Transcript Slide 1

Recent results from the CERN SPS
on quarkonium production
in p-nucleus and nucleus-nucleus collisions
Summary
• Quarkonia resonances can be measured as nice peaks above a “flat” dilepton continuum;
no problem with backgrounds or “particle identification”, if we have good mass resolution
and vertexing capabilities (to clean event sample at the SPS; to evaluate beauty feed-down at LHC)
• But: J/y suppression... J/y enhancement... with respect to what?
• Before we can discuss “new physics” anomalies in nuclear collisions, it is crucial to define
the “normal expected behaviour”, on the basis of measured p-nucleus and light-ion data
• And we must learn how to relate the normal behaviours for different energies and y windows
Carlos Lourenço, 4th International Workshop on Heavy Quarkonium, June 27–30 2006
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Quarkonia studies in heavy-ion collisions: why? how?
Basic idea: in the presence of new physics (formation of a QCD medium with
deconfined quarks and gluons) the centrality dependence of quarkonia production
yields will be very significantly affected
→ we have a “signature”
Prediction: above certain consecutive thresholds, the y’, the cc and the J/y
resonances (besides the Upsilon states) will “dissolve” in the formed medium
→ we have more than a simple signature; we have a “smoking gun”...
However, ...
What happens to the charmonia states in the presence of “old physics”?
Do we understand the basic properties of J/y and y’ production in pp and p-A
collisions?
In A-A collisions, do we have a robust and well understood baseline with respect
to which we can clearly and unambiguously identify patterns specific of 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 aspects surely existing in nuclear collisions?
→ We need accurate p-A data and a robust theory to extrapolate the p-A patterns
to A-A expectations...
2/21
Charmonia studies at the CERN SPS
Measurements of J/y and y’ production have been made in the last few years
at the SPS by the NA50 and NA60 collaborations, in p-A and A-A collisions.
Charmonia production yields have been
presented either in relative terms, with
respect to the yield of high-mass
Drell-Yan dimuons, or as absolute
production cross-sections per target nucleon.
Results have also been obtained in what
concerns pT distributions, centrality
dependence of production yields, etc.
NA50 collected p-A data at 400 and 450 GeV,
with 5 or 6 different target nuclei.
More than 3 000 000 J/y events in total.
J/y
Pb-Pb 158 GeV
p-Pb 400 GeV
3/21
The J/y and y’ are absorbed in p-nucleus collisions ...
The J/y and y’ production cross-sections scale less than linearly with the number
of target nucleons (contrary to what happens with high-mass Drell-Yan dimuons).
p-A 400 GeV
J/y
p-Pb @ 400 GeV
sJ/y ~ 105 MeV
y’
NA50 p-A data collected in year 2000,
with Be, Al, Cu, Ag, W and Pb targets
Note: the spA = spp x Aa parametrization leads to extrapolated s(J/y) and s(y’) pp
values which are 10 to 20% higher than those obtained using the Glauber model
4/21
... as a function of the mass number and of L ...
L is the “path length” which the J/y and y’ states traverse in the target nucleus,
from the production point of the ccbar pair to the nuclear surface
Projectile
p-A 400 GeV
J/y
J/y
L
Target
y’
the “r L parametrization”
exp(-r L sabs)
is a good approximation of
the full Glauber calculation
The solid lines are the result of Glauber calculations, assuming that the reduction
of the production cross-section per target nucleon is due to final state absorption
of the charmonia states in the cold nuclear matter it crosses.
5/21
... at 400 and at 450 GeV ...
From a global fit to the 400 and 450 GeV p-A data, NA50 determined the following
absorption cross-sections:
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)
c2/ndf = 0.7
c2/ndf = 1.4
The sabs values derived from y and y/DY are “identical”, indicating negligible (initial
state) nuclear effects in Drell-Yan production at these energies and at mid-rapidity.
6/21
... and is suppressed in Pb-Pb collisions ...
The J/y production cross-sections measured in O-Cu, O-U and S-U are
compatible with the Glauber extrapolation of the p-A data, keeping the same
absorption cross-section, and scaling the curve down from 450 to 200 GeV.
Pb-Pb 158 GeV
But the J/y suppression pattern changes significantly for Pb-Pb collisions...
7/21
... at low transverse momentum
The J/y “central over peripheral ratio”
strongly depends on pT (at the SPS)
 Only the low pT J/y mesons get suppressed !
(NJ/y / NDY) (ETi)
Ri =
(NJ/y / NDY) (ET1)
8/21
J/y production 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
J/y
PHENIX
Low x2 ~ 0.003
0 mb
(shadowing region)
3 mb
9/21
NA50
J/y
〈pT2〉pp (GeV/c)2
... vs. pT ...
pp
pp
pT (GeV/c)
s (GeV)
The increase of a with pT seems to be 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...
10/21
... and vs. xF
a strongly decreases at high xF ... Why is this so? Higher parton densities?
If so, the J/y should be strongly absorbed in d-Au at RHIC energies; and it is not...
E866
11/21
If you have enough models... one should describe the data...
a
1.0
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
-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
1.0
xF
0.9
HERA-B
preliminary
0.8
0.7
E866
E789
E772
NA50
NA3
-0.4 -0.3 -0.2 -0.1
38.8 GeV
38.8 GeV
38.8 GeV
29.1 GeV
22.9 GeV
0
Be/Fe/W
Be/C/Cu/W
H2/C/Ca/Fe/W
Be/Al/Cu/Ag/W
H2/Pt
0.1 0.2 0.3
0.4 0.5 0.6 0.7
xF
0.8
12/21
Normal nuclear absorption of J/y production at RHIC
At RHIC energies, for charm production, the nuclear effects on the parton densities
(according to EKS98) are just in the crossing from anti-shadowing to shadowing, and
have a significant impact on the rapidity dependence of the measured absorption.
PHENIX
No final state
absorption
sabs = 0 mb
sabs = 3 mb
Such a y-dependent effect is not expected to be seen in the SPS p-A data
13/21
What’s known about the J/y dN/dy in SPS p-A collisions?
NA50 measures dimuons within one unit
of rapidity, at around mid-rapidity
The J/y y distributions are not centered at 0,
even for the p-Be collision system !
NA50 p-A 450 GeV
Pythia with EKS98 gives the
same shape for pp and p-Be
All five distributions are well described by
Gaussians of mean y0  -0.2 and s = 0.85
Forcing y0 = 0, the c2/ndf increases from 1–3
to 20–50, depending on the data set (target)
Why is the J/y rapidity distribution changing
from pp to p-Be? Not because of nuclear
effects on the PDFs...
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Nuclear effects on the PDFs and final state J/y absorption
sabs from PHENIX: after accounting for the nuclear effects on the PDFs (assuming EKS98)
sabs from NA50: effective parameter, convoluting nuclear PDFs and final state absorption
→ The numerical values, 1–3 mb at RHIC and 4.2 mb at the SPS, are not directly comparable
Is there really gluon anti-shadowing at SPS energies?
If the EKS98 model is correct, then the absorption cross-section extracted from p-A data
(collected at 400/450 GeV) is not directly applicable to A-A data (collected at 158 GeV).
→ We need to extract sabs from the p-A data collected by NA60 at 158 GeV (in progress)
For now, we can make a rough estimate of the importance of this issue
15/21
Nuclear effects on the PDFs and the J/y absorption (cont.)
For p-Pb collisions, the EKS98 nuclear modification
factor is 1.12 at 450 GeV and 1.06 at 158 GeV
sp-Pb(450)
208 spp(450)
= exp(-r L sabs,conv)
= 1.12 exp(-r L sabs,real)
sabs,conv = 4.2 mb  sabs,real = 5.9 mb
→ The final state absorption increases to
compensate for the anti-shadowing...
This is the sabs value directly comparable
to the PHENIX values, 0–3 mb
Assuming the same sabs at 158 as at 450 GeV:
sp-Pb(158)
208 spp(158)
= 1.06 exp(-r L sabs,real)
= 0.71 (instead of 0.75)
16/21
Nuclear effects on the PDFs: from p-A to A-A
In A-A collisions, the shadowing or anti-shadowing effect is squared (two nuclei) and it
should change with centrality...
At the SPS, maybe the increased initial production yield (anti-shadowing) with centrality
compensates for the higher sabs value, so that the “expected normal nuclear absorption”
curve in Pb-Pb collisions remains approximately the same as used up to now...
Questions:
1) Can the EKS98 model be trusted at the percent level for the gluon anti-shadowing?
2) How can the centrality dependence of the nuclear effects on the PDFs be fixed?
“Give me two parameters and I can fit an elephant,
give me three and I make its tail wiggle”
[Eugene Wigner]
3) When will we have accurate measurements of open charm production in p-A or d-Au
collisions to separate initial state from final state effects? Will it be done at RHIC?
4) How is all this affected by the feed-down sources, which have a higher sabs value?
17/21
Influence of feed-down from higher states
Approximate radii of the J/y, y’ and cc states:
r(J/y) = 0.25 fm; r(y’) = 2 x r(J/y); r(cc) = 1.5 x 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: sgeom(y’) = 7.7 ± 0.9 mb
Assuming 60% / 30% / 10% as the
fractions of direct J/y production and
feed-downs from cc and y’ decays...
Equivalent to the fit with an effective
sgeom(J/y) = 4.2 ± 0.5 mb
It suggests that the J/y, y’ and cc
states are formed immediately as
such and interact with their asymptotic
geometrical cross-section values...
c2/ndf = 1.0
18/21
The y’ is suppressed from p-nucleus to nucleus-nucleus
The y’ suppression pattern in S-U and in Pb-Pb shows a significantly stronger drop
than expected from the Glauber extrapolation of the p-A data
J/y
y’
y’
sabs = 8 ± 1 mb
sabs ~ 20 mb
The “change of slope” looks very abrupt...
19/21
Extra y’ suppression from p-nucleus to S-U and Pb-Pb
Could it be because of melting in the QGP? Yes, it could be...
But it is very unfortunate that the “drop” happens between p-A and S-U/Pb-Pb, when
we change collision systems and energies, from 400/450 to 200/158 GeV.
Poor statistics prevents the NA60 In-In data from defining the y’ suppression pattern.
If the extra (strong) y’ suppression is
due to the dissolution of the bound cc
state by the QGP, Lattice QCD says
that this would indicate that Tc sits
in the most peripheral S-U or Pb-Pb
collisions at SPS energies...
y’
The y’ suppression measurements
deserve more attention...
And have the advantage of not being
affected by feed-down sources
20/21
Take-home messages
A clear interpretation of the charmonia suppression results obtained in heavy-ion
collisions requires a detailed understanding of charmonia production in “elementary”
pp and p-nucleus collisions!
Guidance from theory has been very important...
but significant progress in the field comes from high accuracy measurements
→ RHIC experiments need accurate d-Au data to enter the charmonia suppression
game in a robust way
→ The LHC heavy-ion program must invest in p-A runs
Otherwise, we will say, in about five years from now:
“Just when we were about to find the answer, we forgot the question”
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