A personal and surely biased view on

1
A personal and surely biased view on
only a few of the many
Open questions on quarkonium and EM probes
Carlos Lourenço, Hard Probes 2006, June 915 2006
2
Recent progress
Since Hard Probes 2004 there has been significant
progress in the topics quarkonia and EM probes,
with new results from NA50, NA60, HERA-B and
PHENIX, as well as new developments in the theory
sector However, the number of open questions has
not decreased Optimistic view The scientific
value of a discovery is also measured by the
number of interesting new questions it
raises Pessimistic view Some of todays open
questions are pretty old...
3
Some open questions
Slide from talk at Quark Matter 2001

• The present SPS results are very interesting
  but important issues remain unclear
• Is the ? meson modified by the medium ? ?
  measure the ? ! Is there a centrality
  threshold in the dilepton enhancement ?
• Is the observed ? enhancement a specific
  feature of heavy ion collisions ? Is the ?
  sensitive to flow ? What is the inverse pT
  slope, T, of the ? meson ?
• What is the origin of the intermediate mass
  dimuon excess ? Thermal dimuons ?
• Is the open charm yield enhanced in
  nucleus-nucleus collisions ? How does it
  compare to the suppression pattern of bound
  charm states ?
• What is the variable that rules the onset of
  ?, ?c and J/? suppression ?
• What is the physical origin of the ?
  suppression ? If it is Debye screening, what
  is its melting temperature (? value of Tc) ?
• What fraction of J/? come from ?c decays ?
  Does it change from p-Be to p-Pb ?

NA60
NA60
?
PHENIX STAR
?
?
?
HERA-B
?
4
What are we missing ?
Slide from talk at Quark Matter 2001

• NA49
• A detailed centrality dependence of the Pb-Pb
  data
• A careful comparison of the existing 40, 80 and
  158 GeV data
• A solid baseline reference from hadron
  production in h-A collisions
• NA57
• A robust p-A (and light ion) reference line
• A scan of the energy density region between
  p-Pb and Pb-Pb
• NA50
• Accurate low ET Pb-Pb data to compare with the
  normal absorption curve
• High precision p-A data for the J/? and ?
  suppression patterns
• CERES
• Improved resolution, statistics and signal to
  background
• A study of the excess as a function of the
  energy density

Done!
Done!
by NA60
Done!
5
Low mass dimuon production

• Are there in-medium modifications of the r
  meson?
• If so, do we see a shift of the r pole mass or a
  broadening of its width?
• Are these observations related to chiral
  symmetry restoration?
• ? Needed improved statistics, mass resolution
  and signal to background ratio and to study
  the signal versus collision centrality and pT

see NA60 talkS. Damjanovic
CERES
?
NA60
6
Intermediate mass dimuons

• Is the observed excess due to thermal dimuons
  from a QGP?
• Or is the open charm yield enhanced in
  nucleus-nucleus collisions?
• ? Needed to measure muon offsets with 50 mm
  precision to separate prompt dimuons from D
  meson decays

see NA60 talkA. David
dN/d?
NA50
?
centralcollisions
NA60
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J/y suppression

• What is the physics mechanism behind the
  anomalous J/y suppression?
• ? Needed compare the J/y suppression pattern
  measured in In-In collisions with the Pb-Pb
  pattern, as a function of several physics
  observables, and with theory predictions,
  previously tuned on the Pb-Pb data

see NA60 talkR. Arnaldi
NA38 / NA50
J/y
y
From ET bins
?
NA38 / NA50 / NA60
8
J/y absorption

• What is the normal nuclear absorption curve at
  the energy of the heavy-ion data?? Needed a
  study of the J/y production in p-A collisions at
  158 GeV
• What is the impact of the cc feed-down on the
  observed J/y suppression pattern?? Needed a
  study of the nuclear dependence of cc production
  in p-A collisions

see NA60 HERA-B talksat Hard Probes 2008
9
Towards the future
Slide from talk at Quark Matter 2001
?

• Higher energies improve the signals but also
  increase the backgrounds !
• ? no replacement for systematic and careful
  studies
• Do not underestimate the importance of the
  baseline reference ? take accurate p-A and
  light ion data
• To improve our understanding, we need
  falsifiable models ! ? when a model is
  disproved by data, we have learned something
• Heavy ion physics is a complicated field ?
  to study the critical nature of QCD we must keep
  a critical mind

?
?
?
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Examples of critical questions
Do we have, after 20 years of high-energy
heavy-ion collisions, convincing evidence of
anomalous J/y suppression? Do we have a logical
case which convinces, beyond reasonable doubt,
people outside of our community? If so, is the
anomalous suppression smooth or step-wise?
With one or two steps? Is the y suppressed
from p-A to S-U/Pb-Pb because of newly produced
hadronic or deconfined matter? Is the cc not
suppressed in S-U while the y is? Do we really
understand the charmonia production mechanisms,
their formation times, and their interactions
with normal hadronic matter (in pp and p-A
collisions)? Should the Glauber formalism, with a
single sabs variable, describe J/y
absorption? What are the ?s, pT and xF
dependences of the normal nuclear absorption of
the J/y, y and cc states? What is the normal
nuclear absorption of the cc at 158 GeV? How can
we ensure that significant progress will be made
on these issues in the coming 10 years? Will the
LHC experiments solve all remaining open
questions?
Easy to get into trouble by asking such questions
!
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Since this talk is not a summary talk...
I will not try to summarise the many new results
presented at this conference I will just mention
a few points which deserve further emphasis

• The elementary pp and p-nucleus (or d-Au)
  collisions
• The pT dimension of the J/y absorption and
  suppression
• The y suppression

12
p-A data on low mass dimuons production
NA60 collected data in 2002,with Be, In and Pb
targetsMuch more statistics in 2004
M
see H. Wöhri talkat Hard Probes 2004
Already in p-nucleus collisions, f production is
enhancedwith respect to the w
13
The p-A input to the low mass dimuon analysis

• Not yet well known
• B(w ? mm)
• w form factor
• r-w interference
• etc
• How do we go from pp and p-A to A-A? What is
  the equivalent of Glauber for light meson
  production?
• Is there a normal nuclear enhancement of low
  mass dimuons? It will be with respect to that
  baseline that we will be able to separate the
  anomalous excess from the cold nuclear matter
  effects

see NA60 talkat Hard Probes 2008
14
The p-A input to the intermediate mass dimuon
analysis

• Not yet known
• Why is the charm cross-section derived from
  the NA50 p-A dimuon data two times higher than
  the world average from hadronic decays? ? Use
  CLEO or PDG form factors instead of Pythias (Y.
  Zhang)
• What is the real DY contribution at these low
  dimuon masses and how does it increase from pp
  to AA? ? Jianwei send us your code!
• Is there a normal nuclear enhancement of
  intermediate mass dimuons? It will be with
  respect to that baseline that we will be able to
  separate the anomalous excess (thermal
  dimuons?) from the cold nuclear matter effects

15
The p-A input to the understanding of the
quarkonia data
Lets open an historical parenthesis
16
The Matsui and Satz paper... 20 years ago
In 1986 J/y suppression is proposed as a signal
of the QCD phase transition from confined
hadronic matter to a deconfined partonic plasma
17
The J/y is suppressed in heavy-ion collisions...
NA38 finds J/y suppression with respect to the
dimuon continuum in the mass range 1.72.7 GeV
(very poor statistics of high mass dimuons) 1)
from p-U to O-U and S-U 2) in O-U and S-U, from
peripheral to central
ET gt 85 GeV
Note p-U and O-U data collected in 1986first
results shown in Moriond 87 and inQuark Matter
87... very fast data analysis!No attempt was
made to understand thephysics contributions to
the continuum
18
The p-A input to the understanding of the
quarkonia data
End of the historical parenthesis
19
20 years later...
Some good progress on the understanding of
p-nucleus collisions - NA50 collected much
better data, with up to 6 targets, at 400 and 450
GeV - Excellent statistics for the J/y and quite
reasonable for the y - The continuum is
understood as Drell-Yan dimuons and D
decays Some of the remaining problems - The
NA50 p-A data were collected at 400 and 450 GeV
not at the energy of the heavy-ion data (158
GeV) NA60 collected a very small p-A data
sample at 158 GeV - The cc nuclear dependence is
not yet known this is important because of its
feed-down to J/y New data exists analyses in
progress HERA-B at 920 GeV NA60 at 400 GeV
see NA60 HERA-B talksat Hard Probes 2008
20
The J/y is absorbed in p-nucleus collisions...
NA50 saw that the J/y is absorbed in p-A
collisions, with respect to the high-mass
Drell-Yan dimuons or in terms of production
cross-sections per target nucleon
Glauber calculation? exp(-r L sabs)
see G. Borges talkat Hard Probes 2004
The lines are the result of Glauber calculations,
assuming that the drop is due to final state
absorption of the charmonium state in the cold
nuclear matter it crosses
21
More from fixed-target p-A collisions
1015 times more data at 450 GeV
22
Feed-downs from the y and the cc
Which fractions of the J/y yields are due to
feed-down from y and cc decays?New results from
HERA-B
see HERA-B talkP. Faccioli
(7.0 0.2 0.4BRs)
7 of the J/y mesons observed by HERA-B are due
to y decays
23
Feed-downs from the y and the cc (cont.)
21 5 of the J/y mesons observed by HERA-B are
due to cc decays Lower than the 3040 values
coming from earlier data (inc. earlier HERA-B
data) Based on 1300 ccs reconstructed in the
dimuon channel lt10 of the full statistics!
see HERA-B talkP. Faccioli
Of the observed J/y mesons 7 are from y
decays, 20 from cc decays? more than 70 are
directly produced
24
Absorption of J/y production versus p-A 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
25
Absorption of J/y and y production versus xF and
pT
a strongly decreases at high xF Not clear why
this is so higher parton density? Why then the
J/y is not strongly absorbed at RHIC?
and it increases with pT(a la Cronin effect?)
26
If you have enough models... one should describe
the data...
Negative xF HERA-B data rule out all models of
cold matter J/y absorption We dont need A-A
data to find anomalies...
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

see HERA-B talkP. Faccioli
BK
Vogt final state absorption
HERA-Bpreliminary
E866 38.8 GeV Be/Fe/W E789 38.8
GeV Be/C/Cu/W E772 38.8 GeV H2/C/Ca/Fe/W NA50 29.1
GeV Be/Al/Cu/Ag/W NA3 22.9 GeV H2/Pt
!!!
xF
27
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 y dependence of the
measured absorption.
PHENIX
No final stateabsorptionsabs 0 mb
sabs 3 mb
Such a y-dependent effect is not expected to be
seen in the SPS p-A data
28
Whats known about the J/y dN/dy in SPS p-A
collisions?
The J/y y distributions are not centered at 0,
even for the p-Be collision system !Why is there
a significant change from pp to p-Be but not from
p-Be to p-W ?
NA50 measures dimuons within one unitof
rapidity, at around mid-rapidity
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 13 to 2050,
depending on the data set (target)
(NA50, to appear in Eur. J. Phys. C)
29
Nuclear effects on the PDFs and the J/y absorption
Is there room for (anti) shadowing at the SPS? Is
the p-A absorption extracted from p-A data
directly applicable to A-A? Not taken care of,
but absorption does a splendid job from pp to
peripheral Pb-Pb sabs at RHIC after removing
the nuclear effects on the PDFs assuming EKS98
1-3 mb sabs at SPS effective parameter,
convoluting absorption and nuclear PDFs 4.2
mb If the nuclear effects on the PDFs change
from 450 GeV to 158 GeV, the value extracted from
the 450 GeV p-A data is not exactly the value to
be used in the In-In and Pb-Pb studies We need to
extract sabs from the p-A data at 158 GeV ! (work
in progress) For now, we can make a rough
evaluation of the importance of this issue
see talk ofR. Granier de Cassagnac
30
Nuclear effects on the PDFs and the J/y
absorption (cont.)
For p-Pb collisions, the nuclear modification
factor is M(158) 1.06 M(450) 1.12
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 comparableto the PHENIX 1-3
mb values
Assuming the same sabs at 158 as at 450 GeV
M(158) exp(-r L sabs,real) 0.71 (instead of
0.75)
31
Nuclear effects on the PDFs from p-A to A-A
In Pb-Pb and In-In collisions, the anti-shadowing
counts twice (two nuclei)and it should increase
with centrality ? compensating for the stronger
sabs value ? Questions 1) Who believes the
EKS98 model at the percent level for the gluon
anti-shadowing? 2) How should we parameterize
EKS98 in A-A versus centrality? Remember
Eugene Wigner Give me two parameters and I
can fit an elephant, give me three and I make
its tail wiggle Can the centrality
dependence of the nuclear effects on the PDFs be
fixed by independent data? 3) How is all
this affected by the feed-down sources, which
have a higher sabs value?
32
What about the J/y transverse momentum dimension?
p-A 400 GeV
pp
ltpT2gtpp (GeV/c)2
ltpT2gt (GeV/c)2
J/y
pp
S-U 200 GeV
(NA50, submitted to EJPC)
Pb-Pb 158 GeV
L (fm)
?s (GeV)
J/y
The average pT of the J/y increases with the
thickness of colliding nuclear matter and with
the ?s of the collisions
Pb-Pb 158 GeV p-Pb 400 GeV
33
Absorption of J/y production versus pT in p-A
collisions
NA50
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...
34
The transverse momentum view of J/y suppression
(NA50, in preparation)
The J/y RCP strongly depends on pT (at the
SPS) Significantly different from what happens
with light hadrons... ? Only the low pT J/y
mesons get suppressed !
see talk ofC. Blume
35
No extra J/y suppression from p-nucleus to O-U
and S-U
The J/y yields 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,both in terms of integrated
absolute production cross-sectionsand in terms
of centrality dependence of the ratio y/DY
see G. Borges talkat Hard Probes 2004
But the J/y suppression pattern changes
significantly between S-U and Pb-Pb...
36
The y is suppressed from p-nucleus to S-U...
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
The change of slope looks very abrupt...The
sabs of the y increases from 450 to 158 GeV?
Maybe, but not from 8 to 20 mb...
37
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 isdue to the dissolution of the
bound ccstate by the QGP, Lattice QCD saysthat
this would indicate that Tc sitsin the most
peripheral S-U or Pb-Pbcollisions at SPS
energies... The y suppression
measurements deserve more attention... And have
the advantage of not being affected by feed-down
sources
y
38
y vs. J/y suppression from p-nucleus to S-U and
Pb-Pb
Are the y and J/y departures from the expected
normal nuclear absorptionoccurring at the same
energy densities? Should we ignore the S-U data
points? Asymmetric collision system, different
energy, different isospin corrections, etc
39
J/y suppression S-U vs. Pb-Pb vs. In-In

• S-U points seem to be 10 too high
• In-In and Pb-Pb patterns do not seem to be
  overlapping
• Important to get a Pb-Pb suppression pattern
  with much better accuracy

40
J/y suppression Pb-Pb vs. Pb-Pb

• Patterns vs. ET and EZDC are perfectly
  compatible
• The error bars must be taken seriously

see L. Kluberg talkat Hard Probes 2004
41
J/y suppression In-In vs. In-In
Who was complaining about the plateau in the
central In-In data? It was not easy... but after
a lot of checks and improvements, the plateau is
gone... A challenge to the in medium formation
modelsit works already at the SPS... in In-In
but not in Pb-Pb
42
J/y suppression the first 30 years
19861995 NA38 believes that the J/y
suppression pattern is consistent with
QGP formation in S-U collisions
19962005 NA50 believes that the J/y
suppression pattern is consistent with
QGP formation in Pb-Pb collisions
(but not in S-U collisions...)
Prediction for the next 10 years
20062015 NA60 believes that the J/y
suppression pattern is consistent with
QGP formation in In-In collisions
(but not in S-U or Pb-Pb
collisions...)
Are we nearly there yet ?
43
Back to the ideal world
Predicted patterns, before any data points were
available.? It must be very easy to discriminate
between the two!
J/y survival probability
normal nuclearabsorption
anomaloussuppression
We need new insight into how to rule out one of
these scenarios, or both
44
A new quarkonium state found at CERN the elmouton
In 1997, Helmut predicted the existence of a new
particle
The elmouton was now observed at CERN
The measurement was not expensive at all in
fact, it was quite sheep
45
Too much data?
ColorfulGrass ofCERN
skin
Several elmoutons have been seen, with different
masses, colors, spin... Can any of the models
describe all experimental data?
X
46
Theory versus data the no new physics model
normal nuclearabsorption
All data sets overlap, showing the expected
normal nuclear absorption pattern!
47
Theory versus data the new physics model
anomaloussuppression
All data sets overlap, showing the expected
anomalous suppression pattern!
48
A more detailed theoretical framework
direct J/ysuppression
y, ccsuppression
gluonanti-shadowing
B decays,shadowing
J/y survival probability
recombination
We need new and improved measurements over a
bigger range in energy density! A next generation
experiment must be established...
49
Improved experimental data
Once again, the model prediction (?) describes
the new measurements... There is a BIG difference
between the measurements are compatible
with...and the measurements show beyond
reasonable doubt that...
50
Hot and dense QCD Matter... matters
The study of high-energy heavy-ion collisions is
very difficult much more than the studies of
ee- or pp interactions. The measurements are
done in exceedingly difficult conditions (not
always valued) and often must (should) be redone
in significantly improved conditions. Remember
extraordinary claims require extraordinary
evidence or at least a second good look. There
are no miracles solid progress requires people,
effort... and beam time.
The End