Given enough models… at least one should describe the data

Slide shown at HP06

Nuclear effects on the PDFs

The EKS 98 model gives significant anti-shadowing for charm production at the SPS

Similar for p-Pb at 450 GeV and Pb-Pb at 158 GeV… but ~6% more J/ mesons are produced, per nucleon, in Pb-Pb than in p-Pb, if both are taken at 158 GeV

Initial state nuclear effects 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, if we would ignore final state absorption

sabs = 0 mb

sabs = 4 mb

sabs = 7 mb

EKS98

0<y*<1

pp

p-Pb

J/y

p-Pb

nDS

DeFlorian and Sassot

Eskola, Kolhinen and Salgado

Frankfurt, Guzey and Strikman

nDSg

EKS98

FGSl

FGSh

FGSo

EKS98 is not the only available model of nuclear effects on the parton densities

DeFlorian and Sassot predict no anti-shadowing for J/yproduction at SPS energies while the FGSo parameterization predicts stronger anti-shadowing than EKS98…

The new EPS08 model gives more anti-shadowing at the SPS than EKS98…

Absorption of y’ and cc states

Approximate radii of the J/y, y’ and cc states:

r(J/y) = 0.25 fm; r(y’) = 2 × r(J/y); r(cc) = 1.5 × r(J/y)

Geometrical cross-sections of the J/y, y’ and cc states:

sgeom(J/y) = 1.96 mb; sgeom(y’) = 7.85 mb; sgeom(cc) = 4.42 mb

NA50 data: sabs(y’) = 7.7 ± 0.9 mb

or sabs(y’/DY) = 8.3 ± 0.9 mb

coincidence?

(no nuclear effects considered here)

c2/ndf = 1.0

Feed down influence on J/y absorption

[Figure made by G. Borges]

We can redo the Glauber calculations assuming 60% / 30% / 10% as the fractions of direct J/y production and feed downs from cc and y’ decays...

And fixing the abs of each of the three states to their geometrical values

The result is perfectly equivalent to a fit with a free effectivesabs(J/y)

coincidence?

(no nuclear effects considered here)

From qualitative hints to more detailed calculations

Let’s express the abs values of the three charmonium states in terms of the value of the directly produced 1S state, called “J” to distinguish it from the observed “J/” (affected by feed down), assuming that they scale with the square of their radii:

with r(c) / r(J) = 1.44 ; r(’) / r(J) = 1.8 [values from H. Satz]

This is a guess… but it is better to assume an answer based on an educated guess than to ignore the existence of the question…

The generic survival probability for the state J (or ’, or c) is then given by:

And assuming 60% J, 30% c feed down and 10% ’ feed down, the survival probability to be compared with the J/ data is:

Now we can fit the existing J/and’ data with a single free parameter:

The charmonium production cross section in p-A collisions

FJ

Kth

SAabs

fjp(x1,Q2)

FjA(x2,Q2,b,z’)

A(b,z’)

Rj(A,x2,Q2)x1, x2

fraction of charm-anticharm cross section below 2mD

K factor to match the magnitude of the LO and NLO cross sections

survival probability for nuclear absorption

parton density in the proton; j = g, q, qbar

parton density in the nucleus

nucleon density in the nucleus

modification of the parton densities in the nucleus

parton momentum fractions

- The calculations were done with several PDF sets and nuclear effects models:
- GRV LO 94, GRV LO 98, CTEQ6L, MRST2001LO;
- non-modified and modified by EKS98, nDS, nDSg, etc

Existing J/ and ’ cross sections in p-A collisions

From the measurements of NA3, E866 and HERA-B, respectively at 200, 800 and 920 GeV, we calculated the mid-rapidity ratios between the heavy and light targets of J/ (and ’) “per nucleon cross sections”

200 GeV : p-Pt / pp = 0.737 0.026 for the J/

800 GeV : p-W / p-Be = 0.8713 0.0263 for the J/ and 0.8032 0.0274 for the ’

920 GeV : p-W / p-C = 0.903 0.031 for the J/

1 data point

2 data points

1 data point

From the NA50 measurements, at 400 and 450 GeV, we calculated the J/ and ’ cross section ratios, between the heavy targets (Al, Cu, Ag, W, Pb) and Be

10 data points

8 data points

Extraction of the absorption cross section

For each energy and target, the calculations were made with several N-PDFs and for abs values between 0.0 and 8.0 mb, in steps of 0.5 mb

Comparing the calculations with the data we derive the “best” abs and its error

EKS98

abs (mb)

4.880.29 EKS983.750.27 nDSg

3.390.26 none

200 GeV

abs : insensitive to the PDF set but very dependent on the nuclear effects model

Our calculations vs. the NA50 values

Using GRV LO 94 and no nuclear effects, we get abs(J) 3.34 0.25 mb

abs(’) (0.45/0.25)2abs(J) 10.8 0.8 mb

NA50 obtained abs(’) 10.0 1.5 mb

abs(J/) ≈ [ 0.6 + 0.3 (0.36/0.25)2 + 0.1 (0.45/0.25)2 ] abs(J)

5.2 0.4 mb

NA50 obtained abs(J/) 4.6 0.6 mb

Conclusion: if we use the same inputs as NA50, we get the same values

(with a smaller error because we make a global fit of the J/ and ’ data pointswith one single free parameter, while NA50 made two independent fits)

158 GeV

Significant drop of abs with collision energy

The J/y and ’ absorption, at mid-rapidity, weakens with increasing collision energy

(open circles)

(closed circles)

Assuming a power law function, we can extrapolateabs to lower and higher energies

abs at 158 GeV is ~50–60% higher than at 400–450 GeV !

(maximum c.m.s. energyof the J/ – N collision)

Cold nuclear matter effects at RHIC energies

PHENIX

RdAu

Mid-rapidity data point : R(dAu) = 0.84 0.20 (stat. and syst. errors added in quadrature)

Extraction of abs from the PHENIX mid-rapidity d-Au data

The calculations were redone with several N-PDFs and for abs values between 0.0 and 8.0 mb, in steps of 0.5 mb

None / nDSg

abs extrapolated to RHIC energies

The extrapolation from fixed-target energies matches well the PHENIX d-Au data

Much more accurate RHIC data needed to verify the functional form of the energy dependence

PHENIX

(EKS98)

Effect on the ’ “suppression”: magnitude and shape

The ’ suppression pattern shows a significant and abrupt drop between the “normal extrapolation” of the 450 GeV p-A data and the S-U / Pb-Pb patterns

’

’

abs = 8.3 ± 0.9 mb

Preliminary

But this “step” happens between data sets collected at very different energies…and will disappear if the ’ abs increases significantly from 450 to 158 GeV !NA60 p-A data at 158 GeV will soon address (and hopefully answer) this question

Effect on the J/ “suppression”: magnitude and shape

The relative comparison between the In-In and Pb-Pb J/ suppression patterns will not change, because they were both taken at the same energy, 158 GeV, preciselyto minimise the number of “free parameters” in their comparison

But there will be a common decrease of the magnitude and of the slope’s steepness

Quarkonium melting by QGP : thresholds steps

In the QGP phase the heavy quarkonium states are “dissolved”, at successive temperature thresholds

The feed-down from higher states leads to a “step-wise” J/suppression pattern

’

c

J/ cocktail (in pp):

~ 60–65% direct J/

~ 25–30% from c decays

~ 10% from ’ decays

The In-In J/y suppression pattern versus a step function

1

Measured / Expected

A1

A2

Npart

Step position

Step at Npart = 86 ± 8

A1 = 0.98 ± 0.02

A2 = 0.84 ± 0.01

2/ndf = 0.75 (ndf = 8-3 = 5)

Taking into account the EZDC resolution, the measured pattern is perfectly compatible with a step function in Npart

Maybe there is even a hint of charm “coalescence” in the most central collisions

Is the step in Npart or in another variable?

Npart is convenient to compare In-In and Pb-Pb data: derived from the same EZDC using the same Glauber formalism (except for the nuclear density functions)

If the “real variable” driving charmonium suppression is not Npart the measured smearing is the convolution of the detector resolution with the “physics smearing”

The detector resolution is 20 (in Npart), while a fit to the measured pattern gives 19: the “physics smearing” is negligible with respect to the ZDC resolution…

The In-In data indicates a step in the J/ suppression pattern and suggests that “the physics variable” is Npart or a variable very strongly correlated to Npart

1

Measured / Expected

A1

A2

A3

Npart

Step positions

What about the Pb-Pb suppression pattern?

Steps: Npart = 90 ± 5 and 247 ± 19

A1 = 0.96 ± 0.02

A2 = 0.84 ± 0.01

A3 = 0.63 ± 0.03

2/ndf = 0.72 (ndf = 16-5 = 11)

-12% : ’ ?

-21% : c ?

If we try fitting the In-In and Pb-Pb data with one single step we get 2/ndf = 5 !

the Pb-Pb points rule out the single-step function and indicate a second step

We urgently need a much more accurate Pb-Pb pattern

Summary and outlook

The J/ and ’ final state “normal nuclear absorption”, determined by p-A data, is insensitive to the PDF set used but is significantly affected by the nuclear effects model assumed: none, EKS98, nDSg, etc

The latest PDF sets are mature, constrained by a wealth of data (DIS, DY, etc), while the nuclear effects on the gluon densities have not yet been measured…

We must measure the open charm nuclear dependence, versus pT and y

All existing J/ and ’ p-A data can be described by Glauber calculations using one single abs parameter, with the c and ’ values fixed by geometrical scaling and the “observed J/” value fixed from the feed down fractions

The fitted abs values show a significant decrease from 200 to 920 GeV ;the value extrapolated to 158 GeV is 50–60% larger than previously assumed…

We must re-evaluate the SPS J/ and ’ suppression patterns, with N-PDFs and the increased abs : the “anomalous suppression” will decrease in magnitude and will become less steep

Acknowledgements:

This work started after discussions withHelmut Satz and Bob Thews, in May 2006

Previous related work:

- Hard Probes 2006 talk, by CL

- Quark Matter 2006 talk, by RV

References of the data points:

NA3, NA50, E866 : published papers

HERA-B : values reported at HP06

PHENIX : preprint arXiv: 0711.3917

Work done in collaboration with:

Ramona Vogt and Hermine Wöhri