1. SPS charm(onium) and bottom(onium) measurements
E. Scomparin
INFN Torino (Italy)
Introduction
Past heavy quark and quarkonium measurements: NA38/NA50 (Helios-3)
Present heavy quark and quarkonium measurements: NA60
What remains to be learned ?
Conclusions

2. Heavy quarkonia
Matsui and Satz prediction (1986) at the origin of the whole field
No experiment was explicitly intended for charmonia detection
Even NA38 (proposed in March 1985) was aiming at the study of
thermal dimuon production
Experimental facts
Relatively small cross section s=20 GeV, BµµJ/~10 nb)
J/µµ channel relatively clean
Need large luminosities and a very selective trigger
NA38 happened to be in a very good situation to study charmonium
(its ancestor, NA10, studied high mass Drell-Yan and  production)

3. Charmonium production: nuclear collisions at fixed target
The question to be answered by studying charmonium in heavy-ion
collisions at the SPS
Is (at least part of the) suppression of charmonia that we observe
in the data NOT due to usual hadronic processes ?
Study carried out by NA38/NA50/NA60 at the SPS from 1986 until today
Basic facts
Essentially the same experiment, although with very significant upgrades
Large set of results with very good statistics
(Lots of) systems studied, including:
p-p, p-d, p-Be, p-C, p-Al, p-Cu, p-Ag, p-W, p-Pb, p-U, O-Cu, O-U,
S-U, In-In, Pb-Pb
Similar (but not identical) energy/kinematical domain between various data sets
Very significant contributions (in a slightly higher energy range) by:
E866
HERA-B

4. The NA38/NA50/NA60 experiments
Based on the same muon spectrometer (inherited by NA10)
no apparatus-dependent systematics
MUON FILTER
BEAM TRACKER
TARGET
BOX
VERTEX
TELESCOPE
Dipole field 2.5 T
BEAM IC
not on scale
NA60
NA50
Many updates in the target region, in parallel with the availability of radiation hard detectors

5. pA collisions: the reference
Glauber fit to BµµJ/ at GeV
J/abs= 4.48  0.42 mb
Main problem:
extrapolation to 158 GeV/c
S-U data (200 GeV) should not be
used (absorption sources different
wrt pA might be present)
Obtain normalization (J/pp)
at 200 GeV
using only pA data
assuming J/abs does not
depend on s
High statistics 400/450 data: J//DY ratios
Obtain J/abs= 4.18  0.35 mb

6. Expected (J/)/DY at 158 GeV
As it is well known, NA50 uses Drell-Yan as a reference process
to study J/ suppression
Is (J/)/DY equivalent to J/ cross section per N-N collision ?
 Yes, Drell-Yan A-dependence measured
DY =  (stat.)  (syst.)
Start from J/ pp/DYpp @ 450 GeV (1.4% error)
Rescale to 200 GeV
J/  see previous page (7.8% error, SU not used)
DY  LO calculation (2.5 % error)
Rescale to 158 GeV
J/  fit a la Schuler to measured J/ cross sections (1.5% error)
DY  LO calculation (negligible error)
Use Glauber (with neutron halo) to calculate centrality dependence
of expected J/ /DY
Include experimental smearing on centrality determination (ET, EZDC, Nch)
Direct measurement of J/ /DY at 158 GeV would significantly
decrease such errors (NA60)

7. J/ /DY in Pb-Pb collisions at 158 GeV
Final NA50 set of data
Old reference
(include S-U in the determination)
New reference
(only p-A collisions are used)

8. Compatibility of data sets
Older data sets considered not as reliable as recent ones
1996 high statistic data: biased by reinteractions (thick target)

9. Study of various centrality estimators
Pattern consistent with ET-based analysis
Departure from normal nuclear absorption at mid-centrality
Suppression increases with centrality

10. What about S-U ? Absorption curve calculated using p-A data only
S-U data found to be in agreement
(once rescaling are performed) with
p-A extrapolation
Peripheral Pb-Pb collisions
No indication for a sizeable extra-absorption in S-U wrt p-A

11. New charmonium studies : NA60
Is the anomalous suppression also present in lighter nuclear systems?
Study collisions between other systems, such as Indium-Indium
Which is the variable driving the suppression?
S-U
In-In
Pb-Pb
Npart
L (fm)
pure Glauber calculation
Study the J/ suppression pattern
as a function of different
centrality variables, including
data from different collision
systems
What is the normal nuclear absorption
cross-section at the energy of the heavy
ion data?
Study J/ production in p-A collisions at 158 GeV
What is the impact of the c feed-down on the observed J/
suppression pattern?
Study the nuclear dependence of c production in p-A

12. NA60: In-In collisions Set A Set B
5-week long run in 2003 – 158 GeV/nucleon
~ 4×1012 ions on target
~ 2×108 dimuon triggers collected
Two muon spectrometer settings
Set A (low ACM current)
Good acceptance at low mass
Used for LMR and IMR analysis
Set B (high ACM current)
Good resolution at high mass
Used for J/ suppression,
together with set A
Set A
Set B
Raw +-
invariant mass
spectrum
mµµ (GeV/c2)
Events/50 MeV
Centrality selection: use
spectator energy in the ZDC
charged multiplicity in the vertex
spectrometer

13. The J/ / DY analysis (NA50-like)
Combinatorial background from 
and K decays estimated from the
measured like-sign pairs
(<3% contribution under the J/)
Signal mass shapes from MC
PYTHIA and GRV 94 LO p.d.f.
GEANT 3.21 for detector simulation
reconstructed as the measured
data
Acceptances from Monte Carlo
simulation:
J/ : 12.4 % (setB); 13.8 % (setA)
DY : 13.2 % (setB); 14.1 % (setA)
(in mass window 2.9–4.5 GeV)
J/y y’ DY
Background
Charm
without matching
6500 data set
no centrality selection
ho messo I valori senza matching
Multi-step fit
a) M > 4.2 GeV : normalize DY
b) 2.2 < M < 2.5 GeV: normalize the charm (with DY fixed)
c) 2.9 < M < 4.2 GeV: get the J/y yield (with DY & charm fixed)

14. Comparison with NA38/NA50 results
“anomalous suppression” present in Indium-Indium
Normal absorption curve based on the NA50 results
Uncertainty (~ 8%) at 158 GeV dominated by the extrapolation from
the 400 and 450 GeV data
How to get a more accurate suppression pattern ? Do not use Drell-Yan

15. Study of the J/ centrality distribution
Compare the centrality distribution of the
measured J/ sample with the distribution
expected in case of pure nuclear absorption
EZDC (GeV)
dNJ/y/dEZDC
Use matched J/ sample
Inefficiencies introduced by the cuts,
used in the event selection, affect in
a negligible way the J/ sample
(or are not centrality dependent)
εvertex dimuon > 99.5 %
εvertex
Main advantage
 Much smaller statistical errors
Main drawback
 No intrinsic normalization, if
absolute cross sections are not
known
Work in progress to obtain dJ//dEZDC

16. Comparison with expected yield
Data are compared with a calculated J/ centrality distribution
Use J/abs= 4.18  0.35 mb
Ratio (Measured / Expected)
normalized to the standard
analysis (~7% error)
Nuclear
absorption
dsigma Psi/dEZDC / dsigma DY /dEZDC
Onset of anomalous suppression in the range 80 < Npart < 100
Saturation at large Npart
EZDC(TeV)

17. Comparison with previous results
The S-U, In-In and Pb-Pb data points do not overlap
in the L variable
The J/ suppression patterns are in fair agreement in the Npart variable
S-U most central point ?

18. Other variables related to centrality
very preliminary
Bjorken energy density, estimated using VENUS
NA50 Pb-Pb
NA60 In-In
NA50 Pb-Pb
NA60 In-In
A more significant comparison requires Pb-Pb points with reduced errors
Work in progress inside NA50 to have a non-DY analysis for the 2000 data
 Results expected soon

19. Comparison with theoretical models
Good accuracy of NA60 data  quantitative comparisons possible
Consider models
formulating specific predictions for In-In collisions
previously tuned on the p-A, S-U and Pb-Pb
suppression patterns obtained by NA38 and NA50
J/ absorption by produced hadrons (comovers)
Capella and Ferreiro, hep-ph/
J/ suppression in the QGP and hadronic phases
(including thermal regeneration and in-medium properties of open
charm and charmonium states) Grandchamp, Rapp, Brown, Nucl.Phys. A715 (2003) 545;
Phys.Rev.Lett. 92 (2004) ; hep-ph/
c suppression by deconfined partons when geometrical
percolation sets in Digal, Fortunato and Satz, Eur.Phys.J.C32 (2004) 547.

20. Comparison with theoretical models
Satz, Digal, Fortunato
Rapp, Grandchamp, Brown
Capella, Ferreiro
No quantitative agreement with any model

21. One more model L. Maiani @ QM2005
Maximum hadronic absorption (Hagedorn gas)
not enough to reproduce In-In and Pb-Pb

22. Summary on charmonium at the SPS
Anomalous J/ suppression
Established fact in Pb-Pb (NA50) and, more recently, in In-In (NA60)
Not present in S-U collisions (NA38)
Onset around Npart = 100
Does S-U show an incompatibility with Pb-Pb and In-In ?
No final word from theory on the interpretation of the results
SPS+RHIC systematics  great opportunity
Other interesting results
Suppression concentrated at low pT in PbPb (see QM05)
Anomalous ’ suppression identical in S-U and Pb-Pb (vs L)
Already sets in for peripheral S-U collisions (see
News to be exepcted in the near future
NA50: non-DY analysis  more meaningful comparison with NA60
NA60: use full statistics for analysis  ~ factor 2 more

23. Can SPS go beyond charmonium ?
NA50 measured  A-dependence in p-A at 450 GeV
= 0.98  0.08
 production not accessible in A-A at present SPS, s too low

24. Bottomonium in A-A at the SPS ?
In the framework of the upgrade of CERN machines
the SPS+ concept is presently under discussion
 Availability of ~1 TeV protons from ~2014 onwards
Pb ions at ~ 400 GeV/nucleon
(s ~ 28 GeV)
Various possibilities: 
(3S)
b(2P)
(2S)
b(1P)
(1S)
Study J/ suppression vs. s (not possible at present SPS energies)
(2S)
c(1P)
J/
Study suppression of  states (depends on
available luminosity)
J/
Needs NA60 upgrade  first discussions are now taking place

25. Heavy quark production
Relatively comfortable cross section (tot~ 20 s=20 GeV)
However
D0  K
Difficult to single out in the high hadronic multiplicity
(attempt by NA49,no signal, nucl-ex/ )
D0  µX
Full reconstruction of the decay topology impossible
Important background (combinatorial+Drell-Yan)
Negligible contribution in the low-mass region
Sizeable contribution in the intermediate mass region
First studies by NA50, important progress with NA60
Pb-Pb
Npart=110
Npart=381

26. p-A shape analysis: m, y, pT, cos spectra
M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443
Dimuon differential distributions in the
region –0.5<yCM<0.5, cosCS<0.5
consistent with a superposition of
Drell-Yan + open charm
Absolute cross sections found to be consistent with direct measurements
of open charm production

27. Extrapolation to A-A collisions
Pb-Pb
Npart=110
Npart=381
Assumption: DY and open charm behave as hard processes A scaling
M.C. Abreu et al., NA50,
Eur. Phys. J C14(2000)443
Excess of dimuon yield: Data/Sources ~1.3 in S-U, ~1.7 in Pb-Pb
Smoothly growing with centrality
Enhancement of known sources
Nature of the excess
New sources appear

28. Enhancement of known sources
M.C. Abreu et al., NA50, Eur. Phys. J C14(2000)443
Factor 3 enhancement
in central Pb-Pb
Excess not compatible with background shape
Compatible with an an enhancement of open charm (m,pT spectra)

29. Thermal production?
R. Rapp and E. Shuryak,
Phys. Lett. B473(2000) 13
Explicit introduction of a
QGP phase
Initial temperature:
Ti=192 MeV
Critical temperature:
Tc=175 MeV
Fireball lifetime:
14 fm/c
(increasing to Ti=221 MeV
still good agreement)
L. Capelli et al.,NA50, Nucl. Phys. A698(2002) 539c
Good description of the mass spectra in the two approaches
for central Pb-Pb events
Only way to solve the puzzle: discriminate between
prompt and displaced dimuon sources

30. ! or NA60: detector concept Matching in coordinate and momentum space
hadron absorber
Muon
Other
and tracking
Muon trigger
magnetic field
Iron wall
2.5 T dipole magnet
NA50 spectrometer
beam tracker
vertex tracker
targets
Matching in coordinate and momentum space or !
Improved dimuon mass resolution
Origin of muons can be accurately determined

31. Compare slopes and momenta
Muon matching
Muons from muon spectrometer
Vertex spectrometer tracks
Compare slopes and momenta
Define a matching 2
Re-fit matched tracks
With this procedure
Combinatorial background can be reduced
A certain level of fake matches is present (new kind of background)
Vary the cut on the matching 2
improve the signal/background ratio

32. Vertex resolution z ~ 200 µm along beam axis Good target ID
(down to very peripheral events)
Dispersion between beam track and VT vertex
Vertex resolution (assuming sBT=20 mm) 10 20 30
 (m)
Number of tracks
x ~ y ~ µm
in the transverse direction
(by comparing beam impact
point on the target and
reconstructed interaction point)

33. Offset resolution Resolution of the impact J/ parameter of the track
Weighted Offset ()  100
Offset resolution (m)
Resolution of the impact
parameter of the track
at the vertex (offset)
40 – 50 µm
(studied using J/ events)
vertex  impact < c (D+ : 312 m, Do : 123 m)
Prompt dimuons can be separated from open charm decays
Define weighted offset  to eliminate momentum dependence of offset
resolution (offset wighted by error matrix of the fit)

34. Weighted offset distribution of the expected sources
Prompt contribution  average of the J/ and  measured offsets
Open charm contribution  MC distribution, after smearing

35. Background subtraction
Combinatorial background
Dominant dimuon source for m<2 GeV/c2
NA60 acceptance quite asymmetric 
Cannot use
Mixed event technique developed  accurate to ~ 1%
Fake matches background also rejected with a mixed event approach
Less important in the intermediate mass region
1% error in the
comb. background
estimate
10% error on
the signal

36. IMR: is an excess present ?
Open charm and Drell-Yan generated with PYTHIA
Drell-Yan normalization fixed using the high mass region
Open charm normalization: use
 NA50 p-A result (better control of systematics related to  channel)
 World-average cc cross section (based on direct charm measurements)
(differ by a factor ~ 2)
Excess
World-aver. norm.
Excess
NA50 norm.
data
prompt
charm
prompt+charm
Answer: Yes, an excess in the IMR is clearly present
(same order of magnitude of the NA50 result)

37. Is the excess compatible with the NA50 observation?
Can we describe the measured mass spectrum by leaving the open charm normalization as a free parameter, as done by NA50?
~ 2 in terms of NA50 p-A normalization
Results of fits reported in
terms of DY and open charm
scaling factors needed
to describe the data
Answer: Yes, we can describe the In-In data with a
“charm enhancement” factor around 2 in “NA50 units” (to be compared with ~ 3 for PbPb in NA50)

38. Check NA50 hypothesis using muon offsets
Fix the prompt contribution to the expected DY
Can the offset distribution be described with an enhanced charm yield?
Dimuon weighted offsets
Kinematical domain
1.2 < M < 2.7 GeV/c2 0 < yCM < 1 |cos| < 0.5
Answer: No, the fit fails
Charm is too flat to describe the remaining spectrum…

39. (and the charm yield is as expected from the NA50 p-A dimuon data)
Alternative options
Try to describe the offset distribution leaving both contributions free
Dimuon weighted offsets
Answer: Two times more prompts than the expected Drell-Yan provides a good fit
(and the charm yield is as expected from the NA50 p-A dimuon data)

40. Is the prompt yield sensitive to the charm level?
Fix the charm contribution to either of the two references, and see how the level of prompts changes
“world average”
“NA50 p-A mm”
Dimuon weighted offsets
Answer: No, both options require two times more prompts than the expected Drell-Yan !
(the charm contribution is too small to make a difference)

41. Mass shape of the excess
Fix the DY and Charm contributions to expected yields
The mass spectrum of the excess dimuons is steeper than DY (and flatter than Open Charm)

42. Centrality dependence of the excess
Relative excess: (Data – Sources) / Sources
very preliminary
Excess per participant:
(Data – Sources) / Npart
Faster than linear increase with Npart

43. Summary on open charm at the SPS
Serious study much delayed with respect to charmonia investigations
First generation experiments
Excess in the intermediate mass region
Connession with open charm possible (NA50)
Could not be proved
Second generation experiment (NA60)
Equipped with accurate vertex detector
Present understanding: open charm yield in A-A follows Ncoll scaling
What next ?
Update NA60 results (full statistics, more accurate alignment)
Run NA60 with PbPb (after 2010)
If the IMR excess is not charm, then what can it be ?

44. Conclusions Long and fascinating history (started 19 years ago!)
Many interesting results, both recent and (relatively) ancient
Still interesting now, when higher energy domains are opening up ?
Surely yes!
Finding a consistent
description of phenomena
occurring in various energy
ranges is an important
challenge, that deserves
being investigated
Future of heavy-ions at SPS ?
Still not defined, but
Heavy-ions can be available once LHC has been commissioned
SPS+ will be built in case LHC luminosity upgrade is approved
Some of us are starting to think about a new dimuon experiment at SPS
Encouragement, suggestions, participation are very welcome !